Galvanizing - Electrochemical Protection for Steel

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The source of the term “galvanizing” is not really related to protecting steel from corrosion. The term was derived from the name of Italian physiologist Luigi Galvani, who discovered the effects of electric current on the nervous system of dead frogs.

During the early years of electrical science, the metal most widely used for producing galvanic electricity was zinc. And then in 1837, Sorel, a French scientist, applied for a patent in France for a process that involves dipping steel in molten zinc, which was termed “galvanizing,” honoring Galvani who died in 1798.

The Electrochemical Series of Metals

When metals are in electrical contact, one metal (the anode) will lose electrons and oxidize, while the current generated will prevent oxidation of the other metal (the cathode).

All metals have an electrochemical pecking order that determines if they will function as an anode or a cathode to other metals in the sequence. The relative position of the typical metals in the electrochemical series is shown in Table 1.

Table 1. The relative position of common metals in the electrochemical series

All voltage values with regards to copper sulfate half-cell.

Electrochemical Corrosion and Galvanized Coatings

Although tables of electrode potentials are beneficial for highlighting the hazards of electrochemical corrosion between various metals, such tables can be misleading. The difference in potential between various metals is the main driving force that offers the corrosion current, but it is not a dependable guide when it comes to the type and rate of corrosion that occurs at a particular point of contact.

The degree of bi-metallic corrosion also depends on the duration of wetness (bi-metallic corrosion can only take place when a conductive solution is present), the conductivity of the electrolyte, and the ratio of the areas of metals in contact. Furthermore, the presence of oxide films on the surface of one or both of the metals can considerably prevent bi-metallic corrosion.

Acceptable Bi-Metallic Contact

In general, galvanized surfaces can safely be in contact with several types of aluminum alloys, stainless steel 304 and 316F, chrome steel (>12% chrome), and tin, if the ratio of the area of zinc to metal is 2:1 or more, and oxide layers are found on the aluminum alloys and stainless steels. Bi-metallic corrosion rates are considerably reduced if electrical resistance is high due to the presence of insulating films or other non-conductive membranes.

Bi-metallic corrosion will not occur at the points of contact between galvanized coatings and other metals that are not exposed to moisture. This is important for a galvanized reinforcing bar and uncoated rebar, where the points of connection are deep inside the concrete mass and are kept in an inert environment after the curing of concrete.

Stress to the galvanized coating can also be decreased by using stainless steel fasteners on hot-dip galvanized items in well-drained atmospheric exposure conditions. This decrease is realized because of the very high zinc/stainless surface area ratio and the short periods of exposure of the assemblies moisture in Australian weather conditions.

Electrochemical Protection and Coating Mass

Under all circumstances where zinc is corroded sacrificially to safeguard exposed steel, the anti-corrosion performance will be governed by the mass of available zinc. Corrosion rates of zinc coatings required to cathodically protect uncoated steel under severe environments (marine/saltwater) could be 25 times as high as the usual zinc corrosion rate.

Table 2 illustrates the mass of zinc available on either side of a 2-mm thick hot-dip galvanized steel article compared to a Z275 galvanized sheet and continuously galvanized tube.

Table 2. Coating mass required for various galvanized products

As shown in Table 2, batch hot-dip galvanized coatings contain up to nine times the mass of zinc available for electrochemical protection. An additional advantage of post-fabrication galvanizing is that it eliminates the unavoidably exposed steel areas on pre-galvanized items, that mostly place high corrosion stresses on the zinc coating.