A photoelectrochemical investigation of conversion coatings on Mg substrates

The structure, morphology and composition of conversion coatings grown in stannate bath on pure Mg were studied using potential–time, polarization curves, X-ray diffraction, scanning electron microscopy and photocurrent spectroscopy. The coating is manly constituted by crystalline magnesium–tin hydroxide, whose morphology and distribution depends on the conversion bath composition and temperature. The photoelectrochemical investigation allowed to estimate the band gap value of MgSn(OH)6 and flat band potential. A sketch of the metal/passive film/electrolyte junction formed during conversion on the metal substrate is reported to account for the overall photoelectrochemical behaviour.