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How Metallic Coatings Protect Steel
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 April 2006 Feature Sidebar

How Metallic Coatings Protect Steel

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Steel is a very versatile product. It comes in many sizes and types, and is applied to many end uses, including steel buildings, automotive panels, signs, and appliances. The strength, formability, and low cost of steel are some of the reasons for its widespread use. Unfortunately, steel is prone to rusting, a phenomenon that causes the surface to become unsightly and, over time, may contribute to product failure. For this reason, steel is protected by a variety of methods ranging from internal alloying (stainless steel, for example), to coating with paints and/or metallic coatings.

Corrosion is an electrochemical process that, in the case of steel, oxidizes the iron in the steel and causes it to become thinner over time. Oxidation, or rusting, occurs as a result of the chemical reaction between steel and oxygen. Oxygen is always present in the air, or can be dissolved in moisture on the surface of the steel. During the rusting process, steel is actually consumed during the corrosion reaction, converting iron to corrosion products, that is, it is simply returning to its original and lower-energy form of iron oxide.

In the case of most low-carbon steel products, iron oxide (rust) develops on the surface and is not protective because it does not form a continuous, adherent film. Instead, it spalls, exposing fresh iron to the atmosphere which, in turn, allows more corrosion to occur. This characteristic of steel is very undesirable, both aesthetically and from the aspect of service life. Eventually, often sooner than desired, the steel is corroded sufficiently to cause degradation in the service life, i.e., loss of structural strength, perforation and intrusion of water, etc.
Fortunately metallic coatings can be applied to steel in a very cost-effective manner to confer sufficient corrosion protection so that it can be used for a multitude of demanding applications.

Metallic coatings protect steel in two principle ways. First, like paint, they provide barrier protection (assisted by the corrosion product film which provides additional temporary protection), and secondly, they provide galvanic protection in most instances.

Barrier Protection

The main mechanism by which galvanized coatings protect steel is by providing an impervious barrier that does not allow moisture to contact the steel, since without moisture (electrolyte) there is no corrosion. The nature of the galvanizing process ensures that the metallic zinc coating has excellent adhesion, abrasion, and corrosion resistance.

Galvanized coatings will not degrade (crack, blister, and peel) as do other barrier coatings such as paint. However, zinc is a reactive material and will corrode and erode slowly. For this reason, the protection offered by a galvanized coating is proportional to its thickness and to the corrosion rate (see Figure 2). It is therefore important to understand zinc’s corrosion mechanism and what factors affect the rate.

Other metallic coatings, such as aluminum, also provide good barrier protection for steel sheet. Why is this the case with aluminum? Similar to steel and zinc, aluminum reacts in air to form an oxide film on its surface. However, contrary to the behavior of iron oxide, and similar to what happens with zinc, the aluminum oxide film that forms does not spall off, and remains as an intact, very tightly adhering film on the surface of the aluminum. By preventing exposure of fresh aluminum to air and moisture, this intact film stops further corrosion of the underlying aluminum. The oxide remains as a stable non-corroding film, rendering the surface passive in most environments.

Corrosion Product Film Protection

Freshly exposed galvanized steel reacts with the surrounding atmosphere to form a series of zinc corrosion products. In air, newly exposed zinc combines with oxygen to form a very thin zinc oxide layer. When moisture is present, zinc reacts with water, resulting in the formation of zinc hydroxide. The final corrosion product is zinc carbonate, which forms from zinc hydroxide reacting with carbon dioxide in the air. Zinc carbonate is a thin, tenacious, and stable (insoluble in water) layer that provides protection to the underlying zinc, and is the primary reason for its low corrosion rate in most environments.

Galvanic Protection

The second shielding mechanism is zinc’s ability to galvanically protect steel. When base steel is exposed, such as at a cut edge or scratch, the steel is cathodically protected by the sacrificial corrosion of a zinc-bearing coating. This occurs because zinc is more electronegative (more reactive) than steel in the galvanic series, as shown in the box below.

Galvanic Series of Metals and Alloys
Corroded End - Anodic (Electronegative)
Magnesium
Zinc
Aluminum
Cadmium
Iron or Steel
Stainless Steels (active)
Lead
Tin
Copper
Gold
Protected End - Cathodic (Electropositive)

In practice, this means that zinc-bearing coatings will not be undercut by rusting steel because the steel adjacent to the coating cannot corrode. Any exposure of the underlying steel, due to severe coating damage or a cut edge, will not result in corrosion of the steel until the adjacent metallic-coating has been consumed. Unless relatively large areas of steel are exposed there is minimal effect on the overall service life of the coating.

The distance over which the galvanic protection of zinc-based coatings is effective depends primarily on the environment. When completely and continuously wetted, especially by a strong electrolyte, for example, seawater, relatively large areas of exposed steel will be protected as long as any zinc remains. In air, where the electrolyte is only superficial or discontinuously present (such as from dew or rain), smaller areas of bare steel are protected. The “throwing power” is nominally about 0.125 inch (3.2 mm) for a pure zinc coating, although this can vary significantly with the type of atmosphere and amount of zinc in the coating.

If the coating is consumed, why use it? In the case of zinc-based coatings, the rate of consumption is considerably lower than that of the steel (by at least a factor of 10 for pure zinc coatings). In this way, a thin coating of zinc can protect steel for a long time (see Figure 2). For example, in a rural atmosphere, where the number and concentration of pollutants in the air is generally quite low, zinc might corrode at a rate of 1.0 micron/year (0.04 mil/year), while low-carbon steel in this same environment might corrode at a rate 10 times as high or more. As discussed earlier, zinc forms an adherent, protective oxide/carbonate film on its surface similar to the oxide film on the surface of aluminum. This film helps to prevent contact between the environment and fresh zinc, and the rate of corrosion is kept low.

The film that forms on the surface of zinc is not as protective as the aluminum oxide film on the surface of aluminum-based coatings. One reason is that zinc oxide/carbonate is susceptible to dissolution if the moisture is sufficiently acidic. This is good and bad. It is good in that, if the oxide film were totally protective, the zinc would no longer offer galvanic protection to the steel at exposed areas. Rusting would therefore occur at scratches and other exposed areas. The downside of the somewhat incomplete protection of the oxide film on a galvanized sheet is that the coating does corrode and is eventually consumed. In the case of aluminum, the oxide is such a good barrier that the metal, while close to zinc in the galvanic series, offers little galvanic protection to iron in most atmospheric environments.

In total, metallic coatings of many types can be applied to steel very cost-effectively to provide outstanding, long-term corrosion protection for use in a multitude of demanding applications. Sacrificial metallic coatings are unequalled in their ability to provide both overall barrier protection plus sacrificial galvanic protection at exposed areas. The result is the unparalleled combination of steel’s strength and versatility with the long service life provided by metallic coatings over a broad range of applications in construction, metal buildings, automobiles, appliances, and many other end uses.

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