Selective corrosion of duplex stainless steel bars in acid

Abstract

The selective corrosion of the phases comprised in four duplex stainless steels (DSS) is studied in acid media (H2SO4 + HCl solutions). The work considers traditional UNS S32205 grade as well as other economical‐advantageous grades (UNS S32001 and UNS S32304). The samples of the lean DSS come from bars formed by hot working (HW), while those of the S32205 come both from HW bars and bars obtained by cold working. The amount of strain‐induced martensite in the austenitic phase of DSS that the processing has induced is highly dependent on the DSS composition. The change in the relative corrosion resistance of the different phases comprised in the microstructure with the potential is considered. The influence of the composition of the DSS is discussed. It has been checked that, at potentials close to the corrosion potential (Ecorr), the corrosion of the ferritic phase is more favored than that of the austenite. Increasing potentials in the anodic activation region tends to favor the selective corrosion of the austenite. The trend to selective dissolution is less marked for the least alloyed DSS. The strain‐induced martensite in the austenitic phase exhibits a corrosion resistance clearly higher than that of the ferrite at Ecorr or potentials close to Ecorr. The possible influence of TiN precipitates in the corrosion development is also analyzed.

Selective corrosion of duplex stainless steel bars in acid. Part 2: Effect of the surface strain and numerical analysis

“…The UH of the surface is always higher than the UH of the bulk of the DSS bars. These differences are due to the high amount of dislocations and defects in the microstructure near the surface, but also, as it has already be shown for these materials , to the higher amount of strain‐induced martensite and the reduced grain‐size in the regions close to the surface.…”

“…Bearing in mind the careful study carried out about the effect of polarization in the corrosion behavior of the bulk materials of the bars and the results of other researchers in quite similar materials , the physical meaning of the peaks for the bulk material can be easily suspected. The peak that appears at low anodic overpotentials should correspond to the preferential corrosion of ferrite, while the peak at high anodic overpotentials should correspond to the preferential corrosion of the austenite.…”

“…The chemical composition and other relevant characteristics of the four DSS bars studied can be found in the experimental chapter of the first part of the present study .…”

“…The working electrodes were 2‐cm long samples from the bars whose transverse section was polished to 0.3 µm. Other details about studied DSS samples whose results are also analyzed in the present article can be found in the first part of the work . Polarizations were performed starting from −550 to 400 mV versus SCE at a sweep rate of 1 mV/s .…”

“…In the first part of this work , results about the corrosion behavior of the different phases present in the cores (the least strained region) of the DSS bars have been presented. It has been checked that ferrite dissolution tends to occur at E corr or at potentials slightly over E corr .…”