Effect of Polishing on Electrochemical Behavior and Passive Layer Composition of Different Stainless Steels.

Krzysztof Rokosz,Rainer Fluch,Grzegorz Solecki,Jouko Lahtinen,Gregor Mori,Marianne Kapp

doi:10.3390/ma13153402

Abstract

In the present paper, the effect of different polishing methods (mechanical and electrochemical) on passive layer chemistry and the corrosion behavior of stainless steels is investigated. It was found that CrNiMo austenites have a substantially better corrosion behavior than CrMnN ones. The nickel is enriched underneath the passive layer, while manganese tends to be enriched in the passive layer. It was also noted that immersion of manganese into an electrolyte preferentially causes its dissolution. It was found that high amounts of chromium (27.4%), molybdenum (3.3%), nickel (29.4%), with the addition of manganese (2.8%) after mechanical grinding, generates a better corrosion resistance than after electrochemical polishing. This is most likely because of the introduction of phosphates and sulfates into its structure, which is known for steels with a high amount of manganese. For highly alloyed CrNiMo steels, which do not contain a high amount of manganese, the addition of phosphates and/or sulphates via the electropolishing process results in a decrease in pitting corrosion resistance, which is also observed for high manganese steels. Electropolished samples show detrimental corrosion properties when compared to mechanically polished samples. This is attributed to substantial amounts of sulfate and phosphate from the electropolishing electrolyte present in the surface of the passive layer.

Highlights

To smoothen steel surfaces, mechanical polishing and electropolishing can be performed
For the three lower alloyed steels, surface treatment has no effect on pitting potential, while for the (a) highest alloyed 27Cr29Ni3Mo, there is a substantial difference of almost 0.2 V
27Cr29Ni3Mo stainless steel, 91.6 s p.l.—passive layer; int.—nickel‐enriched the passive layers were less noble than the ones after mechanical treatments. This can be explained by interface between passive layer and bulk metal; bulk—bulk metal

Summary

Introduction

Mechanical polishing and electropolishing can be performed. Mechanical polishing can introduce cold deformation, residual stresses, and debris into the steel surface. On the other hand, electropolishing will dissolve more active sites, such as chromium poor segregations and nonmetallic inclusions, especially manganese sulfides. It has often been found that electropolished steels have better corrosion properties than mechanically polished ones. Lai [1] in their investigation of the surface condition of stainless steel 316L by electropolishing in a sulfuric acid-phosphoric acid mixed electrolyte. Through electrochemical tests (polarization scans and electrochemical potentiokinetic reactivation (EPR) tests), they found a substantial increase in corrosion properties in sulfuric acid after electropolishing. Hryniewicz, Rokosz et al [2,3,4,5] found a Materials 2020, 13, 3402; doi:10.3390/ma13153402 www.mdpi.com/journal/materials

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