Enhancing adhesion and anti-corrosion performance of hot-dip galvanized steels by sandblasting/phosphating co-treatment

The substations outdoor steel structures employed in aggressive marine environments can accelerate corrosion damage and cause incredible degradation of performance. Hot-dip galvanizing and organic coating dual-coated anticorrosion system is currently the most effective and efficient protection strategy. In present paper, sandblasting and phosphating technique were applied to the surface of zinc plating, the effect of various grit-blasting and phosphating technology conditions on the adhesion performance and corrosion resistance of duplex-coated system were systematically investigated. Results revealed that the bonding strength of the duplex coating after grit-blasting and phosphating pretreatment was 3.25 and 2.71 times higher than that of the untreated, respectively. In particular, sandblasting and phosphating coprocessing of duplex coating could furtherly improve the adhesion behavior and corrosion resistance, which mainly due to their synergistic effect. Sandblasting can rough the surface of galvanized coating and generate many pits and scratches. Thus, phosphating can form more needle-like zinc phosphate crystals in those positions, anchoring and pinning firmly the interface between galvanized coating and organic coating. Meanwhile, the phosphating film still acted as an anti-corrosion physical barrier to hinder the intrusion of corrosive medium and protect galvanized steels from storage rust before painting for a long time.

Effect of under layer metallic coating composition on phosphating and the corrosion performance for automobile applications

Automobile coating system consists of a metallic underlayer followed by a phosphate coating and, lastly, multilayer organic coating. In this work, the effect of the underlying metallic coatings, namely, a Mg-Al-Zn alloy coating (Magizinc) and a conventional galvanized Zn coating on the phosphate coatings formed thereon, and its corrosion performance was investigated. The corrosion resistance offered by the phosphate coating formed on the Magizinc coating was higher than the phosphate coating on the galvanized Zn coating (a reference coating employed in the study) in NaCl solution, as revealed by potentiodynamic polarization, electrochemical impedance spectroscopy, and salt-fog tests. In-depth characterization of the phosphate coatings was carried out using scanning electron microscopy and glow discharge optical emission spectroscopy. It was revealed that the phosphate crystals formed on the Magizinc coating were more fine-grained, compact, and crack-free as compared to that formed on the galvanized coating and contained Mg aiding 4-10 times increase in the corrosion resistance as determined by the electrochemical studies. However, it only improved marginally against the appearance of red rust in a salt-fog test over the unphosphated Magizinc coating. The phosphate coating on Magizinc marginally improved the adhesion of an epoxy primer coating applied on the phosphated Magizinc coating and significantly (>3.5 times longer exposure) retarded the deterioration of the epoxy primer coating in the salt-fog environment in comparison with the similar studies carried out on the phosphated conventional galvanized zinc coating. Notably, phosphating the Magizinc coating caused a ten times reduction in the H pickup compared to that in the galvanized coating under identical phosphating conditions, suggesting the former coating lowered the propensity for hydrogen embrittlement in the steel.