Background The derivation of the term ‘galvanising’ has absolutely nothing to do with protecting steel from corrosion. The name comes from the Italian physiologist, Luigi Galvani, who identified the effects of electric current on the nervous system of dead frogs. In the formative years of electrical science, zinc was the most widely used metal for producing galvanic electricity. In 1837, French scientist Sorel took out a patent in France for a process of dipping steel in molten zinc and provided the process with the name ‘galvanising’ in honour of Galvani, who died in 1798. The Electrochemical Series of Metals When metals are in electrical contact, one metal will give up electrons and oxidise (the anode) while the current generated will prevent oxidation of the other metal (the cathode).
All metals have an electrochemical pecking order that determines whether they will act as an anode or a cathode to other metals in the Series. Table 1 illustrates the relative position of the common metals in the electrochemical series.
Table 1. Relative position of common metals in the electrochemical series
Metal / Alloy Potential (Volts) Reactivity Magnesium -1.55 More Reactive Zinc -1.10 Aluminium -0.86 Cadmium -0.77 Cast Iron -0.68 Carbon Steel -0.68 Stainless Steel -0.61 Lead -0.57 Solder -0.52 Tin -0.49 Copper -0.43 Aluminium Bronze -0.41 Less Reactive
All voltage values with respect to copper sulphate half cell
Electrochemical Corrosion and Galvanised CoatingsTables of electrode potentials are of value in drawing the attention to the dangers of electrochemical corrosion between dissimilar metals but such tables can be misleading. While the potential difference between metals is the prime driving force providing the corrosion current, it is not a reliable guide to the rate and type of corrosion occurring at a particular point of contact.
The severity of bi-metallic corrosion also depends on the ratio of the areas of metals in contact, the duration of wetness (bi-metallic corrosion can only occur in the presence of a conductive solution) and the conductivity of the electrolyte. The presence of oxide films on the surface of one or both of the metals can greatly inhibit bi-metallic corrosion.
Acceptable Bi-Metallic ContactIn general, galvanised surfaces may safely be in contact with most aluminium alloys, stainless steel 304 and 316F, chrome steel (>12% chrome) and tin, provided the area ratio of zinc to metal is 2:1 or higher and oxide layers are present on the aluminium alloys and the stainless steels. Bi-metallic corrosion rates are greatly reduced if electrical resistance is high due to the presence of insulating films or other non-conductive membranes.
Where the points of contact between galvanised coatings and other metals are not subject to wetness, no bi-metallic corrosion will occur. This is important with galvanised reinforcing bar in contact with uncoated rebar. The points of connection are inevitably deep within the concrete mass and after curing of the concrete, are maintained in an inert environment.
The use of stainless steel fasteners on hot dip galvanised items in well drained atmospheric exposure conditions will also cause minimum stress to the galvanised coating, because of the very high zinc/stainless surface area ration and the short periods of wetness to which the assemblies are exposed in Australian weather conditions.
Electrochemical Protection and Coating MassIn any situation where zinc is corroded sacrificially to protect exposed steel, the mass of available zinc will determine the anti-corrosion performance. Corrosion rates of zinc coatings required to cathodically protect uncoated steel in aggressive environments (saltwater/marine) may be 25 times as high as the normal zinc corrosion rate. Table 2 shows the mass of zinc available on both sides of a 2 mm thick hot dip galvanised steel item compared to Z275 galvanised sheet and continuously galvanised tube.
Table 2. Coating mass required for various galvanised products
Coating Class / Item Total Coating Mass g/m2 Hot dip galvanised plate (AS1650) 900 g/m2 Z275 pre gal sheet (AS1650) 275 g/m2 Continuous gal tube (AS1650) 180 g/m2 Continuous gal tube (TS100) 100 g/m2
As this table illustrates, batch hot dip galvanised coatings have up to 9 times the mass of zinc available for electrochemical protection. There is the added advantage that post fabrication galvanising eliminates the unavoidable exposed steel areas on pre-galvanised products that place high corrosion stresses on the zinc coating in the first place.