A Chemical Sensing Plate for Simultaneous Measurements of pH and Cl− Concentration Inside Crevice of Stainless Steel

     The decrease in pH caused by the hydrolysis of metal ions inside crevice is widely accepted to be the predominant factor affecting 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. Kaji et. al. ¹ fabricated a glass plate with a sol-gel SiO₂ sensing layer, which contains a pH indicator or a Cl⁻-sensitive florescent dye, and the H⁺ and Cl⁻ distributions in the crevice solution for stainless steel were visualized. They demonstrated the crevice corrosion was initiated when the H⁺ and Cl⁻ concentrations inside the crevice reached the conditions for metastable pit formation. However, their experiments were not simultaneous measurements for pH and Cl⁻ concentration, so that the combination between the pH decrease and the Cl⁻ accumulation remains unclear. In this study, a simultaneous measurement technique for pH and Cl⁻ was developed and applied to the visualization of pH and Cl⁻ distribution inside crevice in the initial stage of crevice corrosion.

     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 use to measure Cl⁻ concentration. ¹ 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). A metal-organic complex (a fluorescent dye) was used as the sensing dye for pH measurements. 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 sensing plate fabricated in this study was colorless and transparent to visible light, and hence the surface appearance of the steel could be observed through the plate clearly.

     The fluorescence images of the sensing plate placed on a Pt sheet were taken by a fluorescence microscope. The solutions with different pH and Cl⁻ concentrations were used as calibration standards. Figure 1 shows the fluorescence of the Cl⁻-sensitive dye (quinine sulfate) in the sensing layer on the SiO₂ plate. As seen in this figure, the brightness of the blue fluorescence depends on Cl⁻ concentration but is independent of pH. Figure 2 shows the fluorescence of the H⁺-sensitive dye in the sensing layer on the SiO₂ plate. In this case, even though Cl⁻ concentration slightly affects the brightness; however, the brightness of the green fluorescence mainly depends on pH. Therefore, pH can be measured by previously obtaining the Cl⁻ concentration from the brightness of the blue fluorescence.

     A Mn-containing austenitic stainless steel (Fe-18Cr-10Ni-5.5Mn) was used for crevice corrosion tests. Crevice (3 mm × 3 mm) was made between the surface of the stainless steel and that of the sensing plate. ¹ Crevice corrosion tests were carried out in a naturally aerated 0.01 M NaCl solution (298K) of pH 3.0 adjusted with HCl, and the specimen was polarized at 0.3 V (vs. Ag/AgCl, 3.33 M KCl). Figure 3 shows the surface appearance of the initiation site of crevice corrosion (micro pit) and the fluorescence images corresponding to pH and Cl⁻ concentration. It was confirmed that both the pH decrease and the Cl⁻ accumulation occurred at the initiation site of crevice corrosion.

Reference:

1. T. Kaji, T. Sekiai, I. Muto, Y. Sugawara, N. Hara, J. Electrochem. Soc., 159, C289-C297 (2012).