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Hot dip galvanizing provides both barrier and cathodic protection

Hot dip galvanizing provides both barrier and cathodic protection which is why small uncoated steel surfaces are not significant since they will not influence the overall corrosion control life of the coating.

True or False?

This is a misleading and somewhat irresponsible statement frequently expressed without due consideration of the technical facts.

Hot dip galvanizing provides control from corrosion essentially by way of barrier protection in the form of a largely impermeable film of zinc and iron / zinc alloys which gradually wastes away over a period of time. For this reason, the initial thickness of the applied coating will determine its overall protective life in a given environment. To illustrate, a hot dip galvanized coating 100µm in thickness can provide up to 80 years maintenance free life in an inland rural environment (corrosion categories C1, C2 and C3) whereas the same coating situated in aggressive conditions such as the spray zone close to the ocean is unlikely to survive longer than five to ten years. This is where duplex protection (zinc plus paint) is capable of providing remarkably extended resistance to corrosion attack.

The second line of defence provided by hot dip galvanizing is of course the mechanism of cathodic protection provided by zinc when in contact with steel. Corrosion is an electro-chemical reaction aptly described by the theory of the corrosion cell, which principally constitutes the fundamental teaching for students who are studying corrosion in all its aspects.

It states that for corrosion to occur, there are four requirements namely an anode, a cathode, an electrolyte and a continuous electrical circuit. Eliminate any one of these requirements and corrosion ceases. In the case of a hot dip galvanized coating, the zinc constitutes the anode and steel the cathode. Corrosion theory teaches us that the anode (zinc in this case) is attacked while the cathode (steel in this case) is protected. What this tells us in practice is that at a small uncoated steel surface, the hot dip galvanized coating (zinc anode) will be preferentially sacrificed to protect the uncoated steel (cathode). The most beneficial aspect of this corrosion mechanism is that, unlike most organic paint coatings, corrosion cannot creep underneath a hot dip galvanized protective film as long as the coating remains intact.

What then is the downside of this well documented and proven theory in the case of exposed underlying steel? The answer is simple. As described by way of the corrosion cell, it is the anode that corrodes while the steel is protected at an exposed surface. This naturally means that the surrounding zinc coating is sacrificially corroded at a faster rate than necessary in order to protect the exposed steel.

Clearly, uncoated surfaces are to be discouraged but since we do not live in a perfect world there is no such thing as a perfect coating. Hot dip galvanizing specifications provide for the acceptance of minor defects in the coating the quantity and size of which is limited. Unlike most organic coatings it is not possible for corrosion to creep underneath a hot dip galvanized coating. For this reason, the value of zinc and its ability to protect steel cathodically should not be underestimated.