Corrosion on Steel Bolts Holding Aluminium Plates

"It only takes a little moisture to make a battery with steel and aluminum". - Well said.

No place in Chennai is free of humid dry atmosphere.

Ambience in Chennai is hot saturated most of the time with salty humidity, an ideal condition for galvanic corrosion.

krishnan,

i agree with your comments, unfortunately this problem is in one of our sites in north-western part of India not chennai .

Thank you

Do you want to see corrosion of the Aluminum?

What Steel and Aluminum are you dealing with?

Something to read, but my understanding is that the less noble metal preserves the noble one.

You might have too mild steel bolts but look at the bright side, your Aluminum sheets are fine!

http://aluminumsurface.blogspot.com.au/2009/04/corrosion-between-anodized-aluminum-and.html

http://www.aluart.net/Corrosion%20Resistance%20of%20Continuously%20Anodised%20Aluminium.pdf

JNB,

The assembly is supporting soalr modules, but, i don't think there would be any volatage induced into the assembly.

do you think, there may be a possibility of induced voltage??

Thank you

Measure it and you will know!

Me thinks solar panels are there to create energy so there is a slight chance that.

I don't have the experience to know- you'd have to check yourself.

Keeping in mind that this is likely DC, #19 seems to outlay the possibilities well.

The aluminum may or may not be contributing to the corrosion. Inspect closely for pitting in the aluminum. Any pitting in the aluminum may not be as readily noticeable as the corrosion in the steel fasteners because it lacks the stark change in color and any corrosion has a large area in which it might occur, so the changes may be more subtle.

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Even if the passivization layer has held up and protected the aluminum so far, this doesn't preclude the possibility that a future scratch on the surface might change that in a short amount of time.

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Another thing to note is the possibility that the aluminum is not (currently) playing a major role in the corrosion. Steel (such a broad category) has been known to be quite happy playing games of corrosion, even when no one else is there to share in the game. Stress gradients are sometimes sufficient to differentiate cathodic from anodic, allowing the games to commence.....the stress in those fasteners isn't uniform throughout, is it?

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Fasteners are frequently coated which adds new variables to possibilities for corrosion.

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You have mentioned that the surrounding is dry. How dry? Can you tell us a little more? What else might be in the atmosphere? Exhaust?

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What alloys of steel and aluminum are these? What treatments have these alloys undergone? What types of coatings?

The first thing to look at is galvanic action, then some of the more fanciful suggestions.

I worked on a prills tower with a steel structure and aluminum plates on the inside. In some cases we used aluminum bolts with insulating grommets in the steel and others with steel bolts and insulating grommets in the aluminum.

'...then some of the more fanciful suggestions.....'

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Would you specify which of the previous suggestions you deemed to be fanciful?

A simple example to illustrate the situation taken from internet

But it looks contradictory to OP's query!

Agree - this seems contradictory to expected two metal corrosion. Would expect the aluminium to corrode, not the steel.

Perhaps this is good old oxidation (rusting), or perhaps chloride corrosion.

OP - Pictures would be great, can you post some close up images of the corrosion??

Yes, I can post some close up images on how to avoid galvanic corrosion.

I once knew someone who by merely touching corrosion resistant steel could produce a rusty (high quality, or smeared either) fingerprint on the optic rails of a research grade spectrometer. This was not viewed by small humor in the lab group, since the man was the group director.

I think whoever put the PV system to work in NW India was sweaty (or had eaten something with a lot of salt in it) and this started the corrosion of the steel bolts. If the bolts are not electrically bonded to the aluminum (if insulating bushings are used), this is precisely the result I would expect. This is usually a surface coating of iron oxide, and not likely to corrode out the bolt any time soon. Rain will eventually remove the offending chlorides.

NTR,

Lyn is correct in his comment about putting steel and aluminum next to each other creates a battery. I have worked on projects that used aluminum as the source for emergency batteries by the stored energy in the aluminum plates.

Your comment about not seeing this with anodized aluminum is likely due to anodizing converts the surface of the aluminum to Al2O3 (aluminum oxide) which technically is a ceramic. So, this would slow the reaction of the steel bolts attached to the aluminum plates. But, it is doubtful it would stop it from happening.

Isolating the steel bolt from the aluminum plate is the only way to stop the corrosion. There are many ways to do this, so you may find it helpful to do some searching on bolts coatings, isolation methods and non-ferrous type washers. Good luck.

Should take out steel bolts immediately. Theer is no point saying ambience is dry air. Even small amount on consistent enough to lead corrsoion not going into detaiuls of RH of the ambience maintained throughout the year.

I think, generally, you are correct, there isn't much point to 'saying ambience is dry air', unless perhaps you are speaking figuratively about someone who tends to be wordy in their attempts at deadpan humor.

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_{...something with which I am not unfamiliar of being accused.}

Hi ntr

I agree with everything said so far.

Boats generally use stainless bolts and rivets on their aluminium masts; works well.

Whilst they use very expensive 316, 304 would be adequate and much cheaper.

Tony

Tony,

Are you sure about that?

Corrosion resistant steel is even more cathodic than carbon steel, putting it farther away from Al on the charts.

Of course corrosion resistant steel, alone or properly protected, is better in salt water.

guys, this looks nice

Thanks for all the comments and suggestions.

okay, as suggested, i will look into it again. and will get back to you.

keep sharing your views.

NTR

Thanks for the reply. Another excellent reference to find a solution for your problem

http://books.google.co.in/books?id=3nKrEgFJk-gC&pg=PA293&lpg=PA293&dq=corrosion+of+steel+bolts+holding+steel&source=bl&ots=UTUlT5_NnX&sig=fmieW2332eZMTl_edfJgbk2hx7k&hl=en&sa=X&ei=oo5pU8jkOIKHuASoyoDoDw&ved=0CEUQ6AEwBQ#v=onepage&q=corrosion%20of%20steel%20bolts%20holding%20steel&f=false

If no success to find a solution, contact

http://www.cecri.res.in/

Direct contact between different alloys can lead to galvanic corrosion at the joint.

In assemblies which are subject to shock and vibration, this corrosion is exacerbated by the presence of humidity. Jointing compounds are therefore required which both inhibit corrosion and at the same time, are effective insulators.

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Are you talking about corrosion of selected or all steel bolts in the entire assembly? If the attack is concentrated in specific locations, we need to find out the reason for presence of electrolyte in that vicinity. When we use steel bolt, corrosion of base metal (aluminum sheet) is marginally expected. This depends on the electrolyte (surrounding atmosphere). The electrolyte may be present in the form of rain, dew, snow, high humidity, ocean salt spray, or even air pollution. If it is saline type near the coastal side Aluminum corrosion is preferential in the initial stage and the resultant corrosion product aluminium oxide sticks very firmly to the surface of the metal, preventing further attack while being resistant to corrosion itself. The best approach should be: first, to identify the affected portions in the entire sheet; secondly, to isolate the cathode and anode surface using non-conductive washers/caps or exploring the options for coating the bolt; Thirdly, explore the reasons for any current interference.

Polarity reversal of an aluminum-steel couple has also been found to occur in natural environments. Consider using Galvanized steel instead normal steel.

What do you mean, "It's opposite to ... anodised corrosion"?

It means he would suspect the Aluminum to rot away!

No surprises. Unprotected steel will rust in open air.

Start fixing dissimilar metals together without isolation and corrosion gets interesting.

Stainless steel fasteners are normally used for the application quoted.

As others have noted, galvanic corrosion should sacrifice the aluminum. Something in the atmosphere is attacking the steel. The aluminum is protected by its oxide layer. Maybe analysis of the corrosion would be useful.

I suspect that there are times during or near Monsoon season, the humidity is higher and at times temperature drops below dew point. I also suspect this is not a pristine rural environment, but that you are near elevated SOx and NOx in the environment, or that this solar panel is down wind from a volcanic area.

Galvanic series does not "reverse". However, given that the steel is bare, and the aluminum still has an oxide coating over almost it's entire surface, it is possible that under the "right" conditions of elevated "electrolyte", a hygroscopic acidic oxide will form on the aluminum due to NOx, SOx. This will lead to oxidation to Ferrous iron on surface where in electrical contact with the hygroscopic layer (only conductive when near the dew point of the system (which might be considerably higher than normal atmospheric dew point). Once Ferrous acid sulfate is formed, this will continue to ferric sulfate, and auto-catalytic corrosion will continue from there with noticable iron oxide formed.

Alternate mechanism: there might be a stray ground potential, and the (I can only assume) steel frame is conducting back through the bolts, and to the aluminum plates, which may be (or not) grounded, or the contrapositive argument still applies. The only condition needed is for a ground loop to exist. With current flowing through the steel, the points of contact are resistive and tend to build up potential that might force the steel into a driven anodic potential with respect to the aluminum. Or the potential difference between zones of the steel could force a local anode, and a separate local cathode.

As to a remedy, you could apply a water repellent material to the surface of the steel bolts, such as calcium stearate, or even zinc stearate, dispersed in a light oil that will eventually vaporize. This reduces tendency to collect dirt.

Perhaps this can help.

http://www.aluminumfastener.com/hexhead.html