Galvanised steel in contact with galvanised iron??

[Black Cat]

Since becoming responsible for the local hall, I have prepared a list of works that need doing and organised them into a priority listing - the first priority is getting new flashing at the ends of the gables and new guttering. The roof is corrugated iron and seems pretty sound, though needs a bit of fixing down here and there, so replacing the cladding for that is waaaaaaaaaaay down the list after complying toilets and kitchen facilities.

So I asked the local plumber (who is a bit of scheister) to prepare a quote for gal flashing, but agreed colourbond guttering was fine. I have now been informed that Gal steel has a similar reaction when in contact with gal iron to that which occurs when colorbond drains onto gal (or is in direct contact like the flashing would be).

Seems to me a bit of a furphy (and not mentioned on any Bluescope tech specs I have read, which is most of them), so I am hoping a professional roofer can provide me with some insight into this latest scaremongering ...

Thanks.

[barney118]

Firstly the term iron is used loosely and should not be called as such.iron has 4% carbon and is brittle, steel has 0.4% carbon. Galv is 100% zinc coated and it easy to identify by large spangles about the size of 20 cent piece. Zincalume is a combo of zinc and aluminum coating and is also the base used for colorbond. You will get corrosion when you mix zincalume and gal as the aluminum reacts. But I wouldn't see much of an issue with colorbond as the paint separates the reaction except for screw holes. It would be better to use gal on gal. Check to see if you have identified it properly.

[Black Cat]

No worries about ID - the roof has been around a lot longer than colorbond. But thanks for confirming my suspicions that this was a bit of nonsense cos I showed him up knowing about the colorbond/gal issue (and the tech specs recommend against that particular combination - as do my own observations of accelarated corrosion where the two are in combination).

Thanks Barney!

[The Roofer]

Hi Blackcat,

You have to be careful here, as you said "the first priority is getting new flashing at the ends of the gables and new guttering"

Water running from a product such as Zinc/Alum, Colourbond, terracotta, fibreglass, aluminium, acrylic or any inert surface onto a Galvanised surface will result in failure of the Galvanised product eventually. As a rule of thumb - stick to colourbond or zinc/alum at the low point and never above any galvanised material. So your colourbond or Z/A gutter is OK but the top flashings (eg gables as you mentioned) etc should be Galvanised to matc h the roof!

The galvanic reaction involves the CO2 in the air with moisture to form carbonic acid. That's also part of the sacraficial protection that zinc/alum gives itself to cut edges and also is the reason why some lead pencils (carbon - graphite) leaves an indelible mark because of this virtual electric cell when wet!

[Black Cat]

Thanks Roofer - I have to say I was most enthralled at the graphite pencil advice, as it creates the option for ornamental rusting of decorative things (not roofs). Will stick to gal flashings in this instance, and go for colorbond gutters. We can replace all when the roof finally gives up the ghost.

[shauck]

So your colourbond or Z/A gutter is OK

Wouldn't that depend on if the downpipes are gal or not?

[Moondog55]

So you can't use aluminium flashing with either gal or zincalume??

[Black Cat]

Bluescope publish a great tech manual on line Moondog - it gives the gradients between the metals so you can work it out for yourself. It was the gal to gal question that threw me - I think the chap had just had a garbled memory of our first conversation about colorbond to gal.

And Shauck, the downpipes are all pvc so no worries there (apart from the one I had to glue back together today ...).

[The Roofer]

shauck,

Yes! The downpipes should always be Z/A or C/B also - same rule applies - stick to colourbond or zinc/alum at the low point and never above any galvanised material!

[The Roofer]

So you can't use aluminium flashing with either gal or zincalume??

Moondog,
You can use aluminium above Z/A or C/B but not Galvanised!

[barney118]

I am not sure where the roofer has obtained his information from....co2 causing corrosion?? The issue is whenever you have 2 dissimilar metals together they will react, but the process is increased when an electrolyte is present, ie H20 aka water. The problem is metals usually give up an electron in the process and depending on where they sit in the redox potentials for metals. the electron potential for Al is -1.67 and Zinc is -0.76 so the potential between the 2 is 2.43 volts. The Al is more anodic and will become the anode of the 2 and corrode first or give up its electron.
The important point here is the relative surface areas, when you have a large cathode (aka gal Zinc roof) and a small anode ( aluminium flashing) the driving force is great and the corrosion of the anode will be rapid.

[Moondog55]

Moondog,
You can use aluminium above Z/A or C/B but not Galvanised!
What about under and below??

[Moondog55]

I think the roofers use of CO2 was that carbonic acid is the hydrogen ion donor in this reaction if I remember my tech-school chemistry, pure water is non-reactive in a lot of instances.
I remember being told at school that the old lead-washered roofing nails were supposed to be used with a bitumenised felt washer but no-one ever did to my knowledge

[The Roofer]

A bit late - but here goes - I was mainly talking about acceptibility of drainage from an upper surface to a lower surface, not the acceptability of direct contact between metals. I will find the tables and post a link shortly.

1. Moondog - Can you use Aluminium below & under Z/A and C/B? Answer YES!
2. Moondog - Can you use Aluminium below & under Galvanised? YES! (But you can't use Aluminium, Z/A or C/B above Galvanised)

3. Barney118 - "I'm not sure where the roofer has obtained his information from....co2 causing corrosion? " Sorry Barney - I find it hard to describe the reactions that occur on the roof! But here goes -I am talking mainly of the roof catchment area being inert - such as glazed tiles, C/B, Z/A, glass, etc which with rainfall - the surface of these materials (inert) does not take any metal salts down into the catchment area (gutters) and so it is virtually still high purity rainwater (but with CO2 dissolved in it). Rainwater and condensation (on the exterior of a roof) droplets will take in or dissolve CO2 to form a weak corrosive carbonic acid and when on an inert roof - nothing occurs - but on a galvanised roof (ZINC only) the zinc reacts with the carbonic acid (CO2 is removed) and a ZINC oxide is formed that helps build up in a Galvanised gutter (actually becomes stronger). But the carbonic acid from an inert roof falling into a Galvanised gutter actually causing localised corrosion of the galvanised gutter (ZINC) by removing the Zinc coating! I hope to get this info link shortly! Barney - all the other info you gave in regard to electron potential and relative surface areas to work out corrosion potential also applies.

And with the lead pencil (carpenters pencil) - it is graphite and some clay (minor). Graphite is nearly pure carbon (as are diamonds & coal etc) and when carbon is placed in contact with metals it forms an electric cell when wet - and acting as a battery it will chew at the surface and leave a mark that will not come off! Rusting will occur later.


When choosing flashings - both accepability of drainage and direct contact of metals have to be considered.

[Danny]

Dissolved co2 in rainwater is also the reason rainwater is naturally acidic, particularly during thunderstorms.

This page off my signature web site gives professional information about galvanic reaction and roofing materials.

[The Roofer]

Direct Contact between Metals in roofing! (http://steelproducts.bluescopesteel.com.au/files/CTB-12.PDF)

Acceptability of drainage for roofing & inert surfaces (http://steelproducts.bluescopesteel.com.au/files/dmfile/CTB25june2008.pdf)!

[barney118]

As the bulletin states, do not use gal (zinc only) under(down pipes etc) any of the 4 types of roof tested.

Rain water does dissolve C02 when it rains which lowers the PH to become acidic and forms a perfect electrolyte.

The metal ion gives up one its electrons in the process on the anodic side and forms an oxide to protect the steel, hence zinc is used as sacrificial anodes in a number of places, ships, water pipes, and roof coatings as it will corrode in preference to steel (iron Fe). That's why the use of zinc /al Zincalume is far better as Aluminum forms one of the strongest oxides.

[The Roofer]

Thanks Barney118