CORROSION CHART

Marine Environment

When building within one kilometre of the coast, please refer to the Building Code of Australia. The BCA provides ‘standards’ for coatings on external structural steelwork in relation to the distance from salt water and the distance from breaking surf. For instance, coastal envrironments within 100 metres of breaking surf are classified as a severe enviroment and BlueScope Lysaght® recommends the use of COLORBOND® Ultra steel these situations. This affects compliance and warranties.

Material Compatibility

Due to phenomenon known as galvanic or bi-metallic corrosion, some commonly used metals can cause accelerated corrosion when used with ZINCALUME® steel, zinc/aluminum alloy coated and COLORBOND® prepainted steel.

Corrosive Path

This metal guide chart lists commonly used metals in a “corrosive path”. If any of these two metals are in damp contact or a runoff situation, the metal higher on the table will sacrifice itself i.e. corrode to protect the metal lower on the scale. When it comes to the flow of water from one metal to another, the simple rule is to remember that you can run water downhill but not up. For example, zinc to copper is OK but copper to zinc spells trouble.

Other Flashing Materials

Roof penetration flashings are available in a wide range of rubber and aluminium materials, and these can be commonly used.

The most popular flashing materials are COLORBOND® steel or ZINCALUME® steel sheet formed to suit the application. While zinc-coated steel can be used with ZINCALUME® steel or COLORBOND® steel, its generally lesser life span makes it impractical in the long term.

Copper

Copper is incompatible with both ZINCALUME® steel and COLORBOND® steel, either in contact with or where water can flow from it, such is as often experienced with hot water system overflows.

Painting the outside of the copper pipe is recommended. Hot water discharge pipes should be extended beyond the roof, preferably to ground.

Lead

Lead is the only metal generally considered to be compatible with zinc-coated steel but not with ZINCALUME® steel or COLORBOND® steel.

ZINCALUME® steel, in contact with or receiving of run off water from lead is prone to corrosion. Pure zinc or aluminum are suitable alternatives although neither are as workable as lead.

In the event of roof renewals where lead already exists and its re-use is desirable, the ZINCALUME® steel or COLORBOND® steel must be insulated from the lead by a suitable barrier.

This can be achieved by painting the underside of the lead or preferably both surfaces to ensure complete electrical separation. Plastic film can also be used provided it is robust enough and will not tear, e.g., polyethylene damp-course.

Corosion Chart

The further apart the two metals on this chart, the faster the corrosion will take place.

+VE Anodic
ZINC
ZINCALUME
ALUMINIUM
CADMIUM
HIGH STRENGTH ALUMINIUM
SOFT SOLDER
LEAD
TIN
BRASS
COPPER
BRONZE
SILVER
-ve Cathodic

ACCEPTABLE DRAINAGE FROM ONE SURFACE TO ANOTHER

LOWER SURFACE	UPPER SURFACE
	GAL	ZINCALUME	COLORBOND	S/STEEL	ALUMINIUM	COPPER	ZINC	LEAD
GAL	YES	NO	NO	NO	NO	NO	YES	Y/NC
ZINCALUME	YES	YES	YES	Y/NC	YES	NO	YES	NO
COLORBOND	YES	YES	YES	Y/NC	YES	NO	YES	NO
S/STEEL	Y/NC	Y/NC	Y/NC	Y/NC	Y/NC	Y/NC	Y/NC	Y/NC
ALUMINIUM	YES	YES	YES	Y/NC	YES	NO	YES	Y/NC
COPPER	Y/NC	Y/NC	Y/NC	YES	YES	YES	YES	Y/NC
ZINC	YES	NO	NO	NO	NO	NO	YES	Y/NC
LEAD	Y/NC	Y/NC	Y/NC	Y/NC	Y/NC	Y/NC	Y/NC	YES

NOTE :	YES	CONTACT AND DRAINAGE ACCEPTABLE
	Y/NC	DRAINAGE ACCEPTABLE BUT NOT DIRECT METAL CONTACT
	NO	NO DRAINAGE ACCEPTABLE AND NO DIRECT METAL CONTACT

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