Question

Can someone tell me "what Electrochemical Factors affecting corrosion behaviour of austenitic stainless steel" ? and a general explaination for each of the factor?

Answer 1

Corrosion is the electrochemical degradation of a material especially metallic alloy due to interaction with its environment. This phenomenon is characterized by the removal of atoms from the metal to form deleterious compounds in the presence of water and gases. The effects and consequences of corrosion has been known to man since the earliest metallurgical times and has been a constant drain on his productive activities, as such, purposeful attention have been focused on the problem both by scientists and engineers. It is a major industrial dilemma which results in millions of dollar lost each year on mechanical breakdowns, maintenance and repairs. Much of this loss is due to the corrosion of iron and steel. Stainless steel is a versatile material with wide spread applications in numerous corrosive conditions. Austenitic stainless steels are an extraordinary family of stainless steel alloys that have exceptional corrosion resistance and equally impressive mechanical properties which allowed them to be used as a construction material in various aggressive environments such as heat exchanger systems, drilling platforms, wastewater treatment, and desalination plants. The most popular austenitic grades have been type 304. The corrosion initiation process and propagation mechanism on stainless steels is of great importance due to its complication and insidious nature. The corrosion resistance of stainless steels is through the formation of thin, passive protective films on their surfaces which coats the base alloy. The film formation occurs instantaneously within an oxidizing environment. Despite these attributes they are vulnerable to specific corrosion types in astringent atmospheres such as pitting and uniform corrosion. Acidic solutions are specifically aggressive to this film causing severe pit formation and growth in addition to general corrosion. Pitting is a localized form of corrosion in which metallic dissolutions occurs at vulnerable sites and/or regions on the metal surface resulting in the formation of cavities in passivated metals or alloys that are exposed to aqueous, nearly neutral solutions containing aggressive anions. It generally occurs in chloride, halide or bromide solutions. If a fault in the passive layer or a surface defect results in the local destruction of the former, dissolution of the steel underneath leads to a build-up of positively charged metallic ions, which in turn causes negatively charges (e.g. chloride ions) to migrate near the defect. The highly corrosive nature of aqueous mineral acids on most metals requires degree of restraint to achieve economic maintenance and operation of equipment, minimum loss of chemical product and maximum safety conditions. In many industries, maintenance costs can be related directly to the cost of corrosion. Mineral acid solutions such as hydrochloric  acid are widely used for various treatments of materials in industry.

Stainless structures and fasteners can fail as a result of stress corrosion cracking (SCC) in coastal atmosphere. In order to have a better understanding of the marine atmospheric SCC mechanism, the corrosion electrochemistry have to be studied.

• Metal ion concentration: Increase of metal ion concentration in sea water if the steel is used for marine purposes will cause corrosion
• pH: The drop in pH making it more acidic and hence many H+ ion. The surface of stainless steel will always be well wetted in real marine atmosphere due to the high humidity, which facilitates the development of corrosion process.
• Adhered substances: The alien adhered substances simulate the conditions of stainless steel structures in practical environments.
• Products of corrosion: The corrosion products of carbon steel that blown to the surface of stainless steel can act as corrosion accelerator.