Corrosion _ February 2010

INSTITUTE OF ENGINEERING TECHNOLOGY - KATUNAYAKE

CORROSION – 1.1.3

Materials that are not stable interact with the environment (atmosphere) and result in chemical and micro structural changes and degrade (reduces) the properties.

Metals – are not stable highly reactive and oxide in the normal atmosphere and corrodes in the presence of chemicals high temperature humidity etc.

Corrosion is defined as degradation of a metal by an electrochemical reaction with its environment metals react in dry environment. Metals react in dry environment (gases) or wet environments (aqueous solutions).

Dry environment (gases) – dry corrosion or oxidation (with O2 oxygen in air)

Wet corrosion - Wet corrosion or corrosion (aqueous corrosion)

OXIDATION

Fe++ Non metal oxide FeO Fe Fe++ + 2e- METAL e ½ 2O2 + 2e O--

Fe Fe++

Fe++ e O2 (Air)

LOOSELY ATTACHED ELECTRONS

Net Reaction Fe + ½ O2 Fe ++ + O-- FeO

Most metals react with O2 in the air and form a thin layer of oxide on the surface at room temperature. The formation of this layer of oxide is very important. It protects the metal surface from any harmful corrosion reactions taking place any further ie the oxide film is passive.

The oxide films formed on some metals may completely prevent corrosion. These are alloys of aluminium, chromium, titanium, nickel and stainless steel.

Marine Engineering Department of 14


These oxides.

(1) Do not contain any cracks or pores (holes) (2) Have a low conductivity (electrons and ions do not move through the oxide at

a fast rate) and (3) Adhere (bond) well to the metal.

Al2O3 aluminium oxide – very thin layer 5 – 10 microns.

The oxide films that do not completely prevent are iron & steel Na (sodium) Mg magnesium these oxides

(1). Contain cracks and pores (holes)(2). Have a high conductivity (3). Do not adhere (bond) well to the metal

Pores Cracks

Metal oxide

Fe2O3 – Thick oxide layer

Corrosion is a complex subject and lot of research is being done.

But formation of galvanic cells is the main cause of corrosion, and these can be formed in the near pure boiler water, sea water or other electrolyte.

The galvanic elements are provided by, dissimilar metals or mill scale. Scale oxide film on the surfaces of the metal or the differences in surface structure, inclusion composition of the metal, salts, bacteria, oil degradation products in contact with metal surface.

Aqueous corrosion (corrosion)

Corrosion of metals in liquid environments. Proceeds due to the formation of an electro chemical cell. The requirements for this are.

(1). An anode and a cathode (2). The anode and cathode must be in contact



(3). They must be in a liquid containing ions (a electrolyte) which will conduct an electric current.

Liquid environment (electrolyte)

Oxidation Current Reduction Process Process

Flow

Electricity

Flow Anodic CathodicRegion Metal Region

Schematic illustration of metal corrosion

MOST IMPORTANT AQUEOUS ITEM ON BOARD

Water - Composed of hydrogen and oxygen atoms. But also contains hydrogen ioxs and hydroxyl ions.

Hydrogen - A hydrogen atom which has lost its electron.

Written as H+ - indicates excess + ve electrical charge or H – e also indicate loss of the electron.

Hydroxyl ion - Compound of oxygen and hydrogen which has gained and electron.

Written as OH- indicates an excess of negative electrical charge or OH + e indicating a gain of an electron

PH Values - Relative concentrations of above ions in water is important.

The product of hydrogen and hydroxyl ion concentration in water at approximately 250 C must always be equal to 10-14 gm ion / litre of solution.



If hydrogen Ion concentration is greater than hydroxyl ion concentration then water is acidic if concentrations are equal then water is neutral if hydroxyl ion concentration is greater then water is alkaline.

Concentration 10-5 (H+) x 10 -9 (OH-) Solution acid

Equal 10-7 (H+) x 10 -7 (OH-) ,, Neutral

Minus sign is more than is less hydrogen ions. 10-9 (H+) x 10 -5 (OH-) ,, Alkaline

Hydrogen ion concentrations 5, 7 & 9 indicates the degree of acidity or alkalinity.

PH value P-for power H-for hydrogen 10x PH value is the logarithm of the reciprocal of the hydrogen ion concentration.

e.g. 10-5 = 1/105

Reciprocal = 105

Logarithm = 5

PH values range from O to 14 ie from very acidic to very alkaline. (ie from very acidic to very alkaline)

If water temp is increased the hydrogen ion concentration increases. (ie increase in acidity decrease in alkalinity)

If you add chemicals to water – IT alters the hydrogen ion concentration ie PH value.

Acids – Lower PH value

Alkalis – Increase ph value

In electro chemical corrosion pH value is very important.

What happens when iron is in contact with water containing hydrogen ions – result corrosion.

The hydrogen ions in contact with the metal surface become hydrogen atoms by taking an electron from the metal. The resultant metal ion (caused through loss of electrons) combines with the hydroxyl ions in contact with the.



Metal surface and form metallic hydroxide which is soluable in the water depending upon the PH value. Therefore the metal is corroded the action is similar to a battery action where current is caused to flow from anodic to cathodic regions, the migrating irons in the electrolyte (water) and the electrons in the metal form the circuit.

The hydrogen which has formed on the surface of the metal due to the combination of hydrogen and metal electrons may form a polarizing layer upon the metal’s surface.

This will prevent further corrosion. But there will pre dissolved oxygen in the water which will combine with hydrogen to form water and no polorisation will occur and corrosion will continue. Also if water is acidic enough the hydrogen can leave the surface of the metal in the form of hydrogen gas again preventing polarization and continue corrosion. That is the reason for boiler water to be alkaline and with litre or no dissolved oxygen content.



O2 + H2O in the environment hydroxide or Oxide corrosion products

Hydroxyl OH– ions

Metal ions dissolve

Less active Electron Cathodic Flow Active anodic area area

Net reaction Fe++ + 2 (OH-) Fe(OH)2 Rust

Fe Fe++ + 2e-

Oxidation reaction – the actual corrosion process occurs

At the cathode ½ O2 + H2O + 2e- 2 (OH-)

(in neutral / alkaline solutions)

2H+ + 2e- H2 (in acidic solutions)

On a single piece of metal, anodic and cathodic areas can form due to localized differences in the environment. However most of the corrosion failures occur when two different metals are in contact in an electrolytic solution.

Three fore it is the electrode potential difference between two deferment metals which causes corrosion of one metal.

Sea water is circulated, heated stored within a vessel. It is a strongly corrosive medium because it is a good electrolyte. With dissimilar metals in seawater galvanic action results and the more anodic metal corrodes.

Salts dissolved in seawater Nacl, Mgcl2 Mg So4 (CaSO4, Ca(HCO3)2



GALVANIC SERIES OF MATERIAL IN SEA WATER

Titanium More positive (√ ) Noble end of Craphite table Monel Metal Stainless Steel (With Oxide Film) Cathodic Inconel Nickel 70 / 30 Curro nickel Gunmetal Aluminium Bronze CopperAdmiralty ForassMaganese Steel (Without oxide film) Cast Iron Milk Steel Zinc Aluminium Anodic Magnesium Base end of table

This is also known as the EMF (Electromotive force) series. Steel is anodic to bronze in sea water so steel can give cathodic protection to bronze.

If two metals are in contact the metal with the most positive E0 will always be the cathode.

The difference in electrode potentials for the two metals represents the driving force for current flow and accelerated corrosion of the anodic metal.



Galvanic corrosion

Rust Fe (OH)2

Moisture and O2

Fe++ (OH)- From Air

Iron anode Copper (Cathode) Anode

Fe Will Corrode

Generally, the cathodic metal corrodes very little or not at all. Anodic metal corrodes rapidly when the metals are in contact the larger the difference in electrode potential the larger the effect.

Cathodic Protection

Sacrificual anodes are used deliberately to give cathodic protection to the more expensive material eg. Iron anodes can give protection to brass tubes and plates of a condenser.

Magnesium anodes to give protection to steel plates in tanks.

Znc anodes are used to give protection to the tube stack and tube plates in coolers when the cooler covers are cast iron.

Practical methods and points to reduce galvanic effect

(1). Using materials which are close to each other in the galvanic series.

(2). The main component or item of the system to be made from the more noble metal (that is the metal to be protected)

(3). To provide a large area for the less noble material (metal) so that although the corrosion is increased and fast it is spread over a larger area (gives It more longer life)

(4). Graphite grease not to be used in sea water system as graphite is more noble it may cause corrosion of the less noble bronze and steels if they are in contact with it.

Pitting corrosion



May be over a large are ie pitting type of corrosion or it may be localized ie pitting type corrosion. This type of corrosion is caused when there is a large cathodic area and a small anodic area the intensity of the attack on the anode is very high. The large area difference.

Caused by, mill scale, oxide films acid pockets of water scale from salts, pores and crevices, oils and gases and ingress of metals into the boiler. This is dangerous in boilers as the rate increase with the temperature increase where metal surfaces are very hot. Therefore failure may take place.

Dust

Cathode Cathode

Fe

Electron Flow Electron Flow

Anode Corrosion PIT

If there is Cl in the environment in edition to O2 + H2O

Fe++ + 2Cl FeCl2 can also form

But FeCl2 is soluble in water and

FeCl2 + 2H2O Fe(OH)2 + 2HCl

Graphitization of cast iron

Cast iron contains 2 – 3.5% carbon which is mainly in the form of graphite flakes. (or spheroids) embedded in a metal matrix. In contact with sea water the metal matrix corrodes away the graphite is exposed. This is called graphitsation and stimulates corrosion of metals. So if there are bronzes and braze etc it can have a bad effect because is more noble.

Eg. Cast iron sea valves

Velocity of sea water



If the velocity relative to the material increases the rate of corrosion increases there may be some limiting valves.

Reason – (1). Increase supply of oxygen (2). Erosion of the protective oxide film formed by corrosion

Stress Corrosion

A metal consists of crystals or grains the atomic arrangement is regular (ie it has a patters) there is also amorphous (structure less) (ie no proper structure) metal surrounding the crystal structure. When sea water is in contact the weak structure corrodes. If stresses are locked up within the metal they can partly be relieved by this corrosion which causes more amorphous material to be exposed to corrosive attack and if this continuous failure of the metal will take place.

This is mostly found in brasses, but also occurs in aluminium alloys, stainless steel.

Dezincification / Dealuminification

Brass – alloy of copper and zinc in sea water – zinc is anodix to copper and it corrodes leaving a spongy mass of copper this is called bronze dezincification.

To prevent – arsenic is added Zinc content is made less than 37%.

Similar attack - Aluminium bronze - called De aluminification To prevent – 4% to 5% nickel added to bronzes

Fretting Corrosion

Two surfaces in contact with each other undergo slight oscilatory motion (microscopic) (ie movement can be hardly seen by eyes) relative to one another.

Egs :- Those which have been shrunk hydraulically pressed, mechanically tightened one to the other.

Relative motion causes removal of metal and metal oxide film. Removed metal powder combine with oxygen and form oxide powder – in the case of ferrous metals the oxide powder is harder and will cause wear. Removed oxide film would be repeatedly replaced increasing the damage.



Eg. (Tie bolt) (A- frame)

Factors that effect damage caused by fretting corrosion.

(1). If amplitude is large and frequency is high damage is increased.

(2). Load carried by the moving surfaces

(4) . Damage is reduced if oxygen is low

(5). Hardness of the metal surfaces

Corrosion fatigue

If a metal is in a corrosive environment and is also subjected to a cyclic stress. It will fault a much lower stress concentration than that normally required for fatigue failure.

This is due to the weakening of the amorphous material corrosion and stress relief. Eg. In boilers cycle stress set up in Boiler Tubes – due to variations of temps , due to vibrations of the tubes ,fluctuations in thermal condition ie thermal pulsations.

Factors that effect corrosion

(a). Temperature differences in the system which causes potentials to change in the same material.

(b). Stresses in the structure of the metal can be different in various locations which can cause different effect electrical potentials in these locations.

(c). During manufacture of the metal crystal formation various regions can vary due to uneven temps etc.

(d). Impurities in the metal

(e). Flow of oxygen to the cathode

(f). Flow of carbon diode to the anode and cathode

(g). Hydrogen ion concentration of the solution



METHODS OF CORROSION PREVENTION

In the case of some metals once the oxide is formed naturally on the exposed surface of the metal. The metal underneath is rendered passive and oxygen does not come in contact for further corrosion to take place. If the oxide coating is damaged corrosion will start again. For the metal to be protected therefore the oxide coating must be intact. But the best form of protection is provided by application of suitable films or coatings which adhere to the metal surface.

Surface preparation before application of the coating is very important is the applied film or coating will come of the metal surface.

Surface preparation

Dirt, Rust, Oil, Grease and other impurities must be removed (eg. By wire brushing, chipping, remove oil using chemicals) surface preparation of hull of ship in dry dock.

Egs. Sand blast or grit blast the surface to form a rough surface (the hull of the vessel is prepared in this manner. There are standards to which the hull can be blasted (Eg. SA1 SA2 – SA2.5 – SA 2.5 is the best as it indicates the metal surface is bare metal)

Selection of a primer

The initial coating of paint should contain corrosion preventing inhibitors.

There are selected to suit the metal id Fe, Al etc.

Red lead oxide – Manufacturer advice

Application of coatings for surface protection paints (organic coatings0



The most important factor is the preparation.

The selection of the primerTop coating or coats (multiple coats)

When the hull is painted in the dry docks paint coating thickness is measured. Dry film a wet film thickness – no of coats to last till the next docking.

Metallic coatings

Than metal coatings provide a relatively good barrier between metal and its environment.

Hot dipping Old method – the component is immersed in a bath of molten metal and a metal coating is formed on it.

The coating metal should be of low melting point example zinc, Tin, Lead, Aluminium.

Electroplating

The component to be coated is immersed in a solution of the metal to be plated in an electrolytic cell. Then a direct current is passed between the component and the other electrode (coating metal) to make the component cathodic and the metal to be plated anodic due to the chemical reaction that occur, a coating deposits on the component.

A later of sheet metal put on (bonded) at high temperatures to by hot rolling two sheets of metal together.

Inorganic Coatings

Enamel is a mixture of powdered metal oxides (TiO2, ZnO2) this is sprayed on to the component at high temperature and then cooled. The components are reheated to form a shiny protective film.

Anodizing



Anodizing is the process of forming a surface oxide film by electrochemical oxidation. It is used as a surface finishing technique particularly for aluminium forms an AL2O3 layer that is about 10 – 100 m in thickness. Different colours can be obtained by adding dyes to the electrolyte.

Stress corrosion brass – referred to as season cracking.

If a brass sheet is bent and shaped cold work is applied to the bend and shaped areas and the metal will be unequally stressed as between un worked and highly worked areas. This sets up a galvanic couple between adajacent areas of unequal stress heat exchanger tubes.

Stress relieving after cold working process is necessary.

Eg. Of Metals – Brasses, Aluminium Alloys, Stainless Steel


Corrosion _ February 2010

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