The activation/depassivation of nickel–chromium–molybdenum alloys: An oxyanion or a pH effect—Part II

Significance Statement

The present work was performed by Ajit Mishra in his PhD research work under the guidance of Dr. Dave Shoesmith at University of Western Ontario (Canada) and the research was published in two parts in ElectrochimicaActa journal (Vol. 100, (2013), p. 118 and Vol. 102, (2013), p. 328). Currently, Ajit Mishra is working as a Staff Engineer at Haynes International (Indiana, US) and his work is primarily focusses on alloy development, failure analysis and corrosion testing of nickel-based alloys used in various industries.

In recent years, many studies on Ni-Cr-Mo or Ni-Cr-Mo-W alloys have concentrated on low pH conditions, either in bulk acidic solution or under crevice corrosion conditions when a low pH environment can exist beneath the crevice former. However, not much work has been performed in above neutral to alkaline solution. In a high pH environment, corrosion resistant Ni-Cr-Mo or Ni-Cr-Mo-W alloys experiences a high corrosion rate, a phenomena known as “Caustic Dealloying”.To date no convincing explanation of this behavior has been offered. In this research work, comprehensive electrochemical and surface analytical techniques were utilized to unravel the mystery of high corrosion rate in above neutral to alkaline environment which can also lead to failure of material from stress corrosion cracking (SCC).The effects of parameter, such as alloy composition, pH and temperature were also studied for commercially available corrosion resistant nickel-based alloys.

     

Journal Reference

Electrochimica Acta, Volume 102, 15 July 2013, Pages 328-335.

A.K. Mishra, D.W. Shoesmith.

Department of Chemistry, University of Western Ontario, London, ON N6A 5B7, Canada and

Surface Science Western, 999 Collip Circle, London, ON N6G 0J3, Canada

Abstract

The depletion of Cr and Mo in the passive films grown on commercial Ni–Cr–Mo(W) alloys in carbonate/bicarbonate solutions has been studied potentiodynamically and by X-ray photoelectron and Auger spectroscopy. Depletion in these two key alloying elements occurs at lower potentials than expected on all the alloys studied. It has been shown that the key feature leading to this behavior is the buffering of surface pH to >8.6, when the surface deposition (or retention) of insoluble protective Mo(VI) species does not occur as would be expected under the acidic local conditions generated by Cr(VI) and Mo(VI) dissolution. The presence of a small solution concentration of Mg2+ ions leads to the accumulation of a Mg(OH)2 deposit when the surface pH is maintained sufficiently alkaline.

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