Zinc-bearing paints can also be used as a coating on galvanized steel structures in marine environments or for those immersed in seawater for added corrosion protection. Thus, they make repairs fairly straightforward. This type of coating provides the least long-term corrosion protection but is the easiest to apply in the field. Zinc-bearing paints can be used as a primer or finish coating. (For more about galvanizing, read: Hot-dip vs Cold Galvanizing: What’s the Difference?) The outer layer is relatively thin and thus the corrosion resistance is limited.Ĭold galvanizing involves applying zinc paint, either by brushing it on or by spraying zinc dust/oxide mixed with a binder. The coating's structure consists of successive layers that are enriched in zinc until the outer layer is pure zinc.
Hot-dip galvanizing of steel is accomplished by immersing the part continuously, or in batches, in a bath of molten zinc. Zinc-rich coatings are used in a processes known as hot-dip or cold galvanizing to prevent corrosion. Therefore, zinc metals and components coated with zinc-based products can have a significantly longer service life than other metals when exposed to the atmosphere. (For more on corrosion rate, read: Corrosion Rate Conversion: Simple Ways to Convert Data Between Common Corrosion Units. The amount of pollutants present will impact the corrosion rate. In these cases, the corrosion rate can be as much as four times greater than in coastal environments without such pollutants. However, when sulfur-type pollutants are present, they will react with the zinc carbonate and moisture to form a water-soluble zinc sulfate. Zinc's corrosion rate is 1/30 that of steel. While the carbonate layer has protective properties, zinc is a reactive metal and will slowly corrode over time. This layer acts as a protective barrier that prevents air and moisture from contacting the underlying substrate and prevents further corrosion and deterioration. Unlike iron oxides, which flake off easily, zinc carbonate is resilient, chemically stable, and adheres firmly to the metal's surface. This layer then reacts with moisture to produce zinc hydroxide, which further reacts with carbon dioxide to create zinc carbonate. Zinc, on the other hand, reacts with oxygen to form a thin oxide layer. This cycle repeats itself until the iron is fully consumed by the corrosion process. Hydrated iron oxides are brittle and flake away easily, thus exposing more of the underlying metal to the atmosphere for further corrosion. This corrosion product, commonly called "rust," is known for its reddish-brown color and its damaging effects. Iron, for example, reacts with water and oxygen in the atmosphere to form hydrated iron (III) oxide on the metal's surface. However, this element does not rust like most other metals.
Like all metals, zinc corrodes when exposed to air and moisture.
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