Metallic coatings are used for corrosion protection in two different ways. In one approach, the metallic coating has a greater corrosion resistance than the substrate, thereby functioning as the cathode in the electro-chemical reaction. The metallic coating serves as a durable, corrosion resistant outer layer, while the substrate provides strength and load bearing capabilities. The other approach uses a metallic coating that has a higher tendency to corrode than the substrate, serving as the anode in the electro-chemical reaction. The two most common anodic metals used are zinc and aluminum. The metallic coating will corrode preferentially and provide protection to the substrate at breaks or imperfections in the coating. Both types provide a protective barrier for the substrate.
Metal coatings can be applied by using several techniques such as electroplating, electroless nickel plating, thermal (flame) spraying, cold spraying (ultra sonic), hot dipping, chemical vapor deposition, and ion vapor deposition. The most relevant methods used on equipment found on Navy ships are discussed below.
Thermal spraying refers to a process by which a metal wire or powder is melted and sprayed onto a surface to form a coating. In the past, the spraying of molten metal was referred to as metal spraying or metallizing, flame spray aluminum, or wire sprayed aluminum. Flame spray aluminum coatings, which uses aluminum powder, are seldom used on Navy ships due to poor, low quality coating results.
In terms of surface preparation, thermal spraying is expensive. A white metal blast surface preparation procedure (SSPC SP-5) is required prior to applying a thermal spray coating. Thermal spray coatings don’t have the ability to penetrate, seal, or encapsulate surface contaminants and as such the surface must be extremely clean.
The most common method of thermal spray coating application used on Navy ships is wire sprayed aluminum. This method uses an electric arc to pass current through the tips of two wires. Heat from the electric arc melts the wires and an air jet propels the molten metal onto the surface. The extremely high temperatures in the arc zone result in coatings with high cohesion and strength.
Hot-dip galvanizing is a process where a zinc coating is applied to a steel structure to prevent corrosion and provide a protective barrier. Galvanizing causes the zinc to form a metallurgical bond with the steel when a series of zinc-iron alloy layers form. These layers are extremely resistant to abrasion. Atop the zinc-iron alloy layers, a layer of impact-resistant pure zinc forms. The zinc serves as a sacrificial anode and will corrode preferentially to the steel substrate.
Hot dip galvanizing involves the following steps. First, the item is cleaned to remove soil, grease and other surface contaminants. Next, the item goes through a pickling process using a dilute acid solution to remove surface rust and mill scale. The steel item is then dipped into a liquid flux bath to remove oxides. Zinc ammonium chloride solution is the most common type of fluxing bath. At this point, the steel structure can dry after it is extracted from the bath or can go directly into the molten zinc bath at temperatures in the range of 815-850° F (435-455° C). The steel item is then removed from the bath and the excess zinc is removed by draining, vibration, or centrifuge. The item is then air dried or may receive a treatment referred to as quenching, where the item is immersed in a water tank which may contain additional treatment additives. Finally, the item is inspected for the desired coating thickness and surface conditions.
Electroplating is an electrodeposition process that forms a layer of metal on top of another metal. The coating can be for decorative purposes or provide corrosion resistance. Zinc, chromium, tin, and nickel are some of the metals most commonly electroplated on steel and other ferrous alloys. Ferrous and non-ferrous alloys are also frequently electroplated with aluminum, brass, bronze, cadmium, copper, gold, iron, lead, platinum, and silver.
In electroplating, the structure to be plated is submerged in an electrolyte solution containing metal ions of the metal that’s to be deposited. The structure is then made the cathode in an electrolytic cell causing metal ions in the solution to “plate” out on the cathode surface. Electroplating is regulated by controlling the potential, current, temperature, residence times, and the composition of bath solutions.
Zinc can be electroplated onto steel and other ferrous alloys to provide sacrificial protection. This process is known as electrogalvanizing. This method of zinc coating application is often preferable to hot dipping because of improved surface finish and finer control of dimensions. Electrogalvanized steel surfaces lack the spangle (surface crystals) found in hot-dipped coatings and can easily be prepared to receive decorative finishes. The pure zinc coating deposited on the substrate is highly ductile and, because of its excellent adhesion, electrogalvanized steel has good working properties. The zinc coating can remain intact even after severe deformation.