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SUBMTTED BY-Koushik Das1 st sem,Roll no. 10School Of Industrial FisheriesC.U.S.A.T
Corrosion, fouling & boring of the hulls of fishing vessels are serious problems of “high economic significance”. Fouling in marine environment is initiated by the micro- organisms like bacteria, fungi & algae to the surface of the exposed material known as “Bioflim”. More than 4000 species of animals and plants which are include bacteria, diatoms, algae, fungi, protozoan, etc. are recorded from fouling communities (Venkatesan et al.,2006)
Corrosion is the term used to described the conversion of a metal to a non-metallic state. Corrosion is defined as the destruction of a metal by chemical or electro-chemical reaction with it’s environment.Metal + Environment Metal Compound + Free Energy(Fe; Zn) (H2 o; O2; H2S) (Metallic Oxide, Hydroxide, Sulphide) (Fe2 O3, Fe(OH)2, Fe(OH)3, ZnS)
TYPES OF CORROSION:- UNIFORM/ GENERAL CORROSION GALVANIC/ TWO METAL CORROSION CONCENTRATION CELL/ CREVICE CORROSION PITTING CORROSION MICRO-BIOLOGICALLY INDUCED CORROSION STRESS CORROSION EROSION CORROSION FILLIFORM CORROSION SELECTIVE CORROSION
In this type of corrosion take place uniformly over the surface of the metals. Uniform corrosion can be prevented by selecting corrosion resistant alloys, apply chemical inhibitors & coating cathodic protection. Metals such as steel that do not form passive films show uniform corrosion.
Corrosion which occurs when dissimilar metals are in electric contact in presence of an electrolyte such as seawater is termed as galvanic corrosion. Galvanic corrosion can be prevented by selecting the alloys of similar electro-chemical nature (making the area more actively corroding material ‘anode’ larger than the ‘cathode’).
It is a localized corrosion which may occur in small areas of stagnant solution in crevice, joints & under corrosion deposits. This can be prevented by closing the crevices with non-absorbent materials & preventing build up of scales or any solids on the surface of material.
It is a highly localized corrosion occurs on the metal surface. It occurs as a process of local anodic dissolution where metal loss is the presence of small anode & large cathode. Pitting corrosion can be prevented by increasing the velocity of media or removal deposits of solids form exposed metal surface. Suitable selection of alloys, chemical inhibitor & de- aeration of aerated environment will help to reduce pitting.
In the micro-biologically induced corrosion, micro- organisms like bacteria or fungi are involved in the degradation of material. The type of organism involved in the process depend on the substrate and the environment. It is a severe problem in the case of cooling water & submerged pipelines in soil.
Stress corrosion cracking may be occurs at stresses well below the yield point of the alloy. Generally, corrosion does not lead to the mechanical failure of structures or component because the reduction in cross sectional area is controlled either by choosing a corrosion resistant alloy or by applying suitable protective coating.
Interaction with particulate matter present in the fluid, flow of liquids in tubes especially at bends can leads to mechanical damage of the internal surface of tubes leading to erosion corrosion.
This is a process in which one of the materials of a binary alloy selectively dissolves leaving a spongy mass behind. An example is the corrosion in brass and bronze materials.