The marine environment is hostile to most engineering materials, a combination of in-service wear and exposure to marine environment leads to an accelerated material degradation. Insufficient or poor protection of the substrates further assists the accelerated material degradation in marine environment. There is a direct relationship between the material-state of a ship and its operational capability, readiness, and service life. The current state-of-the-art practice is to use paint-based coatings to maintain the material-state of ships. However, the protection offered by paint coatings is usually brief due to inherent permeability and low damage tolerance of these coatings. For this reason, the paint coatings require renewal at regular intervals, typically less than 5-years, to maintain a minimum level of protection from the marine environment. The need for regular painting of ships results in a significant negative impact on the through-life availability, operational capability/readiness, and the cost of maintenance/operation of naval ships. Therefore, the fleet owners and operators should look beyond the conventional paint-based coatings to achieve significant breakthrough improvements in maintaining and enhancing the material-state of naval ships. Metallic coatings, if selected and applied appropriately, will outperform the paint coatings in the marine environment. Historically, the cost and performance of metallic coatings, mainly thermal metal spray (TMS) coatings, prevented their widespread use in the marine industry. The TMS coatings also have their own inherent application and performance related limitations that are widely reported in the literature. However, the cold metal spray (CMS) coating process can overcome the application and performance related limitations that are typically associated with the TMS coatings, therefore creating an opportunity for widespread use of metallic coatings in shipbuilding and fleet upkeep/maintenance. In this paper, the ability of low-pressure (LP-CMS) coatings to repair and reclaim damaged marine components, and application of functional coatings to improve in-service damage tolerance of the damaged/new components is investigated. The results of the investigation show that two LP-CMS coatings, Al-alloy and CuZn-alloy, can be used to repair and preserve both new and damaged components. The accelerated salt-spray and natural immersion corrosion testing of the LP-CMS coatings showed that each coating will be better suited to a particular operational environment, i.e. CuZn-alloy coating performed well in both immersion and atmospheric corrosion environments, whereas Al-alloy coating performed well only in atmospheric corrosion environment.
Formation of Out-burst Structure in Hot-dip Zn-5 mass % Al Alloy Coating
