Masao Fujii1 and Makoto Kumada1
1Tamano Laboratory, Mitsui Ship Building and Engineering Co., Ltd, Tamano
A few environmental factors affecting pitting in stainless steel such as halogenions, pH and temperature were studied by measurements of polarization characteristics, the pitting potentials and characteristics of pits formed. The results obtained are summarized as follows:
(1) Halogen ions accelerate the anodic dissolution. Especially, Cl-, Br- and I- ions cause pits.
(2) Increase of Cl- ion concentration accelerates the anodic dissolution and shifts the break through potential Vc and the pitting potential Vp in the negative direction.
(3) Upraise of temperature accelerates the pitting dissolution.
(4) Increase of pH values decrease the critical Cl- ion concentration required for the formation of pits.
(5) It was found from relation between number of pits and amount of charge that the following four processes existed in the formation of pits: [I] initiation of pits nuclei, [II] increase of number of pits nuclei, [III] saturation of number of pits nuclei and [IV] growth of pits.
(6) Amount of charge necessary for the first initiation of a pit corresponds to the one that serves to transport the minimum amount of Cl- ions to the alloy surface required for the initiation of a pit nucleus.
(7) r=k · t(b+1)/3 was obtained by calculation as the radius of a pit.
(8) The calculated values of the radius of pits agree with the measured ones.
(9) From the discussion on the growth of pits, it was found that in the case of decreasing Cl- ion concentration a few large pits formed, and that in the case of increasing Cl- ion concentration many small pits formed.
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