A component which may appear suitable may prove unsatisfactory in service because it is incompatible with another material or substance in contact with it.
This incompatibility can occur when the metals are in electrolytic contact or when water from one metallic surface discharges onto another. When a noble metal dissolves in water and flows over a less noble one, the more noble metal deposits on the less noble metal and create corrosion conditions.
The similarity of metals is indicated by their relative position in the galvanic series. The more dissimilar the metals, the greater the corrosion potential in a galvanic circuit
Generally, run-off from metals higher on the table to those lower will not cause corrosion, but run-off in the other direction may do so.
Metals such as aluminium, stainless steel, and Zincalume® form an inert surface that does not produce soluble salts and run-off from them will not result in dissimilar metal corrosion. However, because these surfaces are inert, potential for run-off to create inert catchment corrosion on unpainted zinc or galvanised steel must be considered.
Stainless rivets will cause corrosion to Z AZ and ZA coated products.
Where the use of dissimilar metals is unavoidable, a non-absorbent inert material can be used as an electrolytic separator. Long-term corrosion resistance depends on the separation remaining effective.
Examples of separation materials are inert plastic tapes, polythene or silicone sealant, and in the case of fasteners an EPDM sealing washer.
Where gutters and spouting are made from materials incompatible with roof the cladding, there can be contamination from immersion of the sheet ends if the gutter is poorly drained. Special provisions, such as ensuring there is a 10 mm drainage gap between spouting and fascia should be made to avoid immersion of coated steel roofing into copper gutters. Discharging the gutter into a rain head with leaf deflector can also help. Low front spouting can be considered, but that creates aesthetic issues and may contribute to early corrosion in marine areas. No part of copper gutters should be in contact with coated steel roofing or flashings.
The electrochemical tables or galvanic series scales, often quoted in technical literature as a measure of corrosion, show the electro-potential between pure metals, not between their oxides, carbonates or chlorides.
Although theoretically correct, these tables can give a misleading indication of the performance of different materials in contact.
This series only applies to pure metal. Under certain conditions, some metals react with the environment or chemicals to form a passive surface, which renders them less active, so that any ranking can be misleading. "Passivity" becomes an important phenomenon in controlling corrosion rates.
However, the Electrochemical series table is still a useful indicator of electrode potential. The further apart two metals are in the electrochemical series, the greater the potential difference between them.
Metals termed anodic, active, negative, or less noble corrode in preference to metals that are deemed more cathodic, noble, positive or less active. The less noble metal becomes the anode and is subject to corrosion. The greater the potential difference, the more corrosion there will be of the less noble metal; i.e., on the anode.
The difference in nobility is why zinc can protect a steel substrate.
Different electrolytes can lead to different rankings, and metal alloys may display more than one potential than that which applies to their "active" state.
The exposed surface ratio of anode and cathode determines the rate of dissimilar metal corrosion. For instance, if a fastener which has a small surface compared to the cladding becomes the anode, its current density will be high, and the fastener will corrode quickly; e.g., an aluminium rivet in a copper sheet. When the opposite is the case, the effect is not so great.
The 4.9.2A Electrochemical table shows zinc is more active than steel. Contact between steel and zinc, in the presence of moisture, will cause the zinc to corrode or sacrifice itself, to protect the steel.
Timber is generally acidic, although some timbers—such as cedar—are more acidic than others. The interaction between preservative treated timber and metal depends on the moisture content of the timber, the time of wetness and the type of treatment. The corrosion rates of metal in contact with wet CCA-treated timber and with untreated radiata are similar.
Neither AZ coated steel nor aluminium should be used as a flashing embedded in concrete or masonry.