Contents1. Hardness of zinc-iron alloy layers.
A hot dip galvanized coating consists essentially of two parts, one an alloy layer of zinc and iron in contact with the surface of the basis metal and formed by reaction between the zinc and the iron or steel and the other layer of unalloyed zinc. Both contribute to the corrosion resistance and life of the coating.
Photomicrograph of a section of a typical hot dip galvanized coating showing
various alloy layers metallurgically bonded to base steel.
2. Cathodic protection.
The molten zinc in the galvanizing bath covers corners, seals edges, seams and rivets, and penetrates recesses to give complete protection to areas which are potential corrosion spots with other coating systems. Where there is damage or minor dicontinuity in the sealing coat of zinc, protection of the steel is maintained by the cathodic action of the surrounding coating.
| When discontinuity or damage in the zinc coating exposes the underlying steel, the cathodic protection which zinc provides for steel comes into action and ensures that the exposed steel does not corrode. | Most organic coatings and paint films depend entirely on their sealing ability to protect steel from corrosion. They offer no protection to bare steel exposed by failure, damage or discontinuity in the coating film. Corrosion starts and spreads rapidly beneath the coating. | |
3. Long Protective life of galvanized coatings.
The metallic zinc of a galvanized coating is strongly resistant to most corrosive environments, with a rate of corrosion between 1/10 to 1/13 that of steel, depending on the nature of the corrosive environment. The excellent corrosion resistance of zinc in the atmosphere and in most natural waters is due to the formation of a protective layer or patina which consists of insoluble zinc oxides, hydroixdes, carbonates and basic zinc salts, depending on the environment. When the protective patina has stabilized reaction between the zinc coating and its environment proceeds at a greatly reduced rate resulting in long coating life.
* These lives have been calculated on the assumption that 10% of the original zinc remains when rusting of the basis steel begins, which has been demonstrated for coatings of reasonable uniformity.
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