Application of the Microelectrode System to a Study of Pitting Corrosion of Austenitic Stainless Steel

Abstract

Electrochemical study with a microelectrode of pitting corrosion of type 304 stainless steel in 1 N-H₂SO₄ containing 0.5 N-NaCl at 25°C has been carried out under a microscope.
By using a microcapillary whose tip was 5 microns in outer diameter on a micromanipulator, the electrode potential distribution inside a single pit was measured. Potentials inside the shallow pit developed potentiostatically at 600 mV (SCE) are uniformly +450 mV and the protection potential of the pit was estimated as +250 mV from potentials outside and inside the pit.
A single pit, which had grown potentiostatically to a predetermined size, was brought up by the galvanostatic method. The mouth periphery of the pit soon became repassivated and a deep pit could be obtained. During the galvanostatic growth the electrode potential difference between those determined from macrocapillary (outside) and microcapillary (near the pit bottom) was about 150 mV. Both potentials changed gradually to less noble values with time and then a sudden potential drop was observed. The potential at the bottom jumped from +150 to −150 mV. The latter is the potential corresponding to the current maximum in the active region on the polarization curves, determined using a microcapillary, of the activated steel in 1 N-H₂SO₄ containing NaCl of more than 3 N.
The polarization curves show also the dissolution reaction at the surface inside the pit growing potentiostatically is the diffusion-controlled one in the passive potential region.