Laser-assisted surface modification and organo-silane coatings for corrosion resistance of magnesium alloy AZ91D

Fundamental understanding of the role of surface films that form in aqueous environments containing hydroxide and chloride, on the corrosion behaviour of pure magnesium was developed. The equivalent electrical circuits for electrochemical impedance spectroscopy that modelled the corrosion mechanism of magnesium in aqueous environments were developed and verified. Laser-assisted surface modification and organo-silane coatings were investigated for improvement of corrosion resistance of AZ91D. In spite of the change in corrosion morphology, laser-assisted surface modification does not help in improving the overall corrosion resistance of the alloy. Role of specific alkaline surface pre-treatments on the corrosion resistance performance of Bis-1, 2-(TriethoxySilyl) Ethane BTSE coated AZ91D alloy has been investigated. A specific pretreatment before BTSE coating was found to improve the corrosion resistance of the coated substrates, as evidenced by the potentiodynamic polarization results and corroborated by EIS analysis. Silane coatings were thus developed using Bis (Tri-Methoxy Silyl Propyl) Amine (BTMSPA), Bis (Tri-Methoxy Silyl Propyl) Tetrasulphide (BTMSPT), Bis-(Tri Ethoxy Silyl Propyl)-Ethane-Dithiol-Carbamate (BTESPEDC), and (2-Di Ethyl Phosphato Ethyl) Tri Ethoxy Silane (DEPETES). The silane coatings developed using BTMSPA and BTMSPT did not improve corrosion resistance. A silane coating developed from novel long chain BTESPEDC silane provided considerable improvement in corrosion resistance, which was attributed to the presence of hydrophobic thiol group and improved cross-linking between the silane molecules. A silane coating developed using mixture of BTMSPA and DEPETES upon alkali treated AZ91D provided an improvement of at least three orders of magnitude in the corrosion resistance of the coated substrate. EIS analysis revealed high resistance to ingress of electrolyte, and improved cross-linking within the film as the primary reasons of improved corrosion resistance due to the coating of this silane mixture.