The influence of solution chemistry on the deposition and corrosion protection of 3-mercaptopropyltrimethoxysilane coatings on magnesium alloy AZ91

The widespread use of Mg as a light weight construction material has been limited because of its susceptibility to corrosion. The purpose of this project was to characterize the influence of coating bath chemistry on the deposition of 3-mercaptopropyltrimethoxysilane (MPTS) films onto Mg alloy AZ91 and determine its corrosion protection. For MPTS to bind to the alloy surface, it must undergo hydrolysis followed by condensation. These reactions are affected by solution pH, the availability of water, the solvent, MPTS concentration and the surface to be grafted. The reactions occurring in solution were monitored with 1H-NMR and ATR-FTIR. At pH 4, hydrolysis was observed to occur at a faster rate with minimal oligomerization. The solution effects on film deposition were studied with ATR-FTIR, ellipsometry and SEM/EDS. With these techniques, the films were found to be macroscopically non-uniform. There is speculation that the silanes initially physisorb to surfaces and curing is required for cross-linking of Si-O-X bonds. Therefore, the adhesion of MPTS films to Mg alloy AZ91 before and after curing was characterized with ATR-FTIR and ellipsometry. The corrosion protection of the film was determined with potentiodynamic polarization measurements in a corroding media of 0.1M phosphate and 0.1M sulfate at pH 7. The corrosion products were characterized with ATR-FTIR and SEM/EDS. These studies demonstrated that both solution concentration and prehydrolysis of the coating bath have a significant effect on the properties of organosilane films on Mg alloys.