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Consider Corrosion

Posted by on in Technical Support
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Corrosion issues should be considered for solar panels and framing systems on roofs, as it is a harsh environment up there, AND the general expectation is that these things should last for 25 years, at least! In this context, corrosion is a real issue over time.

Some common sayings we all hear: "Dissimilar metals corrode!" and "Only use stainless-steel with aluminium, as they don't corrode". Well, they are only 'sound-bites', and there is a LOT more to it. Suffice to say, a well-designed system with selected materials will give the longest life.

Consider the opening picture. We see STEEL screws driven into ALUMINIUM !! But also note that the frame was over 10 years old at the time - where is the corrosion? Well, there certainly is no run-away corrosion on any of those parts nor the underlying zincalume roof.

Consider also that STEEL screws are used into ZINCALUME roofs all the time, even in coastal areas with salt spray. So evidentially it gives the lie to any blanket statement about 'dissimilar metals'. Why does it not corrode? A number of reasons.

One, surfaces are COATED - there is a lot of high tech in the selection and application of coatings. In the 15 years I have been doing this, I have seen significant improvements in screw coatings, so a modern steel screw would perform even better than those in the opening photo.

Two, catch-points - screw-heads and washers do not allow any spot for dirt/salt to build up. It is when dirt/salt catches in a tiny pocket, and gets wet, that a corrosive galvanic cell is set up. Even two metals the SAME will corrode if water/dirt/salt gathers in a cath-point between two faces.

Three, no high velocity water. This is where rainwater concentrates and runs a lot faster over a reactive surface. This is why gutters draining onto a lower roof always use 'spreader-pipes' (T-shaped with holes), to slow the water and direct it into a number of smaller, slower outlets. Thankfully, most designs of framing do not naturally lend themselves to concentrating and accelerating gathered rainwater.

Notice I do not mention the importance of neoprene washers being "galvanic isolators' and 'stopping corrosion' - because they don't! Let me explain - the neo washers under roof-screw heads do not extend down the shaft of the screw, and the shaft is in intimate contact with the zincalume sheet at all times, thus it is NOT galvanically isolated. The reason the neos are there is more to provide sealing against moisture penetrating the roof, but they also prevent water/dirt/salt catchpoints, and provide a little strain relief between the screw head and roof interface when the roof structure moves and vibrates (eg wind and storms).

So what about Galvanic Tables showing reactive electrolytic voltages between metals? Yes, a table I have reproduced below. These voltages are for BARE METAL - not coated metals!! Once metals are coated, it is the material of the coating that is primarily considered, so in the original picture example, we are talking about how the final aluminum-polymer coating of the tek-screw reacts against the aluminium oxide coating of the aluminium - not much as you can see. Also note that stainless-steel has over SIX times the reactivity with aluminium than does steel and iron! Once again, when coatings are considered, the galvanic potential would be a mere fraction of that.

I must add that this is by no means the whole story in the wonderful, exciting world of materials corrosion. I have discussed only the faces of metals, abutting, and exposed to the atmosphere. There are also other reactions occurring around electrical circuits that are made up of strings of metals attached to each other in realtion to earth, and temporary circuits created when water flows and gathers dirt and ionic particles of metals and transports them to a dissimilar metal surface. In my experience, they are not significant if minim common-sense design is used, so I shall leave these for another story.

b2ap3_thumbnail_corro-table.PNG

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Chris Hart has been employed in the field of IT majoring in hardware and power issues for over 20 years, followed by 15 years designing and supplying solar battery systems for domestic and commercial markets. He has qualifications in electronic engineering and management.

Comments

  • Guest
    Colin Tuesday, 09 June 2015

    Hello.. Our home in Osborne (just past Semaphore/Port Adelaide) has saltwater on both sides with a corrogated "galv" type metal roof. We had problems with rust of the sheets where the antenna was installed because of incorrect isolation materials (according to the installers), resulting in the sheets getting replaced in the end. We have been wanting to get solar panels installed, but these past issues concern us. Is it standard for the solar to use the correct gear for the harsh coastal area we are in, or do we need to order a special type of solar? As you seem to understand about these issues (it goes right over my head :( ), do you think solar would be feasible for us in the longterm?? - we don't want our roof to disappear!! Sincerely, Colin.

  • Guest
    servicenow asset management training Tuesday, 12 March 2019

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