Influence of Solution pH, Anion Concentration, and Temperature on the Corrosion Properties of Alloy 22

Abstract

We introduce an acid addition technique for the rapid assessment of the influence of solution pH, anion (such as chloride) concentration, and temperature on the dissolution of metals. We demonstrate the technique with the characterization of the dissolution of alloy 22 (Ni-22Cr-13Mo-3W-3Fe) exposed to chloride-containing hydrochloric, sulfuric, and nitric acid environments as a function of pH (from pH 5 to pH -1) and temperature (25-90 degrees C). A combination of electrochemical techniques (electrochemical impedance spectroscopy and linear polarization resistance) and atomic force microscopy are used to characterize the influence of the various solutions on the dissolution of alloy 22. Below 50 degrees C, corrosion rates are less than 1 mu m/yr independent of acid type, pH, and temperature. In contrast, between 50 degrees C and the upper explored limit of 90 degrees C, dissolution rates in sulfuric and hydrochloric acid scale approximately linearly with temperature at rates that depend on the solution pH. In nitric acid, corrosion rates are lower at comparable pH values due to the oxidizing effects of nitrates. An increase in the open-circuit potential in conjunction with electrochemical impedance spectroscopy data suggests that nitrates promote a stable passive oxide film that inhibits alloy dissolution in all environments tested. (c) 2006 The Electrochemical Society.