Stress corrosion cracking behavior of Inconel 600 in primary waters

Stress corrosion cracking is a process categorized into many phases. After an incubation period that can last from several hours, short cracks can begin on the surface of a sample. Cracks are just found when their size is higher than detection threshold of the method used and the beginning stage is hence spread with the passage of time. After several hours then in a quick propagation regime where the velocity of the deepest cracks increase by an order of magnitude. The velocity of the shortest cracks slows down and can even stop because of the screening effects if the long crack density is high. It is widely accepted that the transition between the two propagation modes occurs when the stress intensity factor becomes higher than a critical value.

For large crack densities, cracks can coalesce that increases the crack propagation and can hence reduce dramatically the time to component failure.

Stress corrosion cracking of Inconel 600 is usually intergranular with a brittle aspect. The fracture surface can sometimes show a pseudo-intregranular appearance cracking covering about 10% of the fracture surface. The pseudo intergranualr cracking is featured by microfacets at the vicinity of the grain boundary often on grain boundaries developing on angle of less than 45 degree with respect to the applied loading.

Primary water stress corrosion cracking is a complex process and however it has been studied widely over the last 30 years, the cracking process is a matter of discussion. To fuel this discussion, the parameters effecting primarily water stress corrosion cracking are of major concern.

Stress corrosion cracking of austenitic stainless alloys in pure or chlorinated water was first performed. The general behavior for Iron-chromium-nickel alloys in both conditions. Intergranular stress corrosion cracking is a phenomenon involving the synergistic combination of a susceptible alloy, wide environment. The influence of these parameters on Inconel 600 is studied.

A general study of the stress corrosion cracking of austenitic stainless alloy in pure or chlorinated water was conducted. High temperature stress corrosion cracking occurs in alloys containing high iron concentration in chlorinated water whereas alloys with high nickel concentration are sensitive to IG SCC in both pure and chlorinated water. Inconel 600 and other nickel rich alloys are sensitive to SCC in pure water than Iron rich alloys. Alloys containing nickel concentration between 20% to 65% offer good resistance to stress corrosion cracking in high and low temperatures. Therefore preferred types of Inconel bar Inconel 600 and Incoloy 800.

In primary water condition, high stress corrosion cracking dependency on chromium concentration with continuous enhanced stress corrosion cracking resistance from 3 – 17% where no cracking is found. With increase in chromium content resistance to stress corrosion cracking of Inconel 600 increases in primary water condition. It is found that threshold chromium concentration is not confirmed at which stress corrosion cracking is prevented.

It is hard to explain the advantageous role of chromium, it is independent from any mechanistic consideration.