Chemical Pickling of 304 Stainless Steel in Fluoride- and Sulfate-Containing Acidic Electrolytes

Abstract

The mechanism and kinetics of pickling of high-temperature oxidized 304 stainless steel in HF-H2SO4 electrolyte were investigated by surface finish, weight loss, corrosion potential, and dynamic electrochemical impedance spectroscopy measurements. A three-stage pickling mechanism was proposed. In the first pickling stage, the low-frequency resistance is low because the chromium-depleted layer exposed is active, and thus a significant weight loss takes place. Corrosion kinetics is related to the duration of the active state and the corresponding corrosion potential. Both are influenced by composition of the electrolyte. The oxide scales become undercut by the removal of the underlying metal. While the exposed alloy becomes enriched in chromium, an active-to-passive transition initiates the second stage. A dense passive layer that develops on the surface during that second stage increases the low-frequency resistance. A local attack at grain boundaries and at honeycombed recesses proceeds during the third stage while the remaining exposed area is cathodically protected. (c) 2005 The Electrochemical Society.status: publishe