Corrosion of marine infrastructure is relentless. The continual exposure to moisture in the presence of chlorides can rapidly degrade steel structures. The use of paint as a barrier coating for corrosion control requires near constant maintenance. Conversely, a duplex zinc coating, that is, a metallic zinc coating with a paint top-coat, offers both barrier and sacrificial corrosion protection mechanisms, with much longer lifetimes between maintenance compared to paint only coatings. A paint top-coat over a metallic zinc base layer protects the zinc from initial corrosion. The metallic zinc base layer similarly protects the paint from underfilm corrosion at scratches and holes. In this way, duplex zinc coatings provide more than twice the life of the corrosion protection provided by one coating system alone. The synergy between the zinc and the paint coatings provides protection far superior to either system used independently. Duplex zinc coating systems have decades of proven performance protecting marine structures from corrosion.
Metallic zinc coatings have a long history of providing corrosion protection for steel in marine environments, including port facilities, ships, offshore oil platforms and wind energy structures. Metallic zinc coatings provide not only the barrier protection usually associated with paint coatings but also galvanic protection in the event that there is any disruption in the protective coating, such as mechanical damage, local removal of the coating for maintenance or structural modifications, including the installation of bolt holes and welding. The zinc will corrode preferentially to protect the steel.
Zinc coatings can be applied by hot-dip galvanizing or by thermal spraying. Zinc coatings applied by hot-dip galvanizing involve the full immersion of the steel piece into a bath of molten zinc, which ensures complete coverage of all surfaces, inside and out.1 Zinc thermal spray involves projecting drops of liquid zinc onto the surface of the steel using compressed air. With thermal sprayed zinc coatings, there is no size limitation to the part to be coated, and the technology is fully portable, allowing easy field applications.2 Thermal sprayed zinc is also classed as a 'cold' technology and will not change the mechanical properties or the geometry of the steel being coated.
