Crevice corrosion resistance of Hastelloy and Inconel alloys

Inconel and Hastelloy grades have major applications in corrosive environments. Chromium concentration in both of these alloys and molybdenum in Hastelloy alloys have a crucial role in providing corrosion resistance. Addition of molybdenum to Hastelloy modifies the nature of surface from one comprising a susceptible nickel oxide to the one possessing highly corrosion resistant surface comprising mainly chromium oxide. Oxides of chromium decrease the crevice rate and enhance the resistance to it.

How does crevice occur?

 Crevice usually occurs when one material is placed in contact with another. The second material could be a component for example connection like a fastener made from the same material or different material. We consider second material as an elastic band that develops crevice where deposits of mud and oxides and other solid particles that leave precipitates on the surface of alloy test sample. Corrosion due to crevice is caused by exposure to the atmosphere or by retaining water, the other surface beyond the crevice can draw off and dry. Crevice corrosion by metals is a similar mechanism to pitting.

Consideration of crevice corrosion is essential during material selection, particularly when the environment is highly corrosive. Various alloys like stainless steels, Inconel and Hastelloy alloys are widely used in aggressive conditions, whilst there is some sensitivity to crevice and pitting corrosion, the extent of corrosion and degradation is based on the proportions of alloying elements and the concentrations of the corrosive media.

Both Inconel and Hastelloy alloys samples are immersed in water bath corrosion testing equipment at 60oC for a month. Final sample preparation included polishing with 600 grit papers thereafter degreasing in a detergent solution and drying prior to immersion in the electrolyte bath.

Every sample is tightened with an elastic band at the center before being kept in a beaker comprising 3.5% solution of sodium chloride. A lid was placed on every beaker with sufficient pressure to clamp the elastic hand used to suspend the sample to prevent cutting the elastic band. The effect of the proportion of alloying elements in these alloys can only be observed when crevice corrosion occurs at or below 60oC. The extent of surface hardness, the materials and tightening procedures are used to create crevice effect the test results.

Temperature has an essential role in crevice corrosion. With increase in temperature, the beginning of crevice corrosion and thereafter its propagation in the crevice and mass transfer are accelerated. Nickel alloys are not much used in the corrosive conditions because they soon lose their passivity and experience crevice and pitting corrosion.

Microscopic observation by using scanning electron microscopy shows a crevice corrosion region in the Inconel sample. The difference between the component of the sample surface unaffected by the crevice corrosion and the part influenced by the crevice corrosion is easily observed.

The development of a passive layer on the surface usually occurred in both alloys tested in 3.5% sodium chloride solutions. When molybdenum precipitates on the surface as an oxide within a pit it delays and prevents further pitting.

Hastelloy although shows no sign of crevice corrosion on the sample surface. Hence performance of Hastelloy wire with chromium and molybdenum additions attains better crevice corrosion resistance as compare to Inconel alloy. The crevice corrosion temperature for Inconel and Hastelloy is issue to extrapolate from these outcomes and become more complex corrosion process as compare to it is in room temperature.