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 itself ensures that the metallic zinc coating has excellent adhesion, abrasion, and corrosion resistance. Galvanized coatings will not degrade (crack, blister, and peel) as with 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. It is therefore important to understand zinc`s corrosion mechanism and what factors affect the rate.
Freshly exposed galvanized steel reacts with the surrounding atmosphere to form a series of zinc corrosion products. In air, newly exposed zinc reacts with oxygen to form a very thin, tenacious 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.
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 the zinc coating. This occurs because zinc is more electronegative (more reactive) than steel in the galvanic series, In practice, this means that a zinc coating will not be undercut by rusting steel because the steel adjacent to the zinc 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 zinc 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 is effective depends on the environment. When completely and continuously wetted, especially by a strong electrolyte, e.g., 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 in [3.2 mm], although this can vary significantly with the type of atmosphere.
If the coating is consumed, why use it?
In the case of a zinc coating, the rate of corrosion that it undergoes while protecting steel is considerably lower than that of the steel (by at least a factor of 10). In this way, a thin coating of zinc can protect steel for a long time. 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 0.04 mil/year [1.0 µm/year], while low-carbon steel in this same environment might corrode at a rate 10 times as high (0.4 mil/year [10 µm/year or]), or even higher. The primary reason for the reduced rate of zinc corrosion versus the rate for steel is that, as it corrodes, 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.
Recall that steel typically does not form a protective film in that the oxide layer spalls, constantly exposing fresh iron to the environment.
The film that forms on the surface of zinc is not as protective as the aluminum oxide film on the surface of metallic aluminum. 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 of steel would therefore occur at scratches and other exposed areas. The downside of the somewhat incompleteness of protection of the oxide film on a galvanized sheet is that the coating does corrode and is eventually consumed.
Other Galvanically Protective Coatings
Among the commercially available metallic-coated steel sheet products, zinc (galvanize) offers the most galvanic protection. Zinc-5% aluminum alloy coated behaves similarly with respect to the level of galvanic protection that it provides. Steel sheet with a 55% aluminum-zinc alloy coating offers somewhat reduced galvanic protection versus a galvanized or Zn-5% Al alloy coating.
What does this mean about the relative performance of these products?
As with most things in life, everything comes with a price. Galvanically protective coatings are consumed by corrosion eventually. That is why a galvanized sheet has a definite life span, i.e., time before corrosion of the steel begins. Thus, the amount of zinc applied to the steel during manufacture, described as the coating weight [mass], is important to the life of the product. Coating weight (mass) is expressed using terminology such as G60 [Z180], G90 [Z275], G200 [Z600] etc. per ASTM Specification A653/A653M. G60 [Z180] means that the coating weight [mass] is 0.60 oz/ft2 [180 g/m2 ], minimum (total coating on both sides of the sheet). This coating weight [mass] can be translated into thickness; a G60 [Z180] coating of zinc is about 0.00055 in. [0.014 mm] per side of the sheet. For a galvanized coating, the rate of corrosion is typically linear in most environments, i.e., twice the coating thickness translates to twice the “life”.
However, environments differ. For example, a G90 [Z275] coating will exhibit a longer life in a rural atmosphere than in a polluted industrial atmosphere, but still the rate is linear in both environments. Twice the coating thickness or coating mass will give twice the coating life.
What about Galvalume which is 55% Al-Zn alloy coating?
Galvalume is a product that clearly exhibits less galvanic protection than a galvanized coating, at least after some time of exposure. A 55% Al-Zn alloy coating, being less galvanic in nature, is less reactive. This is one reason why the life of this coated product is considerably longer (in most cases) than a galvanized coating of comparable thickness. This behaviour of a 55% Al-Zn alloy coating is the reason why it is being used successfully for bare roofing applications.
Of course, there are applications where the highly galvanic nature of zinc is desired.
Also, there are other considerations that need to be taken into account when selecting a product for a specific application.
Life of a Galvanized Coating in Different Environments
We have established that a galvanized coating protects steel: 1. By two mechanisms – barrier protection and galvanic protection, and 2. By a linear rate of corrosion for any specific environment type, and 3. For considerably different time periods depending on the specific environment
What is the life of the product in specific applications? The answer to this question is very complex. There are many other applications for galvanized sheet in which the corrosion rate might be different. These include: contacts with water, buried in soil, contact with concrete, sheltered areas on buildings such as under eaves, ductwork inside buildings, etc. For each of these and other applications, the corrosion performance depends on many of specific aspects of the application. For example, when used in contact with concrete, how often does the concrete/galvanized sheet interface get wet? When buried in soil, what is the pH of the soil and the soil permeability, the oxygen content, etc? When used for ductwork, does the product experience condensation periodically or on a regular basis? Are there pollutants in the condensate?
The life expectancy of galvanize sheet is now more reliably determined using the Zinc Coating Life Predictor (ZCLP) softwares.