What caused this corrosion?

[jerryk]

I got a couple of LFP batteries to repair and when I cut them open to my surprise there was a lot of corrosion.  No signs of moisture and the two batteries are the same model and vintage but different owners.  I have worked on this brand of batteries a lot and have never seen this before.  The first picture is of what the BMS's usually look like when I cut them open.  Always nice and clean.  The next three pictures show the BMS's of the two with corrosion and also show what I think is the culprit. 

Notice the battery packs without corrosion uses a silver colored silicone to pad the packs, whereas,  the battery packs with the corrosion are using a (new)blue silicone for padding and stability.  My theory is that the blue silicone is acid based and when sealed in the battery case the vapors settled on the copper and metal connectors and caused this.

Any other ideas of what caused this?

Jerry

[amyk]

Good theory. Any noticeable pungent acidic smell remaining behind? Acid-cure silicone is very easy to detect with the nose if it's uncured or still "fresh" and offgassing.

[floobydust]

I see moisture ingress from condensation so I am wondering the battery pack's ambient temps and humidity.
The black screw has speckles and droplets pattern on it. If the heatshrink is sealed really well verses some ventilation.

Although some silicones off-gas acetic acid, I don't see it because this mystery corrosion is not attacking the lead/tin/zinc stuff like silicones would do.
It seems to be blue from chlorine and likes ENIG and it likes to form more on one end of the PCB.

So I think it's a moisture problem plus the batteries off-gassing or it can be the environment i.e. if there is any H₂S or sulfur gas wherever the pack is located although that likes to react with copper.

[tooki]

Although some silicones off-gas acetic acid, I don't see it because this mystery corrosion is not attacking the lead/tin/zinc stuff like silicones would do.
It seems to be blue from chlorine and likes ENIG and it likes to form more on one end of the PCB.

Surely blue is from copper, not chlorine?

[2N3055]

LPF electrolyte contains Lithium hexafluorophosphate (LiPF₆).  It can release all kinds of nasty things when decomposed...
And it decomposes (by hydrolysis) when exposed to humid air...
So if there is slight leak from battery, that could explain some of the corrosion.

[CapLeaker]

My circuit boards look similar when they come directly from the coastline (humid and salty air). Everything corrodes around here.

[tooki]

Notice the battery packs without corrosion uses a silver colored silicone to pad the packs, whereas,  the battery packs with the corrosion are using a (new)blue silicone for padding and stability.  My theory is that the blue silicone is acid based and when sealed in the battery case the vapors settled on the copper and metal connectors and caused this.

Any other ideas of what caused this?

Acetic acid cure silicones (acetoxy cure) is a good theory, the distribution of corrosion on the boards suggests to me board contamination. Right at the top of the list for me would be flux residues from a flux that requires thorough cleaning, which nowadays largely means the water-soluble organic acid fluxes. Those are very common in mass production, as they are easy to wash off without solvents.

Perhaps the board assembled switched from no-clean to water-soluble flux?

[tszaboo]

Notice the battery packs without corrosion uses a silver colored silicone to pad the packs, whereas,  the battery packs with the corrosion are using a (new)blue silicone for padding and stability.  My theory is that the blue silicone is acid based and when sealed in the battery case the vapors settled on the copper and metal connectors and caused this.

Any other ideas of what caused this?

Acetic acid cure silicones (acetoxy cure) is a good theory, the distribution of corrosion on the boards suggests to me board contamination. Right at the top of the list for me would be flux residues from a flux that requires thorough cleaning, which nowadays largely means the water-soluble organic acid fluxes. Those are very common in mass production, as they are easy to wash off without solvents.

Perhaps the board assembled switched from no-clean to water-soluble flux?

Yes, I think silicone sealants, the wrong type (not RTV), can absolutely wreck electronics.
I haven't seen the long term effects because I banned that thing from our production.
But I've seen leaked batteries or water ingress. It doesn't really look like that.

[jerryk]

I can almost rule out moisture infiltration.  These batteries are used in small aircraft.  A typical installation is under the seat or in the boot cowl area.  If moisture makes it to the battery the pilot has bigger problems than a wet battery. 

Both batteries did smell when I opened them.  My first instinct was lithium smell but started to wonder if it was the silicone smell as I examined them.  One of the batteries had minor signs of swelling on one cell pack and the cell is dead.  The other three are good and tight.  The other battery pack pictured is perfectly tight, square, and all cells are within the LFP voltage range and charge evenly.

I pealed back the cell connector board on the "good" pack and there is some minor corrosion on the connector board terminals.  You would think if it were a lithium leak that caused the corrosion there would be more corrosion on the cell connector board than the BMS.  This is the first case of this I have seen and two changes in manufacturing that I see is the silicone used to pad the battery has changed and the pack itself is from a different manufacturer than previous  versions.

I have seen plenty of swollen cell packs in these batteries and no leaks or corrosion anywhere.  So it's either the new cell pack manufacturer has sealing issues or the new silicone has acidic properties.  One more novelty is why are these same corrosion deposits just on the negative wire on the battery?   

Jerry

[floobydust]

I'm not buying it. The corrosion is quite odd, in both the locations and which materials got attacked, and how much there is concentrated in spots.
Why is the silicone at the yel wire takeoff also blueish? To me this is a droplet that landed there (pun not intended).
I think it's some contamination, a mist or condensation forming, at one end of the component as if gravity acting along one edge.

The battery enclosure would also be "breathing" due to heating/cooling.
In a few products, I have seen moist air get sucked in to an enclosure, condense, accumulate and corrode.

For moisture I have used Gore-Tex vents in bigger enclosures, and for sulfur (H₂S) I have placed adsorbent cartridges inside an enclosure.
I don't know what the deposits are on the PVC black wire.

Although some silicones off-gas acetic acid, I don't see it because this mystery corrosion is not attacking the lead/tin/zinc stuff like silicones would do.
It seems to be blue from chlorine and likes ENIG and it likes to form more on one end of the PCB.

Surely blue is from copper, not chlorine?

I thought chlorine, there is mention of chlorine doping in the battery construction.  But many papers on using chlorine to recycle lithium batteries, so I might have it mixed up.
Very strange to see it on gold.

[twospoons]

  These batteries are used in small aircraft.

So they're subject to frequent and rapid changes in air pressure which could easily force humid air into the pack, which cools and condenses. I don't think you've ruled out moisture at all.

[antenna]

That shade of blue reminds me of a carbonate.  A drop of acid can confirm carbonates by fizzing. Carbonates usually form from leaked hydroxides that react with CO2 in the air.  Not sure if hydroxides are in this recipe or not.

My grandparent's breaker panel had a corrosion issue which I posted about here, and in that situation, it turned out to be chlorides from road salt on the floor and high humidity from a stuck floor heat thermostat.  Efflorescence did the rest.  In the case of the panel corrosion, I determined chlorides by electrolysis of a sample with MMO electrodes and noted the smell of chlorine.

[thm_w]

https://earthxbatteries.com/product/etx680/

They claim IP66 rated, maximum altitude 50,000 feet. But looking at the quality of manufacture, its good, but not amazing. I suspect as mentioned above its still possible for moisture to get in (eg the black wire goes through a grommet in the side of the case).

Did you ask the manufacturer?

[amyk]

Both batteries did smell when I opened them.  My first instinct was lithium smell but started to wonder if it was the silicone smell as I examined them.

They're very different - lion electrolyte has a sweetish solvent type smell, while acid-cure silicone smells like very strong vinegar.

[tooki]

Although some silicones off-gas acetic acid, I don't see it because this mystery corrosion is not attacking the lead/tin/zinc stuff like silicones would do.
It seems to be blue from chlorine and likes ENIG and it likes to form more on one end of the PCB.

Surely blue is from copper, not chlorine?

I thought chlorine, there is mention of chlorine doping in the battery construction.  But many papers on using chlorine to recycle lithium batteries, so I might have it mixed up.

Just to clarify, I’m not saying that chlorine definitely isn’t involved, just that blue isn’t a sign of chlorine, but of copper: tons of copper compounds are blue or green. So if you see blue on electronics, it’s almost guaranteed to be a copper compound.

Very strange to see it on gold.

Not at all strange, since it’s just very thin gold plating over copper. The copper is what is attacked, and then the copper compound creeps along and can end up on top of the gold, whether by going the long way around, or through a microscopic hole in the plating.

[floobydust]

Where I saw problems using silicone was when it was not fully cured yet the product had been assembled and pretty much sealed up. So the acetic acid attacked the metals, solder joints etc.
With the big long blobs this battery pack has, it would take a long time to cure. I believe it cures by reacting with moisture in the air.

What exactly the salt/acid is and its source, would take some detective work.

[tooki]

Where I saw problems using silicone was when it was not fully cured yet the product had been assembled and pretty much sealed up. So the acetic acid attacked the metals, solder joints etc.
With the big long blobs this battery pack has, it would take a long time to cure. I believe it cures by reacting with moisture in the air.

Yep, exactly. And since that moisture has to diffuse through the silicone, it can only go so deep within reasonable time.

For an application like that, one would probably be better off using a two-component silicone or polyurethane where polymerization is not reliant on moisture at all.

There are also nifty “dual-cure” 1-component compounds where an external influence (typically UV light) acts as the polymerization initiator, but which have a secondary curing method (moisture, heat, or self-sustaining polymerization) that continue after the UV light is removed. This allows for the adhesive to cure even in areas the UV light cannot reach. There are also UV-initiated adhesives where UV is the initiator, but the actual polymerization occurs after a delay, meaning you can activate the adhesive with UV before joining together the parts, so that light-inaccessible areas cure quickly anyway.