Behavioural approaches of Nickel alloys to NaOH solutions

Corrosion resistance of pure nickel-chromium alloys is determined to find their usefulness in the different temperature-concentration ranges with respect to general corrosion rate and stress corrosion cracking susceptibility.

In boiling 10% NaoH at 103oC, different alloys such as Monel 400, Inconel 600, Inconel 625, Hastelloy C276, Hastelloy C4, C22 and C2000 are tested to determine their low corrosion rates irrespective of nickel concentration. In a solution of boiling 50% NaOH at 14oC and 70% NaoH boiling at temperature of 182oC, the tested alloys show nominal corrosion rates. Inconel 600 and Monel 400 are the commonly used materials to handle hot concentrated caustic. They also offer supreme corrosion resistance at all concentration as anticipated.

In boiling 50% NaOH an anticipated advantageous effect of increasing nickel concentration is observed. It is found that critical nickel concentration for corrosion resistance in this solution is almost half. Although 20 to 30% nickel concentrations also offer a wide enhancement as compare to stainless steel 304.

Nickel alloys containing iron and chromium such Inconel 600 is distinguished by its excellent resistance to uniform surface corrosion and to stress corrosion cracking in NaOH solutions. During testing Inconel 600 in 10% NaOH solutions at temperature of 325oC and pressure of 275 MPa, where there is no sign of cracking after 100 hours.

With the systematic evaluation of the damage to components made from Nickel and Inconel 600, the sensitivity to stress corrosion cracking of the both alloys is observed for the given temperature and concentration limit. Application of nickel is larger than Inconel 600.

Carbon concentration and heat processing have a remarkable effect on the stress corrosion cracking behaviour of Inconel 600. Outcomes of stress corrosion cracking tests following the slow strain rates in 10% NaOH solutions at 288oC are observed. Alloy 600 pipes containing carbon are tested.

The percent part of intergranular fracture on the fracture surface of the failed samples was accounted as a measure of the sensitivity to stress corrosion cracking . For the samples in the as-delivered state, the intergranular fracture part that is the sensitivity to stress corrosion cracking, reduced with increase in carbon concentration to reach a minimum 0.05%C. Resistance to stress corrosion cracking significantly increased by after heat processing and carbon concentration had eventually no influence.

Inconel 600 with modified carbon concentration and heat processing and test temperature did not have a considerable effect on the stress corrosion cracking behaviour.

For most of the tested alloys, the U bend samples are the more critical test condition as compared to the C ring samples. The ferritic and martenisitic stainless steels showed significant resistance to stress corrosion cracking, although the excellent general corrosion and the risk to embrittlement restricts their user under specific conditions. For the austenitic alloys, more highly stressed U bend samples showed that the resistance to stress corrosion cracking increased with increase in nickel concentration. The alloys with low nickel concentrations could not perform in the concentrated NaOH solutions.