Visualization of pH Distributions inside the Crevice of Stainless Steel in Corrosive Environments

Abstract

Stainless steels are known for its excellent corrosion resistance. However, stainless steels sometime suffer from crevice corrosion in chloride environments. The distribution of pH inside the crevice is recognized to be the important factor in crevice corrosion of stainless steels [1, 2]. This study focused on visualization of pH inside the crevice in corrosive environments.An imaging plate was prepared by a sol-gel method. Ethyl violet was used to assess the pH values. The fabricated imaging plates were calibrated by 0.01 M NaCl solution at pH 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0. The imaging plate was immersed in the 0.01 M NaCl solutions, and the color change was observed by an optical microscope. Type 304 stainless steel (Fe-18mass%Cr-8mass%Ni) was used as the specimens. The size of the specimen for the crevice corrosion test is 3 mm × 3 mm×17 mm. The specimen surfaces were polished with a 1 µm diamond paste. The scan rate of potentiodynamic polarization measurements was set to 23mV min- 1. The specimen for the polarization measurements is 17 mm × 25 mm × 3 mm in size. The electrode area was 1 cm2. The electrolyte used for the potentiodynamic polarization was 0.01 M NaCl (pH 3.0). The electrolyte was deaerated before the polarization measurements. The reference electrode was Ag/AgCl (3.33 M KCl). The counter electrode was Pt. The crevice corrosion tests were performed under a potentiostatic polarization. The applied potential was 0.3 V. The crevice was made between the steel surface (3 mm × 3 mm) and the imaging plate. Optical microscope images of the surface were taken every 30 seconds. The pH values inside the crevice were calculated from the R (red), G (green), and B (blue) values of each pixel in the images.In calibration, the color of the imaging plate changed from violet to yellow, suggesting that G values of imaging plate mainly responded to pH. The pitting potential of the specimens was 0.4 V. Therefore, 0.3 V was suitable for the crevice corrosion test. Because 0.3 V (Ag/AgCl (3.33 M KCl)) was lower than the pitting potential. The visualization of pH in the crevice corrosion was conducted. The pH decreased during the incubation, initiation, and growth of crevice corrosion. The pH was the important factor in the initiation and growth of crevice corrosion.[1] T. Kaji, T. Sekiai, I. Muto, Y. Sugawara, N. Hara, Visualization of pH and pCl Distributions: Initiation and Propagation Criteria for Crevice Corrosion of Stainless Steel, J. Electrochem. Soc., 159 (2012), C289-C297.[2] M. Nishimoto, J. Ogawa, I. Muto, Y. Sugawara, N. Hara, Simultaneous visualization of pH and Cl- distributions inside the crevice of stainless steel, Corros. Sci., 106 (2016), 298-302.