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Avoiding Galvanic Corrosion

Ampster said:
My guess is the bolt stretched the aluminum leaving the unstretched portion " out of phase" with the rest of the internal aluminum threads. I may never know if my studs get stretched because I locked them in with Loctite Red. I guess that is reason number 8 why I prefer SS studs inserted almost fully into the internal threads.
;)
Click to expand...

I agree with Ampster. I didn't do any torquing until the M6 stud was sealed in with Loctite Red (just backed off 1/4 turn from the bottom) and dried. All the studs feel rock solid but, as said, the use of conductive and corrosion resistant Ox-gard meant I didn't have to torque them much. I'm not keen on using a torque wrench - I found it better to judge by feel. When the stack is topped off with a spring washer under the nut you just need to nip them up.
 
Once a thread is damaged like that, your better off oversizing it, or installing a threaded insert. You can try running a bottoming tap down the hole, but you will probably remove either the top or bottom half of the threads, if they are partially sheared.

In this application you really need to engage every possible thread in the terminal, and use a torque wrench whenever possible.
 
Ugh .. I may Helicoil these, then. Any concern with using cutting oil (which would be left behind on the aluminum) and/or having Stainless Steel 304 (apparently?) inserts in the aluminum?

Edit: although since I'm probably about to order studs from McMaster-Carr, I see they also have Helicoil inserts.. SS 18-8 ...
 
Last edited:
Oil isn't going to cause any problems unless there is a pool of it in the hole. Take lubrication into account if you are using a torque wrench.
 
Just degrease it, if your Tap Magic or whatever cutting fluid for aluminum doesn't simply evaporate away.
For one, don't want to keep track of different torque requirement for one bolt. For another, want the thread lock to stick.
 
For what little it's worth, I did assemble the cell with a blown-out terminal into a working pack and ran it up to 280A for 20 minutes. That terminal is was heating any more than any of the others (and they are all cool in 4p4s with ample 160mm^2 times 4 bus bars) that my FLIR camera can discern, even though the approximate torque was about half of all the others.

What I could not test is whether the limited torque that I could still develop would be enough to hold the bars sufficiently firmly over time and the inevitable jostling that comes from driving down the road. Probably not, but maybe that red locktite would help there.
 
I just ordered a Helicoil repair kit and the SS16 inserts .. I'll have everything by Monday ... This will be scary to do ... I'll keep you posted ...
I also have an open discussion with SoK about the situation -- we'll see what they say.
 
I use a Permatex #80078 zinc/copper anti-seize lubricant on all battery connections on my sailboat. If it's good enough for the US Navy then I guess it's good enough for me. I haven't had a problem with corrosion since 2012.
 
I received the studs and nuts. Before I went down the road of drilling/tapping/helicoil, I filed down the surface of the terminals (the aluminum had erupted out and caused an uneven surface) followed by 1000 grit sandpaper and then de-ox. I managed to get the studs all the way in and put everything back together and now I have stellar performance. At .5C (50 amps) the difference in volts between the cells went from 7 to 8 mv and everything remained cold. Awesome! And, whew!

Thanks to everyone here about the studs method -- I really prefer this over the bolts and have first-hand experience with what happens otherwise! I'm so glad I learned my lesson on the 100Ah cells that came with the SoK I've been testing and not the 280Ah cells for my final build.
 
Haugen said:
My big question is whether the cross section of the bolt will withstand the pull force applied at that torque. I'm pretty sure it will, but how do I calculate it?
Click to expand...
I spent some time looking for this answer and came up with the calculators that get me what I was looking for.
Start at Engineer's Edge clamping force calculator.

Bolt Torque, Axial Clamp Force, Bolt Diameter Calculator

This fastener bolt torque calculator will Estimate the unknown torque, diameter, axial force applied and coefficient of turning friction for the given conditions.
www.engineersedge.com
Notice that the torque is in N/mm instead of Nm, so 8Nm would be entered as 8000. Use 6mm for the major diameter.
Their tables show 0.30 for the friction coefficient for aluminum.
Click the button for find axial clamp force.
This gives you the Newtons of force.(4166.667N) This value needs to be translated into how much force per meter squared.
We're looking for the clamping force exerted on the terminals. They are 15.5mm in diameter Find the area of the 15.5mm terminal in meters^2. Of course pi * r^2, so square 0.00775m radius and multiply by pi.
Last, divide the clamp force (4166.667N) by the area and you should get a value somewhere in the 200kPa range.
Every spec I have seen for 6061 aluminum has a strength with a minimum of 240kPa.
Leaving a margin, I plan to use 7.0 to7.5Nm to leave plenty of margin for defects in the threads with plenty of clamping force.
I think that is the right math.
Does someone want to check me on it?
 
You should use the cross sectional area of the bolt as it'll be the weak link (assuming the threads aren't). Also I think you have some zeros error because I find 22.1 MPa instead of 2xx kPa.

So for the bolt that's 28.27 mm² and with 4167 N that's 147.4 MPa of stress.

Also 6061 properties (especially the yield strengh) depends on the temper: https://en.wikipedia.org/wiki/6061_aluminium_alloy#Properties and we can see even T4 isn't good enough, you need T6 to handle that stress properly ;)
 
cinergi said:
Thanks to everyone here about the studs method -- I really prefer this over the bolts and have first-hand experience with what happens otherwise! I'm so glad I learned my lesson on the 100Ah cells that came with the SoK I've been testing and not the 280Ah cells for my final build.
Click to expand...

Blue loctite didn't hold. I went with permatex brand red thread locker and those SS18 studs are nice and locked in. I sanded (1000 grit) the tops of the terminals, sprayed deox on it, and torqued to 7Nm. Amazingly easy to work with the studs as far as getting the lugs on and screwing down the nuts. On, off, on, off -- no problems. Threads like butter. I created a 24v battery out of the cells from the two SoK's and draw 41 amps and the voltage difference between the cells (as seen by the Chargery) doesn't change at all. Awesome! I'm sold sold sold!
 
I have a question about galvanic corrosion. Ive heard a lot about using products that contain zinc, and have read that stainless/aluminium is far worse than zinc/aluminium. So, I have an old truck, and the body is entirely aluminium. The aluminium is fixed with mostly stainless screws with a few galvanized screws. Bearing in mind I live in the UK which is plenty wet, there is minimal oxidation between the aluminium/stainless, but on the galvanized screws the zinc had corroded away in 6 months leaving rusty screws. What gives?
 
Colonel.lp said:
I have a question about galvanic corrosion. Ive heard a lot about using products that contain zinc, and have read that stainless/aluminium is far worse than zinc/aluminium. So, I have an old truck, and the body is entirely aluminium. The aluminium is fixed with mostly stainless screws with a few galvanized screws. Bearing in mind I live in the UK which is plenty wet, there is minimal oxidation between the aluminium/stainless, but on the galvanized screws the zinc had corroded away in 6 months leaving rusty screws. What gives?
Click to expand...
Zinc is the sacrificial metal in this case and is the first to oxidize by design. Once the zinc is completely depleted the rest of the metal will follow. This is why large chunks of zinc are used as anodes on boats. Where dissimilar metals are exposed to water the zinc is attached so it corrodes away first or is sacrificed.
 
You basically have two solutions if you can't prevent galvanic corrosion in the first place:

- use non-corrodable metals like stainless steel
- use a sacrificial metal so it corrodes first (but that needs periodic replacement of course)

So as @Mango boat said you saw the second solution in action but without them replaced before the zinc was depleted so not really useful.
 
Over time the aluminum in contact with stainless on that vehicle will puff and turn to powder. There are a lot of variables though, as the aluminum alloy has a major effect.
 
Unless you are submersing these in an electrolyte, galvanic corrosion is really not an issue if they are assembled correctly.

Datapoint of n=1 (we also have many more banks out there with zero issues)

On my own vessel I've been running Winston cells, with SS bolts, since 2010. The cells were manufactured in 2009. I apply a bit of dielectric grease each year, polish up the busbars between cells & re-torque. In 10 years on the ocean not so much as a hint of corrosion. We also live just feet from the ocean so these cells never catch a break, even in the off season, and again, zero terminal corrosion.

I can vividly, early on (2007 ish), remember being told by some internet experts that cells with aluminum & copper would not last a year.... Well, these were made in 2009, have over 1600 cycles, almost all to 80% DoD or deeper, and they still deliver over the 400Ah face value rating when capacity tested.
 
Thanks for all the info guys. Just wondered why zinc is used between contacts when it can corrode. My vehicle is 30 years old, so that's a lot of wet time, and the aluminium/stainless reaction has been so minimal that I'm not going to worry about using stainless studs into aluminium terminals, but I'm going to avoid any product containing zinc and just use pure silicone grease, have read many articles attesting to its use in electrical contacts.
 
Considering how damaging this is. I am surprised it is not more of an issue.
I do have a question however.
What if one uses that thermal paste used for heat transfers on electronics, such as CPU's and mosfets etc?
Some are electrically conductive and some are not.
Maybe a slight layer of that will help keep the aluminum safe?
 
Colonel.lp said:
Thanks for all the info guys. Just wondered why zinc is used between contacts when it can corrode. My vehicle is 30 years old, so that's a lot of wet time, and the aluminium/stainless reaction has been so minimal that I'm not going to worry about using stainless studs into aluminium terminals, but I'm going to avoid any product containing zinc and just use pure silicone grease, have read many articles attesting to its use in electrical contacts.
Click to expand...
It could be that the zinc has been preventing the aluminum / stainless connections from corrosion. I think I would replace the zinc coated ones with new zinc coated ones.
 
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