An inexpensive holder for corrosion experiments was developed to avoid crevice corrosion during electrochemical tests for studying pitting corrosion of stainless steels and other corrosion-resistant alloys. The working principle follows Qvarfort's electrochemical cell. Therefore, this holder enables measurements of pitting potentials of metals susceptible to crevice corrosion. The sample holder attaches to cylindrical or prismatic coupons cut from bars or plates. The coupon thickness has to be greater than 10 mm. The holder is suitable for any chloride concentration and solution temperature at ambient pressure. The testing solution can be deaerated or saturated with any gas if needed. The method was validated by cyclic potentiodynamic polarization experiments on alloys UNS S30400, S31600 stainless steel and nickel-base alloys UNS N08031 and N07022. Results show that the pitting repassivation potential decreases linearly with temperature. Pitting repassivation potentials on stainless steels were 100-150 mV higher than the corresponding crevice repassivation potentials obtained by the PD-GS-PD method. Undesired localized corrosion near the holder electrical connection was not evident in multiple replicate specimens tested with the proposed method.
For many corrosion-resistant alloys (CRA's), crevice corrosion under gaskets, paint or O-rings required to define the exposed area and avoid solution contact with electrical connections interferes with measurements of the critical pitting potential (EPIT). This undesired and uncontrolled crevice corrosion occurs at a lower potential than the one for the onset of pitting corrosion [1].
Many approaches have been used to avoid this artifact [2-5]. Wire-loop electrodes have been used to study pitting without the complications of crevice corrosion [3]. Waterline effects are minimized, considering the ratio of the total exposed area over waterline length. This approach is limited to materials in wire form. Results on the pitting corrosion performance of materials in wire form are not directly comparable to other product shapes of the same grade [2]. The flag-electrode configuration also minimizes the effects of the waterline; however, edge attack can be an issue for wrought products (i.e. after mechanical working) even in deaerated environments [2]. Fabrication of flag-electrodes requires machining or complicated metal-cutting steps, which adds time and expenses to the testing program.
