Study of the effect of temperature on the galvanic corrosion between Alloy 31 base metal and its weld in polluted phosphoric acid

Abstract

This work studies the influence of the solution temperature on the corrosion resistance of a high alloyed austenitic stainless steel (UNS N08031) used as Base Metal (BM), the Heat Affected Zone (HAZ) and the Weld Metal (WM) obtained by the Gas Tungsten Arc Welding technique using a Nickel-base alloy (UNS N06059) as filler metal (GTAW). Electrochemical tests were carried out in a 5.5 M polluted phosphoric acid solution with 0.03 wt% (380 ppm) of chloride ions and 2 wt% of H2SO4 at 25, 40, 60 and 80 degrees C. The potentiodynamic curves of the materials were registered in each condition under study and electrochemical parameters such as E-corr, i(corr), E-b and i(p) were obtained. The galvanic corrosion generated between the BM-HAZ and HAZ-WM pairs was also analysed by means of the Mixed Potential Theory. The lower E-b and (E-b-E-corr) values and higher ip values obtained at higher temperatures indicate that the properties of the passive film formed degraded with temperature. The welding process modifies the characteristics of the passive film, deteriorating them and favouring the loss of passivity of the WM with respect to the BM, as shown by the lowest E-b values and highest i(p) values of the WM.