Rust on Unirac Stainless Steel Mid-Clamps. What's going on here?

[SunNewb105]

Hey guys,

We had 2 ground mounted arrays erected just 3 months ago using Unirac's mounting system. I went back to the panels the other day to find that the mid-clamps were experiencing rust. According to Unirac, these clamps are supposed to be stainless steel. I am confused as to why this is happening and if it's just surface rust or if the integrity of the bolts/nuts are compromised.

I know stainless steel can be susceptible to corrosion if scratched so maybe this happened when the bolts were torqued down (most are experiencing corrosion beneath nut)? We aren't experiencing this sort of rust on any of the end clamps though. Please see the attached pictures and let me know if you guys think I should be concerned and/or have the installer replace all of them (56 panels in total so they would have to disassemble the entire arrays). I am also worried that if it continues, it will leave rust stains on the panels.

Thank you very much for your help!

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[Ampster]

I can't tell from the picture if everything is stainless. If it is stainless the rust maybe from the tools used for assembly or the ocassional surface rust that lower grade stainless sometimes shows. It does not look like it is affecting the integrity of the fasteners.

[SunNewb105]

I can't tell from the picture if everything is stainless. If it is stainless the rust maybe from the tools used for assembly or the occasional surface rust that lower grade stainless sometimes shows. It does not look like it is affecting the integrity of the fasteners.

As long as it's just surface rust and the integrity of the fasteners aren't compromised over time, I really don't care. But how would I be able to tell that being that it's only 3 months old? I actually just went back there and got a magnet to stick to the plate so it must be low quality stainless.vNQ3gWSygs0Zj8fN2-_fvCEquJZuqJ4S2xZ7T2piHFSWxMFVEkj6bejd65SKP_a_Lm5e_I7RNYjwmFEUzQIaGy-2fyyU9pDI6glQr4KszGP5yKzk6zzFvufIv-mzy74hClY0p2bboFEJLHO8MeCdBa_XumEjcoyHAFj5SWkVkynhIag_3Yr6cQhnNlOWrKuB34Xqefo7m3949vVeaCOOclox2FcZZN_LbalGySh-YUdPmk5h6Nem002wVWSbZhw
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[Ampster]

Yes, lower quality stainless will show some surface rust. I used to see it in the harbor on my boat rigging in which the air was occasionally polluted by an oil burning power plant nearby.

[SunNewb105]

Yes, lower quality stainless will show some surface rust. I used to see it in the harbor on my boat rigging in which the air was occasionally polluted by an oil burning power plant nearby.

Thanks for your input. So you don't think it will degrade any further?

[Ampster]

Hard to tell. Give it a few months and if it gets worse call your installer. It is possible they got a bad batch. If they are bad rust will show all over the nut and bolt and take years before the integrity is compromised. In the meantime enjoy the energy from the sun.

[SunNewb105]

Hard to tell. Give it a few months and if it gets worse call your installer. It is possible they got a bad batch. If they are bad rust will show all over the nut and bolt and take years before the integrity is compromised. In the meantime enjoy the energy from the sun.

Right, I just don't want them to start leaving rust stains on the panels. What would you suspect is the issue if the rust is fairly localized? It looks like the plate is what's rusting the most.

[SunNewb105]

Anyone else have experience with rusting on mid-clamps and whether they think I should be concerned about this after just 3 months?

[Mike90250]

I see rust on the threads, and I'd use some vinegar and tooth brush and clean one or two up, and monitor for future rusting. Rinse well.

Being that dark color, it looks like some sort of passivated or phosphate coated steel. SS nearly is always bright .

[SunEagle]

I see rust on the threads, and I'd use some vinegar and tooth brush and clean one or two up, and monitor for future rusting. Rinse well.

Being that dark color, it looks like some sort of passivated or phosphate coated steel. SS nearly is always bright .

It is surprising how much ferric material is still used in stainless steel components. Even high integrity SS can be melted with an induction furnace. I use to work for a company that made SS 410 grade components and we used induction furnaces to melt the metal so it could be poured into sand molds.

There are some SS grade materials that are not magnetic based on how much nickel is used in the makeup.

[SunNewb105]

So I spoke with Unirac today who stated that everything from the clamp itself to the bolt/nut are either 302 or 304 stainless steel. They said this shouldn't happen after just 3 months and are trying to replicate the issue.

I'm thinking this may be a defective batch with a higher iron content.

[oregon_phil]

The Unirac GFT spec says the mid and end clamps are 301, 302 or 304, 1/4 hard mill finish. Literature says cold worked 301, 302 or 304 can become slightly magnetic so I'm guessing this is what's going on with the magnets. I agree with monitoring the amount of iron oxide. The fastener had to dig through the original passivation layer to make a solid electrical connection. All of the other components are suppose to be stainless steel too. In theory, the stainless protection layer is suppose to regrow over time.

[SunNewb105]

The Unirac GFT spec says the mid and end clamps are 301, 302 or 304, 1/4 hard mill finish. Literature says cold worked 301, 302 or 304 can become slightly magnetic so I'm guessing this is what's going on with the magnets. I agree with monitoring the amount of iron oxide. The fastener had to dig through the original passivation layer to make a solid electrical connection. All of the other components are suppose to be stainless steel too. In theory, the stainless protection layer is suppose to regrow over time.

Thank you for this. Do you think the corossion is just on the surface and won't continue to get worse? My fear is that if left like this, it will start staining the panels and then we will be screwed.

[bcroe]

That hardware was developed so everything could easily be assembled from the top, on
a roof mount. That is not necessary on a ground mount. For them I have been using
6061 aluminum and 18-8 SS hardware, mounting panels from the back. There is no
trace of rust after 6 winters. Bruce Roe

[J.P.M.]

Several things come to mind :

1.) There are or may be several different mechanisms of corrosion at work, individually and synergistically. I appreciate that's not much help, but it's what I've found to be the most common situation after yrs. of failure analysis. .
2.) Is the installation near the ocean ? Any chlorides in the area ? That and/or other atmospheric contaminants may be attacking the clamps.
3.) There may be some galvanic reaction at work. I once had a mostly stainless piping assembly where the stainless bolting near a pump was corroding but not much else. Long story (Bad motor ground plus the bolting was positive relative to the piping and acted anodic), but the varmint was mostly an improperly grounded motor that was causing galvanic corrosion. Any stray currents in your array ?
4.) Sometimes improper heat treatment of st. stl. can cause carbides to precipitate at grain boundaries with those boundaries them serving as corrosion sites. A process called sensitization. This can happen for thin sections (and lots of other reasons) such as the clamps appear to be.
5.) As a comment, 300 series st. stl. is non magnetic at forming but may and probably will become somewhat magnetic by various processes including cold work/forming as in thread cutting processes.
6.) A long shot: Anyone ever use a wire brush w/steel bristles on it against the nuts/clamps ? Great way to start rust in stainless steel.
7.) there is a product called Evaporust that some folks use. I put some on the bolting on my SDWH pump bolting based on my above mentioned.

Unfortunately, there are too many variables that influence corrosion to nail it down without more investigation or information. If I was to guess, my $$ would be on a combination of: Stuff in the environment (chlorides of some form would be my guess), poor material formulation/heat treatment that sets up intergranular corrosion, and some galvanic corrosion either from Al-304 galvanic action and/or some stray current running around or both acting together.