Corr Control2014

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T. Hodgkiess Nov 2014

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CHAPTER 5: CORROSION CONTROL

There are a number of methods of corrosion control available to combat aqueous

corrosion:-

1) Design

2) Material Selection

3) Cathodic Protection

4) Anodic Protection

5) Modification of the Environment

6) Coatings.

DESIGN

Many corrosion problems can be avoided by incorporation of corrosion principles at

the design stage. Some examples are indicated below.

(i) Every effort should be made to ensure that water does not unnecessarily come

into contact with parts of an installation, i.e.during shut-down periods. Shallow pools

of water are highly aerated and therefore extremely corrosive. Consequently, vessels

or tanks should if at all possible be fitted with proper means of completely draining

the water (Figure 1). Similarly, “horizontal” pipelines should have a slight slope to

make them self-draining.

Figure 1

(ii) Crevices should be avoided since these can promote severe corrosion.

(iii) Attention should be given to the dangers of promoting bimetallic corrosion by

coupling dissimilar metals - which is often unavoidable in large complex installations.

If possible, couplings of dissimilar metals should be electrically insulated from one

another by means of non-conducting washers, gaskets or sleeves (Figure 2).




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Otherwise, it is important to keep in mind the factors, mentioned in Chapter 4,

concerning this phenomenon:-

• Be especially vigilant concerning small anode / large cathode situations

•

Coat the cathode; more important than coating the anode since small defects ina coating on the latter clearly accentuate the small anode / large cathode

situation whereas even an imperfect coating on the cathode leads to a small

cathodic area.

Figure 2: Reducing intensity of bimetallic corrosion

EFFECT OF COATING THE CATHODE ON GALVANIC CORROSION

Small anode (carbon steel) on left: large cathode (stainless steel) with coating on right;Size ratio: 9:1 (cathode / Anode)

Both immersed in seawater and electrically connected

Galvanic current flowing between the two is shown on the Ammeter, A

BOTH ANODE AND CATHODE UNCOATED

Galvanic currents: 300 A (0 days) -> 120 A (84 days)

ANODE UNCOATED / CATHODE COATED




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Galvanic currents: 11 A (0 days) -> (0 – 6) A (84 days)

(iv) Stray currents from electrical installations can promote corrosion and should

be avoided if possible.

(v) Adequate access should be provided, (a), to allow for inspection, (b), so that

water, sludge and other wet rubbish can be removed from plant surfaces and, (c), to

allow room for painting.

(vii) Avoid high velocities, acute bends, sudden changes of section and other

geometrical features which might cause local turbulence and hence erosion or

cavitation corrosion.

(viii) Specify weld procedures in order, for instance, to avoid crevices and to

prevent water ingress (seam welds are clearly superior in this respect than spot

welds).

DESIGN ISSUES SPECIFIC TO SHIPS

Double skin hulls

Following a rash of major losses of large tankers with disastrous oil spills from single-

hulled vessels following collisions or grounding in the early 1990’s, double skins wereintroduced. The rationale was to provide safety against groundings and collisions but it

could be said that this was at the expense of greater vulnerability to corrosion.

A major factor in this sense is the thermal insulating effect of the two hulls

(sometimes called the “thermos effect”). Thus, in the presence of heated cargo, or

other sources of heat, and, in the absence of the cooling effect obtained by direct

contact with seawater, the inner steel plate attains higher temperatures - with likely

higher corrosion rates (including potentially accelerated rates of microbiologically-

induced corrosion) – than would be the case with a single hull. Thus, steel plate

separating ballast space and cargo will be vulnerable to enhanced corrosion rates from

both sides – if corrosive agents (e.g. moisture, acidic gases) are present in the cargo.

Issues in cargo tanks

The warm, moist acidic gases from acidic constituents (including H2S) in the crude

oil or in so-called inert gas often injected into the space above the oil, causeaccelerated corrosion on the underdeck and pitting of the bottom plate.

Issues in ballast tanks:

• also involving higher temperatures and warm, salty air

• less effective removal of water when “emptying”

•

possibly greater amounts of mud/silt left after emptying and consequent SRB-induced corrosion beneath such deposits




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• reduced corrosion margins due to thinner steel because these higher-strength

steels have essentially similar corrosion behaviour (rates) as conventional lower-

strength steels

• greater possibility for hydrogen embrittlement and other cracking (e.g. fatigue)

problems since higher-strength steels have inherently greater susceptibility to

these mechanical deterioration processes.




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MATERIAL SELECTION

This is not a straightforward process because of the need to consider other required

properties of any candidate materials - not least the costs - and also on account of the

many different forms of corrosion and their dependence upon the environmental

conditions.

As a general indication, Table 1 provides an indication of the relative resistances of a

number of engineering metals and alloys to general overall corrosion in low-velocity,

aerated seawater at ambient temperature. [The break in the scale between carbon

steel and copper represents a relatively large change in corrosion rate.]

Table 1: Relative corrosion rates in moving, aerated seawater at ambient

temperatures

However, different situations pertain to localised forms of corrosion with, for

example, some of the most corrosion-resistant materials listed in Table 1, such as

stainless steel and even titanium at high temperatures, being much more prone to

crevice corrosion than certain of the other materials. Another aspect of such

comparisons is that there can be wide compositional and behavioral variations

between different materials belonging to one class. This is particularly true ofstainless steels - as we shall see later in this chapter.

Carbon steel and unalloyed cast ironsThese materials possess extremely-poor corrosion resistance in virtually all aqueous

environments essentially because they exhibit little tendency to form protective oxide

films on their surfaces. Moreover, the corrosion rate of these materials increasescatastrophically with water flow rate - as the following data demonstrates.

Seawater flow, m/s zero 3 40

(ambient temperature)Corrosion rate, mm/year 0.1 - 0.3 0.75 5

Thus if economic or other circumstances dictate the utilisation of these materials, it isessential to employ one (or more) of the methods of corrosion control such as




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cathodic protection, coatings, modification of the environment and (rarely) anodic

protection.

Alloy Steels

Much attention has been devoted, over the decades, to attempts to identify and

produce low-alloy steels with significantly-enhanced corrosion resistance compared

to plain carbon steels. This has led to the development of the so-called "weathering

steels". The latter contain small quantities (< 1%) of chromium, nickel and usually

copper and exhibit good long-term resistance in most atmospheric conditions at

somewhat extra cost.

Upon initial exposure, the weathering steels (often referred to under one of the trade

names 'Corten') appear to be corroding in a similar manner to ordinary carbon steel in

that they develop a 'rust-coloured' corrosion product. However, this layer is more

adherent and therefore more protective than the rust products on carbon steels and,over a year or so, the corrosion rates of the weathering steels slow down to

acceptably-low rates. Although such steel has apparently been employed for small

craft in the USA and lifeboats in the UK, the general consensus is probably that itsimproved performances do not extend to submerged conditions (especially seawater)

and is even uncertain in coastal atmospheres.

On the other hand, considerable improvement in corrosion behaviour can be secured

by alloying steels with 12% chromium or more. Materials in this category are called

"stainless steels" and are considered in detail in the following sections.

Stainless steels

As mentioned above, the presence of more than about 12% chromium in a steel leads

generally to large improvements in corrosion resistance. But there are very large

numbers of commercially-available stainless steels with widely-varying

compositions:-

12 - 30% Cr, 0 - 25% Ni, up to 6% Mo

plus various amounts of other elements such as C, Ti, Nb, Mn, Cu, N.

The actual composition of a particular stainless steel determines the metallurgical

structure, and hence influences the properties, of the stainless steel at ambient

temperatures. Thus the stainless steel may be ferritic, austenitic, martensitic or duplex

(a mixture of austenite and ferrite).

Chromium, the most important alloying element in stainless steels, is a ferrite former.

Iron-chromium alloys with more than about 12% Cr remain ferritic at all temperatures

down to ambient. On the other hand, nickel tends to stabilise austenite and, with

sufficient Ni (depending upon the Cr content), a stainless steel can be produced which

is fully austenitic at ambient temperatures. The effects of other elements present instainless steel, in stabilising either austenite or ferrite, can be summarised as follows:-




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Si, Mo, V, Al, Nb, Ti, W - all ferrite formers.

Co, Mn, Cu , C, N - all austenite formers.

A very approximate guide to the composition ranges of the four types of stainlesssteels is given below:-

Ferritic:- 12-30% Cr, 0-4% Ni, 0-4% Mo, with low carbon

Martensitic:- 11-18%Cr, 0.6% Ni, 0-2% Mo, 0.1-1% C

Austenitic:- 17-27% Cr, 8-35% Ni, 0-6% Mo

Duplex :- 18-27% Cr, 4-7% Ni, 2-4% Mo

CORROSION BEHAVIOUR OF STAINLESS STEELS

All types of stainless steels are characterised by having considerably superior

corrosion resistance in most environments than has carbon steel. This is due to the

presence, on the surface of a stainless steel component, of a very thin, adherent,

protective layer of (chromium-rich) oxide. This excellent corrosion resistance is a

particular feature of all stainless steels in flowing liquids and results in their ability to

be used without danger of erosion corrosion at much higher flow rates than is possible

for the copper-nickel alloys.

On the other hand, stainless steels are rather susceptible to some types of localised

attack. Examples are pitting, crevice corrosion and stress corrosion cracking (the

latter being more likely at higher temperatures - typically above about 80°C - and at

higher dissolved-oxygen concentrations).

Stress corrosion cracking (SCC)

SCC in seawater is more likely at elevated temperatures – typically above about 80°C

– and at higher dissolved oxygen concentrations. It is often found to occur under

external lagging on pipes and vessels where the combination of high temperature and

high chloride {Cl-} concentration (due to partial evaporation of atmospheric-borne seasalts) represent susceptible conditions. (It should be mentioned here also that even

“dried-out” sea salt is highly deliquescent and absorbs moisture fro the air to yield a

potentially highly-corrosive, concentrated salt solution.)

The internal conditions in offshore hydrocarbon production tubular members can also

induce SCC failures in circumstances where H2S is present being derived from sour

crude or the presence of SRB. The H2S present in solution in the aqueous phase can

cause corrosion and cracking associated with hydrogen embrittlement and a particulartype of SCC termed “sulphide stress cracking” (SSC) of stainless steels (and indeed

other ferrous alloys – including low-alloy steels




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Pitting and crevice corrosion

These types of attack are associated with localised breakdown of the protective oxide

film - upon which the corrosion-resistance of stainless steel depends and the

resistance to such attack of the different grades of stainless steel is related to the

severity of the environment. The susceptibility of stainless steels to these forms of

corrosion represents a major factor in the proper specification of the appropriate

grades of stainless steel in marine engineering. The lowest-alloyed stainless steels

(e.g. the 12 - 14% Cr varieties) will be usually satisfactory in atmospheric conditions -

but may be vulnerable at weak points such as sheared edges especially in marine

atmospheres.

The crucial component in marine conditions is the presence of chloride which

possesses a particular ability to break down the protective oxide film. Thus, in

submerged aqueous conditions, the susceptibility to localised corrosion increases with

chloride content of the water. On account of its high chloride-ion content (around2%), seawater represents a general environment in which pitting and crevice

corrosion of stainless steels are a danger. Also, highly-saline seas, such as the

Arabian Gul,f represent more severe environments in this respect than the mainoceans of the world.

Pitting is also more likely in stagnant aerated water. The initiation of pitting

corrosion requires the localised breakdown of the passive oxide film and, in stainless

steels, such a process can be initiated at locations where the integrity of the passive

film is disturbed in some way. The most common such locations are where surface

mechanical damage has occurred during fabrication/installation and at sites of non-

metallic inclusions, such as manganese sulphide, which are present in allcommercially-produced stainless steels.

Crevice corrosion represents by far the most serious of these two types of attack on

stainless steels in marine components and the vulnerability of crevices arises (as

described in Chapter 3) from the changes in water chemistry which can occur within

crevices - notably reductions in oxygen content and in pH and increases in chloride

concentration - all of which will hasten passivity breakdown. Another important

factor is the tightness of the crevice since, if this is not great, water exchange with the

“outside world” will counteract the chemistry changes just mentioned.

The susceptibility of stainless steels to crevice corrosion in many types of marinecomponents arises because a large proportion of such equipment is deployed in

environmental conditions, aerated seawater, that are optimal for high rates of crevice

attack. It is well established and understood that conditions where low oxygen levels

prevail are usually benign in terms of crevice corrosion of say 316L stainless. Thefundamental rationale for this is that the loss of passivity within a crevice does not, in

itself, automatically lead to severe rates of “crevice corrosion propagation”. This has

been discussed in an earlier chapter but is worth repeating here. A rapid propagation

rate - and hence a practical problem - requires a “macrocell” to exist between thecrevice and the component surface exposed to bulk seawater outside the crevice. A

relatively-high dissolved oxygen concentration in the bulk seawater facilitates the

establishment of such a macrocell which then comprises a large and effective externalcathode fuelling high rates of anodic (i.e. metal loss) reaction within the occluded site.




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This is why severe crevice corrosion of 316L components can occur in near-surface

seawater which is saturated with oxygen. On the other hand, in conditions where thesaline water is denuded in oxygen, corrosion within crevices may be initiated but will

not usually lead to significant penetration.

Influence of stainless steel composition

Different grades of stainless steel exhibit significant differences in resistance to these

localised forms of corrosion with better resistance being conferred by increases in

chromium and molybdenum contents of the stainless steel.

In order to provide an indication of the influence of alloy chemistry upon the pittingor crevice-corrosion resistance, the idea of the "pitting resistance equivalent" (PRE) is

often used. This parameter is defined by empirical formulae like the following:-

PRE = % Cr + x % Mo + y % N

The values ascribed to "x" and "y" vary somewhat in different literature sources with

"x" around 3.5 and "y" in the range 16 - 30.

Table 2 lists a number of commercially-available stainless steels together with their

approximate PRE values. To relate these to practical behaviour, Type 304 stainless is

unsatisfactory for seawater applications due to its susceptibility to localised corrosion.

Type 316L is also rather vulnerable in most marine conditions but the

“superaustenitic”, 20Cr / 18Ni / 6Mo and the 25Cr / 7Ni / 3.5Mo (“superduplex”)

stainless steels possess much enhanced performance in ambient-temperature seawater.

Note also in Table 2, the difference between 316 and 316L – the latter has a specified

lower carbon content in order to provide more resistance to intergranular corrosion

(such as “weld decay”) – as discussed in an earlier chapter.

Influence of electrode potential on localised corrosion of stainless

steels

The onset and propagation of pitting attack and crevice corrosion is also driven by

short-term or long-term positive shifts in component electrode potential. This featurecan be demonstrated in a laboratory experiment by conducting an “anodic

polarisation” experiment in an electrochemical cell. The procedure involves placing a

sample of the material in an electrical circuit and noting its natural electrode potential,

Ecorr, before subjecting the specimen to a continuous, measured shift in potential in thepositive direction at a controlled rate using a “potentiostat’. During this

“potentiodynamic scan”, the current flowing in the electrical circuit is continuously

monitored (together with the potential). A plot of a typical potential/current

relationship for 316L in seawater is shown in Figure 3. Whilst a protective passive

film remains established on the surface of the specimen, the measured current will be

very low but, at a sufficiently positive level of potential (termed the “breakdown

potential”, Eb or the “pitting potential”, Ep) the monitored currents rise to much

higher values and, after this event, examination of say a 316L specimen will revealarrays of pits all over the free surface.




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TABLE 2. Some commercial stainless steels with PRE numbers

ALLOY DESI- NOMINAL COMPOSITION

GNATION %Cr %Ni %Mo %C %N %Cu %W PRE

(UNS number) (max)

Type 304 19 9 0.08 19

(S30400)

Type 316 17 12 3 0.08 24

(S31600)

Type 316L 17 12 3 0.03 24

(S31603)

Type 317L 19 14 3 0.03 29

(S31703)

2205 (Duplex)(S31803) 22 5.6 3.0 0.03 0.15 34

904L 20 25 4.5 0.02 35

(8904)

Superduplex 25 7 3.5 0.03 0.25 1.0 1.0 42

(S32550)

Superaustenitic(S31254) 20 18 6 0.02 0.2 0.7 45

(S32654) 24 22 7.2 0.45 0.4 55

Figure 3: Schematic anodic polarisation plot for stainless steel in a salineenvironment




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It should be pointed out that the “breakdown potential” cannot be specified as a fixedvalue - rather its value depends on many factors. However, higher alloyed

“superduplex” and “superaustenitic” stainless steels possess significantly more-

positive breakdown potentials (and hence much superior corrosion resistance) than

the conventional (say “316”) grades. This feature is demonstrated in Figure 4.

During natural exposure of 316L components to seawater, two factors that are well

known to cause positive drifts of electrode potential to extents that cause pitting

corrosion, are chlorination and the growth of biofilms on the surface of the steel.

There have been numerous reported instances of the the establishment of biological

organisms on surfaces being accompanied by “ennoblement”; i.e. a positive shift in

electrode potential of stainless steels. Whilst the colonising of surfaces by biological

organisms (comprising micro-species - such as bacteria and diatoms - and often also

larger-scale fouling involving seaweed and animals) is inevitable and substantial in

the “lighted” regions of the sea, biological fouling is relatively negligible at greaterdepths. Hence the susceptibility of stainless steels to pitting by this route is muchreduced in subsea equipment.

It may seem rather perverse that the main method of preventing biofouling in

seawater circulation systems, namely chlorination, can also cause pitting degradation

by inducing a positive shift in electrode potential but the theoretical basis of this

phenomenon is well founded and practical instances of such problems have been

documented.

Figure 4: Experimental anodic polarisation results for superduplex (UNSS32550) and 316L (UNS S31603) stainless steels in simulated seawater at 20°C




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Effect of temperature on pitting and crevice corrosion

It is well known that the propensity to pitting and crevice corrosion of stainless steels

increases with temperature. Thus fairly moderately-elevated temperatures of 30 -40°C can result in significantly enhanced susceptibility to localised corrosion of

stainless steels. In some circumstances, this factor will be mitigated somewhat as

temperatures approach 100°C on account of the normally extremely-low dissolved

oxygen contents at such temperatures..

A parameter, which is sometimes used to provide an indication of the relativeresistances of different grades of stainless steel to the onset of pitting or crevice

corrosion at elevated temperatures, is the Critical pitting temperature (CPT). CPT

data are obtained from various types of experiments; one approach is to hold a

specimen at a chosen electrode potential and the temperature of the water

progressively increased whilst monitoring the current flowing in the electrochemical

cell. The temperature at which the current displays an abrupt rise is denoted the CPT.

If the specimens investigated are arranged to contain a crevice, the produced data are

termed “critical crevice corrosion temperatures, CCT. CPT and CCT data, by

whatever method obtained, are of only limited use because:-

1. They tend in general to differentiate the different types of stainless steel in

much the same way as PRE numbers and hence provide very little extra

information than PRE data.

2. They cannot be used with any certainty to specify the actual temperaturesat which a particular stainless steel will suffer localised corrosion in practical

circumstances.

Effects of constructional practice on corrosion resistance of stainless

steel

The corrosion resistance of stainless steels depends on the passive oxide film which

forms spontaneously in air and most waters. Fabrication processes can disturb anddamage the film and thus adversely affect the corrosion resistance of the component

in a number of ways;-

(i) Deep scratches along which corrosion san be initiated in service.

(ii) Embedded iron particles arising from the use of steel wire brushes. The iron

particles will subsequently corrode themselves and also, by acting as crevice formers,

can initiate corrosion on the underlying stainless steel.

(iii) Collection and grinding-in of debris especially on the floor of large vessels from

the passage of workmen.




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(iv) Welding which can leave slag from electrodes, arc strikes, weld spatter, heat

tints, all of which can initiate corrosion by damaging the protective film and leavingcrevice-forming imperfections.

(v) Organic contaminants such as grease, oil, crayon or paint markings, adhesive tape,

can promote pitting or crevice corrosion.

The above possibilities dictate that post-fabrication cleaning procedures should be

employed as a matter of good fabrication practice. However, as a safeguard, the

purchaser should make them a contractual item. Possible cleaning methods are

discussed below.

(a) Degreasing: this will remove organic contaminants which themselves can

stimulate corrosion or can reduce the efficiency of pickling treatments. Degreasing

with non-chlorinated solvents is important since any residual chlorides might induce

crevice corrosion or stress corrosion cracking.

(b) Pickling: this is very effective in removing embedded iron and other metalliccontamination. Pickling is carried out by exposure to a nitric acid/hydrofluoric acid

mixture (usually 10% nitric acid/2% hydrofluoric acid). Small objects are best treated

by immersion in acid baths at about 50°C. On-site piping or vessels can be pickled by

circulation of the

acid or by local application of a nitric acid/hydrofluoric acid paste using a paint roller

or nylon brush. Either immersion or rinsing with clean water is advisable soon afterpickling; otherwise corrosion may be initiated..

(c) Passivation: is carried out with nitric acid. The objective is to thicken the

chromium oxide passive film. This treatment is ineffective in removing surface

contamination but is very useful on machined surfaces. Passivation is sometimes

specified after pickling but is perhaps unnecessary because a pickled surface is

already passivated.

(d) Mechanical cleaning: if this is used (say by grit blasting, sand blasting, glass

bead blasting, grinding) instead of pickling unsatisfactory results can be produced -

such as leaving rough profiles, introduction of contaminants or overheating.

Copper-base alloys

Copper and its alloys possess satisfactory corrosion resistance in many circumstancesin natural waters. As is usual with metallic materials exhibiting good corrosion

resistance, this is due to the presence of a protective film on the surface of the

component. One difference between copper and its alloys and stainless steels is that

the protective film is often somewhat thicker on the copper-base materials and hencemore visible at least in the form of a change in the surface appearance.

Summary comments on the corrosion behaviour of a number of copper-base alloysare presented in Table 3 from which two features stand out:-




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• For adequate durability in seawater, copper, “admiralty brass (Cu-30%Zn) and

copper-nickel alloys with less than 10%Ni are suspect in corrosion resistance.

The vulnerability of the brass alloys to dezincification in seawater is well

documented and these alloys should certainly not be considered for critical-

safety components such as through-hull fittings.

• The corrosion resistance of copper-base alloys is extremely flow-rate

dependent. In the absence of suspended solids, the protective film on these

materials is vulnerable to damage at much lower velocities than for some other

materials such as stainless steels or titanium.

Copper-nickel and nickel-copper alloys

Copper and nickel form a continuous series of single-phase alloys (i.e. in which thetwo elements completely "dissolve" in one another in the solid state. From acorrosion-resistance standpoint, the performance increases with nickel content and

thus as one proceeds down the table of alloys listed in Table 3.

Alloys with 10% Ni or greater can be considered for application in seawater with the

actual material chosen being dependent upon the actual severity of the conditions.

For, instance, the higher the seawater flow, the more likely corrosion problems (e.g

erosion-corrosion) will arise with these alloys and, hence, the more nickel is required

in the alloy.

The presence of iron in some of these Cu-Ni alloys confers additional corrosionresistance by promoting the establishment of a more-protective surface oxide filmupon which the alloys depend for their passive behaviour. The best corrosion

resistance of this alloy system is obtained by specifying a nickel-rich alloy such as

"Monel" (approximately 60Ni/40Cu).

Bronzes

The oldest copper-base alloys termed, bronze, are copper-tin alloys that originate for

ancient times – essentially because they were readily castable. The alloys that are of

greater importance in marine engineering nowadays, however, are the “aluminiumbronzes” which are copper-aluminium alloys containing up to 14% aluminium. The

major group of these are two-phase alloys with 8 – 11%Al whose strength can be

further increased by additions of iron and nickel (nickel aluminium bronze: “NAB”).

One important alloy (“NES 833) has the following approximate composition (%):82Cu, 9Al, 4Ni, 4Fe. These alloys are suitable for casting and for the production of

forgings.

Aluminium bronzes possess good corrosion (and cavitation) resistance in marine

environments in which it is also said to be immune from stress corrosion cracking. The

aluminium content of aluminium bronzes imparts the ability to form, very rapidly, an




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alumina-rich protective film which is highly protective with a good resistance to

localised breakdown and consequent pitting in the presence of chlorides.

Marine applications of aluminium bronze include: valves, pumps, fasteners, couplings

and propeller shafts. In particular for propellers, NAB possesses the required critical

properties: good resistance to corrosion fatigue, corrosion/erosion and cavitationerosion, a high strength-to-weight ratio, good castability and tolerance of welding and

local working for repairing damage sustained in service. This latter aspect of

reparability (e.g. straightening a shaft damaged by, say, grounding) may represent an

advantage of aluminium bronze over the competitive stainless steels.

NOTE: The so-called “manganese bronze” is effectively a Cu-Zn (brass) alloy

containing up to 40%Zn and are vulnerable to dezincification.

Table 3: CORROSION BEHAVIOUR OF SOME COPPER-BASE ALLOYS

MATER- ALLOY COMMENTS MAX

I IAL FLOW*

N m/sec

C Copper Generally suitable for fresh water

R doubtful in seawater

E Brasses

A 70Cu/30Zn OK in some waters; marginal in seawaterS Aluminium brass Often suitable in seawater but vulnerable

E (68Cu/30Zn/2Al at high velocity and in presence ofsulphides 1.0-2.0

D Bronzes

Cu-Al Often suitable in seawater:(especially

C (Aluminium bronze) flowing) but susceptible in stagnant water

O

R Nickel-aluminium Further improved resistance in flowing 10

R bronze seawater

R Cu-Ni Good resistance to SCC and sulphides;

E susceptible if ammonia present

S Cu/10Ni Minimum Ni for seawater 3.0-3.2I Cu/10Ni/Fe

S Cu/30Ni 3.5-4.5

T Cu/30Ni/2Fe/2Mn 4.5-5.5

!

* Max flow in ambient-temp seawater to avoid erosion corrosion: where range is

quoted, upper figure is in clean seawater and lower figure is in moderately-polluted

seawater




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Aluminium alloys

Aluminium possesses the great attribute of low density allied with quite good

corrosion resistance in many environments. However, in order to secure adequate

mechanical strength for many engineering applications, aluminium has to be alloyed

and also, very often, heat treated. The resulting alloys vary quite a lot in their

corrosion resistance with some being questionable in durability in marine situations.

The highest strength alloys are the 2000 and 7000 series which were developed

mainly for the aerospace industry but possess relatively poor corrosion resistance in

seawater. The alloys most usually considered for marine engineering applications are

the 5000 (Al-magnesium) and 6000 (Al-magnesium –silicon) grades. Fabrication

processing can leave aluminium alloys with a thin surface zone comprising a

metallurgical structure which is highly susceptible to corrosion initiation.

Consequently, the application of aluminium alloys in seawater requires carefulconsideration and often there is a necessity for the employment of a protective

coating; this might involve an anodising treatment or anodising plus sealing or

anodising plus sealing plus paint coat. Effective surface preparation prior to aprotective coating operation is important for all metallic substrates (see later) but is

especially crucial for aluminium alloys – one feature of which is the need to remove

the thin surface zone mentioned in the previous paragraph.

A further important factor in the use of aluminium alloys is the danger of severe bi-

metallic corrosion of this alloy when in contact with most other engineering metals

(see Galvanic Series in previous chapter).

A strategy designed to alleviate some of the problems associated with welding

aluminium alloy to steel (e.g. aluminium superstructure to a steel deck, aluminium

decks to steel hull) is to utilise a “transition joint. This comprises a transition piece

comprising aluminium alloy explosively-bonded to steel. The steel segment is

welded to the steel structure and similar for the aluminium.

Nickel-chromium-base alloys

There are a number of alloys available with generally good corrosion resistance. The

most superior in this respect are those with high molybdenum content and the most

commonly-known of these are the “Hastelloys”. Two examples are:-

C-276 (UNS N10276): 16%Cr, 17%Mo

C-22 (UNS N06022): 22%Cr, 13%Mo

Another important material is “Alloy 625” (UNS 06625), 22%Cr, 9%Mo

All these alloys possess excellent resistance to pitting and crevice corrosion althoughAlloy 625 is not as good as the other two in these respects.




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Titanium

This metal and its alloys possess excellent corrosion resistance in a wide range of

aqueous environments. Consequently, it sees application in circumstances where the

conditions are severe (flowing, heated seawater for instance). Although titanium has

traditionally been considered to be expensive and not readily available in forms

suitable for the fabrication of many components, the situation has been improving in

these respects.

Commercially-pure titanium has seen significant application in marine equipment.

Alloying the metal yields enhanced strength and probably the most common titanium

alloy is the Ti-6Al-4V , two-phase (alpha-beta) alloy.

Titanium and its alloys possess excellent corrosion resistance in many environmentsincluding seawater. For instance, their resistance to crevice corrosion is maintained

up to considerably higher temperatures than the stainless steels. On the other hand,

the general wear resistance of these materials is limited.

Also care is necessary in bimetallic contacts involving titanium and its alloys. The

reasons for this are twofold:-

1. Titanium is noble to many other materials in seawater (see Galvanic Series in

earlier chapter).

2. Titanium is susceptible to hydrogen embrittlement. As mentioned in an earlierchapter, one possible source of hydrogen uptake in metals is via the cathodic reaction

during corrosion. Thus, if titanium is in contact with another metal and significant

bimetallic corrosion is occurring on the less-noble component, this may lead to

hydrogen ingress into the titanium. For this reason, special care is needed in coupling

titanium to the most electronegative metals, aluminium, zinc, magnesium. Another

possible source of hydrogen is if a titanium component is attached to a structure

which is receiving cathodic protection, since a poorly-controlled CP system - say

pushing potentials down to about -0.9 V (silver/silver chloride) or more negative -

will increase the dangers of damage to the titanium, especially if this component is

also under stress.

Polymers

A major characteristic of polymers is their usual high resistance to corrosion (or

'degradation' as it is most usually termed when discussing polymers) in many aqueoussolutions or weak acids, bases and salts. Hence, there are now many applications (e.g

water pipework for low-pressure and ambient-temperature conditions) where

polymers can successfully replace metallic materials. However, it should be

emphasised that it is dangerous to be too reliant on this general characteristic.

Polymers often do degrade (albeit by quite different mechanisms than those involvedin the corrosion of metallic materials) in certain conditions as summarised below.




19

1. Some polymer-degradation processes can be extremely rapid - leading to

essentially instantaneous failure of the component. Such mechanisms can involve

direct, rapid chemical attack on the polymer, causing destruction of the large

molecules by breaking them into short-chain segments, or solvation processes inwhich the polymer molecules are essentially dissolved. Examples of chemical

environments which can cause such rapid disintegration of vulnerable polymers are

strong oxidising acids. Another example is the rapid solvation effect of some

organic reagents (e.g.acetone and trichoroethane) on PVC. Clearly, the resistance to

such rapid chemical and solvation attack by specific agents varies widely between

different types of polymer.

2. Other polymer-degradation processes are long-term in nature. One such type is

"hydrolysis" in which (OH)- ions from the environment substitute on polymer chains.

Other degradation mechanisms involve "plasticisation" and "environmental (or

stress) cracking". The former causes a gradual softening of the material and the latteris somewhat akin to stress corrosion cracking in metals but can also involve fatigueprocesses. It is well known that the fatigue performance of metallic materials is

highly dependent upon the associated environment and this feature can also apply to

polymers.

3. Another mode of deterioration is that induced by ultra-violet rays as in sunlight.

Polymers that are especially vulnerable to this form of degradation are polyethylene

and polypropylene but PVC can also be susceptible to surface oxidation and chalkingwhen exposed to UV light.

It is therefore just as important to give careful consideration to the chemical resistance

of any candidate polymer as it is when utilising metallic materials. In this respect,

manufacturers' data on chemical resistance, although often seemingly extremely

comprehensive in the sense of the range of environments listed, may sometimes prove

to be misleading because such data has often been compiled from relatively short-

term tests in highly specific conditions. Thus, as is often stated in such data sheets,

this type of information should only be used as a guide, and careful consideration

should be given to carrying out a practical assessment of any candidate material in the

anticipated service conditions prior to its specification for any purpose. This is

particularly true in connection with the behaviour of polymeric components under

load.

Irrespective of the environmental resistance, a major limitation of polymeric materials

is their strength - particularly as the temperature is raised whence rapid reduction in

strength (and/or eventual thermal degradation) occurs. However, the upper

temperature limits for these materials are being gradually raised by advances in the

development of new polymers. Additionally, improved load-bearing capacities can

be obtained by reinforcement of polymers with glass fibres or carbon fibres.




20

CATHODIC PROTECTION (CP)

Recall the earlier discussion, in Chapter 1, of the electrode potential in which we

concluded that, during normal corrosion, a component adopts an electrode potential

more positive than the equilibrium electrode potential. Eo, for reaction,

M = Mn+ + ne- (1)

That is, the metal exhibits a value of electrode potential at which the above reaction is

stimulated in the anodic direction, i.e. to the right. This is illustrated again in Figure 5

below.

Figure 5: The basis of cathodic protection

As explained in Chapter 1, at potentials more negative than the equilibrium value, Eo,

the electrode reaction (1) cannot occur in the anodic direction, i.e.to the right. Thusat such potentials, the metal is immune from corrosion as illustrated in Figure 5. The

basis of cathodic protection is to force the electrode potential of a corroding metallic

component from its naturally-occurring values (more positive than Eo) to a value

equal to or more-negative than Eo. There are two methods of achieving this requirednegative shift in electrode potential of the component:

(i) By using an impressed current from a D.C. generator or A.C. power source

and rectifier (Figure 6a) with an auxiliary electrode (either inert or expendable).

(ii) By connecting the component to a material which is more reactive (i.e. has a

more-negative electrode potential ). This, Figure 6b, is known as the ‘sacrificial

anode method’ because it relies on the sacrificial corrosion of the reactive material in

order to protect the component.




21

Figure 6: Schematic representation of the two methods of cathodicprotection

Impressed current method

As shown in Figure 6a, with the appropriate polarity connection to the DC current

source (most important), electrons flow to the component to be protected thereby

forcing its potential in the required negative direction. This stimulates cathodic

reactions on the component with anodic reactions being stifled. In the absence of CP,

both anodic and cathodic reactions occur on the surface of a naturally-corrodingcomponent but the effect of proper cathodic protection is that the component only

supports cathodic reactions (hence the name "cathodic protection") with anodic

reactions being transferred to the auxiliary electrode which thus serves as the anode in

the overall electrochemical cell. The anodes can, in principle, be any material - forinstance pieces of scrap steel - but are usually made of "inert" materials (e.g. titanium)

to avoid the inconvenience of periodic replacement.

The decrease in electrode potential of the component will be in direct proportion to

the magnitude of the current supplied from the DC source and the larger the surface

area to be protected, the greater is the required current. Thus the method should

include monitoring of the electrode potential in order to avoid "overprotection" (i.e.forcing the potential to levels well below Eo) which:-

• is wasteful in energy

• can cause hydrogen embrittlement in susceptible materials (e.g. some high-

strength steels, titanium).

Sacrificial anode method

By connecting the component to the more active "sacrificial anodes", a bi-metallic

cell is set up in which the sacrificial metal acts as anode, corrodes and thereby

supplies the required electron flow to the component.




22

As reference to the Galvanic Series in an earlier chapter will confirm, for theprotection of structural steel components (by far the most-important application of

CP), zinc-base alloys, aluminium alloys (formulated to avoid the formation of passive

films) or magnesium alloys can be employed. The “target potential” for carbon steel

(corresponding to E on Figure 5) is about – 0.8V (silver/silver chloride referenceelectrode). A simple representation of this approach, to protect a steel water tank, is

shown in Figure 7.

Figure 7: Representation of use of zinc sacrificial anode in a water tank

General points

(i) The two methods of CP have certain attributes and disadvantages and the choicebetween the two must be made on a case-by-case basis.

Briefly, the advantages of the sacrificial anode method are:-

• it can be used when there is no power

• it avoids capital costs for power equipment

• it is relatively foolproof with little supervision required

•

installation is simple.

The disadvantages are:

• the protective galvanic current available depends on the sacrificial anode area

which, say in large structures, may be cumbersome and heavy and may

involve heavy wiring to keep the resistance low and protective current high

• where DC power is available, electrical energy may be obtained more cheaply

than the cost of replacement anodes.

The use of impressed current methods for cathodic protection has the following

advantages:-• permits much greater control over the system




23

• as mentioned above, it is often cheaper to apply than the use of sacrificial

anodes

• heavy leads are not required since resistance losses may be corrected by

increase in impressed current.

The disadvantages using impressed current are:-• continuous DC power must be available

• care must be taken to ensure that the current is never connected in the

wrong direction since this would cause acceleration of corrosion instead of

protection

• more technical and trained personnel are required.

(ii) Cathodic protection systems need to be carefully designed and engineered in

order to ensure that all parts of a component or structure are completely protected

without incurring excessive costs in electricity or sacrificial anodes. It should be

mentioned, though, that, even if the electrode potential of a component, or part of it, isnot lowered sufficiently to attain the target Eo value to stop corrosion completely,

there will still be a benefit secured. This is because any reduction in the electrode

potential of the component will result in lower corrosion cell currents and hence

reduced corrosion rates; this can be deduced from the schematic polarization diagram

on the right side of Figure 5 – a negative shift in potential from Ecorr moves the current

(and hence corrosion rate) down the anodic polarisation curve.

(iii) Because of the requirement for a complete electrical circuit in the CP/structure

electrochemical cell, cathodic protection can only be used to protect those parts of

structures which are totally-submerged in water or soil.

(iv) In practice, cathodic protection is often used in conjunction with some form of

coating such as painting to minimise the current demand and hence the cost because,

thereby, the protective current only serves to protect any exposed parts of the metalsurface.

(v) In seawater, CP often results in the deposition of a “calcareous scale” on the

component being cathodically protected. The scale is either calcium carbonate or

magnesium hydroxide – the deposition of both being promoted by high-pH conditions

(see Water Chemistry chapter). Such conditions prevail on a surface subject to CP

because of the stimulation of the oxygen-reduction cathodic reaction:-

O2 + 2H2O + 4e- -> 4(OH)- causing relatively-high pH on the metal surface. This calcareous scale is beneficial

because it reduces the magnitude of the required CP substantially – hence lowering

both the capital cost and energy costs of the CP equipment.

Applications of CP include the protection of the submerged parts of steel jetties,

offshore structures (e.g oil-production platforms), sub-sea oil-production equipment,

ship’s hulls, the interiors of water-storage tanks, the exterior surfaces of underground

or underwater pipelines.




24

Particular aspects of the use of CP on ships

Clearly, the most important use of CP on ships is for protection of the main steelwork.

Specific aspects in relation to such activities are discussed below.

As mentioned above, only submerged equipment can be protected by CP; hence thesuperstructures/deck regions of ships cannot be so protected.

CP is often employed, in the form of sacrificial anodes, in ballast tanks but, again note

that such a strategy will only be useful when the ship is carrying ballast water and, even

then, only for the part of the tank up to the ballast water level.

Even for the parts of a ship (e.g. underwater part of the hull) where CP is applicable, it

is always used in conjunction with a paint coating. The latter provides the main

protection and the CP is there to provide protection at any (bare steel) sites of paint

damage or poor application.

Since the required current, to shift the electrode potential sufficiently negative for

protection, is proportional to the water flow, positioning of anodes requires care. For

instance, more anodes may be required near the propeller (including the rudder) –

where turbulence is substantial. A critical feature of the positioning of anodes near the

propeller is the need to ensure that the anodes do not themselves cause cavitation.

In order to maintain the propeller and rudder under the influence of the CP system, they

must remain electrically connected to the hull. For the propeller, this is facilitated by

fitting a slipring around the propeller shaft and grounded to the hull. The rudder is

bonded to the ship’s hull by a low-resistance, flexible, copper cable.

The layout of a ICCP system on the hull of a large modern cruise liner is shown in

Figure 8. This system has zinc reference electrodes (labeled “zinc reference cell” on

the diagram) which are robust and therefore often employed on marine equipment. The

protection potential for carbon steel using the zinc reference electrode is about +200 to+250 mV (equivalent to -800 mV using a silver/silver chloride reference cell). The

dielectric shields around the anodes ensure that the current output from the anode does

not short-circuit near the anode and thereby reaches furthermost parts of the hull.

The dielectric shields can take the form of fibre-reinforced polymer plates surrounding

the impressed current anode (Figure 9) and these plates should take up the smoothcontour of the ship’s hull

There may also be additional (say zinc) sacrificial anodes at high-risk areas such as the

rudder which is subject to high turbulence from the action of the propeller.

As an indication of the electrical requirements for an ICP system on a ship, one ICP

organization provides the following data;-

HULL (total wetted surface area = 19700 m2)

19700 m2 with current density of 35 mA/ m2 = 690 Amp

PROPELLER (area = 62 m2)




25

62 m2 with current density of 600 mA/ m2 = 37 Amp

It is worth noting also that, for a correctly functioning system, the level of current

required to attain target electrode potentials is an indicator of the condition of the paint

system.

Figure 8: CP system on a modern large cruise-liner




26

Figure 9: Schematic representation of dielectric shield surrounding an

impressed current anode on a ship’s hull.

Notes: Not to scale, Anode is recessed

Dielectric shield follows smooth contours of the hull

Influence of cathodic protection (CP) on corrosion behaviour of

stainless steel in seawater

The application of CP to stainless, either as a primary strategy or inadvertently via the

connection of the stainless steel to a carbon steel structure for which the CP is

utilised, is beneficial in counteracting the likelihood of problems due to pitting or

crevice corrosion. This is because the CP currents cause the electrode potential of the

component to be shifted in the negative direction. It is clear that this potential changewill prevent the pitting attack associated with ennoblement effects (see Figures 6 and




27

7) but negative shifts are also beneficial in other ways in reducing problems from

pitting and crevice corrosion of austenitic stainless steels such as 316L. Indeed, theuse of CP specifically to confer protection against crevice corrosion of stainless steels

has become very common in the offshore industries in recent years. In some quarters,

it is felt that the choice of 316L stainless together with CP can be regarded as a safer

bet than the use of superaustenitic or superduplex stainless steel.

In order to secure benefits from the application of CP to stainless steels, it is not

necessary to affect a potential shift down to the levels (-0.8V or lower versus

silver/silver chloride reference electrode) required for carbon steel structures. One

company markets specially-engineered marine CP systems geared to impose

potentials of around -0.2V to stainless steel components. However, shifts to the

more-negative values appropriate for carbon steel will simply provide an even greater

margin of safety to stainless steels. Concern regarding the possibility of hydrogen

embrittlement of CP-protected equipment at sufficiently-negative potentials are not a

factor with austenitic stainless steels such as 316L but may be with stressed

components of duplex stainless steels if the CP potential is allowed to go verynegative.

The CP systems mentioned in the above paragraph are usually referred to as “RCP”

(Resistor Controlled Cathodic Protection). They work on the sacrificial CP principle

but with the additional incorporation of a resistor in series with the sacrificial anode.

Such a system, for control of the internal surfaces of a stainless steel pipe carrying

seawater, is shown schematically in Figure 10. The function of the resistor is to control

the “anode output”, i.e. the current supplied by sacrificial corrosion of the anode(usually zinc) and hence to control the imposed potential on the stainless steel pipe to

the required values – usually about –0.2 V, Ag/AgCl as stated in the precedingparagraph. {Recall that the higher the current in the CP circuit, the more negative is the

imposed potential on the component to be protected.)

The point is that zinc anodes, used in the normal mode of cathodic protection of carbon

steel (see earlier), would impose a potential much more negative than that required for

protection of stainless steel and the relatively high current associated with such

conventional, resistor-free design would result in much shorter anode lives and

therefore more frequent anode replacement, Thus the resistor control maintains the

stainless steel in the protective range of potential with the use of low currents and

hence:-

•

low anode consumption rates• longer lengths of equipment (e.g. pipe) protected by a single anode.

It should be pointed out that the design and operation of RCP systems needs to takeaccount of the level of chlorination (if applicable) and the presence of biofouling

deposits.

Also, as with all types of CP equipment, it is important to incorporate referenceelectrodes into the design in order to provide a monitoring capability to ensure proper

operation of the RCP equipment.




28

Figure 10: Schematic representatioin of RCP system for ptotectinf a stainless steel

pipe. “R” = resistor, “A” = anode

Issues associated with “over-protection” in CP operations

If the imposed electrode potential on a structure is allowed to go much more negative

than the “target potential” (about – 0.8V Ag/AgCl for carbon steel – see earlier), this is

called over-protection. Overprotection is more likely with impressed current than when

utilising sacrificial anodes. Overprotection is wasteful in energy (since more negative

potentials require higher impressed currents) and it increases the dangers of .hydrogenembrittlement in susceptible “materials” such as:-

• welds joint in carbon steel

• high-strength steels

• titanium.

This enhanced susceptibility to hydrogen embrittlement is due to the fact that, at the

relatively-negative electrode potentials associated with overprotection, an alternative

cathodic reaction becomes dominant (compared to the oxygen-reduction cathodic

reaction). This alternative is the “water reduction” cathodic reaction:-

H2O + e- ! (OH)- + Hads followed by

2 Hads -> H2 (hydrogen evolution and no problems)

or

2Hads -> 2Hm (enters metal substrate with H-embrittlement danger).

Stray current corrosion associated with cathodic protection systems.

There are certain circumstances where current flowing between the CP anode and the

structure being protected can cause stray current corrosion on other structures or

equipment. This scenario is shown in Figure 11 which represents the application of

impressed-current CP to a structure (“P”) submerged in say the sea or in an

underground location.




29

The diagram also shows a nearby “secondary structure” or component (“S”) which is

not in direct electrical contact with the protected structure.

Indicated on the diagram are the flows of negative (“e”) and positive (arrow) charge

respectively. A portion of the CP current flowing between the CP anode and the

protected structure passes through the secondary component. In parallel with theexample described in the earlier chapter, the sites (“A”) from which the positive

current departs from “S” will be likely to suffer stray current corrosion. This

phenomenon is often termed “corrosion interaction”.

Three practical marine examples of the situation depicted in Figure 10 are:-

• if “S” was an unprotected ship’s hull moored beside a jetty, “P”, receiving

CP.

• alternatively “S” could represent an ROV connector and “P” the subseastructure which it is physically attached to but electrically isolated from

• stray current corrosion on a ship’s rudder (during application of CP) if the

rudder is not electrically connected to the hull.

The third bullet point above demonstrates a basic method of avoiding stray current

problems when using CP. Stray current effects are eliminated if it is possible to

establish a direct electrical connection between the secondary component and the

structure to be protected.

Figure 11: Stray current corrosion of secondary structure (S) resulting from

application oif CP to structure (P)




30

ANODIC PROTECTION

In certain (fairly rare) circumstances, a metal, which is corroding at finite rates under

natural conditions, can be passivated by shifting its electrode potential to an

appropriately more-positive value. This situation arises in specific metal/environment

conditions where, as indicated on Figure 12:-

• a passive film is not stable in the electrode potential region at which the metal

naturally finds itself (i.e. Ecorr)

• but becomes stable at some more-positive potential range.

Figure 12: Electrochemical basis of anodic protection

Thus the metal can be protected by shifting its electrode potential from Ecorr to a value

of Ep (or more positive) where the establishment of a passive film will confer

corrosion protection in the same manner as naturally-occurring films (on, forexample, stainless steel). The required positive shift in electrode potential can be

achieved by using a set-up rather like the one shown in Figure 6a but with the

opposite polarity and this procedure is known as “anodic protection”. Note that this

is the opposite to the situation in cathodic protection. Other crucial differences

between these two approaches to corrosion control are as follows.

(i) Whereas cathodic protection can, in principle, be applied to any corroding metal

in any medium, anodic protection has only restricted application and is especially

unsuitable when chlorides are present (e.g. seawater).

(ii) Anodic protection is inherently risky if proper electrochemical control is notmaintained. This is because shifting the electrode potential of a component in the




31

positive direction from its natural value, Ecorr, results in an acceleration of corrosion

rate. As shown in Figure 11, even these specific metal/environment systems forwhich anodic protection is applicable, exhibit this characteristic for a range of

electrode potential immediately more-positive to Ecorr before an electrode potential is

attained at which the corrosion rate declines due to the establishment of a passive film

on the component surface. Thus, improper control can lead to acceleration ofcorrosion.

Note again, the inherently safer feature of cathodic protection (as mentioned earlier)

in that a negative shift in electrode potential of insufficient magnitude to completely

stop corrosion will nevertheless still bring about a reduction in corrosion rate.

In summary, anodic protection is a useful corrosion-control technique in certain

favourable circumstances but is certainly not relevant to marine equipment.




32

MODIFICATION OF THE ENVIRONMENT

This approach involves:-

• either removing from an environment some component which contributes to

the corrosivity• or making additions of substances ("inhibitors") which slow down corrosion

rates.

An example of the former tactic would be the deaeration of a feedwater to a plant in

order to reduce the oxygen content and hence control the corrosion rate via limitation

of the cathodic reaction.

Enormous numbers of inhibitors have been formulated - often in an empirical manner

- to reduce corrosion rates in specific systems. The mechanism of action of inhibitors

can involve interference with either the anodic or cathodic reaction or both. A well

known cathodic inhibitor is the oxygen scavenger, sodium sulphite which functions asfollows.

2Na2SO3 + O2 --> 2Na2SO4 (2)

Both these methods are important in controlling corrosion in process industries (e.g.

in close-circuit water systems) but their application in marine engineering is rather

limited. Two scenarios where they find some application are:-

• (sometimes) where seawater is being used as a feed (usually for coolingpurposes) to a coastal industrial plant

•

inhibition of corrosion of offshore hydrocarbon production tubing and

pipework.

In relation to the second bullet point above, although corrosion resistant alloys

(“CRAs”) are receiving increasing attention for such components, carbon steel and

low-alloy steels remain the material of choice for many applications and hence

require corrosion protection. The internal environment of the tubulars/pipes usually

contains an aqueous phase together with the oil or gas. Also CO2 and/or H2S may be

present. Temperatures in extreme high-pressure, high-temperature (“HPHT”) fields

may be up to 200°C. Organic inhibitors represent an important means of controlling

corrosion in these components but their effectiveness in the most severe (hightemperature) conditions is perhaps problematical.




33

PROTECTIVE COATINGS

A multitude of these are available for various purposes. They can be classified by

type: metal coatings, inorganic coatings and organic coatings. Table 4 uses this

classification to list a number of the more-commonly used coatings. The coatings

function at least partly by exclusion of the environment from the substrate but someof them are not completely impervious to corrosive agents, such as moisture and

oxygen, even when properly applied. However, they often have a secondary mode of

protection; many paints, for instance, containing inhibitive pigments.

Table 4: Classification of protective coatings

It should be emphasised that a critical factor in any successful coating operation is the

prior preparation of the component surface which needs to be free from from dirt,

grease and moisture.

Hot-dipped coatings

This is one of the oldest, simplest and generally cheapest coating methods. Itinvolves applying a low-melting-point metal to a high-melting-point substrate. The

coating metals are: zinc, aluminium, tin, lead and the most important metals that are

coated by this method are carbon steels and cast iron.

Galvanising

This involves immersion of the component in a vat of molten zinc and this type of

coating has the ability to provide a degree of protection to coated steel components

for a time even at breaks in the coating on account of localised sacrificial protection

provided by the coating (see Figure 13). Galvanising is very-widely used for the

protection of steel in atmospheric conditions (typical examples are electricity-

transmission towers, motorway crash barriers, scaffolding, railings and bolts).




34

Figure 13: Local sacrificial protection to substrate at breaks in a galvanizedcoating

Although galvanising sees some application for submerged water (including seawater)

service (e.g. pipework), it is unlikely to survive for many years in seawater – on

account of the fairly high corrosion rates of zinc in relation to the galvanised coating

thickness – especially in flowing conditions (as in pipes) which can cause a several-

fold acceleration of the corrosion rate. For deck components, however, longer livesare possible- especially if the galvanised coating is subsequently painted.

Cladding

This (Figure 14) involves welding or hot rolling a relatively-thin sheet of a corrosion-resistant material onto a component made from a material of insufficient corrosion

resistance. An example is the cladding carbon steel process vessels (e.g. thermal

desalination plant) with type 316L stainless steel or copper-10% nickel. Another

widely-employed application, in marine engineering, is weld cladding of “Alloy 625”

(UNS 06625), a Ni-22%Cr/9%Mo alloy, onto steel components (e.g. pipework). Ther

have been instances of the legs of offshore platforms being clad with copper-nickel

alloy around the seawater/atmosphere level – where conditions are especially severe

such that paint coatings are less effective. Clad coatings that are applied by hotrolling the substrate and coating metal may yield superior adhesion of the coating.

Figure 14: Cladding




35

Spray coatings

These are applied to equipment by a variety of thermal-spray techniques - one of the most

effective being the “high velocity oxy fuel” (HVOF) process in which the coating material is

fired onto the component at supersonic velocities and high temperatures to produce a coating

usually of excellent adhesion to the substrate and very-low porosity. A wide range of metals,ceramics and polymers can be deposited using thermal spray techniques.

Some thermally-sprayed coatings (e.g. zinc or aluminium) are employed to provide corrosion

resistance. Others, e.g. ceramics and “cermets’ (such as tungsten carbide-cobalt composites)

are utilised primarily to enhance component resistance against wear. Even the latter type

may need to operate in a corrosive environment and, in this respect, process design andcontrol are crucial to the deposition of a “defect-free” coating. If so obtained, good

protection to the substrate can be attained but the inherent corrosion resistance of many

thermally-sprayed coatings is not as good as, say, electrodeposited hard chromium plate (see

below), since the metal matrix of a thermally sprayed coating may not be as corrosion

resistant as chromium. For example, a widely used HVOF-sprayed coating is WC-Co which

comprises tungsten carbide particles distributed within a metallic cobalt matrix. Also there is

a danger of microgalvanic action between WC particles and the Co matrix.

(Chemical) conversion coatings

These are thin surface zones produced on a component not by a deposition process

but, rather, by immersion in a suitable chemical solution. This results in a chemical

reaction that converts the surface to a constitution that is useful for impartingcorrosion resistance. Two such treatments are phosphating and chromate conversion

(or variants of the latter).

Phosphating

As the name suggests, this process involves immersion in a phosphate solution. It

does not confer much benefit alone but is attractive as a pre-treatment operation

because it converts the surface into a condition that greatly improves the adhesion of

subsequent paint coatings. An acidic solution of zinc phosphate has represented a

traditional phosphate treatment but current technology involves more complex

phosphate baths – including Ni. Fe, Mn in addition to zinc.

Chromate conversion coatings

These treatments have been employed traditionally to “passivate” the surface of acomponent by reaction with a chromate solution, into which the component is

immersed for a few minutes. The chemical reaction yields a thin (of the order of tens

of nanometers) surface layer of a chromium-containing oxide that increases the

corrosion resistance (recall the effect of Cr-rich passive layers on stainless steels).The two classes of materials upon which such treatments (often termed,

“passivation”) are used are zinc-base electrodeposited coatings (see earlier) and

aluminium alloys.




36

On account of health & safety concerns with the “hexavalent chromium (Cr6+)”

solutions that have been traditionally employed for these chromate conversioncoatings, the use of this chemical is being phased out. Alternatives, that have been

widely embraced in several industrial sectors, are based on trivalent chromium (Cr3+)

solutions which are considerably less toxic than the hexavalent chromium variants.

Anodising

This is an electrochemical process in which a protective oxide is grown on the surface

to increase durability. This method is widely used on aluminium alloys and also is

applicable to titanium alloys.

The process is undertaken in an electrochemical cell in which the aluminium-alloy

article is made the anode (hence the name of this process). The cell solution

(‘electrolyte”) can be of several types: chromic acid (now not favoured because it is

an hexavalent chromium compound), sulphuric acid, boric acid, tartaric acid. The

resulting oxide layer comprises a thin barrier layer at its base and a columnar pore

structure throughout most of the reminder of the oxide (see Figure 15). To boost

further the corrosion protection provided by the anodized layer, it is usually subjected

to a “sealing treatment’ in which it is dipped in a solution that seals the pores. This

solution can be sodium dichromate (now not favoured because it is an hexavalent

chromium compound), hot water or others – which, incidentally, can contain dyes that

impart a desired colour to the component.

Figure 15: Schematic representation of the cross-section structure of an

anodized coating

Electrodeposition

The electrodeposition process involves placing the component (M2)is placed in a bath

(virtually always aqueous) and made the cathode in an electrochemical cell. The anode is

often a piece of the metal, M1, that is to be coated onto the cathode. With the appropriate

electrochemical conditions, the anodic reaction is:-dissolution of M1 M1 ! M1

n+ + ne-




37

the cathodic reaction is deposition of M1: M1n+ + ne-! M1 onto the component metal, M2

Examples of Electrodeposition

•

Coating of steel components with zinc, or cadmium, or nickel for corrosion resistance.• Coating of steel components with chromium for wear resistance - often called, “hard

chromium plate (HCP)”. From a corrosion resistance standpoint, HCP possesses

excellent inherent corrosion resistance due to chromium oxide passive film. But, in

order to attain high hardness (therefore good resistance to some types of wear), the

HCP has to undergo a phase transformation during electrodeposition and this leaves a

multitude of fine cracks that potentially provide paths for ingress of corrodent to the

substrate. Thus HCP cannot be relied upon to provide good protection against

aqueous corrosion to steel substrates.

• Coating of steel components with a thin chromium layer for decorative purposes

•

Coating of steel components with nickel + top layer of chromium for corrosionresistance

Issues with electrodeposition

Hydrogen embrittlement

In virtually all electrodepostion processes, the deposition is carried out at electrode potentials

more negative than Eo for hydrogen evolution, 2H+ + 2e- = H2.

Hence, in addition to the metal deposition at the cathode, e.g. Cd2+

+ 2e-

! Cd,hydrogen is produced and, any hydrogen atoms that enter the metal substrate can cause

hydrogen embrittlement. This is a particular issue when applying an electrodeposit onto a

component made of high-strength steel (say UTS > about 1100 MPa).

Consequently, the electrodeposition operation is usually followed by a “hydrogen bake-out”

in which the component is placed in an oven at about 200°C for a few hours in order to expel

any absorbed hydrogen.

Additional surface treatments to the electrodeposited coating

Of course, the objective of most electrodeposition processes is to provide corrosion

protection to a (usually steel) article. The durability of the coated component can be further

boosted by a passivation treatment in which the coated article is placed in a aqueous bath

containing chromic acid which forms a very thin (say 1-2 micron) Cr-rich passive surface

layer.

Additionally, the component can be painted (either unpassivated or passivated) to provideanother corrosion resistant surface layer.




Retrofit/repair paint coatings

For many situations, these utilise a different, water-tolerant paint formulation than the paint

on a new structure. And also may comprise a single, high-build coating rather than theoriginal multilayered version. For example, a typical coating might consist of a thick (say

2mm), single layer of a two-component, epoxy-based paint which can cure underwater.Effective paint application is often compromised by difficulties of access and local coating

conditions. For instance, for repair coating of underwater parts of jetties or other offshore

structures, requires an underwater paint or provision of dry-access conditions by means of a

mobile coffer dam.

Pollution/safety Issues

In recent times, tough regulations/legislation have been introduced all over the world which

have necessitated substantial changes to paint coating systems. Some examples are as

follows:-

• coal-tar epoxies, not so long ago, were widely used as marine paints but are

not allowed these days

• paints containing heavy metal (e.g. copper) are not favoured

• volatile organic compounds (“VOCs”), which were considered essential topaint performance, are nowadays severely restricted.

Antifouling coatings

Paint systems for underwater areas of ships and other marine structures also need to

include an additional, outer, “antifouling coating”. This coating is formulatedspecifically to deter settlement of marine organisms/animals that proliferate in the sea

and which can add substantially to the weight of the structures. In ships, they increase

hydrodynamic drag forces and hence impose serious increments to fuel costs.

Corr Control2014

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Transcript of Corr Control2014