ABSTRACT

This paper discusses the development and optimization of a procedure for evaluating crevice corrosion repassivation by the cooling of corroding Ni-Cr-Mo alloys. The procedure consists in 1) potentiodynamic polarization at 0.167 mV/s in the anodic direction until current density reaches 20 µA/cm2; 2) galvanostatic polarization of 4 hours at 20 µA/cm2; 3) potentiodynamic polarization at 0.0167 mV/s in the anodic direction until current density reaches 20 µA/cm2; 4) potentiostatic polarization at the final potential of the previous step for 4 hours; 5) cooling at a constant rate while the same potential of previous step is maintained. The procedure was successfully applied to alloys N06022, N06059, N07022, N06686 and N10362 in 0.1 and 1 mol/L chloride solutions, and alloy N06625 in 0.1 mol/L chloride solutions. Judicious application of this testing procedure is advised for more concentrated chloride solutions. The optimized procedure allowed a low and relatively constant current density over time in step 4. Consequently, current density drop in step 5 is almost purely due to the effect of cooling and repassivation temperature may be determined correctly. Slow cooling in step 5 led to repassivation at temperatures above expected values, but rapid cooling led to repassivation at temperatures below expected values.

INTRODUCTION

Ni-Cr-Mo alloys are among the few metallic materials that may withstand localized corrosion and are virtually immune to stress corrosion cracking in hot chloride solutions.1,2 These alloys are well known in the chemical processing industries and they are also applied in oil and gas, pharmaceutical and flue gas desulfurization industries.3 Handling oxidizing solutions containing chlorides at high temperatures calls for materials able to resist localized corrosion in the forms of pitting and crevice corrosion.1,2 Chemical composition of Ni-Cr-Mo alloys may be tailored to provide the required corrosion resistance in different harsh environments. The localized corrosion resistance of these alloys is ranked by the PRE (Pitting Resistance Equivalent).2 PRE is a function of the weight percentages of the alloying elements Cr, Mo and W (Equation 1). The applicability of PRE, initially developed for stainless steels, to Ni-Cr-Mo alloys has been discussed in recent literature.4,5

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