Corrosion

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CORROSION PROBLEMS IN FERTILISER INDUSTRIES

INTRODUCTION:
Chemical industries encounter failures affecting their productivity leading to enormous financial losses. It is estimated that in process industries the phenomenon of corrosion failure contributes to 60%, while the remaining 40 % accounts for mechanical, operational factors. Therefore, it is necessary to have a through knowledge on various aspects of corrosion in process industries. Fertiliser plants encounter corrosion due to presence of a wide spectrum of corrosive materials. Corrosive materials, which are responsible for corrosion at various stages of fertiliser manufacture, are sulphur compounds, chlorine, carbonates, carbon-di-oxide, ammonia, nascent nitrogen and hydrogen, carbamate etc. The operation of the plant at high temperature and pressure also contributes towards corrosion loss in fertiliser industries.

Generally corrosion failures are classified into uniform corrosion, pitting corrosion, galvanic corrosion, crevice corrosion, inter-granular corrosion, stress corrosion cracking, hydrogen embrittlement, carburisation, sulfidation and metal dusting. Stress corrosion cracking is one of the frequently encountered corrosion problems in fertiliser industries.

Various factors affecting the corrosion are quality of feed stock and pre treatment, presence of impurities like sulphur compounds, chloride etc.

Uniform corrosion is a general attack on the material by forming uniform oxide layer over the material.

Deterioration by pitting is one of the most dangerous and most common types of localised corrosion encountered in aqueous environments. Pitting occurs when the protective film brakes down in isolated spots, where halide salts are in contact with the surface. After initiation, the attack may accelerate because of difference in electric potentials between large area of passive surface and active spot. It can be avoided by proper selection of the material with respect to the environment.

Crevice corrosion is due to the presence of local difference in oxygen concentration or other oxidising agents in the crevice. It occurs due to the presence of crevice between metal and metal or non metal components. Unintentional crevices such as cracks, seams and other metallurgical defects also lead to crevice corrosion. Crevice corrosion progress rapidly.

The best solution for this is to have a design that eliminated the crevice.

Galvanic corrosion occurs when a metal or alloy is electrically coupled to another metal or conducting non metal in the same electrolyte. The three essential components are : 1) Material posssesing different surface potential

2) A common electrolyte
3) A common electrical path
Erosion can be defined as the removal of surface material by the action of numerous individual impacts of solid or liquid particles. Erosion is typically thought to involve the action of dilutte dispersion of small solid or liquid particles entianed in a fluid jet. Stress corrosion cracking is a term used to describe service failures in engineering materials that occur by slow environmentally induced crack propagation. The observed crack propagation is the result of combined and synergistic interaction of mechanical stress and corrosion reactions. The stress required to cause SCC are small, uaually below the macroscopic yield stress, and are tensile in nature. The stress can be externally applied, but residual stress often cause SCC. Intergranular corrosion takes place when the the corrosion rate of the grain boundary areas of an alloy exceeds that of the grain interiors. This difference in corrosion rate is generally the result of difference in composition between grain boundary and grain. The difference in corrosion rate may be caused by number of reasons. A phase may precipitates at at a grain boundary and deplete the matrix of an element that effects its corrosion resistance. A grain boundary phase may be more reactive than the...
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