With the ongoing problems with my swollen Pylontech batteries, and our Zwartkops substation burning down last week, I though it was time to give my panels a quick clean, just to get rid of the worst dust that has accumilated.
Imagine my surprise when I saw that my galvanised garage roof is starting to rust, at the bottom of each of my panels
Checked the rest of the roof, and no other rust patches, just exactly where every panel will shed water. Every one of my 18 panels has one rust spot.
The only explanation I can think of would be higher acidity in rain water, plus any dew accumulating overnight will attract any airborne pollution
For now, I will wire brush the spots, use a quality rust converter to neutralize the rust, and then cover with several layers of roof paint.
If I recall my school chemistry knowledge you can prevent this by applying a DC voltage between the panel and the roof.
This is also called cathodic protectionā¦
I loosened the hold down clamps and moved all the top row panels upward with about 20 cm, allowing me to walk between the rows and to get down and inspect under the top row dripping spots. All the panels had at least one rust spot under them. Made sure to retighten all clamps.
Used Rust-Oleum Rust Reformer spray to cover all the rust areas and will leave it to completely dry for a few days, before covering the areas with a red oxide protective layer. Once that has dried for a week or two, I will then cover all the spots with a light gray roof paint, making sure to also paint the portions below the panels.
However, a long term fix is needed, to stop rainwater dripping off the panels, onto the metal roof. Initial thoughts are to add PVC guttering under the drip areas, with the gutter outflows then piped down the side of the garage. Still looking into how I will fix the guttering to the underside of the panels and if this is even feasible.
Iām thinking more in the line of removing the cause. It is galvanic corrosion. The water acts like an electrolyte. Butā¦ unlike your normal run of the mill galvanic corrosion, the metals are not in direct contact with each other.
This makes me wonder if 1) enough aluminium ions make its way down with the water, or 2) the water runs a long enough stream/droplet to make a ācircuitā between the two.
If it is the latter, insulating your rails from the zinc will help (the circuit would be open).
You could also look into a sacrificial material of sorts, but then youād look at something like magnesium, which isnāt easy to obtain. Or more zinc (corrosion to the underlying metal only starts once the zinc is all consumed, so a simple zinc strip might do it).
Indeed!
I would be inclined to place a strip of polycarb sheeting under the area where it drips.
These are made in the different profiles so get the identical one for your roof.
This would create a longer trip for the rainwater.
Iām fairly sure that the trickle of water needs to be just that, not drops. Only then will it conductā¦
Iāve never seen corrugated iron roofing grounded. Should it be? I mean, to the same place as your electrical installation? Honest question, I do not know.
That was my first thought as well, that they really should both be grounded, which will in theory place them at the same potential.
But that is also exactly the scenario with a boat, where the boat is earthed to the earth on the shore power side, creating a long loop with the ocean in the middle as electrolyteā¦ and the resulting corrosion that comes from that. And the way we fix that is precisely to break that loop, either with two diodes (so it needs to exceed 0.6V to conduct), or an isolation transformer.
Mostly, the bit I am unsure about is whether you need a stream of water (so that the electrolyte bridges the gap between the metals), or if drops is sufficient. The galvanic corrosion I am familiar with would need a stream of waterā¦
As far as I am aware, you would be required to earth your metal roof if you have PV panels on them, but I could be remembering it wrong, Iām sure @Rautenk will know.
If both the roof and the panels are earthed, you have a situation very similar to what happens with boats connected to shore power. By connecting the boat (and itās hull) to the same earth as the shore, you turn the sea into an electrolyte, and the first thing taking a hit is the bronze propeller.
In your case the rain water is the electrolyte, closing the circuit.
So if we used the same solution as for the boat, you could earth the panels through a galvanic isolator (same as for the boat). This blocks small DC voltages, but passes the larger ones. This is probably overkillā¦
But, I think mostly the solution here is to prevent the circuit from closing by draining the water away before it hits the zinc, or adding a thick hot dip sacrificial zinc strip right where it does hit. Essentially a zinc run-off strip mounted right below the panels.
This may be the easiest to implement, so for now it is what I will do. The odd heavy rain that may overflow the gutter below the panels should also not be as contaminated with aluminum, seeing as it will only be in contact with the panel frame for very short time.
