The metastable pitting potential and its relation to the pitting potential for four materials in chloride solutions

• The metastable pitting potential Em is defined according to the testing conditions.
• For four materials tested, linear relationship exists between Em and Eb values.
• As chloride concentration changes, the Em and Eb values change in the same order.
• The metal passivity determines the nucleation of both metastable and stable pitting.
• The Em value can be taken as a parameter characterizing pitting tendency of metals.

The metastable pitting potential for four materials, Q235 steel, X70 steel, pure iron and 316L stainless steel in chloride solutions were measured by slow scan potentiodynamic polarization. For the four materials, good linear relationship exists between the metastable pitting potential Em and pitting potential Eb. The close relationship between the two parameters is attributed to the passivity of the metals in environments, which plays the key role in both the nucleation processes of metastable pitting and stable pitting. Higher passivity results in higher average pitting potential and larger difference between average Eb and Em values.