Impact of electrolyte contamination on surface finishing and pitting corrosion of electropolished 304 stainless steel

摘要

The aim of the process of electropolishing (EP) stainless steel based on anodic dissolution of themetal surface is to enhance corrosion resistance and to improve the aesthetical properties of thesurface of the polished element. Due to economic reasons, EP processes are conducted at thetemperature of 30-70˚C while preserving the sustainability of the bath. In industrial conditions, thebaths usually consist of: concentrated solutions of H_3PO_4 and H_2SO_4 acids and organic additives.Sulphuric(VI) acid ensures an adequate conductivity of the bath, while phosphoric(V) acid is mainlyresponsible for the polishing of metal surface. During the EP process, the electrolytic bath isenriched with metal (iron, chromium and nickel) ions that originate from the dissolution of theanode. If the electrolyte has been used for a long time, this may result in increased roughness anddeteriorated gloss of electropolished surfaces. Monitoring the changes in the composition ofprocessing baths during electropolishing of austenitic stainless steel on an industrial scale allowsus to determine the conditions for the EP process. This, in turn, enables to obtain the best qualityproducts while at the same time minimising the costs of bath replacement and disposal taking intoaccount the environmental impact by mobile speciation of heavy metals.The paper presents the results of laboratory tests considering the influence of long-termcontamination of a bath that contains a mixture of H_3PO_4 and H_2SO_4 on surface finishing andpitting corrosion of electropolished 304 stainless steel. Potentiodynamic measurements werecarried out using the Solartron SI 1287 Electronic Interface towards a Saturated Calomel Electrode.The pitting corrosion resistance tests of steel in a 0.5M solution of sodium chloride demonstratedthat, although the electrolyte was strongly contaminated with iron, chromium and nickel ions,electropolishing (process parameters 8 A/dm~2, 15 min, 55°C) did not reduce the pitting potential,although the roughness and gloss of the samples were significantly lowered. After the EP processin electrolyte that was not contaminated with metal ions samples were characterised by roughnessRa=0.11 μm and gloss 955 GU. On the other hand, in samples that had been electropolished inelectrolyte contaminated with iron (over 6% by mass), chromium (less than 2%), and nickel ions(less than 1%) these values were, respectively, Ra=0.29 μm and 200 GU. The study also presentsresults of tests related to the removal of nickel and chromium compounds from used industrialbaths.