PREVENTION OF TRANSPASSIVE DISSOLUTION OF AUSTENITIC STAINLESS-STEEL COUNTER ELECTRODES IN NICKEL BASED ELECTROLYTIC COLORING SOLUTIONS FOR ANODIZED ALUMINUM


CAKIR A.

WERKSTOFFE UND KORROSION-MATERIALS AND CORROSION, vol.43, no.1, pp.10-16, 1992 (Journal Indexed in SCI) identifier

  • Publication Type: Article / Article
  • Volume: 43 Issue: 1
  • Publication Date: 1992
  • Title of Journal : WERKSTOFFE UND KORROSION-MATERIALS AND CORROSION
  • Page Numbers: pp.10-16

Abstract

In low nickel ion containing, weak acidic, electrolytic coloring solutions for anodized aluminium, austenitic stainless steel counter electrodes dissolve transpassively. The role of nickel and cobalt ions in preventing the transpassive dissolution of AISI Type 304 and 316 austenitic stainless steel counter electrodes was investigated. Potentiodynamic polarization method was used. The test solutions were a buffer (29.66 g/lt boric acid), a buffered supporting electrolyte (32.6 g/lt boric acid + 26.18 g/lt magnesium sulphate heptahydrate + 14,95 g/lt ammonium sulfate) and the coloring solutions (supporting electrolyte with different nickel (27.6-101 g/lt as heptahydrated sulfate) and cobalt (33-166 ppm) content). The oxidation of bivalent nickel ions starts 200 mV lower than the transpassive dissolution of stainless steel. However, even a small amount of cobaltous ions (165 ppm) in the solution produces a much steeper oxidation peak on the top of the nickel oxidation current. In order to transpassively dissolve a stainless steel electrode in cobalt containing nickel solutions, anodic current must be greater than the combined oxidation peak of nickel and cobalt ions, otherwise the working potential of the stainless steel electrode is always kept at the passive region hence the transpassive dissolution is prevented.