Determination of critical pitting temperatures for NiCrMo alloys using electrochemical noise measurements

The determination of critical pitting temperatures (CPT) in various test solutions like ferric chloride solution according to ASTM G48 or “Green Death” solution is a common test method for the comparative assessment of the pitting corrosion resistance of highly alloyed steels and NiCrMo alloys. In addition to the well-known disadvantages of standard methods, like long test times, subjective examination, and large scatter, for the highest alloyed NiCrMo alloys no stable pitting corrosion can initiate even at the highest test temperatures. This paper describes the limitations of standard test methods and shows how these problems can be solved by an alternative test solution and an adjusted test method. By capturing and examining the current noise under potentiostatic conditions during continuous heating in a 4.5 M calcium chloride solution the transition from metastable to stable pitting corrosion as a criterion for CPT can be detected in a reproducible way.