galvanic corrosion in metal roofing.
Metal roofing materials can be susceptible to galvanic corrosion and you can damage your roof if you do not understand the basic facts of what it is and how to prevent it.
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What is galvanic corrosion?
At a first glance, if you are not familiar with the term you could guess that this has something to do with corrosion of galvanised steel. It could indeed be that, but it is a lot broader subject entirely.
- The term Galvanise derives from the name of an Italian anatomist Luigi Galvani (active around the 1790's) who was doing experimental work when he passed an electrical current by accident through a dead frog and it's legs contracted.
- While Galvani did not discover the process that now bears his name he laid the groundwork for others to later discover electrolysis.
- Electrolysis consists of using an electric current to transfer atoms from one metallic electrode to another via a conducting liquid (electrolyte).
- So in the case of the early days of commercial galvanisation of steel, a zinc anode transfers particles of zinc to the surface of a steel plate via an electrolyte. An acid bath.
- Electrolysis has maybe hundreds of different applications from decorative to protective coatings in the building products field alone.
So this electrical action is well understood has been used for a long time for many purposes.
We have long known that the various combinations of iron and steel corrode in damp conditions. Then it was noticed that if for example steel was placed in contact with, say copper then it would corrode at a lot faster rate than normal.
Scientists being what they are, soon found out the reason for this
accelerated corrosion. It is an accidental variation of electrolysis and they called it
galvanic action or
galvanic corrosion.
- Electrolysis is a beneficial manufacturing process.
- Galvanic corrosion is a similar but unwanted process that costs millions.
- For galvanic action to take place you only need two different metals to touch in the presence of slightly acidic moisture.
- One of the metals will move to the other accelerating the corrosion in the process.
- If we take steel as our base, then metals like Copper, brass, bronze, lead and nickel are said to be "Nobler"
- Metals like aluminium, zinc, (or zinc base alloys) and magnesium are said to be "less noble".
- Galvanic corrosion happens in the "Less Noble" metal and passes material to a "Nobler" metal.
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Galvanic corrosion - The Galvanic series
A list of metallic elements that has been sorted in relation to their so called nobility, their resistance to corrosion when in contact with an electrolyte, is called a Galvanic Series.
Here is a short Galvanic series in no great detail.
- Gold..............At the top the "Nobler, non corroding metals.
- Graphite
- Silver
- Stainless Steel (316 passive)
- Monel
- Stainless Steel (304 passive)
- Nickel (passive)
- Lead
- Copper
- Nickel (active)
- Stainless Steel (316 active)
- Stainless Steel (304 active)
- Iron
- Steel
- Tin
- Aluminium
- Galvanized Steel
- Zinc..............At the bottom the less noble corrosive metals.
Note! Stainless steel is low carbon steel with varying amounts of chromium added. The addition of the chromium into the steel allows SS to form a chromium based oxide on the surface in the presence of oxygen. This is known as the passive film. SS that has been altered by working, heating etc. or that is not in an oxygen rich environment (say sea water) can lose the passive film and is known as active SS, which makes it more susceptible to corrosion.
I found this out to my cost one day when grinding near some SS handrail wires. Where ever a spark had touched the wire the SS became rusted and had to be replaced.
Galvanic corrosion - How does this affect my metal roofing.
- Modern steel roofing has excellent protective coating systems. The old galvanised sheeting is replaced with paint based coatings, or zinc and aluminium alloys. A trade name in Australia is "Zincalume"
- All modern metal roofing materials can give thirty years protection at least if they are fixed according to the manufacturer's instructions.
- If you let your steel roofing contact a copper water pipe in the presence of a touch of moisture then steel will leave your sheeting and ordinary rust will be formed in the steel roofing faster than it would if the copper wasn't there. Don't be fooled by the fact that the steel is pre finished with paint, we are talking about a copper pipe that sits in a hole that has been drilled through the steel roofing. Even microscopic scratches in the coating will allow the action to start.
- Likewise if you have galvanised (zinc coated) metal roofing with brass fittings touching to roof, in the presence of moisture, then the protective zinc will disappear exposing the base metal to galvanic corrosion.
- Galvanic corrosion will not take place in dry arid conditions.
- Conversely it will be a greater problem in wet and humid climates, and it will be worse on the coast where salt spray also comes into the equation.
Not all galvanic corrosion is bad of course. The boating industry has long used sacrificial zinc anodes on steel hulls, attached to motors and near propellers to stop corrosion happening where it could be harmful and encourage the inevitable corrosion to happen in the zinc blocks that can be easily replaced.
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Galvanic corrosion - Ways to prevent galvanic action taking place when fixing metal roofing.
I have had more years than I care to remember working with steel roofing materials, so of course what follows is slanted towards steel, but galvanic action can and does affect other metal roofs like lead, copper or zinc. Be it standing seam, clip fixed or screwed, find out as much as you can about your material of choice and follow the manufacturer's guidelines.
- Galvanic corrosion does not take place in dry conditions. The roofing should be designed and built so that it sheds rainwater totally. There should be no small pockets or water lying for hours around penetrations, gutters etc.
- Use the manufacturer's recommended fixings. In particular us the neoprene or EPDM seals to roofing screws if they are specified.
- As a general rule use fastenings of the same material or coated with the same material as the roofing sheeting.
- Always use sarking (silver foil vapour and heat barrier) directly under steel based sheeting. This forms a barrier between other metal parts of the roof on the inside of the roofing, like galvanised bracing straps, box and valley gutters etc.
- Use the same materials for the roof sheeting and the ridge, hip and barge flashings.
- If you are laying say a copper or a zinc roofing on top a ply underlay be sure to isolate the steel nails that are fixing the ply from the roofing material. A layer of roofing felt (asphaltic felt)is a normal choice.
- ALWAYS, ALWAYS! On any roof but particularly steel,clean the roof down at the end of every day. Do not leave scraps of roofing, swarf, or broken drill / screwdriver bits on the roof.
- If you are adding a new roof to an older type of roof make sure that the different materials have an isolation barrier between them. For example if you are tucking a new corrugated "Zincalume" roof under an old corrugated galvanised iron roof then lift up the old sheets and sandwich either a foam or plastic layer between them. Better still is to design the addition so that there is a complete break between the two or to simply replace the old galvanise iron.
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Galvanic corrosion - Prevent galvanic corrosion after the roof is built.
- Any metallic pipes going through the roof should be isolated from the metal roofing. That is usually in the case of electrical and plumbing pipes by means of a rubber or EPDM type grommet. Other wise a small sleeve of similar material wrapped around the pipe where it passes through the sheeting and held in place with the appropriate type of silicone or sealer.
- Larger pipes like sewerage vent pipes, kitchen extractors etc. should use "Dektite type fittings with the EPDM gaskets, or purpose made gaskets should be used to isolate the pipe and then the flashing material from the roofing.
- Any subsequent additions should use the same type of metal roofing material.
- Use the standard recommended roofing screws to fix all accessories and extras on a roof even if SS screw are provided. (Unless of course the roofing is SS.)
- Lead sheet is a useful material for flashing around odd shapes, but it should have no place on a modern steel roof. If in repair work you want to take advantage of an existing lead flashing then put a barrier between it and the sheeting. Roofing felt or similar is good. At the least a good coat of bitumen based paint may suffice.
- On a non copper roof immediately fix any water leaks from copper pipes, solar collectors etc. These can deposit a corrosive copper solution onto the metal roofing.
- Any add ons to the roof like TV aerials, Satellite dishes or fixings for permanent access ladders should have rubber type gaskets between them and the metal roofing.
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Galvanic corrosion - Other incompatible roofing materials
- Use the recommended sealer for making any joints in modern roofing flashings etc.
- Use NON-ACIDIC CURE silicone for the modern "Zincalume based metal roofing products.
- ACIDIC CURE silicone like wet area silicone, glass silicone, should not be used on steel roofs!
- Carbon. Would you believe it? Ordinary pencils are not
compatible with Zincalume. Look at the galvanic series list, graphite is right at the top and zinc at the bottom. I am not saying that pencil marks will make holes in your roof, but I have noticed that after a short while they are hard to remove. They bond to the surface. This is more of a problem of course if you are fixing clear Zincalume sheets for wall cladding. I like using water based kids
marking pens. They wipe off easily.
- Chlorine While on the subject of spas, if you store your pool chlorine in an enclosed steel garden shed, sooner or later the shed will rust out. More likely sooner if you leave the lids off the chlorine.
- Touch up paint. Not really incompatible, but in the past people
would buy packs of touch up spray, and spray fist sized patches everywhere
there was a scratch. The end result was that after a couple
of years or so the touch up paint would fade and the whole thing looked
like a mangy dog.
The current wisdom is don't paint small
marks, they won't cause rust. For larger damaged areas, change the sheet.
In other words, be careful when you are fixing and you will get a better job.
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Galvanic corrosion - Conclusion
Please don't think that because of all these warnings on this page that you may be better off not using metal roofing on your house. On the contrary, I believe it is the best roofing material available today. From the smallest job to the largest it is cost effective and does its job efficiently.
Follow the guidelines, fix it correctly and look after it. Then rest easy, knowing it will look after you and yours for years to come.
