Studying the onset of galvanic steel corrosion in situ using thin films: film preparation, characterization and application to pitting

This work reports about a novel approach for investigating surface processes during the early stages of galvanic corrosion of stainless steel in situ by employing ultra-thin films and synchrotron x-radiation. Characterized by x-ray techniques and voltammetry, such films, sputter deposited from austenitic steel, were found representing useful replicas of the target material. Typical for stainless steel, the surface consists of a passivation layer of Fe- and Cr-oxides, a couple of nm thick, that is depleted of Ni. Films of approximate to 4 nm thickness were studied in situ in an electrochemical cell under potential control (-0.6 to +0.8 V vs Ag/AgCl) during exposure to 0.1 M KCl. Material transport was recorded with better than 1/10 monolayer sensitivity by x-ray spectroscopy. Leaching of Fe was observed in the cathodic range and the therefor necessary reduction of Fe-oxide appears to be accelerated by atomic hydrogen. Except for minor leaching, reduction of Ni, while expected from Pourbaix diagram, was not observed until at a potential of about +0.8 V Cr-oxide was removed from the steel film. After couple of minutes exposure at +0.8 V, the current in the electrochemical cell revealed a rapid pitting event that was simultaneously monitored by x-ray spectroscopy. Continuous loss of Cr and Ni was observed during the induction time leading to the pitting, suggesting a causal connection with the event. Finally, a spectroscopic image of a pit was recorded ex situ with 50 nm lateral and 1 nm depth resolution by soft x-ray scanning absorption microscopy at the Fe L-2,L-3-edges by using a 80 nm film on a SiN membrane, which is further demonstrating the usefulness of thin films for corrosion studies.

Automated summary

This work presents a novel approach for studying surface processes of stainless steel during galvanic corrosion using ultra-thin films and synchrotron x-radiation. The films, deposited from austenitic steel, were found to be representative replicas of the target material. By monitoring material transport and leaching of Fe and Ni, the study provides insights into the early stages of corrosion events.