In order to know the electrochemical behaviours of Ti in the crevice, the weight loss, corrosion potential and galvanic current were measured for Ti in the crevice and on the open surface in boiling acid NH
4Cl solution. Also, anodic polarization measurement for Ti in the crevice and on the open surface was conducted in boiling acid and neutral NH
4Cl solutions. The results obtained were as follows.
In boiling acid NH
4Cl solution containing a small amount of the oxidizing agent, titanium undergoes crevice corrosion in narrow gaps at points of contact with nonmetallic materials due to the depletion of the oxidant in the crevice.
In acid and neutral solutions containing more than 5 N NH
4Cl, the current of Ti in the passive region in the crevice was much higher than that of Ti on the open surface, and a granular corrosion product mainly composed of TiO
2 (rutile) was observed after anodic polarization measurement.
In anodic polarization of Ti in the crevice in boiling 5 N NaCl solution, a similar phenomenon to that in NH
4Cl solution was observed, but not in Na
2SO
4 solution. Potential decay curves indicate that the corrosion product of Ti in the crevice in chloride media is less stable and easily activated than that of Ti on the open surface.
The results mentioned above suggest that the passive film formation is hindered and oxychloride TiO
xCl
y is formed on Ti in the crevice in boiling neutral solution with higher Cl
− ion concentration (5 N, 10 N) due to the depletion of H
2O in the crevice, promoting crevice corrosion.
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