Corrosion and its types

Study of corrosion is important because Materials are Precious resources  Engineering design is incomplete without knowledge of corrosion  Applying knowledge of corrosion protection c

Corrosion and its types

What is Corrosion

Corrosion may be defined as the gradual destruction of metals by the chemical or electrochemical reaction with the environment During corrosion, the metals are converted to their metallic compounds at the surface The loss of materials due to corrosion has become a great problem The most common example for corrosion is the rusting of iron when it is exposed to atmospheric conditions The rusting is due to the formation of hydrated ferric oxide on the surface Another example is the formation of green film of basic copper carbonate on the surface of Cu, when exposed to moist air containing CO2.

Corrosion Metallurgy in Reverse

4Corrosion: Metallurgy in Reverse 2

2Fontana, M.G., Corrosion Engineering 3rd ed 1986, New York: McGraw-Hill.

Study of corrosion is important because

 Materials are Precious resources

 Engineering design is incomplete without knowledge of corrosion

 Applying knowledge of corrosion protection can minimize disasters

 Corrosion- may contaminate stored food, dairy products , etc

 Corrosion products cause pollution

 Artificial implants for the human body ?

Theories of corrosion

1 Direct Chemical Corrosion or Dry Corrosion

2 Electrochemical Corrosion Or Wet Corrosion

Direct chemical corrosion or dry theory of corrosion

 This type of corrosion occurs mainly through the direct chemical action of atmospheric gases such as O2, halogens, H2S, CO2, SO2, N2, H2 or liquid metals on metal surface in the absence of moisture

There are 3 types of chemical corrosion

Oxidative corrosion

 It is brought about by the direct action of O2 present in the

atmosphere on metals at low or medium temperature in the absence of moisture to form metallic oxides Alkali and alkaline earth metals are rapidly oxidisesd even at low temperature At high temperature almost all metals except (Ag, Au, Pt, Pd) are attacked.

Liquid metal corrosion

 It is due to the action of following liquid metal at high temperature on solid metal or alloy This type of corrosion is found in nuclear power plants Corrosion takes place either due

to the dissolution of solid metal by the liquid metal or due to the penetration of liquid metal in to the solid metal.

Corrosion by other gases

 CO 2 , SO 2 , Cl2,H 2 S, F 2 also have corrosive actions on metals.

 The extent of corrosion depends on the affinity of the metal and the gas involved in corrosion.

 If the film formed is protective then extent of corrosion decreases, Cl 2

 If the film is non protective it destroys the metal Chlorine gas attacks Sn to SnCl 4 which is volatile and leaves the metal exposed for further attack.

 H2S gas attacks steel forming FeS layer which is porous in nature.

ELECTROCHEMICAL CORROSION

OR WET CORROSION

This type of corrosion occurs when :

solution

This corrosion is due to the existence of separate anodic and cathodic areas between which current flows through the conducting solution

the anodic and cathodic areas One part of the metal becomes anode and the flow of electron current between the anodic and cathodic areas takes place through an electrolyte in contact with it The anodic reaction involves the dissolution of metal to metallic ions with the liberation of electrons Hence corrosion always takes place at the anodic area

 At anodic area, M Mn+ + ne- (oxidation)

 The cathodic reaction consumes ne- with either by the evolution of H2 or absorption of O2 depending on the nature of environment.

Evolution of hydrogen

Consider the corrosion of iron The anodic reaction is the dissolution of

Fe as Fe2+ ions with the liberation of electrons

these electrons are taken by H+ ions present in acidic solution forming

Absorption of oxygen

 In neutral or slightly alkaline medium, the cathode reaction takes place with the absorption of atmospheric O2 In this type, the electrons liberated from Fe flow from anodic to cathodic areas where these ne- are received by O2 dissolved forming OH - ions

 ½ O2 + H2O + 2e- 2OH- (reduction)

 The liberated Fe2+ ions at the anode and OH- ions at the cathode diffuse and combine to form the first corrosion product namely Fe(OH)2.

composition Fe3+2O3 2H2O which is known as rust

TYPES OF ELECTROCHEMICAL CORROSION

 Galvanic corrosion

 Concentration cell corrosion

a) Drop corrosion b) Water line

Galvanic corrosion

Concentration cell corrosion

surface exposed to an electrolyte of varying concentrations Metal in contact with lower concentration will act as anode and undergoes corrosion It is observed in chemical plants and storage tanks and also in marine structures like ships Differential aeration corrosion is the most important

Differential aeration corrosion

 This type of corrosion occurs when one part of the metal is exposed to a different air concentration from the other part This causes a difference in potential between differently aerated areas It is found that less oxygenated part acts as anode more oxygenated part acts as cathode.

 Corrosion of metals partially immersed in a solution is due to differential aeration corrosion Consider a Mg rod partially immersed in NaCl solution The areas of Mg rod above and just below the water line are more oxygenated and will act as cathode The remaining part of the rod which is well under the solution is less oxygenated and act

as anode and undergone corrosion.

Pitting Corrosion

formation of pin holes, pits and cavities on the metal surface It is due to the breakdown or cracking of the protective film on the metal at specified points This gives rise to the formation of small anodic and large cathodic areas Once a small pit is formed the rate of corrosion will be increased

Crevice corrosion

etc

region because oxygen supply to these areas is restricted compared to exposed areas which become the cathode

Microbiological corrosion

Erosion corrosion

 It is the relative movement between a corrosive fluid and metal surface.

 All types of equipments exposed to moving fluids are subjected to this type of corrosion

 Examples ship propellers, hydraulic turbines, pump impellers, diesel engine cylinders.

Soil corrosion

 A) Acidity of the soil

 B) Moisture and electrolyte

 C) Micro-organisms

 D) Differential aeration

 Examples : underground structures and pipelines for oil gas and water

Examples of stress corrosion

 Season cracking: It is observed in cold drawn brass articles.Corrosion cracking of brass takes place only in presence of traces of ammonia or amines

 Caustic embrittlement:

 Fe/Conc NaOH // Dil NaOH/ Fe