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Disimilar Metals

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.

Galvanic Series

Galvanic Series
MagnesiumActive (Anode)
Zinc 
Galvanised Steel 
Aluminium 
Mild Steel 
Cast Iron 
Lead 
Brass 
Copper 
Bronze 
Monel 
Nickel (passive) 
Stainless Steel 304 (passive) 
Stainless Steel 316 (passive) 
Silver 
Titanium 
Gold 
Graphite 
PlatinumNoble (Cathode)
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
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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.

 

Lead in Contact with AZ

Lead in contact with coated or uncoated AZ will cause premature corrosion.

 

Cladding in Contact with Stainless Steel Rivet

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 metals incompatible with roof cladding, there can be contamination from splashing or immersion of the roof sheet ends into the poorly drained gutter. Special provisions—such as painting the inside of a copper spouting, or an apron flashing—should be made when using copper or lead gutters and spouting with coated steel roof cladding. See Fasteners. The top of copper spouting should not touch coated steel cladding, tiles or flashings.

 

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Revision Category: 
1 - Minor Errata
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Editing for clarity.

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