Pitting development and how to prevent this type of corrosion
Pitting is a type of corrosion that causes the development of pits on a metallic surface. It is a type of widely localized however intense corrosion, dangerous as the actual loss of metal is nominal considering the total mass of metal which may be affected. Meanwhile the system damage by perforation is resulted by pitting corrosion. The pits can be small and sporadically distributed across the metal surface or very close together to create an appearance of the general corrosion of the metal. For stainless steels, conditions in which crevice corrosion occurs, pitting is also developed. As the food industry is concerned, it is almost widely caused by chloride based conditions specifically at low pH values.
Several studies are conducted to find the cause of pitting corrosion and all feature a common factor which is that there is damage found in the adherent oxide layer. It causes ionic flow and the creation of an electrochemical cell. There is although, no specific theory which states the reason for the layer cracking. The metal dissolution during the pitting can be due to a surface scratch, dislocation or other defects. Although pit propagation occurs in a way similar to that found with crevice corrosion Similar to crevice corrosion, the pits are usually undercut and on vertical surfaces may create an extended morphology due to gravitational effects.
The onset of pitting corrosion occurs in few days however usually takes several months for the develop of considerable pits. It makes the evaluation of the pitting propensity of a specific condition extremely tough to find, and there are no short-cut lab test methods. Techniques and test solutions are implemented to rank alloys which involve 6% ferric chloride solution. Another chemical method using ferric chloride states the temperature at which the solution causes pitting within 24 hours, the outcomes are stated as critical pitting temperature. Although, both of these methods are used to state the sensitivity of a variety of alloys rather than stating the function of a material in a service condition. Electrochemical methods are also used. Similar to crevice corrosion, the chemical composition of an alloy has a deep effect on the resistance of a material to pitting corrosion.
Inconel and Hastelloy bar alloys are found to be fine resistant to pitting corrosion. They contain nickel as a primary agent. In heat resistant applications, nickel enhances stabilization of austenite and in gamma prime super alloys, nickel combines with other alloying elements to develop inter-metallic phases that serve as alloy strengtheners. Low nickel heat resistant alloys are featured by supreme oxidation resistance, however their resistance is insufficient to prevent carbon corrosion. Nickel when present in alloy by +35%, it prevents the sensitivity of carburization. One method is the limiting carbon diffusion ate in high nickel alloys. Carbon in reducing conditions at temperatures below 850oC enhances metal dusting in various alloys. Nickel based alloys such as Inconel 718 and Inconel 625 also offer suitable performance in preventing pitting corrosion.
