The association of potentiokinetic reactivation and electrochemical pitting tests on a nitrogen bearing 19 Cr-17 Mn steel with its thermal history

The double loop electropotentiokinetic (EPR) technique was used to study the transformations in a 19 Cr-17 Mn steel. This technique revealed that the transformation of delta ferrite and other microstructural variations during cooling after solution annealing influenced the stability of the passive film formed on a nickel substituted stainless steel exposed to aggressive media. The pitting susceptibility of the Cr---Mn steel was found to have increased significantly after being subjected to furnace cooling following solution annealing. The presence of delta ferrite in the austenite matrix was found to have a benign influence on pitting resistance. The formation of a sigma phase was found to be responsible for the deterioration of corrosion resistance. This paper explains the variation in the EPR test results by correlating the changes in the alloy microstructure brought about by thermal history with the changes in electrochemical responses, the latter being directly influenced by the matrix and grain boundary Cr content of the different phases formed.