Corrosion from dissimilar materials usage may have two origins:
- Contact between different metals, producing a galvanic cell which causes the more active metal to corrode.
- Water run-off from particular materials on to a metal, causing corrosion.
- Surface oxide, relative surface areas, water purity and environmental factors can influence the outcome, so the consequences may not relate strictly to the well-publicised Galvanic Series.
- The following table shows which metals and materials can be used together in a roof and/or gutter installation and which should be avoided.
If dissimilar metal usage cannot be avoided then contact and/or water run-off must be avoided by insulating surfaces. Separation by rubber seal or coating the surfaces and maintaining the coating as an effective barrier for the life of the roof will be required. For further important information refer to the MRM code of practice at www.metalroofing.org.nz.
Contact with non-metal materials and water run-off from them can also cause corrosion problems. Well-known examples are:
- Inert catchment – where water running from a non-zinc surface onto unpainted galvanised steel can cause rapid consumption of galvanising. The guilty surfaces include glass, plastic including GRP sheeting, painted or unpainted Zincalume, painted galvanised steel, concrete tiles and butyl rubber. (The effect is often seen on the unpainted interior surface of galvanised gutters where rust spots will appear at each water drip point).
- Timber – particularly copper treated including treated timber walkways. Any contact with wet timber should be avoided.
- Lime cement and concrete.
- Wet insulation.
- Soot or sulphur.
- Carbon (lead pencil or some black sealing washers), which causes Zincalume® to corrode.
- Galvanised netting must not be used directly under aluminium roofing. Where the galvanised netting has not been correctly isolated and has made or can make contact with the underside of the aluminium roof, pitting of the aluminium will occur. Either avoid using galvanised netting or isolate contact with an inert strip such as Dimond purlin protection strip or install over a vented drainage mat. Building paper cannot be relied upon as an inert strip, especially in severe marine environments.
- When stainless steel fasteners are used through aluminium roofs an oversize clearance hole around the fixing and a profiled metal washer with an EPDM seal must be used. If the fastener is not isolated from the roofing, any moisture, especially salt laden air, creates a corrosive cell between the stainless steel and the aluminium which results in rapid corrosion of the aluminium. Alternatively aluminium fasteners can be used (with an oversize clearance hole around the fixing and a profiled metal washer with an EPDM seal) into non-copper treated timber in place of stainless steel fasteners.
- Clouts or staples must not be allowed to make contact with aluminium roofs.
Dissimilar Metals Guide
Example – Zincalume: to check the compatibility of Zincalume with other material, locate Zincalume along the top (horizontal axis) and check the water run-off and contact columns for compatibility with other materials. This indicates, for example, that water run-off from Zincalume onto unpainted galvanised steel must be avoided, but that direct contact between Zincalume and galvanised steel is acceptable.
