Investigation of the stress corrosion cracking susceptibility of annealed and heat treated alloy 625 castings and forgings in sea water

Abstract

Alloy 625, the nickel based superalloy commonly called Inconel* 625, was investigated for its susceptibility to stress corrosion cracking in sea water using the slow strain rate tensile test method. Four microstructures of the alloy commonly found in end products were investigated. Bimetallic couplings with other metals were simulated with a potentiostat at plus and minus one volt with respect to a saturated standard calomel electrode (SCE). Baseline tests were conducted in air and sea water without applied potential. The response of the alloy to cathodic protection of minus three volts SCE was also investigated on the two most commonly used microstructures, as cast" and "forged/annealed". The different microstructures developed were characterized with a scanning electron microscope (SEM). The gage lengths, fracture surfaces, and sections of test specimens were also examined with a SEM. The data from the slow strain rate tensile tests were compared with data from standard tensile tests performed on the same processed material. The results from this investigation indicate that Alloy 625 is not susceptible to stress corrosion cracking in the normal sea water service environment where temperatures are close to ambient. However, the results indicated that Alloy 625 is susceptible to the hydrogen embrittlement form of stress corrosion cracking when subjected to potentials that produce hydrogen evolution. This embrittlement leads to intergranular cracking.