Dachromet Coating and Reusing Fasteners

Indi

Indi

Member
In the manual it says the green coating on lots of the original fasteners is Dachromet, a corrosion preventive coating used to prevent contact (Galvanic?) corrosion.

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Possibly this coating wearing off is the reason many of the 10.9 high tension bolts are said by the manual to be once use, even though they are not stretch bolts and are structurally ok to reuse. This would mean that by reusing the old fasteners, the female aluminum threads would be gradually weakened, as the electrons transfer to the more noble ferrous bolts, as well as increasing the risk of the fastener seizing in...
Even if this is the case there is not much to do with the information as none of the genuine VWAG replacement bolts seem to come with this green coating on, even though the part number matches the original.
I've been using Aluminium grease to reassemble everything so hopefully the combination of the grease sealing to keep water or any other electrolyte out of the threads, and a little bit of extra sacrificial aluminium is enough to prevent any damage.

Just thought it was interesting and wanted to share, I don't know if this is the case or not but seemed like a potential explanation.
 
One of my pet subjects, reuse of 10.9 fasteners and one of my pet hates, aluminium oxide resulting from galvanic corrosion all in one thread 😊.
There is nothing conclusive that I have found so the more thoughts on these subjects the better. This is where I got to:
I speculated some time ago on here that the general Audi instruction to replace fasteners was to replenish the green coating. I got no reply.
I think that green coating, or Dachromet, is no longer allowed due to EU environmental legislation. Audi no longer supply.
A waterproof dielectric grease reduces galvanic corrosion but Porsche advise against lubricating Dachromet. Can’t get a reason why, even from America where they can still get Dachromet.
I thought torques were specified for dry bolts with Dachromet (as Porsche) but a well respected member advised Audi torques are specified lightly oiled.
Very confusing so I started doing fasteners my way. Not had any repeat failures but too early to be conclusive.
 
Here is something I am confident of, over the years I have made the mistake of greasing steel fasteners into aluminium with copper grease. This should in theory increase galvanic corrosion but it doesn't to any noticeable extent.
They are all coming out fine, they get a clean and go back with a more suitable grease. I concluded one of the main factors in preventing galvanic corrosion is to keep salty water out.
 
Progress with my A2 is currently on hold while I fix a Porsche and get it out of my lockup so the A2 can go in.
Progress is slow due to galvanic corrosion of aluminium. To give you some idea of the problem an A2 is an aluminium car with a few steel components that cause a few problems. A Porsche is a steel car with lots of aluminium components that caused lots of problems.
It means I became very interested in Dachromet only to find that it would not be on my car and Audi are unlikely to have supplied it since 2007, scroll down to specifics about surface coating https://www.ejot.co.uk/Technical-information/Surface-Coatings/p/VBT_OBERFLAECHEN#:~:text=The following anti-corrosion surfaces,coating from the company DACRAL)
 
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I would say it’s almost certainly better using “new” bolts, without the green coating, rather than old bolts that have been used several times and the coating may have worn off. Dachromet was banned in the EU because it contains hexavalent chromium as mentioned, but Dachromet isn’t the green coating, it’s the zinc flake finish underneath. The “green” is probably a sealer or lubricant as @Little Dog mentioned.
Almost all bolts you buy today that would have been finished in Dachromet, are now finished in another zinc flake finish, without chromium (probably Geomet). I don’t think it’s unreasonable to think that VAG assessed this finish and deemed the “green” lubricant/sealer surplus to requirement, hence why the bolts now are just dull grey. They will provide very similar corrosion resistance to Dachromet bolts in “as-new” condition.
 
donp38 said:
How about dielectric grease?
Click to expand...
Durlac is the best option if you’re worried. It is specifically designed to prevent galvanic corrosion. There is a normal version and a “green” version which is slightly less horrible for the environment.

If you decide to use this, please check the MSDS and take appropriate safety precautions. Chromates are restricted for a reason, and even with “green” Duralac there are precautions.
 
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I have some Duralac on order, thanks for the tip @a-zwo . It looks like a good tool in controlling galvanic corrosion. However all the evidence I have in front of me is that dissimilar metals will coexist until an electrolyte is introduced; then things go horribly wrong. The key is to keep the electrolyte out.
 
I was wondering about wether bolting a magnesium sacrificial anode to it would work, it should protect anything made of aluminium and steel as long as it’s all connected or earthed even if it’s not directly touching

 
I didn’t explain why I’m so interested in Duralac. When our A2 steel screws react with aluminium because the thread is a through hole that gets wet and the furthest threads become solid with aluminium oxide. We then heave the screw out ripping out the female thread and we fix it by putting in a coil. Coils are stainless steel that will, in contact with aluminium, cause galvanic corrosion. No problem I thought the aero industry coil all aluminium threads. Yes but I found that they use specially plated coils to prevent galvanic corrosion.
I thought Duralac may be useful in the absence of aero industry plated coils?
 
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It has a slight “thread locking” effect because it does dry out. However, it’s much less of a thread locking effect than galvanic corrosion.

Given that, I would say using it on thread inserts it’s a perfect application.
 
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