COP v3.0:durability; compatibility

4.10 Compatibility 

Materials comprising the building envelope should not be considered in isolation, as their performance can be affected by contact with or run-off from other materials.

This reaction is caused by either their relative places in the electro-chemical series or by the mineral composition of their surface moisture.

4.10.1 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.

4.10.1A 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

 

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.

 

4.10.1B Lead in Contact with AZ

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

 

4.10.1C Cladding in Contact with Stainless Steel Rivet

Stainless rivets will cause corrosion to Z AZ and ZA coated products.

 

4.10.2 The Electrochemical or Galvanic Series 

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 4.10.2A 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.

 

 

4.10.3 Compatibility Table 

The compatibility table should be regarded as indicative only, due to the many permutations of the environment, the amount of moisture present and the relative size of the components.

The indicator for “use with caution in moderate environments” should be interpreted as a warning that it could be unsuitable when there is a risk of continued moisture or other contaminants.

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.

 

4.10.3A Butyl Rubber in Wet Contact with Cladding

Butyl rubber in wet contact with coated aluminium and steel has been found to accelerate corrosion.

 

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.

When 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 13.3 Fasteners. The top of copper spouting should not touch coated steel cladding, tiles or flashings.

Neither AZ coated steel nor aluminium should be used as a flashing embedded in concrete or masonry.

4.10.3B Interactive Material Compatibility Tool

This online tool interactively provides an interpretation of the information in the 4.10.3C Material Compatibility Table Table.  Simply select the two materials in use to view compatibility.

 

4.10.3C Material Compatibility Table

Material water flows from
Aluminium
pre-painted aluminium
AZ coated steel
zinc or zinc coated steel
pre-painted AZ steel
Copper/brass
Stainless steel
Lead
Plastic/glass
concrete/plaster wet
concrete/plaster dry
Wet timber
Cedar
Butyl rubber
AluminiumContactYYYYYNCNYNCNNC
 Run ontoYYYNYYYYYYYYYY
pre-painted aluminiumContactYYYYYNNNYNCNNN
 Run ontoYYYNYYYYYYYYYY
AZ coated steelContactYYYYYNNNYNCNNN
Zincalume ®Run ontoYYYNYYYYYYYYYY
Zinc or Zinc coated steelContactYYYYYNCYYYYNNN
(galvanised - Z)Run ontoYYYYYYYYYYYYYY
pre-painted AZ steelContactYYYYYNNNYNYNNN
 Run ontoYYYNYYYYYYYYYY
Copper/brass*ContactNNNNNYYCYCYYYY
 Run ontoNNNNNYYCYYYYYY
Stainless steelContactCNNCNYYYYYYYYY
 Run ontoYYYYYYYYYYYYYY
Lead*ContactNNNYNCYYYYYNYY
 Run ontoCNNYNYYYNCCYYY
Plastic/glassContactYYYCYYYYYCYYYY
 Run ontoYYYNYYYYYYYYYY
Concrete/plaster wet*ContactNNNCNCYYYYYYYY
 Run ontoNCNYNYYYYYYYYY
Concrete/plaster dryContactCYYYYYYYYYYYYY
 Run ontoCYYYYYYYYYYYYY
Wet timberContactNNNNNCCNYYYYYY
 Run ontoYYYYYYYYYYYYYY
Steel*ContactCNCNCNNCYCYNNY
 Run ontoYYYYYYYYYYYYYY
Cedar*ContactNNNNNYYYYYYYYY
 Run ontoYYCCYYYYYYYYYY
Butyl rubberContactCNNNNYYYYYYYYY
 Run ontoCCNNCYYYYYYYYY

 

4.10.3D Material Compatibility Key

YSuitable
NNot suitable
CMay need separation. Use with caution in severe or moist environments
*May cause staining, but not corrosion
Note:
  • Runoff and contact effects may vary according to the relative size/area of the two materials.
  • Most incompatible materials will not react if moisture can be eliminated from area of contact.
  • Use with caution - may mean separation required, or unsuitable in severe environments, or when in wet contact.
  • Wet concrete includes uncured concrete, fibre-cement, or within plaster walls.
  • Dry concrete includes cured concrete not exposed to rain.