Corrosion and its types

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 CO Corrosion: Metallurgy in Reverse2 Corrosion Metallurgy in Reverse Fontana, 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 Direct Chemical Corrosion or Dry Corrosion 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 O , halogens, H S, CO , 2 SO , N , H or liquid metals on metal surface in the absence of 2 moisture There are types of chemical corrosion Oxidative corrosion  It is brought about by the direct action of O 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  CO2, SO2, Cl2,H2S, F2 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  If the film is non protective it destroys the metal Chlorine gas attacks Sn to SnCl 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 : A metal is in contact with a conducting liquid Two dissimilar metals or alloys are immersed partially in a conducting solution This corrosion is due to the existence of separate anodic and cathodic areas between which current flows through the conducting solution TYPES OF ELECTROCHEMICAL CORROSION  Galvanic corrosion  Concentration cell corrosion a) Drop corrosion b) Water line  Pitting corrosion  Crevice corrosion  Erosion corrosion  Soil corrosion  Stress corrosion  Intergranular Corrosion  Microbiological corrosion  Selective leaching corrosion  Hydrogen damage  Stray current corrosion Galvanic corrosion Concentration cell corrosion  This type of corrosion is due to electrochemical attack on the metal 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  Pitting corrosion is a localized accelerated attack resulting in the 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  It is produced at the region of contact of metals with metals and non metals  It occurs at the crevice between different metallic objects such as bolts, rivets, nuts etc  When these crevices come in contact with the liquids these become the anodic region because oxygen supply to these areas is restricted compared to exposed areas which become the cathode Intergranular corrosion  When a molten metal is solidified starts at number of randomly distributed nuclei within the molten mass then each of them grow in a regular pattern or atomic to form grains  Grain boundaries are generally more prone to corrosive attack Grain centres behave as the cathode and grain boundaries as anode  Aluminium copper alloys are attacked by this type of corrosion Microbiological corrosion  i) Aerobic condition  Sulphur bacteria causes this type of corrosion  Corrosion takes place by differential aeration mechanism  ii) Anaerobic condition  Sulphur reducing bacteria cause corrosion of iron and steel  The corrosion that occurs in the hull of titanic is of this type   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 Stress Corrosion  It takes place due to the combined effect of mechanical stresses and the corrosive environment of a metal  This type of corrosion takes place in the fabricated articles of some alloys such as zinc brasses, nickel brasses etc  The metal portion under stress becomes anode and other portions become cathode 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 [...]... Concentration cell corrosion a) Drop corrosion b) Water line  Pitting corrosion  Crevice corrosion  Erosion corrosion  Soil corrosion  Stress corrosion  Intergranular Corrosion  Microbiological corrosion  Selective leaching corrosion  Hydrogen damage  Stray current corrosion Galvanic corrosion Concentration cell corrosion  This type of corrosion is due to electrochemical attack on the metal surface... anode and OH- ions at the cathode diffuse and combine to form the first corrosion product namely Fe(OH) 2  Fe2+ + 2OH - Fe(OH) 2  In presence of excess of O2, Fe(OH)2 is changed in to Fe(OH)3 with the composition Fe 3+  2Fe(OH)2 + ½ O2 O 2H O which is known as rust 2 3 2 Fe O 2H O rust 2 3 2 TYPES OF ELECTROCHEMICAL CORROSION  Galvanic corrosion  Concentration cell corrosion a) Drop corrosion. .. 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. .. centres behave as the cathode and grain boundaries as anode  Aluminium copper alloys are attacked by this type of corrosion Microbiological corrosion  i) Aerobic condition  Sulphur bacteria causes this type of corrosion  Corrosion takes place by differential aeration mechanism  ii) Anaerobic condition  Sulphur reducing bacteria cause corrosion of iron and steel  The corrosion that occurs in the... 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... structures and pipelines for oil gas and water Stress Corrosion  It takes place due to the combined effect of mechanical stresses and the corrosive environment of a metal  This type of corrosion takes place in the fabricated articles of some alloys such as zinc brasses, nickel brasses etc  The metal portion under stress becomes anode and other portions become cathode Examples of stress corrosion. .. hull of titanic is of this type   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)... Corrosion  Pitting corrosion is a localized accelerated attack resulting in the 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  It is...Mechanism  Electrochemical corrosion involves the flow of electron current between 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... with metals and non metals  It occurs at the crevice between different metallic objects such as bolts, rivets, nuts etc  When these crevices come in contact with the liquids these become the anodic region because oxygen supply to these areas is restricted compared to exposed areas which become the cathode Intergranular corrosion  When a molten metal is solidified starts at number of randomly distributed ... rust TYPES OF ELECTROCHEMICAL CORROSION  Galvanic corrosion  Concentration cell corrosion a) Drop corrosion b) Water line  Pitting corrosion  Crevice corrosion  Erosion corrosion  Soil corrosion. .. ? Theories of corrosion Direct Chemical Corrosion or Dry Corrosion Electrochemical Corrosion Or Wet Corrosion Direct chemical corrosion or dry theory of corrosion  This type of corrosion occurs... Stress corrosion  Intergranular Corrosion  Microbiological corrosion  Selective leaching corrosion  Hydrogen damage  Stray current corrosion Galvanic corrosion Concentration cell corrosion