The Role of Chloride Concentration on the Pitting Kinetics of Aluminum

Abstract

This work examines the role of chloride concentration on the polarization behavior and pitting susceptibility of aluminum in highly concentrated NaCl brines. NaCl-rich brines are typically present during marine atmospheric corrosion at elevated humidity levels, which include the range from 75 % RH up to 98 % RH, corresponding to 6 M (saturated) and 0.6 M NaCl respectively at 25 oC. Although aluminum pitting is well-studied at chloride solution concentrations below 2 M (> 90% RH), electrochemical insight of aluminum atmospheric corrosion occurring at higher chloride concentrations (> 2 M [Cl-]) is hindered by lack of data. To address this, anodic polarization experiments along with post-mortem microscopy were performed on high purity Al (> 99.9 wt%) in NaCl solutions ranging from 0.6 to 6 M. Results to date show that pitting potentials and pitting transition potentials are logarithmically proportional to chloride concentration within this range, extending the applicability of previously proposed semi-empirical models to higher concentrations.1,2 Measured repassivation potentials, however, deviated considerably from previously reported trends and predictions based on measurements at lower chloride concentrations. The role of pit morphology, IR drop and hydrogen evolution during anodic polarization will be addressed to rationalize these discrepancies.SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. References S.T. Pride, J.R. Scully, J.L. Hudson, Journal of the Electrochemical Society, 141, 3028 (1994).J.R. Galvele, Journal of the Electrochemical Society, 123, 464 (1976).