By
Martin van Leeuwen |
Salty air, seawater, microbial activity, ultraviolet radiation and sulphur from engine fuel can all quickly corrode the steel structure of a passenger ship. Combating corrosion is challenging, but zinc-rich paint coatings that contain fine particles of metallic zinc powder are a very effective first line of defence. The coatings, which typically contain 85-95% zinc powder, have an organic binding system that provides a physical barrier to corrosion. Meanwhile, the metallic zinc particles provide abrasion resistance to the paint and cathodic protection for the underlying steel.
Typically, zinc-rich primer coatings are covered with a multi-layer epoxy system and a polyurethane topcoat for ultraviolet radiation resistance. These additional barrier layers can also contain zinc oxide, which is a white colouring pigment that provides protection against mildew and ultraviolet. It also neutralises the acids that form during paint oxidation.
Alternatively, ship operators can apply a dense metallic zinc or zinc alloy coating to a steel substrate by thermal spraying. The thermally sprayed coating consists of 100% zinc or zinc alloy and, because the process forms a metallic bond between the sprayed particles and the underlying steel, it ensures cathodic protection of the steel.
In maritime environments, thermal sprayed zinc-aluminium alloy metallic coatings provide even better corrosion protection than zinc alone. They’re often used with an organic paint coating on top to ensure maximum durability and minimum maintenance.
Thermal sprayed metallic zinc or zinc alloy coatings require the same steel surface cleanliness as organic paint coatings and a rough substrate for excellent adhesion. They don’t require any drying time and, if they are used as a part of a duplex coating system, they don`t need any further surface preparation. However, they should be overcoated with an organic paint coating immediately after spraying to improve maintenance and achieve the lowest lifecycle cost.
Thanks to their versatility and multiple benefits, marine coating systems with metallic zinc, metallic zinc alloys and zinc oxide guarantee maximum durability and minimum maintenance.
Martin van Leeuwen is the manager of technology and market development at the International Zinc Association
This article was first published in the Spring/Summer 2020 issue of Cruise & Ferry Review. All information was correct at the time of printing, but may since have changed.
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