L-f i I 1

"echnische Universiteit :indhoven

Faculteit der

Technische Natuurkunde Vakgroep Deeltjesfysica, groep Plasma- en Atoomfysica

CLEANING OF IRON

ARCHAEOLOGICAL ARTEFACTS BY CASCADED ARC

PLASMA TREATMENT

Masters Thesis by

René Severens

Report number: VDF /NT 93 - 16

U nder supervision of

ir. Mark de Graaf and dr. ir. Richard van de Sanden

Research leader: prof. dr. ir. Daniël C. Schram

'In science, we resembie children, here and there picking up a pebble at the Coast of [( nowledge, while the vast Ocean of the Un­known extends itself before our eyes. '

John Newton (1725-1807)

Preface

Th ere is a peculiar attraction to investigating an exotic subject such as this ( and exotic it was, invalving the unlikely combination of electrochemistry, metallurgy, nuclear and surface analysis techniques, plasma physics and archaeology). In my quest for The Answer through the dungeons of the unknown, batteling my way through vast piles of hooks, I frequently encountered other players, on a quest of their own, who, after becoming enchanted with my mission, guided me on my path. Some even took me by the hand and walked with me. To them, I owe much.

First of all, I thank Mark, supervisor and companion, who came along all the way, keeping up good spirits with his unrelenting enthusiasm. I am very grateful for all the fruitful discussions I have had with Daan ( and they did not always concern Physics ), but most of all I appreciate the trust he has put in me. Richard has madesure that, before he left for sabatical, I got properly started.

Leo van IJzendoorn, who has taught me everything I know about HElS, was the first to join up, recognizing the fun of studying such an uncommon problem. He and Frans Munnik, the expert on PIXE and workstations, have not once denied a request for analysis time, even if it involved working overtime. Sjaak Beulens I thank for doing the XPS measurements at the SERC Daresbury laboratory, and for making sure I received all the data files.

Great appreciation and respect I have for Ries 'no problem' van de Sande, whom I suspect to already have the answer, the design already drawn up in his mind, when I'm still thinking of the problem. I thank Hans Dalderop for letting me use the XRD machine and helping me preparing the pellets, and also for keeping me informed about the latest developments in materials research.

I also wish to express my gratitude to the various corrosion experts for all their advice: Dr. Janssen and Mrs. Dr. Visscher of the chemica! engineering department, and Ir. Blekhorst of Hoogovens Research. Of course, I extend my appreciation to Henk Kars and Ronnie Meijers of the Rijksdienst voor Oudheidkundig Bodemonderzoek, who deposited the problem with us in the first place and providedus with some extra-ordinary objects.

Finally I thank all my friends, notably René, Roger, Edgar and Maarten, for their loyalty and companionship.

René Severens Eindhoven, August 1993

Abstract

Upon excavation of an iron object, the rate of corrosion is greatly increased by the ample supply of oxygen. For succesfull conservation it is, apart from the removal of the dirt crust consisting of soil compacted together with iron oxides, required tostop the corrosion process. A crucial step in achieving that goal is the removal of chloride infections.

By making use of a reducing atmosphere such as a hydrogen plasma (it being preferred over molecular hydrogen gas consiclering the higher chemical activity of the atoms ), it has been observed that the dirt crust becomes easily removable due toreduction of the oxides. Furthermore, the hydrogen can reduce iron chlorides to metallic iron.

The results of a study upon the mechanism of oxide reduction and chloride removal will he presented. For this, a number of samples (steel platelets originating from a shipwreck, heavily corroded platelets from contemporary steel, pellets pressed from powdered 15th­century coffin nails and pellets pressed from hydrated ferrous chloride) have been treated in an argon, hydrogen or argon/hydrogen mixture plasma, and in a vacuum oven. Their composition has been analyzed using HElS, PIXE, XPS and XRD.

It will he argued that the oxide reduction is a direct result of the chemical activity of the hydrogen, but that the bulk of the chlorides is removed by evaporation of volatile components, which are formed either by hydrolysis of soluble components or by thermal decomposition; only the removal of the last fraction ( typically 1%) may again he the result of hydrogen reduction.

Contents

1 Introduetion 6

2 Theory 8 2.1 Corrosion: an electrochemical proces 8

2.1.1 The reactions .... 8 2.1.2 Chloride infections 11

2.2 Hydrogen-invoked reduction 13 2.2.1 Reduction of oxides . 13 2.2.2 Reduction of chlorides 17

3 Experimental 19 3.1 Plasma reactor 19

3.1.1 Samples 19 3.2 Material Analysis 21

3.2.1 High Energy Ion Scattering (HElS) 21 3.2.2 Proton Induced X-ray Emission (PIXE) 22 3.2.3 X-ray Diffraction (XRD) ......... 22 3.2.4 X-ray Photoelectron Spectroscopy (XPS) . 23

4 Results 25 4.1 Characterization of untreated samples . 25 4.2 Oxygen reduction . 32 4.3 Chloride removal 35 4.4 Structural changes 37

5 Discussion 40 5.1 Oxygen removal . 40 5.2 Chloride removal 42 5.3 Structural changes 44

6 Conclusions and suggestions 45 6.1 Conclusions ......... 45 6.2 Suggestions for further research 45

4

6.3 Practical consequences and suggestions

Bibliography

A Inorganic compounds A.1 Narnes and crystallographic data A.2 The Fe-0 phase diagram A.3 The Fe-C phase diagram .

B Data for XPS, PIXE and HElS

C Calculation of activation energy

D On the influence of recirculation on the H 0 -density D.1 Observations ................. .

D .1.1 dissociation degree in the are . . . . D.l.2 dissociation degree in the expansion .

D.2 Discussion . D.3 Condusion .................. .

E Conservation and dechlorination techniques: a short review

5

46

47

50 50 51 52

53

54

56 56 56 56 57 60

61

Chapter 1

Introduetion

A major problem for conservation of iron excavations from soil, mud or water is that these objects suffer from fast corrosion. Especially the presence of chloride infections, appearing as yellowish dropiets on the surface of the metal, the so-called weeping, endaugers their stability. It is obvious that for conservation to he succesfull, the corrosion process must he stopped. The removal of chloride infections is a crucial step in achieving that goal.

There is a wide variety of conservation techniques currently being employed, ranging from storage in alkaline environment to electrolysis [1], the most popular chloride removal technique being the alkaline sulphite treatment, first proposed by North and Pearson [2]. The method, however, is quite laborious and takes 3 to 5 months; its details, including that of other techniques, are summarized in appendix E.

One of the techniques also investigated by North and Pearson is a heat treatment in a hydrogen gas atmosphere at 400 oe [3]. Reekoning that atomie hydrogen is chemically more active and can penetrate more easily into metals, Daniels et. al. succesfully used a hydrogen glow discharge at a gas temperature of less than 100 oe for cleaning silver artefacts [4]. Although their attempts to use a glow discharge for chloride removal from iron artefacts were unsuccesfull, the idea was picked up by Vepfek who used an RF glow discharge (27 MHz, 4 kW) at gas temperatures of 400°C [5] - [10]. His work has led to a commercial machine for conservation of iron archaeological artefacts, in which already over 14,000 objects have been treated.

The procedure currently employed by Vepfek consistsof two steps. First, the excavated objects are treated in a mixture of hydrogen and methane, resulting in a partial reduction of the oxides in the hard incrustation consisting of mainly silicates and calcium compacted together with migrated oxides. The latter becomes soft and can be easily removed by a scalpel. When compared to conventional laborious and brute crust removal techniques involving hard abrasives and sand blasting, more details of the original surface can he revealed after reduction in the plasma.

Secondly, the artefact is treated in a mixture of hydrogen, methane, nitrogen and argon to remove chlorides and to passivate the surface against postcorrosion by carburizing and nitriding. The pretreatment typically takes 2 h and the final treatment 20 h, which is significantly faster than the alkaline sulphite treatment. Final proteetion is clone by dipping

6

INTRODUCTION 7

in a molten aliphatic hydracarbon wax for 10 h, which can he polymerized by treatment in a N F3 plasma. The treated artefacts are subjected to 100% RH (relative humidity) and 45 oe for six to nine months: it is claimed that only 1-2% of the artefacts show postcorrosion associated with the presence of chlorides (the weeping).

Since it is not easy tooperatea stabie glow discharge when a voluminous metal object is to he treated (difficult RF coupling; possible arcing phenomena in the DC case), we decided to investigate treatment of iron excavations in a setup where the plasma is created outside of the treatment chamber: an expanding cascaded are plasma. This configuration has several other advantages: the souree features a large flux of atomie hydrogen, and in the expansion, where artefacts are to he treated, the electron temperature is low (between 0.15 and 0.4 eV), resulting in a 'gentle' treatment. Results of preliminary tests exposing various artefacts (compasses, knives, an axe) from various eras (ranging from Roman to 18th century) obtained from Rijksdienst voor Oudheidkundig Bodemonderzoek to the cascaded are flowing thermal plasma for only 20 minutes are published in [11].

To understand the essence of the problem it is necessary to have somebasic knowledge about the corrosion process. This is taken care of in chapter 2, which also gives a description of reduction mechanisms. Chapter 3 describes the experimental setup (i.e. plasma reactor and samples) and the analysis techniques used. Chapter 4 gives a resumé of the results that were obtained, and they are discussed in chapter 5. The conclusions and its implications are summarized in chapter 6. A review of dechlorination techniques currently employed is given in appendix E.

Chapter 2

Theory

The first part of this chapter gives information concerning the origin and the dangers of chloride infections. In the second part, the importance of several process parameters for hydrogen-invoked oxygen and chloride reduction is explained, and some bonuses and maluses of using atomie hydragen are mentioned. The chapter ends with explaining the functioning of chloride remaval due to hydralysis and evaporation.

2.1 Corrosion: an electrochemical proces

Definition: Charaderistic for the occurrence of corrosion is that at the surface of an elec­trically conductive material the anodic or oxidation reaction of a redox system takes place simultaneously with the cathadie or reduction reaction of another redox system, where the absolute value of the anodic or oxidation current is equal to the absolute value of the cathadie or reduction current. This implies that the net current for the total surface is equal to zero [14).

The rusting of iron ( the formation of a hydrated oxide in the presence of oxygen and water) must be distinguished from dry oxidation. The rate of oxidation decreases for increasing thickness of the oxide layer (e.g. layer thickness 16Á after a day and 35Á after a year). The rate of rusting, which is a specific type of corrosion, sametimes remains constant over very long periods. The cause of this difference is that dry oxidation forms a homogeneaus oxide layer over the surface that passivates and protects the metal against further oxidation. In the case of rusting however, the layer formed does not proteet the metal from further attack [15).

2.1.1 The reactions

When a metal, e.g. iron, is brought into contact with a liquid capable of containing metal ions, an equilibrium will occur [13)-[17):

8

E f

0

E t

~

-ff~~ ---------Eev- -------­.,.

---- • i

THEORY 9

Figure 2.1: Polarization curve: current densities for the forward and backward reactions as a function of potential. At the equilibrium or Nernst potential, the currents are balanced. The curve on the right is a Tafel-plot, giving the current densities on a log scale [13).

ox

Fe :;:::= Fe2+ + 2e­red

For non-noble metals, there is always a surplus of electrons in the metal. This gives rise toa potential difference between the (positive) solution and the (negative) metal: the Nernst or equilibrium potential (Fig. 2.1). The metal can however also serve as an active medium on which a redox system already present in the solution can react, e.g.:

red

0 2 + 2H2 0 + 4e- :;:::= 40H-ox

This redox system has a different equilibrium potential.

Gorrosion can only occur if the equilibrium potential of the redox system is more positive than of the metal-ion system.

The potent i al the met al (=conductor) will obtain, the corrosion poten ti al, will he between the two equilibrium potentials. For the redox system this implies a negative deviation from equilibrium potential, thus favoring the reduction reaction, whereas for the metal-ion system, there is a positive deviation from equilibrium potential, favoring the dissolution of the metal (Fig. 2.2).

Instead of an equilibrium we now have a stationary situation in which there is a con­tinuous reduction of oxygen, while an electrochemically equivalent amount of iron passes into solution, as long as there is iron and oxygen available, and a medium for the exchange of electrons and ions. The rate at which corrosion occurs, is given by the corrosion cur­rent which is determined by electron conductivity of the iron and corrosion products, ionic conductivity of the solution and diffusion of oxygen towards the active region.

E

• i j iar

,: ,.

E

• I I

E -------------...,:------------ev. r : · .

Eev,m

-i

. I 1kr,'

, /

I

-------~-No

"" .,."

" "/

/ ,

I• . I 1km'

I I

-1{-.J-"--------­Jl·: . I

."' I .,: I

icor log icor

-log i

THEORY 10

Figure 2.2: Polarization curve of a corrosion system. Eev,m represents the equilibrium potential of the metal-ion system, Eev,r that of the redox system. The total system is balanced at the corrosion potential Ecor. The corrosion ra te is determined by the corrosion current density icor [13].

In first instance, the reaction forms a uniform layer of ferrous hydroxide precipitate: Fe(OH)2,1 which is unstable in the preserree of oxygen. It oxydizes to ferric oxyhydroxide FeOOH, either in amorphous form or to-y- or /1-varieties for low and high chlorid content respectively [18]. /1-FeOOH, the most common type of rust in sea water, is usually referred to as akaganeite and can be written FeOOH · Clx with x ~0.05, whereas -y-FeOOH is called lepidocrocite which can be written 8FeOOH·FeOCl. A list of compounds mentioned throughout this report is given in appendix A.

After some time, the corrosion layer looses its homogeneity forming small pits at posi­tions where the local composition of the metal differs from the bulk material (metallurgie or structural differences or at interfaces between different metals ). Heterogerrei ties in the aqueous medium (local differences in pH, aereation, temperature or salt concentration) have the same effect. Ditfusion of oxygen into the pit is more diffi.cult than to the sur­rounding surface, causing the reduction of oxygen to take place preferably at the surface, which is balanced by an increased rate of metal dissalution in the pit. As the area of the arrodie pit is much smaller than the area of the surrounding cathodic surface, the small pit has to cope with the entire cathodic current, resulting in a high local rate of metal dissolution.

Near the cathodic surface, pH rises by production of hydroxide ions, whereas in the arrodie pit, pH drops due to hydrolysis [20]:

1 In chlorinated medium the precipitate formed is known as Green Rust.

Figure 2.3: Enhanced corrosion in a corrosion pit [14].

0

@)

o,

@

@ o,

9

o,

@ @) J o,

FeOH+ + H20 ~ Fe(OH)2 + H+

THEORY 11

I@ o,

I@

o, @

@ 0

o, @

I

The acid production will facilitate dissalution of iron and an autocatalytic process commences.

2.1.2 Chloride infections

However, a positive charge is built up inside the pit. For reasons of electroneutrality, ei­ther the ferrous cations will diffuse outward, or some anion must migrate into the pit. In neutral solutions (i.e. free of chloride anions ), either 0 H- will migrate into the pit, thereby neutralizing the acid, or a ferrous cation will diffuse outward thereby loosing the opportunity to further lower the pit pH. Ditfusion of H+ directly decreases the pH. In chloride containing solutions however, cz- anions will migrate into the pit (Fig. 2.3). In this case, there is no acid neutralization, so pit pH can become very low. The chloride ion concentration will build up until the tendency for inward flow under the potential gradient is balanced by the tendency for outward flow due to the concentration gradient produced. The concentration can rise to 4-6 Molar at which a pH of approximately 4.5 is reached. Furthermore it should be noted that oxygen is less soluble in concentrated salt solutions as present in the pit, so that the pitting effect (preference for dissolution of iron in the pit) is much more pronounced in a chloride containing environment. The well-known fast perforation of steel slate in marine coastal regions illustrates the hazards of the pitting effect in chloride containing environments.

THEORY 12

After excavation, the supply of oxygen is greatly increased. First of all, this oxygen is used to oxydize any remairring ferrous hydroxides and green rusts into ferric oxyhydroxides. Hereafter, corrosion will take place at an increased rate since the diffusion of oxygen is no longer the rate-limiting step.

The ferrous chloride in the pit undergoes a reaction which can be summarized :

In great a bundance of chlorides ( sea water), these are incorporated into the ferric oxy­hydroxide to form {3-FeOOH. The precipitation of {3-FeOOH causes local stress which could eventually cause fracture. The acid produced by the reaction will again enhance the dissolution of iron, thus autocatalyzing the proces. It can can only stop if all chlorides are consumed by the precipitation of {3- FeOOH. The pH in the pit drops until the solubility of {3-FeOOH becomes significant (pH<2) and ferric ions will remain in solution, giving rise to the weeping of the objects.

Perrous chloride is highly hygroscopic. Experimentally it has been observed that when FeCl2 • 4H20, the crystals that are formed by evaporation of the water from a solution offerrous chloride at room temperature, is mixed with iron powder and exposed to moist air, a brown solid ({3-FeOOH) and a yellow solution, containing ferrous, ferric and chlo­ride ions and having a pH of 0.4 - 0.9, are formed [22]. A relative humidity (RH) of only 20% is sufticient tostart the reaction. In a Dutch museum, humidity is typically 30-40% RH.

To summarize, the dangers of chloride infections lie in the facts that:

• they provide ionic conduction in any aqueous phase present, thus increasing the rate of corrosion [15]

• they will aid the dissolution of iron by formation of acid-containing pits [20]

• they cause an aqueous phase to exist already at 20% RH [22], whereas for clean steel adsorption of a water film does not occur below 60% RH [15]

Note that these warnings only concern soluble chlorides: if cz- ions are incorporated into {3-FeOOH, they are harmless. However, akaganeite ({3-FeOOH) is unstable with re­spect to goethite (a-FeOOH), which is in turn unstable with respect to hematite (Fe20 3 ),

although the latter is a slow proces. Conversion of {3-FeOOH releases cz- ions, although it remains uncertain whether or not the chloride concentration created in this way is sufReient to attract water from atmosphere to form a solution [1].

THEORY 13

2.2 Hydrogen-invoked reduction

2.2.1 Rednetion of oxides

the reactions

Hydrogen gas is commonly employed as a reduction agent for iron oxides, its reduction rate being typically four times that of carbon monoxide at equal gas pressure and temperature. The process starts at around 350°C. At 440°C its rate has increased by a factor of three, and above 500°C reduction occurs very rapidly [25]. The reactions involved are:

and, depending on the sample temperature,

T < 560° : Fe304 + 4H2

T > 560°: Fe304 + H2

FeO + H2

3Fe + 4H20

3Fe0 + H20

Fe+ H20

The reduction from hematite to magnetite (Fe30 4 ) to wüstite (actually iron deficient: Fe0.950) to iron can only proceed at a temperature above 560°C, wüstite being unstable below that temperature (The Fe-0 phase diagram is given in Appendix A).

The reduction of hematite to magnetite occurs irreversibly, all others are equilibrium reactions. The equilibrium constant I< is defined as being the ratio of the partial pressures of water and hydrogen: I<= [H20]/[H2]. lts dependenee on absolute temperature is, along with the molar reaction heat Q, stated in table 2.1. It is observed that the equilibrium constant increases when increasing temperature, thus enhancing reduction. Note that using a high pressure hydragen atmosphere would have to result in a high water vapour pressure, and the reduction must speed up to maintain it.

reaction log K I Q [kJ /Mole] I Q [eV] I Fe304 + H2 = 3Fe0 + H20 -2632/T + 2.68 48 0.5

FeO + H2 = Fe+ H20 -742/T + 0.44 16 0.16 0.25Fe30 4 + H2 = 0.75Fe + H20 -1510/T + 1.36 167 1.7

Table 2.1: Dependenee of the equilibrium constant I< on absolute temperature, and molar reaction heat Q at 650°C for the first, 700°C for the second and 500°C forthelast reaction [25], where it should be noted that the forward reaction is endothermic.

By continuously flowing hydrogen over the oxide and pumping off the produced water vapour, there is a continuous reduction since the equilibrium dictates that a certain partial pressure of water vapour must be maintained. In this case, the reduction does not proceed step-wise: the reaction product can contain all oxide phases, even up to metallic iron. If

THEORY 14

the size of the object to he treated is small and flow rates are high, the composition of the reduction gas is not influenced by evaporation of reaction products and it therefore does not change with time; machines in which such a situation exists are called differential reactors [38].

Even though the energy required to obtain completereduction from hematite to metal­lic iron is more than for a completereduction from magnetite to metallic iron, in practice, the former proceeds at a higher rate. The effect is due to formation of a highly porous magnetite intermediate2 thus increasing the active surface area [47). At low reduction tem­peratures ( <600°C), the porosity can even he maintained in the metallic phase3 [25].

Important parameters determining the rate of reduction are thus:

• Microstructure of the oxide (porosity and diffusion resistance)

• Gas pressure and temperature

• Gas flow

process sequence

The total reduction process can he summarized in a model of the reduction reaction se­quence in accordance with (38] and (46]:

The following subsidiary processes take place:

• Transport of hydragen across the boundary layer of gas that forms around the object

• Diffusion of hydragen through macro- or micropores to the reaction site

• Phase-boundary reaction, including adsorption of hydragen chemisorption, separa­tion of oxygen from the oxide lattice, the formation and growth of nuclei of reaction products, and finally desorption of water molecules from the solid surface

• Diffusion of water vapour through the macro- or micropores

• Transport of water vapour across the boundary layer

• The further growth of the layers of reaction products may involve reactions in the solid state and diffusion processes in the solid partners

2 According to X-ray densities, magnetite has a 5% lower volume than hematite, but on a macroscopie scale a volume increase of typically 10-15% is observed due to formation of long-stretched pores perpen­dicular to the reduction front, i.e. in the direction of the highest oxygen pressure gradient

3 When reducing iron oxide powders, the described process leads to so-called pyrophoric iron which reacts strongly with water and air: it is ignitable at room temperature. Pyrophorosity is lost by annealing at temperatures above 700°C.

THEORY 15

If the gas flow velocity exceeds 0.3 ms-I, diffusion across the boundary layer, its thick­ness decreasing with an increase of flow velocity, is no longer of influence on the reduction rate [37]. Diffusion through the object is governed by the partiele concentration gradient (of hydragen and water), the porosity factor "Y defined as the volume of the pores relative to the total volume of the object, and the labyrinth factor Ç, the latter dealing with the orientation of the pores relative to the diffusion pathandtheir branching and cross-linking. It has been derived that the diffusion constant is proportional to T 312 and inversely propor­tional to the pressure [38]. However, the porosity "'( and labyrinth factor Ç depend strongly on the amount of reduction, which, in its turn, again depends on temperature (reported values of "Y • Ç for reduction of hematite vary from 0.28 at 650°C to 0.51 at 950°C [38]).

The behaviour of the phase-boundary chemica! reaction (as described previously) as a function of temperature is, according to classic reaction kinetics (Arrhenius law), given by the activation energy:

where

k = rate constant of the reaction A = Arrhenius constant T = absolute temperature in K Ea = activation energy R = gas constant = 8.314510 J /mole K

For most reactions, especially when using limited temperature intervals, the pre-exponential factor n can be put to zero.

For the reduction of hematite (Fe 20 3 ) to iron with molecular hydrogen gas, reported values of the activation energy range from 50 kJ /Mole [38] to 68 kJ /Mole in more recent work [43]. Thus, determining the rate coefficient as a function of temperature in the experiment yields an activation energy for the overall process; comparison to the literature value might give information concerning the importance of a subsidiary process.

Atomie hydrogen

A parameter associated with the equilibrium constant is the standard Gibbs free energy change !:l.G0

, defined as: !:l.G = -RTlnK

R being the gas constant and T the absolute temperature, at unity activity for all par­ticipating reaetauts [40]. A negative !:l.G indicates that there is a loss of free energy and the reaction can proceed spontaneously, whereas a positive !:l.G implies that the reaction requires external energy input. For the oxygen reduction reactions, a negative !:l.G implies that the water vapour partial pressure is higher than the hydrogen partial pressure.

THEORY 16

(kcaUmole)

30

-30

-10

-80

-80

-100

1200 I

400 300 T(K)

-10

-20

-30

-40

-50

-60

-10

-80

Figure 2.4: Gibbs free energy changes of the reduction of iron oxides with molecu­lar and atomie hydragen [8].

Figure 2.5: Gibbs free energy changes of the reaction of iron and sodium chlorides with molecular and atomie hydragen [8].

Figure 2.4 shows the Gibbs free energy changes for several iron oxide reduction reac­tions, where it has to be noted that the curves invalving FeO are only valid at temperatures above 560°C. lmmediately it is seen that atomie hydragen is much more favorable for oxide reduction than molecular hydrogen4

•

However, at high gas pressures, the possibility of hydragen embrittlement exists.

hydrogen embrittlement- Penetration of atomie hydrogen into metal leads to the formation of molecular hydrogen in voids [14] (in steel, hydrogen can react with the carbon atoms to form methane). The hydrogen pressure in these voids can increase up to 100,000 bar, which can cause local deformation or even tot al destruction of the met al (hydrogen blistering), or lossof ductility and mechanica! strength (hydrogen embrittlement). The removal of carbon from steel, often occurring in water vapour-containing hydrogen at high temperature, also results in a loss of mechanica! strength. In some special cases hydrogen directly attacks one of the components of an alloy: the desintegration of oxygen-containing copper is a familiar example.

The effect also (and mainly) occurs during electrolysis reactions, in acid solutions and when heatinga metal object in a high pressure hydrogen environment. In the latter case, the effect is due to the increase in solubility of hydrogen in iron with temperature; after cooling the hydrogen will segregate and form small gas bubbles in the iron matrix [35], [36], [42].

4Thermodynamic tables on Fe203, Fe304 , FeO and H20 are given in [38]

THEORY 17

2.2.2 Rednetion of chlorides

In analogy to the oxygen reduction reactions, iron chloride reduction reactions are [25]:

2FeCl3 + H2

FeCh+ H2

2FeCl2 + 2HCl

Fe+2HCl

The reduction of ferric to ferrous chloride already starts at 100°C, and the reduction of ferrous chloride to iron at approximately 300°C. For the latter reaction, the equilibrium constant /{ =[HCI]2 /[H2] as a function of temperature is given in table 2.3. Again, the equilibrium constant rises with temperature, indicating that the reduction to iron is an endothermic process.

FeCl2 + H 2 = Fe+ 2HCl

Temp [0 C] 702 725 800 925 932 1005 /{ 0.064 0.11 0.297 0.767 0.813 1.63

Table 2.2: Dependenee of the equilibrium constant /{ on temperature [25].

Also the reduction of chlorides is more favorable when using atomie hydrogen, as is seen from the Gibbs free energy changes depicted in Fig. 2.5.

Properties of iron chlorides

I FeCl2 I FeC/3 I melting point [0 C] 670 306 evaporation point [0 C] 1023 317 evaporation heat [kJ /Mole] 134 23.6

Table 2.3: Some properties of iron chlorides [25]. Melting and evaporation point at 1 atm. pressure.

1. Perrous chloride - FeCl2 In the presence of oxygen, lawrencite can be oxidized into hematite and ferric chloride vapour (at a temperature above 317°C), following the reaction:

In the presence of water, hydralysis takes place following the reaction:

THEORY 18

A combination of both water and air can reduce lawrencite topure hematite and hydrochlo­ric acid vapour already at a temperature of 250°C (25).

Hydrated forms offerrous chloride crystals are FeCl2 • 2H2 0 (rokhunite) and FeCh · 4H20.

2. Ferric chloride - F eG 13

Molysite exists as a morromer (FeCl3) and as a dimer (Fe 2Cl6 ). In the preserree of water, it is lost through three possible channels:

2FeCl3 + 3H20

2FeCl3 + 2H20

2FeCl3 + 6H20

Fe203 + 6HCZ

2Fe0Cl + 4HCZ

2Fe(OH)3 + 6HCZ

the latter reaction only occurring at temperatures below 200°C. The iron oxychloride pro­duced by the second reaction decomposes into hematite and ferric chloride vapour5 at temperatures above 350°C:

this temperature decreasing for lower pressures (110°C at 1 mPa). Hydrated forms of ferric chloride crystals include FeCb · 2.5H20 and FeCl3 · 6H20

(hydromolysi te).

3. /3,"'- FeOOH Akaganeite, /3-FeOOH, has the composition FeOOH, Clx with x :::;0.05 and lepidocrocite, "'- FeOOH, has the composition 8Fe00H, FeOCl (18). It could be expected that (al­though this is not yet confirmed in literature), upon dehydration, the chloride is released in for example an FeOCl intermediate:

At temperatures above 350°C, the FeOCl decomposes and the chloride is removed.

5There is also decomposition 2FeCls-+ 2FeCl2 + C/2 , but the chlorine vapour pressure produced by this reaction is two orders of magnitude lower than the FeC/3 vapour pressure caused by evaporation and sublimation.

Chapter 3

Experiment al

The first part of this chapter gives information concerning the plasma reactor and the samples that were used. Hereafter, the techniques used for characterization are explained.

3.1 Plasma reactor

The plasma souree is a flowing thermal plasma in a cascaded are, with three cathocles at the upstream side, a stack of electrically floating copper cascade plates and an anode nozzle. The latter also serves as the conneetion to the vacuum-pumped treatment cham­ber (Fig. 3.1 ), a water-cooled steel cylindrical vessel ha ving a length of 40cm and 40cm diameter. The pressure and voltage drop at each plate can he monitored, as well as the heat transfer from the plasma to the cascade plate walls. As also the input energy is known, the efficiency of the energy conversion to the outflowing plasma can he obtained ( TJ= 30-40 %) . As this energy is preferably carried in the chemical dissociation of hydrogen gas, also the dissociation degree can he determined. In the conditions used for treatment, the dissociation degree in the souree is always near 100%: the emanent hydrogen atom flux can increase to values over 100 A equivalent (~ 1021 H 0 /sec) [11). The dissociation degree in the center of the expansion chamber, where during treatment the sampleholder is positioned, has been determined by means of RF excited active actinometry for a full hydrogen plasma. It is found to he 5-10 % typically. The loss in dissociation degree can he explained by adsorption of hydrogen atoms to the wall and desorption of hydrogen molecules, combined with astrong recirculation flow and mixing (appendix D).

3.1.1 Samples

For analysis, it is required that the area of the samples does not exceed 3x3 cm2 , and their thickness should not exceed 3mm. Sample surface should he as flat as possible and it should also he homogeneous.

Artificially rusted steel platelets, either by immersion in salt water for a few weeks or by daily sprinkling salt water over the metal surface positioned under a slanted angle in

19

expansion chamber

cascaded are

to pumps

EXPERIMENTAL 20

thermo­couple

Figure 3.1: The experimental set up: cascaded are plasma source, low pressure expansion and heated sample holder

the open air, show a homogeneaus corrosion layer of roughly 100 f1m. Unfortunately, the corrosion layer chips off from the underlying metal when exposed to high temperature (such as in the plasma), rendering these samples useless for the experiment.

shipwreck- A 1 mm thick plate originating from a shipwreck provided excellent sam­ples for the experiment. Unfortunately, the area of the plate was only sufficient for a few samples and therefore could not offer the possibility of variation of several treatment parameters.

standard - Heavily corroded steel platelets ha ving an area of 3 x 3 cm2, a tot al thickness

of approx. 3 mm and a rust layer thickness of approx. 1 mm, were cut from a corner profile. These samples, which are flat, temperature resistant and readily available, will henceforth he referred to as standard.

nails and pellets - Furthermore we have used nails from 15th century coffins, of which a large quantity was supplied by ROB. These nails consist of a thin inner core of metallic iron surrounded by a layer with high chloride content, which is again surrounded by a hard layer of magnetite (Fe30 4 ). The outer crust consists of soil (mainly Si02 ) compacted together with migrated oxides (haematite, goethite). Since these nails are neither flat nor homogeneous, they are crushed, pulverized and pressed into pellets for chemical analysis.

ferrous chloride- Finally tests have been clone on pellets pressed from FeCl2 • 4H20.

sample holder - The sample holder consists of a copper plate thermally insulated from the steel arm connecting it to the vessel wall by boron nitride blocks. It is positioned under a slanted angle (30 degrees with respect to vessel axis) so as to have neither shadowing of the sample nor obstruction of the gas flow. A ceramic heating element has been fixed to the backside of the plate. A flat sample can he clamped on top of the plate, a thin sheet

Weeping due to chloride infec­tions ( magnification 20 x )

EXPERIME:'\TAL

Treatment of an ancient knife m a hydragen plasma

Sample holder in a pure argon plasma jet

Figure 3.2: Simulated 2MeV He++ HElS spectrum (Rutherford regime), of a silicon substrate with a SiW top layer. The position marked W corresponds to surface scattering from W­atoms, the position marked Si to surface scattering from Si-atoms. At the layer interface (marked int), it is observed that the Si­concentration increases.

EXPERIMENTAL 21

Energy (MeV) 0.0 0.5 1.0 1.5 2.0 2.5

1 00 ,--------,.----,------,----,------,

D (!)

80

>= 60 D (!)

.':::' 0 40 § 0 z

20

0

-Simulction of Si-W/Si

~4 OoOoO

Si-W oQoOo . Ooooo Int -oooooo Si ooooooo'Wo

1nt

l Si

l 200 400 600 800

Channel 1000

of boron nitride insulating it from the electrically conductive copper; pellets are fixed using a thin metal ring. Sample temperature is monitored using a chromel-alumel thermocouple. Wh en treating nails, they are fixed to a steel pin inserted into the plasma jet. No additional heating is used. Typical operation conditions during treatment are:

Ar-flow = 0 .. 3.5 SLM H2-flow = 0 .. 3.5 SLM P-:hamber = 0.4 .. 2 mbar

Ia re = 30 .. 75 A Pare = 2 .. 12 kW

3.2 Material Analysis

Tsample

Ttreat

3.2.1 High Energy Ion Scattering (HElS)

= 330 .. 560 oe = 10 . .45 min

For material analysis, two techniques are used employing high energy particles created in a 30 Me V Philips AVF cyclotron. An oxygen depth profile is obtained using High Energy Ion Scattering (REIS). Here, a beam of mono-energetic a-particles ( current approx. 50 nA) having an energy of 8.8 MeV is directed towards the sample (one HEIS-measurement has been clone using 3 MeV protons). The energy spectrum of the scattered ions yields the sample composition: by collision with a light element the projectile looses a large fraction of its energy, and collision with a heavy element causes a small energy decrease [27] ( see Fig. 3.2 as an example). The energy loss of a partiele during passage through the matrix is, for given matrix composition, determined by its path length in the sample material, which is proportional to the depth. A depth scale can thus directly be attributed to the energy scale. The concentration of element A can be determined from the height of the step corresponding to scattering from A, provided the collisional cross section for the collision a ---+ A is known. In standard Rutherford Backscattering Speetrometry (RBS), this cross section can be calculated from the Rutherford formula, but at high energies (for oxygen more than 2.5 MeV) also nucleus interactions, described by the Yukawa- potential, play a role. Now, the ratio of collisional cross sections for iron and oxygen is calibrated using

EXPERIMENTAL 22

a magnetite (Fe30 4 ) single crystal [29]. The advantage of using high energies is that it permits the use of resonant scattering: the sensitivity for oxygen is improved by a factor of 30 when using 8.8 MeV a-projectiles.

The range of this technique is limited to approx. 10 pm, and the detection limit for chlorine is roughly 5% atomie concentration for the samples we used.

3.2.2 Proton Induced X-ray Emission (PIXE)

A better sensitivity for chlorine is obtained using Proton lnduced X-ray Emission (PIXE): the detection limit is in the order of parts per million mass concentration [30]. Contrary to RBS, PIXE gives no depth information but an average over the top 5-10 pm.

The target is bombarded wi th a proton beam ( current approx. 100 pA) of 3 Me V energy. These protons cause a.o. inner shell electrans (K or L) to be removed from the target atoms. An electron from a higher shell will fill up the vacancy. The energy thus released is characteristic for the element and can either be used to eject an Auger­electron from the atom, or to emit an X-ray photon. By measuring the energy of the X-ray photon, the identity of the element is obtained. From the peak surface the concentration can he calculated. The program that was developed in the group for Nuclear Analysis Techniques contains a data base in which ionization cross sections, fiuorescence yields, branching ratios, proton stopping powers and X-ray absorption coefficients for all elements are stored. Sample composition is determined self-consistently: from an initial estimated matrix composition, stopping powers and absorption coefficients are calculated. With these values, sample composition is calculated from the measured data. If there is a discrepancy with the estimated composition used for calculation, the new composition is used to calculate stopping powers and absorption coefficients, and so on. The most crucial limitation of PIXE is that light elements (Z~ll) are very difficult to detect, the fiuorescence yield becoming practically zero in favor of the Auger process.

The name of the X-ray line is determined by the shell where the vacancy is created and by the shell from which the de-exciting electron originates. An electron de-exciting to the innermost shell causes K-radiation to be emitted, de-excitation to the n=2 level results in L-radiation, to n=3 M-radiation etc. An index denotes the origin of the de-exciting electron: de-excitation from the L-shell to the K-shell is known as K0 -radiation, and from the M-shell Kp. Multiplet splitting (if observed) gives an extra index: the transition 2p3; 2 ---+ lso is denoted Kal and Ka2 represents the transition 2p1; 2 ---+ lso.

3.2.3 X-ray Diffraction (XRD)

For finding out whether or not specific crystallites are present in a sample, X-ray diffraction is used. At specific angles between the surface and an incident beam of X-rays, there are maxima in the intensity of the scattered X-rays due to interference. The relation between wavelength À of the incident X-ray, the crystal interplanar distance d and the angle of

EXPERIMENTAL 23

diffraction () is, in the case of diffraction, given by Bragg's Law:

2dsin() = n>.,

n being an integer (in practice, n=1 as the probability for second order diffraction is much lower than for first order diffraction). All crystals have characteristic interplanar distances: they will appear as a fingerprint in the XRD spectrum, which can he compared to a standard library [19). Note that the more planes interact in the interference, the stricter the interference condition will he met: decrease of peak width is an indication for increase in crystallite size.

X-rays are produced by electron bombardment (current approx. 40 mA) of a copper anode which has a 50 kV higher potential than a heated filament cathode. The Cu-Ka emission thus obtained has a wavelengthof 1.54059Á(Ka1 ) and 1.54439Á(Ka2 ).

3.2.4 X-ray Photoelectron Spectroscopy (XPS)

Surface analysis by X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spec­troscopy for Chemica! Analysis (ESCA), uses soft X-rays (for this report Al-Ka x-rays were used, having an energy of 1486.6 eV; the beam intensity was 2.8 kW) to create ionization in the sample [31). The emitted electrons have kinetic energies given by:

T = hv- BE- cl>s

where hv is the photon energy, BE is the binding energy of the atomie orbital from which the electron originates, and cl> s is the spectrometer work function: measuring the electron kinetic energy directly yields orbital binding energy. In first approximation the binding energy is characteristic for a particular atom, and thus it is possible to determine element concentrations from the peak areas in an XPS spectrum. Since the binding energy of an orbital is determined not only by the atomie nucleus but also by the contiguration in which the atom is incorporated, it is also possible to draw conclusions concerning the chemica! state of the atom, i.e. to distinguish e.g. Si in metallic silicon (BE= 99.6 eV) from Si incorporated in quartz, Si02 (BE= 103.4 eV).

Although the range of the X-rays (thus of ionization) in a solid is several microns, that of the electrons is of the order of several nanometers. Only those electrons that originate within a few nanometers from the solid surface can leave the surface without energy loss, and it is these electrons that produce the peaks in the spectra. Those that undergo loss processes before emerging form the background. In XPS it is customary to use spectroscopie nomenclature, i.e. ionization of the 2p3; 2 orbital (n=2, 1=1, j=3/2) of iron gives an Fe2p3; 2 XPS peak.

Positive charging of an electrically insulating sample by the removal of electrons causes the apparent binding energy to increase: the emerging electrons must also overcome the sample bias. This effect can he reduced by adding slow electrons to the vacuum space with an adjacent neutralizer; excessive compensation can however shift the apparent binding energy to lower values, as has been observed in the measurements. In this report, the energy

EXPERIMENTAL 24

scale was calibrated on the carbon C1s line: most commercial XPS vacuum chambers make use of an oil diffusion pump, which leads to hydracarbon impurities on the sample surface. The C1s peak position associated with the adsorbate impurity is reported in literature to he 284.8±0.2 eV [31], 284.6 eV [32] and 285±0.15 eV [33]. For our work, a value of 284.9 eV was chosen, the measured C1s peak position being 281.9 eV thus resulting in a shift of the energy scale of 3 eV.

name type typ. range typ. area characteristics HElS ion in, ion out 10 J1m 5 mm2 depth resolution PIXE ion in, x-ray out 10 J1m 15 mm2 sensitive; no Z:::;ll XPS x-ray in, electron out 3 nm 1 cm2 chemica! information XRD x-ray in, x-ray out 10 J1m 1 cm2 only crystallites

Table 3.1: Summary of the material analysis techniques used for this report

Chapter 4

Results

The obtained experimental results are divided into four parts. First, the untreated samples are characterized, yielding their composition to he amorphous ( or poorly crystalline) iron oxide hydroxide with several impurities. The second part mentions the observation of dehydration and of oxide reduction in the preserree of hydrogen, for which sample temperature was found to he important. The third part compares the chloride remaval effectivity of the hydragen plasma treatment to reference treatments in pure argon and a vacuum oven, and the last part concerns observed changes in the mechanica! strengthand appearance of an object due to treatment.

4.1 Characterization of untreated samples

FeOOH

HElS - Fig. 4.1 gives the measured HElS-spectra for the sample originating from a shipwreck, the untreated specimen denoted as 1. The spectra have been normalized with respect to the iron stepheight. Concerning depth profiling, it should he noted that, for the iron concentration, the surface is at the position marked Fe, corresponding with the backscatter energy of an 8.8 Me V a:-particle from an Fe-atom without additional stopping. The depth profile for oxygen starts at 0 and extends to the left of the figure while going deeper into the layer. The bumps on the oxygen step stem from the peculiar behaviour of the collisional cross section for a: --toxygen collisions as a function of energy, due to nucleus-nucleus interactions (for more details, see [28]).

After calibration on Fe30 4 the oxygenjiron ratio was determined to he 2.3±0.15. For the standard sample (spectrum is shown later) this ratio was found to he 2.1±0.1. This leads to believe that the original composition is mainly F eOO H ( either goethite, aka­ganeite, lepidocrocite, amorphous or a combination of these), as expected.

XPS - Fig. 4.2 gives the Fe2p peaks (Fe2p3; 2 centraid at 711 e V, Fe2p1; 2 centraid at 724.4 eV) measured with high-resalution XPS. When camparing this to experimental data from [31] (Fig. 4.3), it is seen that both FeOOH (Fe2p3; 2 at 711.6 eV) and Fe20 3 (Fe2p3; 2

25

Figure 4.1: HElS-spectra of ship­wreck samples, untreated and 30 minutes treatment.

30

25

C'-l ..... 20 c :::s 0 u 0 - 15 ·-~

10

Binding Energy [eV]

5 740 730 720 710 700

Figure 4.2: Measured XPS iron 2p peaks.

RESULTS 26

Energy (MeV)

25or------,2 ______ ,4 ______ ;5-----.

- shipwreck plate untreoted 1 -shipwreck plote oven 2

20 -shipwreck plote H2-plasmo 3

" 11>

>= 15

" 11> N

0 '0 § 0 z

5 e e .. 2 oFe

0~, ~-.---.---,--,---,---~~

0 100 200 300 400 500 600 700 Channel

, ... ·::·.··'•'·'· •.-.; .... :.- '.'~:::;.:.:~--.::··~:.-:·.-..., ... { :·,., .... ~·.;'<~-ill

... ) . ..-=,· .. .;::/~· ·.

.::_ .. ~'i~--..:~.=-"~··.

~.

:._ (c)Fe3

o4

.. ... : :-.· ..

:..:-

:- (d) FetCOOH)2 ~;.

~ .. ~ 718 714 710 706

- BINDING ENERGY (eV)

Figure 4.3: XPS iron 2p3/ 2 peaks for some typical iron corrosion compounds (31].

at 710.8 eV) have the same shape as the observed curve, including the so-called shake-up satellite peak at 719 eV. Here, the ion that remains after X-ray ionization endsin an excited state, reducing the energy of the Auger electron, which leads toanapparent binding energy that is higher than the original peak. Data from (33] reveals that et.-Fe20 3 is also ruled out, as this shows splitting of the Fe2p3/ 2 peak; 1-Fe20 3 still remains a possibility. From comparison of the 01s peaks (Fig. 4.4; there are two peak values: one at 530 eV and the other at 531.3 eV) to the oxygen data from (33] (Fig. 4.5), it is seen that there is a match with the FeOOH curve, where difference in binding energy of the oxide (530 eV) and the hydroxide (531.3 eV) causes splitting of the 01s peak. The condusion drawn from the HElS measurements that the composition is mainly FeOOH is thus confirmed.

50

40

!l 30 § § 32 20

10

0 534

Figure 4.4: pea.ks.

o2-

untreated standard

532 530 528 Binding Energy [eV]

Measured XPS oxygen ls

RESULTS 27

534 532 530 528 -BINDING ENERGY leV)

Figure 4.5: XPS oxygen ls peaks for FeOOH [33].

XRD - X-ray diffraction measurements on untreated samples (powdered top layer as well as unpowdered platelets) show no peaks at all for the untreated shipwreck and standard samples, leading to the belief that the iron oxide hydroxide is amorphous [34]. Strong peaks associated with Si02 were observed when analyzing the powdered crust of 15th century nails (Fig. 4.6), which is, given the fact that these nails are excavated from soil, what can he expected.

20

ö êii

ö êii

40 angle (28)

ö êii

XRD spectra nail ernst + =y-F~03

60 80

Figure 4.6: Measured XRD spectra of crusts of nails before and after 20 mms. plasma trea.tment.

2 " :J 0 u

10'

10'

10'

10'

10'

10'

o 4 6 8 10 a X-ray energy [keV]

Figure 4.7: PIXE spectrum of the un­treated standard sample.

Impurities

RESULTS 28 10'

10'

10'

10'

10'

10°

0 3 4 5 8 X-ray energy [keV]

Figure 4.8: PIXE spectrum of the un-treated pellet from nails.

PIXE - Fig. 4. 7 gives the PIXE spectrum of the untreated standard sample and Fig. 4.8 of the pellet. The figures show apart from the raw counts, also the fitted peak shapes and the fitted background. The Fe-Ka and Fe-K,s lines can clearly he distinguished. Note also the presence of the so-called escape peaks: they are caused by ionization of a silicon atom in the detector crystal by an incoming X-ray photon. When the X-ray photori emitted by the ionized silicon atom escapes from the crystal, the energy at which the original photon is detected is reduced by the charaderistic silicon X-ray energy. For quantitative analysis, the counts in the escape peaks are added to the counts in the primary peaks.

The element concentrations calculated from these spectra are given in table 4.1, giving the atomie concentration normalized to iron, both in parts per thousand.

I element I standard I shipwreck I pellet I Al - - 45 Si 5 5 620 p - 4 60 s 5 10 15 Cl 50 150 15 K 2 - 6 Ca - - 25 Mn 5 5 0.2 Fe 1000 1000 1000

Table 4.1: Atomie element-to-iron ratio for the untreated samples calculated from PIXE

spectra; all concentrations in parts per thousand (0.1% ).

For the standard samples, being the largest statistica! population ( consisting of in total 24 analyzed specimen), cross correlations are made between the several element concentra­tions as determined with PIXE. All but one of these specimen have been treated either in a plasma or in an oven. If there is correlation between two elements, the different atoms

RESULTS 29

are either bound in the same molecule, or their respective molecules are affected by the treatment in a similar manner. The absence of correlation between two elements provides us with an argument to state that these two elements are not part of the same compound, e.g.: I< and Cl are not correlated, therefore the presence of I< Cl is unlikely. The calculated correlation coefficients are given in table 4.2 using the PIXE element-to-iron ratios. It must he mentioned that the manganese-to-iron ratio remains practically constant, indicating that manganese is a constituent of the steel alloy. Correlation with sample temperature during treatment is also stated in the table; the information will he of use later in this chapter. In the table it is seen that the only element correlation of significanee is between chlorine and sulphur; a correlation plot is shown in Fig. 4.9.

I Si I s I Cl I K I Ca I Mn I temp I Si 1.00 0.00 0.00 0.00 0.44 0.00 0.00 s 0.00 1.00 0.65 0.00 0.01 0.09 0.53 Cl 0.00 0.65 1.00 0.03 0.00 0.01 0.76 K 0.00 0.00 0.03 1.00 0.07 0.18 0.07 Ca 0.44 0.01 0.00 0.07 1.00 0.00 0.01 Mn 0.00 0.09 0.01 0.18 0.00 1.00 0.06 temp 0.00 0.53 0.76 0.07 0.01 0.06 1.00

Table 4.2: Correlation coefficients R2 for the standard sample population (24 specimen) using the element-to-iron ratios obtained with PIXE. Also, correlation with sample tem­perature during treatment, if any, is stated.

10

2

1

0.2

0.1 •

w-1

•

• • •• • • •

• • • •

• • ... • • •

p2=0.65

101

: I

Cl

Figure 4.9: Correlation plot for sulphur and chlorine ( standard samples)

Energy (MeV) 0.0 0.5 1 .0 1 .5 2.0 2.5 3.0

14

12

::"2 10 Q)

~ '() 8 Q)

-~ 0 6 §

:IÈ. 4

2

0 100 200 300 400 500 600 700 Channel

Figure 4.10: 4 MeV He++ HElS spectrum of the untreated shipwreck sample. The smooth curve is a simulation for Fe02Cl0 .2

and the depth scale is in J.Lm.

The chlorine is distributed homogeneously through the layer, as can heseen from a 4 MeV o-particle HElS depth profile of the untreated shipwreck sample (Fig. 4.10).

tl.) ..... c s g -:;;:

80

60

40

20

0

RESULTS 30

Ols

XPS scan

- nail crost

1000

Fe2s+ Fe-~M23M45 f Fe-~M45~s

+ Fe2p

800 600

Cis

Ca2s NIs Ca2p + +• + 400

Binding Energy [ e V]

Figure 4.11: XPS scan of (untreated) nail crust.

200 0

XPS - chemica! info. Fig. 4.11 gives an XPS scan of nail crust. It is customary to use a decreasing binding energy scale ( corresponding with an increasing photo-electron kinetic energy scale). Photo-elektron peaks are indicated in the spectroscopie XPS notation (e.g. 01s or Fe2p3; 2), whereas Auger peaks, due to X-ray induced Auger emission, are indicated in the historica! X-ray notation. The latter designates states with n= 1, 2, 3, 4, ... as K, L, M, N, ... respectively, while states with various combinations of 1= 0, 1, 2, 3, ... and j= 1/2, 3/2, 5/2,... are given suffi.xes 1, 2, 3, 4,... etc. The first letter denotes the vacant orbital, the second denotes the origin of the de-exciting electron and the third the origin of the Auger electron: O-KL1L23 is the Auger peak associated with a transition from the 02s orbital to a vacancy in the 01s orbital, where the 02p orbital emits an Auger electron.

For a quantitative estimation of the element composition, the seperate peak profiles have been measured using high resolution XPS. Peak area has been calculated assuming a linear background. The results are given in table 4.4.

For comparison, table 4.5 gives the typical gangue ( =impurity) composition in natural iron ore, in weight percentages, according to [41] and [44].

From the XPS peak positions, it is possible to distinguish different chemica! configura­tions. An attempt has been made to determine the chemical composition of the samples using peak position data compiled in [31] and [32]; the result is shown in table 4.3. Also stated are the chemica! compositions of typical iron ore gangue compounds.

RESULTS 31

line stndrd pellet candidates gangue

Cis 284.9 284.9 The peak at 284.9 eV originates from the hydracarbon adsorbate due to the -286.2 286.2 use of an oil ditfusion pump in order to maintain the vacuum. The peak at 288.7 288.7 288.7 eV is the corresponding shake-up satellite as described by Briggs. The

peak at 286.2 eV is not caused by C02 (291.9 eV), but probably by some iron carbide (metal carbides typically 286-288 eV) due to segregation of carbon. It should be noted that this peak only appears in the spectra of samples ha ving a black surface (af ter heat treatment).

Nis 398.5 400.3 The peak at 398.5 eV is prominently present in the sample that was treated -400.3 in a vacuum oven employing a liquid nitrogen oil trap, but it appears only

in this sample. Due to lack of data, the corresponding chemica! composition is unknown. The peak at 400.3 e V, which is prominently present in the pellets, could be caused by a NH2-configuration{400 eV) or perhaps Na2N202 {400.2), but not by some nitrous oxide (>404 eV)

Ois 530 530 The peaks at 530 and 531.3 eV are due to FeOOH, being the oxide and the var 531.3 531.3 hydroxide respectively, as explained before. The peak at 532.6 eV is due to

532.6 Si02 (532.6 eV).

Nals I071.6 - Most sodium configurations have a Is binding energy of I071.6 eV, amongst Na20 which are NaCl, NaP03 and NaN02. A sodium oxide (I072.7 eV) can be ruled out. However, analysis of several samples showed no correlation between the sodium concentration on the one hand and Cl, P or N on the other.

Al2p - 74.6 Al203 (74.0-74.7 eV) Ah03

Si2p IOO 103.4 Metallic silicon (99.6 eV) is a well-known impurity in commercial steel. The Si02 peak at I03.4 eV is due to quartz, Si02 {I03.4 eV).

P2p - I33.5 A candidate would be (NaP03)3 at I33.5 eV, but, again, no correlation was P205 found between Na and P. P205 {I35 eV) is unlikely, and combinations with halides (Cl, F etc.) are ruled out (>140 eV)

Cl2P3/2 I98.2 I98.2 To our knowledge, there is no literature value for FeOOH·Clx. However, -binding energies for other metal chlorides include I97.9 eV (KCl), I98.0 eV (NaCl), I98.6 eV (FeCh) and I98.8 eV (FeCl3). In the pellets, NaCl is impossible since there is no Na, and KCI is also ruled out since the pellet containing the largest amount of chlorides contained no potassium. Also for the standard samples there is no apparent correlation between Cl and Na or K. Configurations like HCI (207.4 eV), Ch {207.8 eV) and chlorates (>208 eV) can be ruled out.

K2P3/2 293 293.4 KN02 at 293 eV and KCl at 292.5. K20 unknown. K20

Ca2p3/2 - 347.5 CaO {346.5-347.3 eV), CaC03 at 347 eV.NoCaCh (348.3 eV) CaO

Fe2p3/ 2 711 711 As explained before, these peaks are due to FeOOH or ')'-Fe203. No metallic Fe203 iron is observed (707 e V). Possible iron chlorides cannot be distinguished from FeO the main iron peaks: FeCh at 710.5 eV, FeCh at 711.I eV

Table 4.3: Chemica! composition of the samples. The first column denotes the XPS line (note that the splitting of the Al2p, Si2p and P2p is smaller than the line widths, and can therefore not he measured), the second and third its centroid position in the standard sample and the pellet. The fourth column lists candidate compositions, or configurations that are out of the question. The last column lists typical gangue components.

I element I standard I pellet I c 28.2 13.7 N 0.2 0.8 0 55.1 59.1 Na 0.5 0.1 Al - 2.3 Si 0.4 16.5 p - 1.3 Cl 1.0 0.2 K 0.4 0.8 Ca - 1.9 Fe 14.2 3.3

Table 4.4: Element concentrations for the untreated samples calculated from XPS

peaks. Atomie concentrations in % of the total sum.

4.2 Oxygen reduction

dehydration

RESULTS 32

I component I conc.

Si02 1-30 Al203 0.5-5 MgO 0.05-1 CaO 0.2-2 Mn 0.1-0.4 P20s 0.4-0.6 s 0.01-0.5 Na20 0.1-1 K20 0.02-0.04 Cl 0.3-2

Table 4.5: Gangue composition in nat­ura! iron ore, in weight% [41], [44]. Low concentration impurities include Cu, Pb, Zn, Cr, Ni, V, Ti and Sn.

As already mentioned, fig. 4.1 gives the measured HElS spectra for the sample originating from a shipwreck. Part of it has been treated in a vacuumoven (8·10-6 mbar) for 30 minutes at 400 oe, and part of it has been treated in a 25% H2-plasma (Ar 3 SLM, H2 1 SLM, 40 A, 7 kW, 2 mbar, T~ 550 oe) for 30 minutes. The spectra have been adapted in such way that the iron stepheights overlap, clearly showing the reduction in oxygen content.

The oxygen/iron ratio was determined to he 2.3±0.15 for the untreated specimen, 1.5±0.1 for the oven treated specimen and 0.1±0.03 for the plasma treated specimen. This suggests that the original composition, FeOOH, is dehydrated to Fe20 3 in the oven and the oxide is further reduced in the hydrogen plasma. The other samples (standard and pellets) show a similar behaviour. Fig. 4.12 gives the XPS 01s lines of an untreated and a plasma treated standard sample: it is observed that the peak associated with the hydroxide has decreased with respect to the oxide peak, again indicating dehydration.

Dehydration is also observed with XRD (after treatment of complete 15th century nails, there crust was removed and analyzed; the spectra are shown in Fig. 4.6), where after heat treatment peaks associated with 1- Fe20 3 , maghemite, appear1 . Maghemite is the rare

1 Here, it should be noted that these peaks can also be attributed to the presence of magnetite, since magnetite and magheruite have the same lattice constants (appendix A). It is possible that a small part of the magnetite core, having become more brittie as aresult of the treatment, has been removed, together with the crust, for analysis.

RESULTS 33

50 50

plasma treated

40 40 standard sample

"' 30 "' 30 ë ë ::I ::I 0 0 (.)

(.)

..9 ..9 ~ 20 ~ 20

10 10 untreated standard

0 0 534 532 530 528 534 532 530 528

Binding Energy [eV] Binding Energy [eV]

Figure 4.12: XPS oxygen 1s peaks of the untreated an plasma treated standard samples, indicating dehydration.

Energy (MeV)

120,---__ -,2 ______ ,4 _______ 6,_---,

-standerd untreoted 1 1

O -standerd plasma treoted 10 min. 2 -standerd plasma treoted 20 min. 3

-o ~ 8 >--"0

~ 6 0

E 0 4 z

2 s & • 2 oFe

O~L-,---,--,---,---.---.~~

0 100 200 300 400 500 600 700 Channel

Figure 4.13: Infiuence of treatment time ( standard samples)

Energy (MeV) 4 6 2

8,-----~------,------,,----.

-o ~ 4

0

§ 0 z 2

- untreated } -T=433 C !50~ H2l; oven ond Ar identicol -T=467 C 107. H2 -T=476 C 1007. H2) 4 -T=495 C 57. H2) 5

11 6 4 2 00

a e • 2 oFe

0 100 200 300 400 500 600 700

Figure 4.14: HElS spectra before and after plasma treatment (10 minutes) with var­ied composition. Depth scale is in JLm.

cubic variety of Fe20 3 • It can only be formed by dehydration of FeOOH below 500°C (37] or by oxidation of Fe30 4 below 400°C, and is unstable with respect to hematite.

treatment duration

Fig. 4.13 gives the infiuence of treatment time on the oxygen concentration. Here, standard samples were used. Treatment took place with the following conditions: Ar 3.15 SLM, H 2

0.35 SLM, 70 A, 5 kW, 0.4 mbar, 485 °C. The oxygen/iron ratio of the untreated sample was found to be 2.1±0.1 (FeOOH). After 10 min. plasma treatment this ratio drops to 0.65 at the surface increasing to 1.5 at a depth of 1 JLm, again caused by dehydration to Fe20 3 , and after 20 min. it has dropped to 0.1 at the surface and 0.25 at adepthof 1 JLm.

0.8 ,----------._.---------,

0.6

0.2

--~--- --------o--------n • Ar

50%

• 5%~

OL.....--'--~-~~~~---'-_.___.

450 500 400

T substr (oC)

Figure 4.15: The oxygen concentration near the surface ( after treatment, normal­ized to untreated sample) as a function of sample temperature during treatment. Data from fig. 4.14.

RESULTS 34

0.8

,......_ 4oo•c

"0 0.7 ~ ~ 474•c ~ • 433•c

~ 0.6 = :s

0 ..... 0.5 • 467•c

~ ...... 476•c ~ - 0.4 • 4s6•c ~ '-" ,.......,

0.3 0 ..........

• 495•c 0.2

100 0 20 40 60 80

H2 fraction ( % )

Figure 4.16: Same data as in fig. 4.15, but now as a function of hydragen fraction in the input gas.

plasma composition and sample temperature

Fig. 4.14 gives the REIS-spectra for a series where the plasma composition is varied from full argon to full hydrogen. as a reference, an oven treated sample is used. The substrate temper at ure was kept at 465±30 oe for all plasma treatments ( other parameters: total flow 3.5 SLM, 0.4 mbar, 10 minutes). The samples treated in the oven and in a pure argon plasma show only dehydration to an F e203 configuration. In the presence of hydrogen, a further decreasein oxygen content is observed, indicating that chemical reduction of oxides takes place. At the surface ( = top 1 pm), it is the substrate temperature that determines the oxygen reduction. The 'cold' 50% H2-spectrum practically coincides with the argon and oven spectra suggesting that 433 oe is too cold for the reduction of iron oxides to be noticably effective within 10 minutes. At greater depth, the chemical reduction becomes less effective; there, the best reduction was obtained in a full hydragen plasma.

In Fig. 4.15 the oxygen concentration (at the surface, after treatment) is given as a function of temperature, normalized to the oxygen concentration of an untreated sample. This means that [0]= 0.7 .. 0.75 corresponds to dehydration. Fig. 4.16 gives the same data, but now as a function of hydragen fraction in the input gas. It can he observed that there is strong correlation with sample temperature, but that there is no apparent correlation with the hydragen fraction in the input gas (as already can be seen from Fig. 4.15, correcting the data in Fig. 4.16 for temperature would yield a reduction that is practically constant).

Figure 4.17: 3 Me V H+ REIS spec­tra for a pellet made from nails, before and after it was plasma treated. The smooth curve is a calculated simulation ( see text).

pellets

RESULTS 35

Energy (MeV) 2.0 2.5 3.0

120,--,------,-----~~----_,~ 1.5

- pellet untreated

100 =~(~~~oE~~~~ ree~be~H-C-Si/Fe-0-H-C-Si Ll

~ 80

20

or---,----.---.----~~~--~ 400 500 600 700 800 900 1 000

C~o'mel

Fig. 4.17 gives the 3MeV proton HElS-spectra of an untreated and a plasma treated pellet (3.15 SLM Ar, 0.35 SLM H2, 0.4 mbar, 400°C, 45 minutes). The smooth curve is a calculated simulation of 2.4 pm of Feo.1sSi0.10Ü 0.22Co.o3 on 30 pm of Fe0.25Si0 .11 Ü0 .45C0 .04

using high-energy proton collision cross sections for oxygen and carbon and high-energy stopping powers. The composition of the untreated pellet was found to he approximately Fe0.19Si0 .11 0 0.47C0 .08 • lt is seen that the oxygen fraction decreases as a result of the plasma treatment. However, here it is not possible to distinguish between dehydration and oxide reduction because of the presence of an unknown amount of Si02 (it should he taken into mind that also the collisional cross section for silicon is, in this energy range, no longer given by the Rutherford formula, and therefore the amount of silicon specified in the simulation does not correspond to the actual concentration present in the sample).

4.3 Chloride removal

10'

10'

UI 10'

ë :J 0 u 10

2

10'

10'

0 4 6 8 10 X-roy energy [keV]

10'

10°

10'

102

10'

10°

12 4 6 8 X-rcy energy [keV]

10 12

Figure 4.18: PIXE spectra of the untreated standard sample (A) and a 20 minutes plasma treated standard sample (B). The chlorine peak has disappeared.

RESULTS 36

First of all, it must be noted that since PIXE offers no depth resolution, it is not possible to distinguish a (homogeneous) incomplete chloride removal over the ent i re analysis range from a complete chloride removal over part of the range (e.g. a 50% reduction could be the result of a 50% reduction over the full range, but it could also be caused by a 100% reduction over the effective half range, or any intermediate situation).

Fig. 4.18 compares the PIXE-spectra for the untreated standard sample to that of a sample treated for 20 minutesin a 10% H2-plasma with a substrate temperature of approx. 560 oe. It is immediately noted that the Cl-peak has completely disappeared (from the noise the lower detection limit of this particular measurement is estimated to be 100 ppm absolute mass concentration).

hydrogen vs. heat

In order to distinguish chloride removal due to the chemica! activity of the hydrogen from thermal effects, reference experiments have been done in a pure argon plasma and in a vacuum oven (pressure 1 mPa = 10-5 mbar). The resulting chlorine-to-iron ratios as determined with PIXE are summarized in table 4.6 (end of this chapter), along with the obtained sulphur-to-iron ratios.

standard samples

From the data it is seen that a pure heat treatment gives a significant chloride reduction: up to 97% of the chlorides were removed in the oven (sample 24), and up to 99.5% in the argon plasma (sample 13). Fig. 4.19 shows the effect of temperature and duration of an oven treatment on chloride concentration in standard samples. It is concluded that a higher temperature leads to better chloride reduction, but that longer treatment does not automatically imply an improval. It should be noted that the 15th century nails, which had been treated in the oven simultaneously with the standard samples, as well as the standard samples, showed brown spots after exposing them to air for a few weeks; the samples treated at 250° suffered more than the ones treated at 350°C, whereas there was no apparent discoloring of the specimens treated at 500°C.

However, best chloride reduction was obtained in a hydrogen-containg plasma: up to 99.9% was removed (sample 9). Fig. 4.20 shows the equivalent of Fig. 4.15, but now for the chloride content, where the samesamples were analyzed (samples 2-7 and 12 originate from one treatment and analysis batch). It is observed that there was a significant reduction for all samples, and none of them exhibited post-corrosion in air at room temperature. As was the case with oxygen reduction, it is found that adding hydrogen might lead to an improved chloride removal, but comparison to samples from another batch (samples 16, 18 and 24) reveals that sample 12 might as well be a statistica! deviation: more treatments are required to substantiate.

c .s 0.5

~ ~ 0.4 CJ

8 0.3 I

0 13 0.2

.:::: o; 0.1

§ z 0

Figure 4.19: The effect of temperature and treatment duration on the chloride content in standard samples. The sam­ples were treated in a vacuum oven (p= 1 mPa).

other samples

RESULTS 37

0.1 •

O%H2 c 0 0.08 -~ tl c <I)

2l 0.06 0 • • • CJ

ó SO%H2 100%H2 5%H2

-g 0.04 • lO%H2 • 1 20%H2

0 0.02 c

0 440 460 480 500

sample temperature in ° C

Figure 4.20: Chloride content in standard samples after 10 minutes plasma treat­ment, as a function of temperature. The 0% H2 point (pure argon plasma) differs noticably from the other data points.

In the pellets (samples 30-36), there was no substantial chloride removal (typically 50% reduction), and there is no apparent chemical activity of the hydrogen. The shipwreck samples and especially the ferrous chloride pellets however show a noticably better chlo­ride withdrawal in the hydragen plasma compared to the reference treatments.

The general picture we get is that the bulk of the chlorides is removed by a thermal process, but that the best reduction is obtained in the hydragen plasma.

4.4 Structural changes

The treatment implies a number of changes to the structure and morphology of the object. First of all, there is dehydration of the iron oxide hydroxide, as was already mentioned. Fig. 4.21 shows a detail of an object and Fig. 4.22 shows a Scanning Electron Microscope (SEM) micrograph of a sample, both after treatment. The cracks that appear have also been reported by other authors, e.g. [10] and [21]. The dehydration is one of the causes for an easier removal of the incrustation ( desired), but it could also lead to loss of mechanica! stability (brittleness) in objects that are held together mainly by the rust layer. The latter is demonstrated by the observation that when patting ten 15th century nails with the sharp

RESULTS 38

Figure 4.21: Detail of an object after plasma Figure 4.22: Micrograph of a sampie treatment. showing cracks after treatment (for both

plasma and oven treatment).

end of a small hammer, this had no effect on any of the untreated specimens , but after treatment , eight of them broke apart.

Most samples exhibit a black appearance after heat treatment, in the oven as well as in the plasma.

RESULTS 39

standard sample Hydrogen Plasma Treatment

# type time [mins] temp [0 C] press. [mbar] [Cl] x0.1% [S]x0.1% 1 untreated - - - 50 5 2 5% H2 10 495 0.4 2.6 1.4 3 10% H2 10 467 0.4 2.0 2.9 4 10% H2 10 486 0.4 1.5 2.4 5 20% H2 10 486 0.4 1.8 3.8 6 50% H2 10 433 0.4 2.6 3.1 7 100% H2 10 476 0.4 2.6 5.3 8 10% H2 30A 20 465 2 14 3.2 9 10% H2 70A 20 560 2 <0.1 <0.1

10 80% H2 30A 20 360 2 8 1.6 11 80% H2 70A 20 430 2 10 3.2

Reference Treatment 12 Ar 50A 10 474 0.4 4.5 1.3 13 Ar 30A 20 550 2 0.3 0.7 14 oven 15 250 w-5 22 9.6 15 oven 15 350 w-5 5.8 2.0 16 oven 15 500 w-5 1.8 <0.1 17 oven 20 400 w-5 6.9 2.6 18 oven 20 560 w-5 2.2 0.6 19 oven 30 250 w-5 22 13 20 oven 30 350 w-5 5.5 1.0 21 oven 30 500 w-5 3.3 <0.1 22 oven 45 250 w-5 7.7 2.5 23 oven 45 350 w-5 5.4 1.9 24 oven 45 500 w-5 1.6 0.5

shipwreck sample Hydrogen Plasma Treatment

25 untreated - - - 150 10 26 20% H2 45A 20 480 2 2.2 4.2 27 25% H2 40A 30 550? 2 <0.1 1.0

Reference Treatment 28 Ar 45A 20 >600 2 0.8 2.1 29 oven 20 400 w-5 23 1.1

pellets from nails Hydrogen Plasma Treatment

30 untreated - - - 16 8 31 10% H2 45A 45 400 0.4 6.8 7.6 32 10% H2 45A 45 400 0.4 6.3 7.4

Reference Treatment 33 oven 20 560 10 ·:> 6.8 6.4 34 oven 45 250 w-5 12.3 5.9 35 oven 45 350 w-5 6.2 3.9 36 oven 45 500 w-5 9.2 3.9

ferrous chloride Hydrogen Plasma Treatment

37 untreated - - -

I 1900 -

38 10% H2 40A 45 400 0.4 3.4 -Reference Treatment

39 Ar 30A 45 400 0.4 28 -

Table 4.6: Summary of the chlorine-to-iron and sulphur-to-iron ratios from PIXE measure­ments. In all plasma treatments, total flow was kept at 3.5 SLM.

Chapter 5

Discussion

In this chapter, the observations concerning the oxygen rednetion reaction, chlo­ride removal and structural changes are commented.

5.1 Oxygen removal

From Fig. 4.15 it is concluded that for any reduction to occur (apart from dehydration), hydrogen is required. The active particle, H 0 or H2 , is thus far unknown. The effectivity of the reduction is, in our parameter range, influenced by sample temperature (better for higher temper at ure); there is no apparent correlation with the molecular hydrogen fraction in the plasma input gas. The temperature dependenee indicates that there is an activation energy. To test this, an Arrhenius plot is made.

In order to produce an Arrhenius plot for the total reduction process, we define the amount of reduction 1?.. as being the fraction of the oxygen that was removed from the surface (i.e. top 1 J-tm) as a result of the plasma treatment, excluding dehydration:

n = 0.11- [O] 0.71

This means that when the sample is only dehydrated, 1?..=0, but if there was a complete reduction to metallic iron, 1?..=1. It can he derived (appendix C) that the activation energy is obtained from the tangent of ln{lnC~n)} vs. ~·

The plot is shown in Fig. 5.1. The straight line in the plot corresponds to the acti­vation energy for the reduction of hematite by molecular hydrogen as given by [43] (note: activation energy only determines the slope of the curve; the vertical off-set is arbitrary). First of all, it should he noted that the plot consists of only five data points and suffers from statistica! scattering ( especially the low temperature points): to draw any definite condusion would require more data and reproducibility tests. If pushed, however, the following comments could he made:

40

Figure 5.1: Arrhenius plot for the oxide reduction mechanism, where k* repre­sents lnC2n)· The plotted line corresponds to the lit­erature value for the activa­tion energy.

......., :i ~ .........

,....._ * .!.<: '-' .s

DISCUSSION 41

1 r---------------------~

0

-1

-2

• In the middle region, there is reasonable correlation with the predicted curve for reduction with molecular hydrogen. Therefore, it might he argued that the dominant reductive partiele is the hydrogen molecule1

, which is confirmed by the observation that the rate of reduction increases significantly between 400°C and 500°C, matching the description in [25]. Iron oxide reduction with atomie hydrogen is exothermic already at room temperature [8].

However, we found no correlation between the hydrogen fraction in the input flow and the reduction rate, which would have been expected if hydrogen molecules were the reactive particle, since their partial pressure increases linear with the hydrogen fraction in the input gas. This might he an indication that hydrogen atoms are responsible for the reduction: it is well possible that the atomie hydrogen density does not change significantly with the fraction of hydrogen gas intheinput flow. (Basically, it is found that when using a full hydrogen plasma, the dissociation degree in the expansion is only approx. 10% due to recirculation, wall association and mixing. However, when using 10% hydrogen gas in argon, the atomie hydrogen fraction could increase to up to 20%, due to a better dissociation effectivity of the are and less influence of recirculation. The idea is substantiated in appendix D.)

• The reduction with 5% hydrogen in argon proceeded faster than would have been expected on basis of Arrhenius kinetics (perhaps due to a larger fraction of atomie hydrogen reaching the sample?) and the low temperature reduction proceeded slower than would have been expected. Here, it should he noted that for our calculations

1 A literature value for the activation energy for the reduction hy atomie hydrogen, if it exists, is not yet found; however, it is most likely that it is smaller than for reduction hy molecular hydrogen, since a large part of the activation energy is due to the potential harrier that must he overcome for chemisorption to the metal. For atomie hydrogen, there is no potential harrier for chemisorption to metals [38].

DISCUSSION 42

we have assumed that the reduction duration is equal for all samples. However, when treating the samples, total treatment duration has been kept constant, the difference between the two being the time required for dehydration. It is possible that under these particular circumstances, the actual reduction duration was, for the low temperature sample, shorter than for the other samples, leading to an apparently lower rate coefficient. The opposite could he the case for the high temperature point.

Resuming the above, there is some contradictionary information found on the question whether atomie or molecular hydrogen drives the oxygen reduction process. An uncertain factor is the dehydration process, that, especially at the lower temperature treatments, may have severely obstructed the oxygen reduction process. For explicit answers, the experiment must he repeated using fully dehydrated samples and a single plasma condition, where the sample temperature is varied by external means.

5.2 Chloride removal

The general condusion from our observations is that the bulk of the chlorides are removed thermally, were it is found that a higher temperature has a positive effect, but treatment duration is of less influence. However, the rest concentration can he decreased by using hydrogen gas.

To ensure the stability of an object, it is imperalive that the rest concentration of chlorides be as low as can be achieved.

The observation that a significant chloride remaval was obtained with a heat treatment (1 atm. N2-gas), corresponds well with the observations clone by North and Pearson [3]. They have used a differential thermal analyzer (DTA; 10°C/min; max. 980°C) which incorporated a thermogravimetrie balance. The exhaust gases were fed into a rapid scan mass spectrometer. It was observed that after 24 hours of heating in nitrogen at 500°C, 80% of the initial chlorides were removed (as Cl2, HCl and FeCl3). The residue consisted mainly of Fe203, FeCl2, Fe3C, C and Si02.

Main causes for the thermal chloride removal are thought to he, in accordance with the properties of iron chlorides as discussed in chapter 2:

• vaporization of HCl and Cl2 gas

• vaporization of ferric chloride FeCl3

• decomposition of FeOCl

where the FeOCl could he the intermediate formed by dehydration of lepidocrocite or akaganeite:

/3, -r- FeOOH--+ Fe203 + FeOCl + H20 i These reactions are consistent with the data in Fig. 4.19. There it was shown that a

longer treatment does not imply an improved chloride reduction. Just the fact that the

DISCUSSION 43

final temperature is reached during heating, implies that all constituents having a boiling point at a lower temperature than the final sample temperature, are evaporated (provided the vapour does not saturate, which holds in the case of a differential reactor). The major amount of chlorides is therefore removed during heating of the sample.

North&Pearson also found that in a reducing atmosphere (1 atm. hydragen gas), also FeCl2 was reduced to metallic iron after 24 hours of treatment at 400 °C. As this temperature is far too low for volatilization to occur, it was concluded that direct reduction must take place:

FeCl2 + H2 --+ Fe+ 2HCl

The FeCl2 reduction as measured by North and Pearson is given in table 5.2. This ob­servation is similar to our FeCl2 • 4H20- experiment, where we found that treating in an Argon plasma removed 99% of the chloride, but that treating in a hydragen plasma resulted in a rest concentration which was an order of magnitude lower. Also Fig. 4.20 showed better chloride remaval when using hydrogen.

Temp. Exhaust Weight Residue composition {% by weight) Celsius gases loss% FeCl2 FeOCl Fe Fe-oxides Si02 + C H20 R.T. - - 2.3 8.0 15.1 49.7 14.9 8.5 100 H20 2.7 2.6 4.8 10.4 61.0 14.3 8.0 200 H20 13.5 2.8 3.9 7.2 64.0 17.3 1.6 300 H20 14.8 4.4 3.3 8.3 65.0 18.9 -400 H20 + HCI 23.6 0.15 0.35 68.6 9.2 21.6 -500 H20 + HCl 37.1 - - 79.8 2.9 17.2 -

Table 5.1: Thermal decomposition of marine cast iron corrosion productsin dry hydragen [3]. Samples were taken from semi-corroded cannonball. Note that at low temperatures, there is some FeCl3 included with the FeCl2•

The fact that adding hydragen to the plasma leads to a better chloride remaval in our samples could he caused by:

• the reduction of the iron oxide resulting in a more open morphology enhancing dif­fusion of volatiles out of the matrix. We have seen that the best chloride remaval in hydragen was obtained when using high temperatures combined with significant ox­ide reduction ( especially above the critica! point of 560°C, where the oxide reduction reaction forms a wüstite intermediate, chloride rest concentration is deminished).

• the actual preserree of FeCl2 which cannot he evaporated without the preserree of water or free oxygen, but can he reduced by the reaction described previously. Also in the ferrous chloride experiment it was seen that there was a better chloride remaval when using hydrogen: here, there is no oxide to he reduced.

DISCUSSION 44

5.3 Structural changes

It was found that some of the treated objects, either in the plasma or in the oven, show an increased brittleness. This may have several causes:

• Temper brittleness, a reported effect when heating steel to a temperature between 200 and 500°C, being most noticeable above 450°C [25], [39]. The effect is not yet completely understood, but it is suspected that grain boundary precipitation of carbon is involved, thus lowering the energy of surface formation at the grain boundary, leading to an increased chance for intergranular fracture [37].

• Dehydration, leading to cracking and porosity.

• An increase in porosity due to the reduction of the oxides. This can only occur in the hydrogen treated specimens.

• Hydrogen embrittlement, also preelucled to the objects treated in hydrogen.

It seems that, since the the latter two effects only occur in specimens treated in hydrogen but yet the increase in brittleness has been observed also in nails treated in an argon plasma and in the oven, one of the other effects prevails.

The black appearance of the artefacts is, so far, not understood. Segregation of carbon from steel is not to he expected, the moveability of carbon atoms in steel being negligeable below 1100°C [39], and the discoloring already appearing at temperatures as low as 250°C (lowest temperature tested). Perhaps the carbon present in the rust layer appears at the surface of the rust particles as a result of dehydration, or the dehydrated hydroxide has become highly amorphous and light-absorbing. A transition of part of the rust to black magnetite upon dehydration is also not yet excluded as a potential cause.

It has been mentioned by several authors that, in order to preserve the original met­allurgical microstructure in any remairring iron, the maximum temperature during a heat treatment should not exceed roughly 400°C [3] the transformation of martensite to troos­tite, sorbite and perlite becoming significant at that temperature [7]. Some comments on the Fe-C system are given in Appendix A. However, for the oxide and chloride reduc­tion to he effective within the timeframe stuclied here, higher temperatures seem to he required; especially the thermal decomposition of FeOCl speeds up considerably above this temperature [25]. Again, an uncertainty originates from the dehydration process; it is not excluded that after sufReient dehydration, a lower temperature plasma treatment is effective too. Further experiments are required.

Chapter 6

Conclusions and suggestions

6.1 Conclusions

• The reduction of oxides is a direct consequence of the chemical activity of hydrogen. The reduction rateis strongly infiuenced by sample temperature and morphology.

• The removal of chlorides is mainly caused by evaporation of volatile components, which are formed either by hydrolysis of soluble components or by thermal decompo­sition. A higher temperature leads to better chloride removal, but treatment duration is of less infiuence.

• The use of hydrogen increases the chloride removal performance. The effect is due to direct chemical reduction and/or to the increased porosity of the object.

6.2 Suggestions for further research

• Care should be taken to properly condition the experimental parameters:

1. Make use of dehydrated samples in order to be able to fix the actual treatment duration. This can be achieved by e.g. a pretreatment in an argon plasma.

2. If possible, temperature must be stabilized with an external heat source. Con­siclering the difficult practical consequences this implies, it is worthwile to inves­tigate the possibility of pulsed hydrogen supply in an argon carrier jet using a fast flow controller. The argon is used for heating, whereas the hydrogen serves as the reductive gas.

3. In practice it was found to be very difficult to obtain homogeneous, representa­tive (read: old) specimens. This leads to two options:

- Use pure singular crystals to study the chemical activity of the plasma.

- Take the statistica! approach: treat a large amount of samples and test them in a elimate chamber.

45

CoNCLUSIONS AND SUGGESTIONS 46

• It is desired to register the temporal evolution by monitoring in situ the effect of treatment. Options are: active actinometry, passive spectroscopy, mass speetrometry or gas chromatography.

• To gain understanding about the role of hydragen atoms and molecules in the reduc­tion process, an effort should be made to perform a reference treatment in an oven filled with hydragen gas.

6.3 Practical consequences and suggestions

Using high temperatures offers the advantage of significant removal of chlorides, and the disadvantage of enhancing brittleness. Best removal is obtained in a high temperature reducing hydragen atmosphere. In earlier experiments it was found that the latter signifi­cantly facilitates crust removal, which is not the case without hydragen (49].

ft should be noted that, for chloride removal from within the bulk of an object, an open structure is required, which ever treatment is chosen. Without this, it is impossible for the reaction products to diffuse outwards. This implies that it is not possible to remove chlorides from underneath a thick layer of magnetite without affecting the structure of the magnetite crystal.

If dehydration is acceptable, we suggest the following options:

• Fast heating of the object surfacetoa high temperature (perhaps a welding device?) leading to chloride evaporation. If clone properly, the core of the object does not become hot, thus suffering little from embrittlement. This method is extremely fast and cheap, although less effective than the other option.

• Treatment in a reducing environment such as a hydragen plasma, after dehydration of the surface of the object. The porosity of the surface will increase as aresult of the treatment. Without post-treatment this would result in an increased corrosion rate; however, the increased porosity also impraves the adherence of any post-treatment coating (e.g. epoxy resin, wax etc.), thus even improving corrosion resistance. It is also possible to do a post-cleaning treatment with a:C-H, a:Si-H, N2 or mixtures in the same reactor, before air-exposure.

A combination of fast heating and post-treatment could be the employ of an agon plasma. It is more expensive than a welding device, but offers extreme versatility and controllability within msecs.

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47

BIBLIOGRAPHY 48

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[25] Gmelin lnstitut für anorganische Chemie der Max Planck Gesellschaft, Gmelin Hand­buch der anorganische Chemie, teil 59: Fe. Springer Verlag (1931)

[26] Marlina A. Elburg, The technology of early iron production in the central and eastern parts of the Netherlands

[27] W. Chu, J.W. Mayer, M. Nicolet, Backscattering Spectrometry. Academie Press (1978)

[28] W.E. Runt et. al., Phys. Rev. 160( 4), 782 (1966)

[29] R.J. Severens, internal report VDF/NK 92-31 (1992)

[30] S.A.E. Johansson, J.L. Campbell, PIXE, a Novel Technique for Elemental Analysis. (1988)

[31] D. Briggs, M.P. Seah, Practical surface analysis, vol. i: Auger and XPS, J. Wiley&sons (1983)

[32] Perkin-Elmer Corporation Handhook of X-ray photoelectron spectroscopy.

[33] N.S. Mclntyre, D.G. Zetaruk, Anal. Chem. 49 11, 1521-1529 (1977)

[34] I.Suzuki, N. Masuko, Y. Hisamatsu, Corrosion Science 19, 521- 535 (1979)

BIBLIOGRAPHY 49

[35] M. Smialowski, Hydrogen in Steel. Pergamon Press (1962)

[36] R.G. Ward, An Introduetion to the physical chemistry of Iron & Steel Making. Edward Arnold Publ. (1962)

[37] J.S. Kirkaldy, R.G. Ward, Perrous Metallurgy. University of Toronto Press (1964)

[38] L. von Bogdandy, H.-J. Engell, The Reduction of Iron Ores. Springer-Verlag (1971)

[39] American Society for Metals, Heat Treater's Guide. (1982)

[40] 0. Kubaschewski, Mettalurgical Thermodynamics in Encycl. of Phys. Science and Technol., vol. 8 (1987)

[41] R. Matheus, Ph. D. Thesis, Rheinisch-Westfälische Technische Hochschule Aachen, Aachen, Germany (1969)

[42] R. Gosch, Ph. D. Thesis, Technische Universität Carolo- Wilhelmina, Braunschweig, Germany ( 1981)

[43] M. Bergmann, Ph. D. Thesis, Rheinisch-Westfälische Technische Hochschule Aachen, Aachen, Germany (1983)

[44] I.H.M. Ali, Ph. D. Thesis, Rheinisch-Westfälische Technische Hochschule Aachen, A ach en, Germany ( 1984)

[45] J. Vetter, Ph. D. Thesis, Rheinisch-Westfälische Technische Hochschule Aachen, Aachen, Germany (1984)

[46] H.C. Schaefer, Ph. D. Thesis, Rheinisch-Westfälische Technische Hochschule Aachen, Aachen, Germany (1984)

[47] W. de Bruijn, Ph. D. Thesis, Technische Universiteit Delft, Delft, The Netherlands (1990)

[48] M.J. de Graaf, R.J. Severens, D.C. Schram, D.K. Otorbaev, Z. Qing, R.F.G. Meu­lenbroeks, M.C.M. van de Sanden, to he publ. in Proc. llth Int. Symp. on Plasma Chem. ISPC-11, August 22-27 1993, Loughborough, UK.

[49] H.J.M. Meijers, Private Communication.

Appendix A

Inorganic compounds

A.l Narnes and crystallographic data

I stoichiometry I name I lattice constants

a-FeOOH goethite 4.18 2.45 2.69 ,8-FeOOH akaganeite 3.33 2.55 7.47 1-FeOOH lep i doeroei te 6.26 3.29 2.47 8-FeOOH - 2.55 2.26 1.69 (a- )Fe2Ü3 hernat i te 2.71 1.66 1.42 1-Fe2Ü3 maghemite 2.52 1.48 2.95

Fe3Ü4 magnet i te 2.53 1.48 2.97

Fe1-yÜ wüstite 2.15 2.49 1.52 FeCh lawrencite 2.54 5.90 1.80 FeCh molysite 2.68 2.08 5.90 FeOCl - 8.00 3.42 2.53 Fe3C cementite 2.01 2.07 2.11

Table A.1: Common narnes and the three main X-ray lattice constants (in Á) of inorganic compounds frequently mentioned in this report.

50

INORGANIC COMPOUNDS 51

A.2 The Fe-0 phase diagram

•c Atomie Percentage Oxygen so 52 54 56 58 60

.IL, \ J \ Lz+ ~ +' 1$97' I$SJ•f---= ï., \ I L, /f? I\'· zaJO l ~~.60 2s.o1 1 + G

2$.JI/

.( + L, \;2aJ6 Jb.04 -

~! 1424• f /457' -25.60 7.64 .

22.~

I 1/ -

rJ7t· t ZJ./61 2Z!JI ,

-~

JOOO

2800

2600

2400

I I I I -I I

~+€1 € I 7}~ ï I

-2200

i I I €1 +) I

~ 2JJJ I I 1)- I

1' + 1) I + G

2000

800

I I I

I

600

I y a+€ I 400

I I I I 200

IV 560' 23.26 I I 000

1(Fe0) I ay I Fo3 4 F~ES?low 8 00 400

Fe 0.2 0.6 22 2J 24 25 26 27 28 29 JO J/

Weight Pereenloge Oxygen

Figure A.l: The iron-oxygen phase diagram [37]

Legend to Fe-0 phase diagram a ferrite ", hematite

ï austenite L1 liquid iron 8 8-iron L2 liquid iron oxide é wüstite G oxygen gas ( magnetite

Note that wüstite only exists (in stabie form) above 560°C, and that it is non-stoichiometrie (Fe1_y0). Also note the phase transition from a-iron (ferrite) to 1-iron (austenite).

A.3 The Fe-C phase diagram

1600

~

1400

1300

1200

1100

~ .. 0:: .. F ~1000

e ~100

~ 800

~ ::& ~

100

800

0

I 11 I I I

~ ... _ .... -I

ATOMIC PERCENT CARIION

10

I I I

12

I I' I

71::' 1---

"' ---... r-.. ...... "~ 1--~L:

~ ............

""' (•L

r ~ ------- -------la~-----

.I .,,. 1140'C

V f••

I (•r..J!

..... 100

~~ ,I 100

100

•·r ~~ V' u

---~~------- t: -------- .................. <lOl ..... nM:

...... a .... i'!-«•r..J! ...

0.4 0.1 L2 l6 2.0 2.4

WEIGHT PERCENT CARBON

INORGANIC COMPOUNDS 52

14 .. 11

I I I I I

- 2800

-

Fe- F11C SYS1EM- i r.uo

F•-C SYSTEM ---- I_

' // - 2400

'

2200

~ , ' ,'/ L•C

~ ----;' ~- ~~--· ,.,

2000 ... ;;; ---- -- ---:0:: ,z

- 11! :0::

i:! -I 800 :a

w - $

~

1600 ..;

- r--....... 1'--- I -

~ ~

400 Ir ::& w ...

1200

.. ........... r---= -

.......... ~ -I

(,Vj ------- ---

f( ~ ... L--_,........,..•u - 1---

-11 ·--,~i~r - 1--c::::

0 • 10 .. 2D .. .. WEIGKT f'lGNT C1MC1t X 101

I I I I -I 000

4,0 ... , . Figure A.2: The iron-carbon and iron - iron carbide equilibrium constitution diagram [37]

Note that the solubility of carbon in austenite (I-iron) is two orders of magnitude larger than in ferrite (a-iron): austenite can contain up to 2 weight percentages of carbon. The a ~ 1 transition temperature is decreased by adding carbon.

Cementite, the only stable iron carbide known so far, can form several (macroscopie) crystal configurations in iron: they are known as pearlite (ferrite with included lamellae of cementite), bainite (formerly called troostite and sorbite), and martensite. The exact structure depends on the history of the iron, including the casting, ageing, quenching etc. However, they are transformed by so-called martensitic transformations, which do not require ditfusion of carbon and can therefore proceed already at low temperatures (200°C).

Appendix B

Data for xps, pixe and heis

element z XPS PIXE HElS

line BE [eV] range [eV] Ka [eV] (Eaut/ Ein)surf c 6 1s 287 12 0.2560 N 7 1s 402 9 0.3148 0 8 1s 531 4 0.3660 Na 11 1s 1072 2 1041 0.5008 Al 13 2p 74 4 1485 0.5557 Si 14 2p3/2 102 6 1740 0.5689 p 15 2p3/2 133 8 2014 0.6000 s 16 2p3/2 165 8 2308 0.6106 Cl 17 2p3/2 199 11 2622 0.6404 Ar 18 2p3/2 241 0 2957 0.6736 K 19 2p3/2 293 1 3313 0.6678 Ca 20 2p3/2 347 2 3691 0.6744 Mn 25 2p3/2 641 4 5895 0.7506 Fe 26 2p3/2 710 8 6399 0.7541

Table B.1: XPS: position of strongest peak when using Al-Ka souree in binding energy scale, with the range for chemical shifts. PIXE: Ka X-ray energy. HElS: kinematic factor for a-particles under 165° backward scattering.

53

Appendix C

Calculation of activation energy

I List of symbols k rate coefficient t time 0 oxygen concentration T treatment duration Oo oxygen conc. at t=O T absolute temperature H hydragen concentration A Arrhenius constant n fractional reduction R gas constant Ea activation energy B,B* dummy constauts

We start from the definition of the rate constant:

dO --=k·O·H

dt (C.1)

If we assume that k and H do not vary with time, the salution of the above differential equation is:

0 = Ooexp( -kHt) (C.2)

The fractional reduction (which is a function of temperature) we define as being the fraction of the oxygen concentration removed during the total treatment duration:

T

-I dO dt 0 dt

n = Oo , n E [0, 1] (C.3)

By integrating C.1 over the treatment duration and substitution of C.2 and C.3 we get:

n = 1 - exp( -kHt) (C.4)

which is equal to 1

k = Bln( 'R) 1-

(C.5)

54

CALCULATION OF ACTIVATION ENERGY 55

where we have put B = Jt. Substituting C.5 into the Arrhenius equation gives:

Ea 1 Aexp(- RT) = Bln(

1 _ R) (C.6)

and by putting B* = ln~ we thus obtain

(C.7)

so the act i vation energy is calculated from the tangent of ln{ ln( 12n)} vs. ~. Note that, for small R, ln{ln( 12n)} ~ ln(R).

Appendix D

On the infl uence of recirculation on the H 0-density

With a 10 kW DC cascaded are plasma souree a production of up to 5x 1021

H0 Js is achieved in full hydrogen, the dissociation degree being above 70%. After expansion into a low pressure background, the dissociation degree, measured by active actinometry and Rayleigh depolarization scattering, drops drastically. It appears that wall association leads to molecular hydrogen which re-enters in the expanding plasma beam. U sing a simplifi.ed model, it is reasoned that diluting hydrogen with argon does not imply a proportionalloss in atomie hydrogen concentration [48].

D.l Observations

D.l.l dissociation degree in the are

By measuring the power transferred to the plasma (input power minus power transferred to the cooling water) it is possible to calculate a lower limit for the dissociation degree, defined as [H]/([H]+2[H2]), in the are. It is plotted as a function of hydrogen gas input flow in Fig. D.l. The atomie hydrogen flow derived from this is shown in Fig. D.2 a production of up to 5x 1021 Ho js is achieved with 10 kW input power.

D.1.2 dissociation degree in the expansion

The local dissociation degree in the expansion has been measured with RF excited active actinometry. Active actioometry is needed since electrooie excitation is absent due to the low electron temperature: between 0.15 and 0.4 eV. In active actinometry, the electrous are locally heated by an RF probe exciter with typical powers of 10 W. As a result H Balmer lines and the H2 Fulcher rovibrational band are excited and can be spectroscopically observed. From the ratio of their intensities it is possible to calculate the local dissociation degree. Fig. D.1 (bottom curve) shows the resulting dissociation degree at z=250 mm from the are exit as a function of input flow. The background pressure was 130 Pa and the

56

ÛN THE INFLUENCE OF RECIRCULATION ON THE H 0 -DENSITY 57

1022 0 0

0 are

0 ,...., 0.8 u

,...._ !& 'M 0 "' ... ... s ~ ...

u N 0.6 ·~ ... + §: c..

'-" ... ;:;:; ~ s 0.4 0 c 0 :c

0.2 expansion • • •

0 1021 3 4 5 6 7 3 4 5 6 7

H2 flow [slm] H2 flow (slm)

Figure D .1: The dissociation degree in Figure D.2: The atomie hydrogen flow the are and in the expansion. out of the are plasma source, derived

from fig. D.I.

are current was 37 A; the RF probe exciter power was 20 W. Not only has the dissociation degree dropped to a much lower value than in the souree (10% instead of more than 70% in the are), but it also increases with the gas flow, whereas in the source, the opposite is observed. Also Rayleigh depolarization scattering measurements,which are based on the phenomenon that, when making use of polarized incident radiation, the 90° Rayleigh scattered radiation is fully polarized only if the scattering objects are spherically symmet­rie, e.g. in the case of ground-state atoms, but in the case of nonspherically symmetrie molecules, a small component of the scattered radiation is depolarized, confirm the domi­nanee of H2 molecules rather than the H atoms of the source, although they were performed in another (yet similar) apparatus.

D.2 Discussion

The observation that the dissociation degree drops in the expansion leads to the hypothesis that inthereactor the highly dissociated plasma is strongly mixed with the cold background gas. Si nee the molecular hydrogen flow from the are is low, hydrogen molecules must he created in the expansion vessel. Wall association

H + H(adsorped) ~~ H2(desorped)

is known to he effective and to result in hydrogen molecules. Together with a strong recirculation flow, this wall association process will provide a reentry flow of hydrogen molecules into the plasma jet.

In order to understand the observed increase in dissociation degree in the vessel with increasing flow, we consider the following cru de model. First of all, assume the ratio of

cascade are

ÜN THE INFLUENCE OF RECIRCULATION ON THE H0 -DENSITY 58

expansion

H adsorped

Figure D.3: The concept of wall association and recircula­tion.

atoms and molecules to he proportional with the ratio of the circulation time and residence time:

[H] Teire --"' --[H2] Tres

basedon the assumption that the atom, when in the recirculation flow, adsorbs to the wall and associates to form a molecule. The average lifetime of an atom is thus proportional to the time required to complete one circulation. This time is given by:

L A Tiet L Tcirc ~ - ~ -----

Vree Ajet Tree Vjet

where L and A are the vessellength and cross section area respectively, Vjet, Ajet and Tjet

are the jet flow velocity, cross section area and gas temperature, and Vree and Tree are the velocity and gas temperature of the recirculation flow. The equation is based on the consideration that the jet flow and the recirculation flow must he balanced: njetVjetAjet = nreeVreeArec- If we assume the pressure in the jet and the recirculation to he equal, this leads to VjetAjet/Tjet = VreeAree/Tree· Finally, we have used that Ajet ~ Aree ~ A and thus Vjet ~Vree· A typical value for the recirculation time is 5 msec.

The residence time (in our setup typically 0.5 sec) is the time a partiele resides in the vessel before being pumped away, and it is given by

nV pV Tres=~= kTree~

where n is the average partiele density, ~ is the pumping speed in particles per second (which is, during stabie operation, equal to the incoming partiele flow), pand V are vessel pressure and volume respectively, and k is the Boltzmann constant. Consiclering that the

"0

~ 0.8 i

~ c:: 0 ·;;

0.6

§ ~ 0.4 lU

.s ~ 0.2

ÛN THE INFLUENCE OF RECIRCULATION ON THE H 0 -DENSITY 59

0 ~~~~~-L~~~~~~-L~

Figure D.4: The atomie hydragen den­sity as a function of the hydragen frac­tion in the input gas, normalized with respect to the full hydragen case, ac­cording to our crude model. We have imposed the input flow of argon + hy­drogen atoms to be constant; x SLM

argon at Jl=Ü therefore corresponds to x/2 SLM H2 at Jl=l.

0 Ar

0.2 0.4 0.6 0.8

H2 input fraction (J.1)

loss process foratomsis wall association and the loss process formolecules is being pumped out, we thus find:

[H)- ~~ [H2) - Vjet P

K being a proportionality constant ( containing a.o. the dissociation degree from the are, jet gas temperature and the cross section area of the plasma jet). When increasing the input flow at constant vessel pressure (this means that the pumping speed~ must be increased), our crude model prediets an increase of the dissociation degree, as has been observed in the experiments. Note that operation at low pressure also gives a high dissociation degree, provided the cross section area of the jet does not increase too much.

Since the mass of an argon atom is 40 times that of a hydragen atom, its acoustic velocity is lower by a factor six. As a lower velocity leads to a longer circulation time and thus to a higher ratio [H]/[H2], we will extend our crude model to also encompass argon/hydrogen mixtures.

First, we de:fine a: as being the atomie hydrogen fraction leaving the source: a: = [H]/([H]+[Ar]), and J1 as being the molecular hydragen fraction in the input gas: J1 = [H 2]/([H2]+[Ar]). We impose that the total partiele input be constant: [H]+[Ar)=1 (this implies that the total input flow decreases for increasing hydrogen fraction). Furthermore, we assume that all flow veloeities scale with the acoustic velocity, giving an effective jet veloei ty v jet = a:· 6v Ar+ ( 1 -a:) ·vAr = ( 1 + 5a:) ·VAr. The atomie hydrogen density increases

ÜN THE INFLUENCE OF RECIRCULATION ON THE H0 -DENSITY 60

with a (representing more hydrogen input), but decreases with Vjet:

[H] "' a _ 211 1 + 5a 1 + l1J1

where we assumed full dissociation in the are, giving a= 2J1/(1+J1). The result is shown in Fig. D.4, where the atomie hydrogen density is normalized to the full hydrogen case (J1=1). Note that as a boundary condition we have imposed constant partiele input: if J1=0 corresponds to x SLM argon gas, J1=1 corresponds to x/2 SLM hydrogen gas.

From Fig. D.4 it can heseen that, according to our crude model, the atomie hydrogen density does not decrease dramatically when diluting hydrogen with argon. In fact, using 4% hydrogen and 96% argon ( explosion limit) results in an atomie hydrogen density which is still 33% of the full hydrogen case. Here, hydrogen enrichment due to the higher pumping speed for argon when compared to hydrogen, has even been neglected, and, more important still, so has the higher dissociation degree in the are for lower hydrogen fractions. The use of small hydrogen fractions offers some other features: ionization is maintained longer, are operation is more simple and more safe and there is more efficient energy conversion to the plasma. If operation is limited to below the explosion limit, costs for installation can he drastically reduced.

D.3 Conclusion

With a 10 kW DC cascaded are up to 5 x 1021 Ho js can he achieved. In the expansion, dissociation is lost due to the presence of recirculating hydrogen molecules, produced by wall association of hydrogen atoms. The highest dissociation degree is obtained using low pressure and high pumping speed. The use of argon/hydrogen mixtures does not imply a proportional loss of atomie hydrogen density, whereas it offers some practical benefits.

Appendix E

Conservation and dechlorination techniques: a short review

Basically, there are two approaches in reducing the rate of corrosion: one is by conditioning of the starage environment, and the other is by rednetion of the amount of chloridesin the corrosion layer [1]. Of course, the choice for one approach does not rule out the other.

1. Storage in dry air Mechanism: at low humidity (i.e. < 20% RH) there is no adsorption of a water film. Corrosion cannot praeeed since the electrolyte required for ionic conduction is nat present. Disadvantage: dehydration can cause cracks. Goes for all starage techniques: only suitable for starage when waiting for more definite treatment.

2. Storage in de-oxygenated atmosphere Mechanism: cathadie rednetion of oxygen impossible. Disadvantage: methad requires air-tight containers with nitrogen: expens1ve. Nat a definite treatment.

3. Storage in alkaline environment Mechanism: hydroxide ions farm tagether with iron ions a passive precipitate on the metal sur­face. Simple realization. Disadvantage: deep in the corrosion layer corrosion can still proceed. Nat a definite technique.

4. Rinsing in de-ionized water Mechanism: soluble chlorides are washed out. Extremely cheap. Disadvantage: Rest concentration of chlorides in the corrosion layer is too high.

5. Alkaline sulphite extraction The most popular dechlorination technique. Objects are submerged in an aqueous salution of 0.5 M Na2S03 en 0.5 M NaOH in air-tight vessels at a temperature of± 50 °C. Treatment takes 3 to 5 months while regularly changing the solution. After treatment the remaining chemieals

61

CONSERVATION AND DECHLORINATION TECHNIQUES: A SHORT REVIEW 62

must me rinsed out thoroughly; they can he fixated using a Ba(OH)2 solution. Mechanism: 1. Sulphite is commonly used to remave oxygen from a salution by transition to sulphate. Since there is no supply of oxygen in the air-tight vessel, corrosion will halt as there is no rednetion of oxygen. The potential gradient in the pit is decreased, thereby disrupting the balance between potential gradient and chloride concentration gradient. Under the latter, chloride will diffuse out of the pit. 2. The alkaline sulphite salution reduces akaganeite and hematite to magnetite, thereby releasing any chlorides incorporated in the corrosion products. Disadvantage: the sulphite cannot he rinsed out completely, causing cracks after submission to air. However, this effect is used to remave chloride infections that are present underneath a hard layer of magnetite in some objects, by repeatedly treating them. In this case, the mechanica! stability of the object is severely damaged.

6. Rinsing in a warm alkaline bath Mechanism: the rust layer swells up by the formation of amorphous iron oxide hydroxide poly­mers, releasing also chloride i ons previously incorporated in the corrosion layer. Disadvantage: The swollen rust layer is very fragile.

7. Electralysis or electrophoresis Mechanism: chlorides, which are concentrated near the metal due to a potential difference be­tween the positive ( anodic) met al and the negative ( cathadie) rust layer, can he driven out by reversing the potential. For this, the metallic core of the object must he connected to the negative pole of a power supply, and a positive counter electrode must surround the object. Disadvantage: water can dissociate forming hydragen gas at the negative pole, leading to hydra­gen embrittlement. To avoid this, the potential difference may not exceed 1 to 2 Volts. However, small potential differences are usually insufReient to remove all chlorides.

8. Boiling or steaming Mechanism: when drying a hot and wet object, soluble chlorides are driven out by osmosis. Disadvantage: the object can break as aresult of boiling, and steaming can induce severe corro­sion.

9. Submersion in various inorganic chemieals Mechanism: treatment with lithium hydroxide in methanol causes the chlorides to he extracted into the salution by the reaction LiOH + cz- -+ LiCl +on-. Treatment takes several months. After drying in air the lithium hydroxide reacts with carbon dioxide to form lithium carbonate, which acts as a alkaline buffer on the metal surface thus protecting it from corrosion. Other chemieals used are sodium benzoate and sodium sesquicarbonate. Disadvantage: rest concentration of chlorides too high.

10. Submersion in various organic chemieals Mechanism: organic solvents have the benefit of featuring lower surface tension than water and therefore have a superior penetration ability. Often dehydrated ethanol is used, to which formalde­hyde ( converts iron oxide hydroxides into magnetite thus also releasing non-soluble chlorides ),

CONSERVATION AND DECHLORINATION TECHNIQUES: A SHORT REVIEW 63

isopropanol ( decreases surface tension) and tri-ethanolamine ( anti-corrosion chemica!) has been added. Disadvantage: the solubility of saltsin organic solvents is appreciably lower than in water, result­ing in long treatment duration. The faster ethanol method (treatment time typically one week) is expensive.

11. Submersion in amines Mechanism: in an alkaline amine solution the iron oxide hydroxides in rust have a negative sur­face charge. The amine ion is positive and can therefore penetra te very well into the rust layer. There it causes dehydration and subsequent conversion to magnetite. Disadvantage: rest concentration of chlorides too high.

12. Submersion in liquid ammonia Mechanism: very low surface tension and high ion mobility, resulting in excellent penetration power. Disadvantage: only those compounds dissolve that have a low solubility in water. The method, which requires special cryogenic equipment, can only heusedas an addition toanother technique.

13. Annealing in air or nitrogen Mechanism: at temperatures of at least 400 oe in air FeCl2 oxidizes into FeCl3 which then evaporates. To prevent severe corrosion and oxidation of iron carbid es (increased brittleness) in hot air, it is preferred to anneal in nitrogen at700 oe. Treatment is fast and simple. Disadvantage: (goes for all heat treatments at temperatures exceeding 200 oe:) metallurgie in­formation, which is necessary to determine the origin of an ore, the castingor smithing process, can be lost due to formation of iron carbides. This information should thus he obtained prior to annealing. Also, the object will dehydrate.

14. Treatment in hydrogen plasma Mechanism: atomie hydrogen has excellent penetration and very strong reducing abilities. Apart from dechlorination the treatment also facilitates removal of the incrustation. By adding methane and nitrogen to the carrier gas ( usually an argonjhydrogen mixture) the metal surface can be carburized and nitrided. Treatment is very fast. Disadvantage: Dehydration, loss of metallurgie information and risk of hydrogen embrittlement. The method requires special equipment.