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Zinc Coating

Zinc is a unique and very useful nonferrous metal. The largest market for metallic zinc is its usage in high performance, corrosion control coating systems. Zinc is resistant to most atmospheric conditions, yet it remains sufficiently reactive to cathodically protect ferrous metals such as iron and steel.

Among the nonferrous metals, zinc is the least expensive and most readily available. While there are other metals that can also be used to protect iron or steel such as magnesium, aluminum and cadmium, none have proven to be as useful, economical or as effective as zinc. In most corrosive atmospheres, when zinc substrates are top coated, they exhibit a much longer service life than the same zinc coating(s) applied over bare iron or steel. ASTM and the Zinc Institute studies demonstrate this. Due to the porosity of the metallizing application and its reactivity to strong acids and caustics, zinc coated iron or steel scheduled for buried service is most effective when sealed or used as a base for a topcoat.

In the electrochemical series, zinc is a less noble metal than iron; it has a more negative electrode potential. If zinc is in conductive contact with iron, and an electrolyte such as water is present, the zinc ions go into solution. Again, in electrochemical terms, the exposed surface of the iron forms a cathode, and the zinc-coated surface of the pipe forms an anode. Zinc ions migrate to the damaged point and form a layer of “scarring,” which stops the corrosion.

Regardless of how the zinc is applied, whether by zinc spray, sherardizing, zinc plating, hot dip galvanizing, etc., corrosion protection of ferrous surfaces is provided in two different ways: as a continuous, long-lasting corrosion barrier and also as a galvanic protective layer.

Spray applied zinc, often also called arc spray, fusion bond or metallizing, is the most common industrial method of applying zinc, and is the method used at AMERICAN. In this process, zinc wire is continuously fed into a high temperature electric arc and melted. Compressed air atomizes the molten zinc, and the resulting spray is deposited in a fashion similar to conventional spray coating application. At this stage, the zinc coalesces with zinc oxide forming at the interface between each droplet. The zinc oxide forms only a small percentage of the coating, and electrical continuity is maintained both throughout the coating and with a ferrous substrate so that full cathodic protection is achieved.

Zinc as a substrate provides a smooth, dull gray surface that is very reactive. Over time the zinc layer on buried pipe changes into a dense, firmly adhering, impermeable and uniformly crystalline layer of insoluble compounds consisting of zinc oxides, hydrates and zinc salts of different compositions.

To summarize, in addition to providing cathodic protection, zinc as a substrate increases the life and effectiveness of the coatings applied over it.

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The exterior of ductile iron pipe shall be coated with a layer of arc-sprayed zinc per ISO 8179. The mass of the zinc applied shall be 200 g/m2 of pipe surface area. A finishing layer topcoat shall be applied to the zinc. The coating system shall conform in every respect to ISO 8179-1 “Ductile iron pipes – External zinc-based coating – Part 1: Metallic zinc with finishing layer. Second edition 2004-06-01.”

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