Crystallization pressure and volume variation during rust development in marine and urban-continental environments: Critical factors influencing exfoliation

Abstract

The rust layer formed on carbon steel exposed to natural marine and urban-continental envi-ronments for up to 50 years was studied. Mineralogical phase composition of the rust layer was evaluated by X-ray diffraction (XRD), akaganeite, goethite, lepidocrocite, magnetite, and amorphous phases were identified. Morphological characterization of the specimens was performed using scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) microanalysis. Mechanical stress generated during the formation of the oxide causes exfoliation-induced breakout of the rust layer. Volume variation generated by structural transformations and crystallization pressure (Δp) of the crystalline phases were analyzed to assess the mechanical stress on the rust and a linear relationship was found between the molar volume expansion ratio coefficient (α) and the Δp param-eter. The highest Δp was yielded by goethite (374.99 MPa), while akaganeite presented the highest α value (3.29).