Quantitative evaluation of global and local chromium contents with the EPR test on ferritic and martensitic stainless steels

Abstract

Abstract The cause of localised corrosion phenomena in stainless steels is often related to the element chromium, which is essential for the formation and resistance of the passive layer. A lower than nominally stated content of chromium occurs when the element is highly concentrated and locally bound in chromium-rich phases or precipitates, mostly carbides, and thus locally or globally absent for passivation. The method of electrochemical potentiodynamic reactivation (EPR) is uniquely suited to demonstrate the effect of chromium depletion on the passivation. Here, the steel surface is initially active in sulfuric acid and passivation is achieved by external dynamic polarization. After passivation, the polarization direction is reversed and in chromium-depleted areas a reactivation takes place, which is detected by an increase in current. The detection limit for a reduced chromium content depends largely on the strength of the sulfuric acid used in the EPR test and the polarization speed. The relationship between the sulfuric acid concentration and the electrochemical parameters from the EPR test is shown here using the example of ferritic Fe-Cr alloys with graded Cr contents in the range of approx. 6 to 18 %. From the passivation and reactivation current densities of the EPR data, models for the detection of global and local chromium contents (detection limits) are derived. The developed models are applied to the martensitic stainless steel X20Cr13 in different heat treatment conditions and the influence on the distribution of chromium in the microstructure, which is responsible for the passivation and thus the corrosion resistance, is shown.