Non-destructive thickness measurement as a tool to evaluate the evolution of patina layer formed on weathering steel exposed to the atmosphere

Abstract

This work considers the magnetic induction method on rough surfaces (ABNT NBR 10443) to determine the thickness of the patina layer formed on weathering steel (WS) sheets exposed for up to two years at São Paulo atmosphere and on WS sculptures exposed for sixteen and twenty-four years at the same site. A good agreement was found with SEM measurements. It was also found that patina thickening followed a two slopes bilogarithmic law, with a steeper slope for short exposure periods, in agreement with increased corrosion rate. For long-term exposure, it was found a low thickening rate (<2μmyear−1), suggesting stabilization of the patina layer. The increase in goethite (α-FeOOH) content over time reinforces the hypothesis of a more protective layer as indicated by Protective Ability Index (PAI) for 24 years old sculpture.