Electrochemical Investigation of the Nucleation and Propagation of Pits in Iron-Chromium Alloys

Abstract

Electrochemical measurements performed on single crystals and polycrystalline samples of Fe 13 Cr and Fe 16 Cr alloys in Na₂SO₄ + NaCl solutions differing in Ci⁻ ion concentration, have shown that during pitting the anodic current I increases with time t:

I∼t^b Depending on experimental conditions, b may vary from 2 to 6. At b = 2 all the pits nucleate simultaneously, so that their number does not change with time of anodic polarisation. At b > 2 the number of pits increases with time.

Galvanostatic polarisation of polycrystalline samples in solutions containing much fewer Cl₋ ions than SO₄ ²⁻ ions does not lead to a steady potential. Under these conditions periodic oscillations of potential occur, several hundred millivolts in amplitude. This phenomenon is probably due to the inhibiting action of SO₄ ²⁻ ions on the nucleation of pits by Cl⁻ ions, and it occurs only at certain concentration ratios of these two ions in the electrolyte. Inhibition of pit nucleation is also revealed during potentio-static and potentiokinetic measurements at potential values several hundred millivolts more positive than the breakdown potential measured galvanostatically.

An interpretation of above phenomena is given for both single crystals and polycrystalline alloy samples.