Let’s talk about galvanic corrosion.

Little Dog said:
@Sootywg the bolt is difficult to pick out in the manual. If it is M12 1.5 by 100mm then the torque is a big one 70 Nm + 90 degrees.

Edit, I see the tap you used was M12 1.25. I don’t right now have access to my manual. It could be a typo on my part on the bolt size. Wait until somebody else comments re the torque. I will check again tomorrow.

In my case there was very little female thread left. My bolt was the same as the one @Howey posted. I had to fight it all the way out with a 6 sided socket and a long breaker.
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You had me worried for a moment. My error in the earlier post. The tap I purchased is indeed M12x1.5 (not 1.25mm) phew.
Had it been a 1.25mm, the thread would have been wrecked and a hell of a lot more swarf and a new console required. For some reason I thought it was 1.25 when I posted.
Removing it required a long breaker to start but then only a 12” socket wrench the rest of the way.
This is the best pic I can get of the thread in the console.
Glad I took the photo, it needs flushing out I think.
 

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Sootywg said:
You had me worried for a moment. My error in the earlier post. The tap I purchased is indeed M12x1.5 (not 1.25mm) phew.
Had it been a 1.25mm, the thread would have been wrecked and a hell of a lot more swarf and a new console required. For some reason I thought it was 1.25 when I posted.
Removing it required a long breaker to start but then only a 12” socket wrench the rest of the way.
This is the best pic I can get of the thread in the console.
Glad I took the photo, it needs flushing out I think.
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You had me worried too but didn’t want to post my concerns. I scaled off your images of your bolt and tap and got M12 1.5 twice thought all would be well.
Your female thread looks a bit rough, it may blow out but I would oil my bottoming tap and run it through again. I guess by the aluminium oxide still being powder your taps were not lubricated?
If not a tub of CT-90 will be useful in the future, keeps the tap lubricated and stops it cutting good metal.
 
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Little Dog said:
You had me worried too but didn’t want to post my concerns. I scaled off your images of your bolt and tap and got M12 1.5 twice thought all would be well.
Your female thread looks a bit rough, it may blow out but I would oil my bottoming tap and run it through again. I guess by the aluminium oxide still being powder your taps were not lubricated?
If not a tub of CT-90 will be useful in the future, keeps the tap lubricated and stops it cutting good metal.
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Paraffin for aluminium if you have it when drilling or tapping.
 
Little Dog said:
I’ll give it a try when coiling. Does it work when chasing and trying to avoid cutting?
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I have resorted to WD 40, others are available.
By chasing do you mean cleaning out the original threads with a tap, if so then yes as it works as a lubricant.
 
Stan H said:
I have resorted to WD 40, others are available.
By chasing do you mean cleaning out the original threads with a tap, if so then yes as it works as a lubricant.
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Yes by chasing I mean cleaning out original threads. There are tools for the job but I use cutting taps so they have to be lubricated. If not a dry tap cuts the original thread oversize.
 
More examples of galvanic corrosion that prompted me to start this thread. This is another example of how the presence of an electrolyte accelerates galvanic corrosion. This is boot floor pan heat shield (not A2) it is secured to the floor pan with studs and pressed steel nuts (very A2 like). The central locating holes, away from road spray, show lite corrosion:

IMG_1032.JPG


Closer to the wheels and spray corrosion is progressing a little more:

IMG_1033.JPG


Aluminium around outer locating holes directly behind the wheels has disintegrated:

IMG_1034.JPG


The outer securing nuts, right hand two, were solid inside the pressing with aluminium oxide and difficult to remove. One has had the corrosion scraped out a little to show the white aluminium oxide:

IMG_1037.JPG


This example appears to support my suspicion that the presence of an electrolyte, UK salty road spray, is a key contributor to galvanic corrosion,

Also the rear of two of the pressed nuts being full of aluminium oxide show how much aluminum expands when it oxidises. I can't illustrate it but the expansion creates pressure and the oxide is hard and gritty too making fasteners difficult to remove. Not nice stuff to have to deal with.

I have been working on fixes. I'll post them later along with more details of how this applies to A2.
 
I started this thread with an example of aluminum on aluminium galvanic corrosion. The presence of an electrolyte and a more noble metal appears to be enough for aluminium to corrode in contact with aluminium. This is important for A2 but I've not yet found an example on the A2.

A double skinned aluminium heat shield corroded between aluminium skins. I have several examples like this that looked reasonable on the outside but when I peeled the outer skin back, it looked distorted and felt crispy, I found aluminium oxide:

IMG_1038.JPG


Corrosion has progressed from the steel through the central hole and out between the aluminium skins. More evidence of the effects of an electrolyte seeping between aluminium components and causing corrosion when steel is nearby.
 
An example that supports Audi's instruction to replace fasteners and the observations in this thread https://www.a2oc.net/community/index.php?threads/dachromet-coating-and-reusing-fasteners.54619/

I'm finding lots of examples of this, this one was available to get a picture. It is a rear bumper, in places it was so badly corroded I had to coil the threads. However where the original plating on steel fasteners has remained intact there is very little corrosion of aluminium:

IMG_1050.JPG


I think the zinc is less noble than aluminium so it will sacrifice itself to prevent aluminium corrosion. Once it is gone from a steel fastener then the more noble steel will corrode the aluminium. Again the whole process will be accelerated if an electrolyte seeps between the zinc and aluminium.

Conclusion, advice from Audi to replace fasteners is good but expensive; if they are still available. More on cheap fixes later.
 
Little Dog said:
More examples of galvanic corrosion that prompted me to start this thread. This is another example of how the presence of an electrolyte accelerates galvanic corrosion. This is boot floor pan heat shield (not A2) it is secured to the floor pan with studs and pressed steel nuts (very A2 like). The central locating holes, away from road spray, show lite corrosion:
..........................................................
I have been working on fixes. I'll post them later along with more details of how this applies to A2.
Click to expand...
I repaired my back in 2016

Fixing the weather / heat shield - another thread

Recently I fixed my weather / heat shield with the help of Matt's (wilco184) excellent thread from earlier in the year: http://www.a2oc.net/forum/showthread.php?30198-Rivnuts . I had some variations in the materials used and fixing. ======= Focus of work ======= The main focus was the...
www.a2oc.net www.a2oc.net
 
@Alan_uk that is a good thread on repairing heat shields, I'll be careful not to replicate but will quote. My A2 is quite good. I bought it new and put rubber washers between the steel fasteners and heat shields. I didn't notice but when it was Audi serviced the technicians binned the rubber washers around the fuel filter. I have corrosion issues there to sort out but did, many years ago, buy new genuine heat shields. It will be interesting to see what I find when I finaly get my A2 into my lock up and get under it.
I have a steel car in there that has numerous galvanic corrosion and rust issues and is refusing to be fixed and leave. At least it is feeding threads that could be useful to members here.
 
Dear @Little Dog.
Please don't omit photos and text that you deem important on the grounds they similar to @Alan_uk post.
The process of innovation and progress starts with a review of existing knowledge on any subject. Referencing demonstrates the breadth of your research. This method of working can identify gaps in knowledge share new understand and make progress.
Your insight on car repair is appreciated.
🙂👍
 
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I’ve got an example here of galvanic corrosion of aluminium from a cycle. I think it is a casting. I have similar corrosion on the A2 space frame castings around the rear axle mounts.
Aluminium through axle to QR converter from a titanium cycle.

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I’ve removed the aluminium oxide and I’m surprised at the depth of the rot.

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I’ll be taking a much closer look at the A2 the next time i’m under it.
 
I know what you mean on the A2 castings contact areas. Noticed some around mine when I refurbished the front and rear suspension last year. Took out the rear axle beam to strip and do all the braking system & refresh the beam.
 
Not been online much recently and there has been zero progress with the A2. I've continued to make observations and take pictures re galvanic corrosion. Galvanic corrosion is caused by a flow of electricity between two dissimilar metals, anything that increases that flow is bad? Apparently not so, there was a time when I blathered everything in copper grease and that conducts electricity. Here are two steel bolts, with copper grease that have been screwed into an aluminium casting. The tapped holes are through holes and the casting receives road spray.

IMG_1091.JPG


They are fine and so is the thread in the casting. I wouldn't now use copper grease but where I have I'm not concerned about it.

I'm not a chemist so don't fully understand but these words from the internet shed some light on what is going on:

"Electrolytes play a crucial role in electrochemical corrosion by serving as a medium for the transfer of ions between the anode and cathode in a corrosion cell. When a metal is in contact with an electrolyte, such as water or a solution containing salts, the electrolyte facilitates the flow of electrons from the anode to the cathode, leading to the corrosion of the metal. The presence of electrolytes can accelerate the corrosion process by providing an environment conducive to the movement of ions and the formation of corrosion products. Additionally, electrolytes can affect the pH and oxygen concentration at the metal surface, further influencing the corrosion rate".

I think it is there in the last sentence, the copper grease stops electrolyte access to the dissimilar metals so the pH and oxygen concentration does not become favourable to corrosion.

Thoughts?

Edit, the copper grease will conduct electrons, an electrolyte conducts ions. Ions cause the damage?

en.wikipedia.org

Electric current - Wikipedia

en.wikipedia.org en.wikipedia.org

So the key in the words from the internet above is the last but one sentence, electrolytes provide an environment conducive to the movement of ions.

Thoughts?
 
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Here is something I’ve been using to help understand what I’m seeing:

IMG_1015.jpeg


It is a galvanic series chart giving the voltage difference between two different metals positioned together both in contact with a common electrolyte. In this situation galvanic corrosion can be expected but only if the voltage difference between the metals is greater than 0.2V.

Zinc is slightly less noble than aluminium so will corrode before aluminium. However steel is less noble than aluminium so when the zinc plating deteriorates on steel fasteners the surrounding aluminium can corrode if wetted with an electrolyte. This adds weight to the Audi instruction to use new fasteners.

Stainless steel is less noble than mild steel and is far less noble than zinc. So it is to be expected that any mild steel fasteners plated in zinc will not last very long if wetted by salty road spray:

IMG_1090.JPG


Just 22k miles on a car dry stored over winter, the mild steel fasteners on a stainless exhaust hanger are turning to dust. Not sure there are many stainless components on an A2; turbo mounting bolts only? However we introduce stainless components, repair coils and corrosion resistant bolts. I will be checking those securing my selector tower, they could be messing up my gearbox case.
 
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Little Dog said:
Here is something I’ve been using to help understand what I’m seeing:

View attachment 121404

It is a galvanic series chart giving the voltage difference between two different metals positioned together both in contact with a common electrolyte. In this situation galvanic corrosion can be expected but only if the voltage difference between the metals is greater than 0.2V.

Zinc is slightly less noble than aluminium so will corrode before aluminium. However steel is less noble than aluminium so when the zinc plating deteriorates on steel fasteners the surrounding aluminium can corrode if wetted with an electrolyte. This adds weight to the Audi instruction to use new fasteners.

Stainless steel is less noble than mild steel and is far less noble than zinc. So it is to be expected that any mild steel fasteners plated in zinc will not last very long if wetted by salty road spray:

View attachment 121405

Just 22k miles on a car dry stored over winter. Not sure there are many stainless components on an A2; turbo mounting bolts only? However we introduce stainless components, repair coils and corrosion resistant bolts. I will be checking those securing my selector tower, they could be messing up my gearbox case.
Click to expand...
Check what grade the stainless steel fixings are as there is more than one. Some grades will still rust. 😱
 
Stan H said:
Check what grade the stainless steel fixings are as there is more than one. Some grades will still rust. 😱
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The corroded fasteners on the stainless exhaust hanger in the picture above were low milage Dacromet coated mild steel, probably grade 10.9 as used extensively in the factory on an A2. I have replaced them with A2 stainless which is less corrosion resistant than A4 stainless. I couldn't get A4 in the sizes I need and I don't know the grade of the stainless hanger components. A2 stainless has to be better than mild steel, there should be no galvanic corrosion between the hanger components and the stainless fasteners.
 
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