Effect of Al Content on the Long-Term Corrosion Behavior of Arc-Sprayed ZnAl Alloy Coatings

Abstract

The corrosion of steel structures in aggressive marine environments is a vital issue that induces significant degradation of their performance and lifespan. Herein, three arc-sprayed ZnAl coatings with varied Al contents of 0 wt.%, 15 wt.%, and 50 wt.% were deposited onto a hull steel substrate. The effect of Al content on the long-term corrosion protection performance of ZnAl coatings left in a chloride-containing solution for 840 h was systematically investigated. The evolutions of open-circuit potential, polarization curves, and electrochemical impedance spectroscopy of different ZnAl coatings during the long-term immersion test were examined. The morphologies and phase constitutions of the corrosion products were characterized. The results indicated that the corrosion rate of ZnAl coatings decreased as the Al content increased, and the ZnAl50 coating exhibited the most superior long-term corrosion protection performance. Moreover, for the three ZnAl coatings with an Al contents varying from 0 to 50%, their corrosion rate increased with immersion time in the initial 360 h due to the formation of the unstable and porous corrosion product ZnO; after 360 h immersion, their corrosion rate decreased with the prolonging of immersion time. This was revealed to be related to the formation of different corrosion products. ZnO and stable Al2O3 were the main corrosion products for the pure Zn coating and ZnAl15 coating, respectively. Al2O3 and powerful layered double hydroxide Zn6Al2(OH)16CO3·4H2O were found to be the dominant corrosion products of the ZnAl50 coating, which was responsible for its remarkable long-term corrosion protection performance.

Changes in Corrosion Behaviour of Zinc and Aluminium Coatings with Increasing Seawater Acidification

“…Carbonates are particularly useful for increasing corrosion protection among those substances [55]. Other research has noted that the formation of layered double hydroxides was observed [16]. These inorganic substances have complex structures containing zinc, aluminium, hydroxy groups, and carbonate groups.…”

“…Generally, zinc coatings show no significant changes in terms of their corrosion resistance in acidic and alkaline solutions (pH = 4-10) and, therefore, seem to be a good choice for undersea constructions [13]. However, to enhance the corrosion protection of Zn coatings in high-chloride and -acid environments, various types of Zn coatings with Al and Mg have been developed [7,8,11,12,[14][15][16]. For instance, 'Galfan' Zn coating, which was developed in 1980, contains around 5%wt.…”

“…However, arc-sprayed Al coatings with Mg have also been found to act as self-healing elements, improving materials' resistance to electrochemical corrosion in a solution of NaCl+CaCl 2 +NaHCO 3 with chloride and carbonate ion participation in distilled water at pH 9. This is due to the passive nature of the corrosion products of Mg 6 Al 2 (OH) 16 CO 3 , formed under an aggressive chloride environment, causing them to fill cavities and create a thin layer of stable oxides. In contrast, Al-Zn coatings only offer barrier protection by depositing less stable corrosion products with poorer adhesion.…”