Metals are widely used in our daily lives from aluminum and copper wires to gold and silver for jewelry and electronics. One of the most widely used metals today is iron, or rather, steel alloys made from the processing of iron. Steel has become one of the most versatile products used in home appliances, car dashboards, logos, buildings, and bridges. Steel, however, is as vulnerable to corrosion as any other metal.
Metal coatings provide protection for steel so that steel can be used in a variety of challenging environments. Metal coatings provide two main types of protection that are shielding protection and current protection in certain situations.
When a metal coating such as zinc is applied to steel, it dries and hardens, creating an impermeable barrier against moisture infiltration, which removes one of the essential components necessary for corrosion since oxidation would not occur without electrolytes (water) and, therefore, it would not rust. Another important aspect of shielding protection is corrosion membrane protection. As mentioned above, some metals, such as aluminum, react with oxygen and form protective oxide films on their surfaces, which are elastic and firmly adhered to the surface of aluminum to prevent moisture from infiltrating and further corrosion, making aluminum an ideal material for sheet metal.
Zinc is often used in coated structural steel, which reacts with oxygen and water in the air to form corrosion products that form a protective layer against the underlying steel.
The reactive zinc that is exposed reacts with oxygen to form zinc oxide, which then reacts with water to form zinc hydroxide. When zinc hydroxide reacts with carbon dioxide in the air, the resulting product is zinc carbonate, and these corrosion film products, like those formed on aluminum, are resistant to water infiltration and adhere tightly to the surface of steel so that it does not fall off as easily as corrosion on the iron.
Zinc, however, is an active metal that slowly corrodes over time. So zinc’s degradation rate is still several times lower than steel’s, and it significantly extends the service life of the steel it is intended to protect.
The second way to protect steel is to sacrifice the coating to provide current protection for the underlying steel. For example, if the underlying steel is exposed due to scratches on the cutting edge or severe coating damage, the surrounding coating will be the first to corrode before the steel begins to corrode.
Current protection, also known as cathode protection, is due to the fact that zinc is more electronegative than iron/steel. When zinc coating is coated, it is first corroded as a sacrificial anode, so the underlying steel is protected as a cathode. This current corrosion will continue until the anode material (zinc coating) is completely consumed.
The coating in the building will inevitably be scratched or damaged, so current protection is especially important. For this reason, zinc coatings are superior to aluminum coatings in protecting steel components. Although zinc and aluminum are very similar in terms of current series properties, the protective oxide film formed by aluminum is difficult to penetrate and will not be consumed by corrosion, so it does not provide current protection for damaged steel; on the other hand, the effective barrier formed by zinc coating is not as protective as aluminum, so it will give priority to corrosion to protect damaged or exposed steel.
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