Corrosion What is Corrosion??? Prepared By Dr. Biswajit Saha

Corrosion Corrosion Can be defined as the gradual destruction of metals by the chemical or electrochemical reaction with the environment. Weakening of the material Loss of metallic properties such as malleability, ductility, electrical conductivity etc Example of corrosion Rusting of iron when it is exposed to atmospheric conditions. The rusting is due to the formation of Fe2O3.xH2O on the surface. Formation of green layer of basic copper carbonate of [CuCO3+Cu(OH)2] on the surface of Cu, when exposed to moist air containing CO2.

(Lower energy, stable state) Causes of Corrosion Most of the metals occur in nature in the combined form such as Oxides, sulphites, carbonates etc Extraction of metal Pure metal (Higher energy) Metallic Ore (Lower energy, stable state) Why the Study of corrosion is important??? Corrosion deterioration of machines and equipments. Reduces efficiency The product obtained may be contaminated and may be leads to several problem including health hazards Corrosion- may contaminate stored food, dairy products , etc Corrosion products cause pollution Engineering design is incomplete without knowledge of corrosion

Chemical Corrosion or Dry Corrosion Types of corrosion Chemical Corrosion or Dry Corrosion Electrochemical Corrosion Or Wet Corrosion Chemical Corrosion or Dry Corrosion 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 ( Direct action of oxygen) Liquid metal corrosion (action of liquid metal at high temperature) Corrosion by other gases (H2S, CO2, SO2, N2, H2 )

Oxidative corrosion ( Direct action of oxygen) Mechanism 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. 2M + nO2 2Mn+ + 2nO2- M2On Alkali and alkaline earth metals are rapidly oxidized even at low temperature. At high temperature almost all metals except (Ag, Au, Pt, Pd) are attacked.

Nature of Oxide formed in Oxidation Corrosion A thin layer of oxide formed at the surface of metal can be Stable oxide: The oxide films on Al, Cr, Cu, Pb, Sn etc are stable and adhere to the surface. Such film forms a protective coating over the metal and prevents further oxidation or correction. Unstable oxide: the oxide film formed on the surface of metal decomposed back into metal and oxygen. Hence in such cases corrosion is not possible. The oxides of Ag, Au and Pt. Volatile oxide: the metal oxide film formed it gets volatilized and metal surface again gets exposed for further attack leading to continuous and rapid corrosion. Mo + 3 O2 2MoO3

According to this rule specific ratio calculated follows When the oxide film is sufficiently porous so that the diffusion of cation M+ and anion O- takes place smoothly then oxidation corrosion takes place continuously. Pilling-Bedworth Rule describe the porous and non-porous nature of metallic oxide which is formed during corrosion According to this rule specific ratio calculated follows Specific volume ration = Volume of metal oxide Volume of metal If the specific volume ratio is smaller, then oxidation corrosion will takes place. Because the oxide films will be sufficiently porous for diffusion of M+ and O2- If the specific volume ratio is more than 1 then it will be non-porous.

Corrosion by other gases Depending on the chemical affinity between metal and gas, form protective and non-protective layer on metal surface. Such as CO2, SO2, Cl2, H2S, F2 have corrosive actions on metals. 2Ag + Cl2 2AgCl If the film is non protective it destroys the metal. Chlorine gas attacks Sn to SnCl4 which is volatile and leaves the metal exposed for further attack. H2S gas attacks steel forming FeS layer which is porous in nature.

Liquid metal corrosion This type of corrosion is found in nuclear power plants. Sodium metal leads to corrosion of cadmium in nuclear reactor. 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. It is due to the action of following liquid metal at high temperature on solid metal or alloy.