Corrosion control is a costly and persistent problem across every class of surface ship, and as ships age, the maintenance costs and effort required to extend their service lives increases. Paint coatings have holidays, low abrasion resistance and poor bond strength. Corrosion initiated at scratches and holes easily lifts the paint film, accelerating attack of the underlying steel. Paint over corrosion only masks an ongoing problem. In contrast, metallic zinc coatings have excellent abrasion resistance and superior bond strength. Furthermore, metallic zinc coatings offer both barrier protection and cathodic protection to steel. The ultimate corrosion protection is to use a metallic zinc base layer with a paint topcoat. The paint adds an extra layer of barrier protection to the zinc, while the zinc protects the paint from underfilm corrosion at scratches and holes. Effective corrosion control significantly reduces maintenance costs and provides the lowest cost of ownership for a vessel. Application of metallic zinc coatings for steel ships will be presented with case studies.
Commercial ships cover a wide range of vessels including tankers, bulk carriers, cargo ships, tugs, fishing boats, passenger ferries and cruise ships. Across all sectors commercial ships contribute significantly to the global economy. Maintenance costs for commercial ships are twofold. First there is the direct cost of corrosion maintenance and second, there is the lost revenue when the ship is not working. Direct maintenance costs for corrosion control of commercial ships is estimated to be $5.4 Billion annually.1 The indirect cost of downtime needed for the corrosion maintenance repair adds another $5.2 Billion for a total cost of $10.6 Billion per year.
Much of this cost is due to simple steel corrosion that could be significantly mitigated with painted metallic zinc coatings. Zinc provides very effective corrosion protection for steel by acting first as a barrier coating, keeping corrosive elements away from the steel, and secondly as a sacrificial anode.2 Painted metallic zinc coatings provide improved corrosion protection performance. A painted metallic zinc coating is called a 'Duplex' coating.3 The paint adds an extra layer of barrier protection to the zinc, while the zinc protects the paint from underfilm corrosion at scratches and holes.
