ABSTRACT
Corrosion protection of structures operating in seawater is usually provided by cathodic protection (CP) by sacrificial anodes, often in conjunction with protective organic coatings. However, these methods have some limitations. Organic coatings, when used without CP, can provide protection to steel only when intact and anodes not only significantly increase the overall mass of the structure, but also have to be replaced periodically.
An alternative corrosion mitigation method is the application of thermally sprayed metallic coatings, such as Al, Zn and ZnAl, which work as a barrier to corrosive environments when intact, and as an evenly distributed sacrificial anode when damaged.
In this work, the behavior of arc-sprayed ZnAl- and Al-coatings was investigated under two conditions – (i) full immersion in artificial seawater (ASTM D1141), and (ii) under droplets of artificial seawater. The first to simulate the immersed zone and the second to simulate above-surface splash zone conditions. The effectiveness of the coatings was evaluated using electrochemical techniques (OCP monitoring, dynamic polarization and LPR). Raman spectroscopy and SEM/EDX were employed to analyze corrosion products.
INTRODUCTION
A promising candidate for corrosion protection of steel structures operating in marine environments is the application of sacrificial, thermally sprayed (TS) coatings. Unlike organic coatings, sacrificial metallic layers provide protection not only when they remain intact (as a barrier layer), but also in case of a damage. 1 Even if steel is partially exposed, for instance as a result of a mechanical damage, TS coatings provide cathodic protection working as an evenly distributed anode.2 Moreover, in comparison to traditional galvanic anodes, TS coatings do not have to be replaced periodically and do not significantly increase the weight of the structure.
The most commonly used sacrificial coatings comprise Al, Zn and their alloys due to their greater electrochemical activity in aqueous media than the steel substrate to be protected. Test performed on thermally sprayed Zn in seawater revealed that the lifetime of the coating is approximately proportional to the thickness of the coating.3 Moreover, the protectiveness of the coating is associated with the formation of the corrosion products, which can hinder dissolution of the coating. An excellent corrosion performance of thermally sprayed aluminum (TSA) coatings in neutral environments is associated with its passive nature.4 A passive oxide film, which forms on the surface of aluminum works as an electrical insulator. 5 It prevents the movement of electrons (produced during anodic dissolution of the metal) from the metal to the oxide/solution interface, which results in the inhibition of cathodic reactions on the coating. Alloying Zn with Al can improve corrosion resistance properties of the coatings as ZnAl alloys combine high anodic efficiency of Zn and high passivity of Al.
