What is Electrolysis on metal roofs?

Electrolysis on metal roofs, aka galvanic corrosion (also called bimetallic corrosion) is an electrochemical process.  We describe electrolysis as one metal corrodes when it comes into contact with another metal that is of dissimilar type.  The corrosion is caused by a self-induced current. This current is created when the two dissimilar metals are in contact in the presence of an electrolyte. Fresh potable water (a weak electrolyte) is one example. This reaction can become stronger if the roof is within close proximity of the marine environment and salt deposits on the roof dissolve into the water.

Three Elements of Galvanic Corrosion

For Electrolysis to occur, there are three elements required:

  1. Dissimilar metals
  2. Metal-to-metal contact
  3. Metals in the same conduction solution (rain water, exacerbated by salt)

Old vs New roofs

Older metal roofs in Queensland tended to be constructed out of galvanised metal sheets giving rise to the phrase “timber and tin” houses.

The term galvanised iron, originated in England with the use of wrought iron sheets as the base metal in the earlier part of the 1800s.

Mild steel sheeting rapidly replaced wrought iron as the base metal following improved steel making and processing methods later in the 19th century. The first Australian galvanising works were set up in Sydney in 1863. Lysaght’s plant in Newcastle began producing corrugated galvanised roofing on 4 April 1921.

The old hand-dipping process provided the metal sheets with excellent weather protection. Sheets of 0.6mm thickness (24 gauge steel) could be curved for bull-nose verandah roofs. Thicker sheets of 0.8mm (22 gauge steel) were also used for roofing although they were less common.

Materials used for hot dip galvanising have not changed. However, early dipping processes tended to make the galvanising layering more uneven with thinly-coated areas more prone to weathering and rusting. On the other hand the coating could be in places very thick (as much as 800 gms/m2).

The use of lead flashings on this type of roof did not cause galvanic corrosion so lead on galvanised roof sheets was and is common.

The new era…

In the 1960s Colorbond or pre-painted roof sheets commenced production in Australia as did Zincalume sheets. Colorbond has a zincalume core with a painted surface to allow for a variety of aesthetically pleasing applications.

Zincalume is a metallic-coated steel product that consists of 55% aluminium, 43.5% zinc and 1.5% silicon. However the entire coating is approximately 80% aluminium. The coating gives it a lifetime of four times that of galvanized steel and is lightweight, with edge protection. Due to the metallic-coating system employed, zinc sheets are incompatible with lead flashing and copper piping.

Dissimilar Metals

Examples of dissimilar metals that cause electrolysis are:

  • Stainless steel self-drilling roof screws used to fix Colorbond painted steel roof sheeting
  • Zincalume steel roof sheets and lead flashing
  • Galvanised roof sheets and Zincalume roof sheets
  • Black marking pencil on bare galvanised and Zincalume steel products

Colorbond roofing has a higher resistance to galvanic corrosion because it has an inert paint layer covering the reactive Zincalume coating.

Below are obvious examples of roofs we assessed where the electrolysis reaction between the dissimilar metals has eaten through the sheets:

metal roof with severe electrolysis reaction

Metal roof with severe electrolysis reaction

Electrolysis under coating

Electrolysis under the coating

 

Conclusion

When installing or replacing metal roofs it is critical that consideration be given to the pre-existing metal sheeting and what is being used to repair it.  While galvanised sheeting is more expensive it is still available and will prevent electrolysis occurring.

Of Note – Do Not Use Lead Pencils on Metal Roof Sheets

In their March 2016 Bulletin, Ambrose Builders highlighted the issues of using a common black led/marking pencil on a zinc roof due to galvanic corrosion.  The Bluescope Technical Bulletin states:

“One unusual example of such galvanic activity is related to the corrosion induced by the use of common black marking pencils on bare galvanized and zincalume zinc/aluminium alloy-coated steel products. Black “lead” pencils contain graphite/carbon rather than lead. This reacts with the metallic coating! It results in indelible marking or fine corrosion of the sheet surface and, in aggressive environments, severe knife like corrosion of the metallic coating”

Roofguard’s solution to electrolysis on metal roofs

Ideally, we prefer to eliminate the electrolysis source on a roof, however that is not always possible or practical solution. The second best option is to manage the risk of electrolysis eating away the roof.

Besides the application of a specially formulated anti-corrosive primers, we have found that installing sacrificial anodes extends the life of the roof and significantly delays the bimetallic reaction on metal roofs.

Sacrificial Anodes are highly active metals that are used to prevent a less active material surface from corroding. They are created from a metal alloy. An anode have more negative electrochemical potential than the other metal it will be used to protect. The sacrificial anode will be consumed in place of the metal it is protecting, which is why it is referred to as a “sacrificial” anode.