Analysis of Convective Mass Transfer by Potential Relaxation: IV . Active, Prepassive, and Transpassive Iron Dissolution at a Rotating Disk

Abstract

Liquid‐phase effective mass‐transfer boundary‐layer thicknesses and salt‐film thicknesses were determined at an iron rotating disk in by fitting short‐time overpotential relaxation data after current interruption to a single mathematical model for surface concentration changes. Steady‐state liquid‐phase mass‐transfer boundary‐layer thicknesses during active and transpassive iron etching were in good agreement with those predicted by the Levich equation. Steady‐state prepassive salt‐film thicknesses, which varied between 0.0012 and 0.0039 cm, were found to be dependent on the disk rotation speed to the −0.57 power and the disk potential to the minus one‐sixth power. For a short period of time during the initial stages of constant‐current transpassive etching, a prepassive salt film was present on the iron surface. When this film dissolved, mass transfer was controlled by a liquid‐phase boundary layer which was unaffected by moderate rates of gas evolution. Salt films, which formed at particular current densities during steady‐state transpassive iron etching, ranged in thickness from 0.0014 to 0.0016 cm and were independent of the applied potential and disk rotation speed.