The decrease in pH caused by the hydrolysis of metal ions inside crevice is widely accepted to be the predominant factor affecting the initiation of crevice corrosion for stainless steels. The accumulation of chloride ions inside crevice is also thought to be of great importance for the initiation of crevice corrosion. ^{1, 2} In this study, a simultaneous measurement technique for pH and Cl⁻ concentration was applied to the visualization of pH and Cl⁻ distributions inside crevice in the initial stage of crevice corrosion of Type 430 stainless steel.

A sensing layer with fluorescent dyes was made on one side of a SiO₂ plate using the sol-gel SiO₂ coating technique. Quinine sulfate (fluorescent dye) was used to measure Cl⁻ concentration. The excitation wavelength was 350 nm. The emitted light was filtered through a 420-nm long-pass filter, since quinine sulfate has a peak wavelength of 451 nm (blue fluorescence). ¹ Tb³⁺-DPA (dipicolinic acid) complex was used as the fluorescent dye for pH measurements. ² The excitation wavelength of this dye was 270 nm, and the emitted light was filtered through a 510–550-nm band-pass filter. In this case, green fluorescence was observed.

The crevice specimens were prepared from Type 430 stainless steel (Fe-16.5Cr). A crevice (3 mm × 3 mm) was made between the stainless steel and the sensing plate. ^1,2 Crevice corrosion tests were carried out in a naturally aerated 0.01 M NaCl at pH 3.0, and the specimen was polarized at 0.0 V (vs. Ag/AgCl, 3.33 M KCl). Before the initiation of crevice corrosion, pH inside the crevice gradually decreased with time. And also, Cl⁻ concentration increased with time. At pH 1.8 and ca. 0.1 M Cl⁻, crevice corrosion initiated, and a large pH drop and a large Cl⁻ increase were observed.

References:

1. T. Kaji, T. Sekiai, I. Muto, Y. Sugawara, N. Hara, J. Electrochem. Soc., 159, C289-C297 (2012).
2. M. Nishimoto, J. Ogawa, I. Muto, Y. Sugawara, N. Hara, Corros. Sci., 106, 298-302 (2016).