TOF-SIMS and AFM analysis of pH effect on the interfacial films on η-phase in aqueous salt solutions

Abstract

The pH effect on the surface and interfacial films on η-phase (MgZn2) in aqueous solutions under acidic, neutral, and alkaline conditions has been evaluated using time of flight-secondary ion spectroscopy (TOF-SIMS), Atomic force microscopy (AFM) and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX). TOF-SIMS depth profile plots reveal that under an acidic environment (pH2) deep corrosion penetration occurs with a dispersion of corrosion products which claims a considerable depth matrix cross-section. Under near neutral environments (pH 6), the corrosion film is seen to be stratified into two layers of different compositions, while in a slightly alkaline environment (pH 10) the film appears not to be distinctly differentiated, whereas in a very alkaline environment (pH 13) a compact film rich in hydroxides develops. TOF-SIMs surface and depth profile maps were consistent with the depth profile plots. SEM and AFM images reveal that the surface roughness increased in with a decrease in pH value from the acidic to the alkaline environments. EDX elemental composition analysis also indicated a severe drop in the zinc content of the film in the alkaline environment. Largely, metallic zinc enrichment occurs following the initial magnesium dissolution whose stability is greatly affected by the near-surface pH of the bulk solution, thus, giving rise to different film structures.