Different types of industrial corrosions that should not be ignored

Pitting

Pitting corrosion is a type of galvanic corrosion in which chromium in the security layer is dissolved leaving behind the corrosion susceptible iron.  Pitting corrosion of stainless steels usually occurs in acid chlorides. They react with chromium to develop the soluble chromium chloride. Hence chromium is eliminated from the passive layer leaving behind the active iron. When chromium is dissolved, the electrically driven chlorides bore in the stainless steel developing a round pit. The residues in the pit are highly corrosive to stainless steel. While using molybdenum or nitrogen as an alloying element, the resistance to pitting in stainless steels enhances.

With increase in molybdenum concentration in an alloy, its resistance to pitting increases at a higher rate. So, molybdenum and chromium based alloys are known to provide outstanding resistance to pitting.

Crevice corrosion

It is another commonly occurring galvanic corrosion that attacks a metal when it is close touch of a crevice agent. Crevice corrosion is easy to identify and determine as when and where it will occur. Similar to pitting condition, a conductive fluid should be available, moreover if chlorides occur , the corrosion rate accelerates. Crevice corrosion is based on the environmental temperature, alloy composition and metallurgical type of an alloy. So, there is a connection between the tightness of a crevice and extensiveness of corrosion. A limit called critical crevice corrosion temperature (CCCT), below which the metal is not attacked.

With increase in difference between critical temperature and service temperature, the chances of occurrence of crevice corrosion increase. Although the effect of temperature is not much clear for crevice, but increase by 60oC or 100oF to CCCT, the pitting can be identified.

Intergranular corrosion

The metallic materials comprise small grains that are oriented in a random manner. Due to their random orientation of the grains, there is a variance among atomic layers where the grains meet which is called a grain boundary.

Carbides are developed when heating occurs, for example welding, heat processing or metal production. Understanding the mechanism of carbide formation can help to find ways to prevent it. For instance, using low carbon grade of stainless steel during welding. Use of these grades is very common in these days because they are made by using argon-oxygen-decarburization refining and all steels are made by using this method as it allows precise control of the alloying elements for example 304L and 316L. The best way is to use a stabilized grade. These grades should be used when the steel is kept for long durations in the temperature range of 800 – 1500oF or 425oC to 800oC.

The stress factor is more fine. Stress should be tensile and exceed the yield strength of the component. When physical pressure is applied the material on a fixed shape, the yield strength exceeds. But situations can be made complicated by stress increasing aspects. Here to overcome these conditions, it is recommended to use high nickel alloys for example Nickel Inconel bars and Monel bars. Corrosionresistant Monel bars are fit for use in stress conditions.