Enhanced corrosion resistance of AA 2024-T3 and hot-dip galvanized steel using a mixture of bis-[triethoxysilylpropyl]tetrasulfide and bis-[trimethoxysilylpropyl]amine

Abstract

The corrosion resistance of AA 2024-T3 and hot-dip galvanized steel (HDG) was studied after treatment with bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane), bis-[trimethoxysilylpropyl]amine (bis-amino silane), and their mixture. Electrochemical tests in neutral 0.6 M NaCl as well as scanning electronic microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX) were performed. The results showed that: (1) hydrophilic bis-amino silane did not offer good corrosion protection on either of the metals. This is probably because the bis-amino silane film tends to be positively charged. This promotes ingress of anions like Cl − ions as well as water into the film by electrostatic attraction. As a result, corrosion readily proceeds at the interface. (2) Hydrophobic bis-sulfur silane performed very well on AA 2024-T3, but failed on HDG. The failure here stems from non-uniform film coverage on HDG owing to an insufficient wetting of bis-sulfur silane solution on the Zn oxide on HDG. Local corrosion initiates at defective sites which are poorly covered by the silane film. (3) A bis-sulfur/bis-amino mixture at the ratio of 3/1 greatly enhanced the corrosion resistance of both AA 2024-T3 and HDG. This substantial improvement is achieved by selectively overcoming the major shortcomings of the individual silanes.