Galvanic Corrosion

[Alb]

As my MG5 is only a couple of months old I was quite surprised to see how much corrosion there was when I removed the rear wheels (it's nowhere near as bad on the fronts for some reason). This isn't oxidisation of the hub but galvanic corrosion caused by contact between the steel hub and alloy wheel.

Because I've caught it early, it cleaned up reasonably well using a soft metal brush.

If this were left unchecked you could well find future difficulty in removing the wheel and you might also experience run out problems if the wheel was unable to sit flat against the hub. The faying surfaces should have an insulating coating to prevent electrical interaction but that doesn't seem to have been adopted by MG (The wheels on my other car have this).

To prevent recurrence I applied a smear of copper grease to the hub:

and also to the wheel contact points:

This should be done sparingly as you don't want to get any on the friction surfaces.

I'd be interested to know whether others have seen the same thing.

 

[Caldosan]

Thanks for that, that’s very informative. I will take a look at mine and pop some copper grease on them as you describe this weekend. Nice one Alb.

 

[Lovemyev]

I have seen seen this on almost every make and model of cars that have this combination of mild steel hubs and alloy wheels fitted.
Only about 3 days ago, I have advised owners to do the very same thing as you, to their cars on a different thread.
The corrosion is caused by having the two dissimilar metals of mild steel and alloy, coming into close contact and then moisture enters between both surfaces and then very quickly starting off the process of the white powder corrosion.
If this is left unchecked, then trying to remove the road wheel at the side of the road, is rendered almost impossible.
I have have carried out the same process as you, on every car I have owned with this combination for the last thirty odd years and it works !.
The area that is very important, is the contact area of the mild steel boss on the hub ( the part you balance the wheel on, while trying to start the road wheel bolts ) and the inside hub of the alloy wheel.
Copper grease is my choice, but any grease will do at a push.
Another good tip, buy yourself a wheel aligning stud.
It makes refitting the road wheels SO much easier.
VAG cars have in the tool kits of their cars as standard.
I will try and upload a photo, money well spent IMHO.

 

[m4rmite]

Apply grease to the face of the hub, not copper grease.

Bmws you had to boot the wheel off so not limited to mg.

 

[Cocijo]

There is much debate about the use of grease on the mating surface for the wheel and also wheel studs. Manufacturers like the mating surfaces and wheel bolts clean and dry and their tightening torques are based on a clean dry mating surface. As I say it’s a subject of much discussion.

 

[Lovemyev]

There is much debate about the use of grease on the mating surface for the wheel and also wheel studs. Manufacturers like the mating surfaces and wheel bolts clean and dry and their tightening torques are based on a clean dry mating surface. As I say it’s a subject of much discussion.

I agree, grease should NOT be applied to the mating surface where the back of the alloy road wheel comes in contact with the mild steel hub.
It will be flung out by centrifugal force and will contaminate your brake linings.
This could result in huge reduction in braking power, or even brake loss.
I advise caution carrying out this process if you are in any doubt what so ever !.
Grease should ONLY be applied to the hub centre boss ( where the cap is protecting the wheel bearing ) and a thin smear inside of the alloy road wheel.
Then, when the road wheel is refitted there is water resistant barrier between the mild steel hub and the alloy road wheel.
This will prevent the white corrosion from forming and will allow the road wheel to be removed easily in the future.
Again, do NOT apply any grease to the rear mating surface of the road wheel, where it comes in contact with the mild steel hub !.
Only the centre cap area folks.
This white corrosion condition occurs on any make or model where a combination of a steel hub and a alloy road wheel is used.
Applying excess grease to the bolts or studs is not recommend because it will cause a hydraulic reaction and then result in an incorrect torque settings being achieved.
Remember, when refitting road wheels, the bolts should be always be torqued to the factory spec’s as we are all aware .

 

[Alb]

Apply grease to the face of the hub, not copper grease.

Bmws you had to boot the wheel off so not limited to mg.

Copper grease is better because you are looking for anti sieze and insulating properties not lubrication. There are other compounds that would do the same job. The stuff Ford used on my Focus is white but I don't know what its chemical composition is.

 

[Alb]

I agree, grease should NOT be applied to the mating surface where the back of the alloy road wheel comes in contact with the mild steel hub.
Grease should ONLY be applied to the hub centre boss ( where the cap is protecting the wheel bearing ) and a thin smear inside of the alloy road wheel.
Then, when the road wheel is refitted there is water resistant barrier between the mild steel hub and the alloy road wheel.
This will prevent the white corrosion from forming and will allow the road wheel to be removed easily in the future.
Again, do NOT apply any grease to the rear mating surface of the road wheel, where it comes in contact with the mild steel hub !.
Only the centre cap area folks.
Applying excess grease to the bolts or studs is not recommend because it will cause a hydraulic reaction and result in an incorrect torque settings being achieved.
Remember, when refitting road wheels, these should be all torqued to the factory spec’s .

You prevent electrolytic corrosion by electrically insulating the dissimilar metals from each other. This has to cover all points of contact to be fully effective although in reality some conductive paths will likely remain (wheel bolts for example). You could have a design which sleeved them but you rapidly get into the law of diminishing returns so it wouldn't be worthwhile in this application.
The key point is doing nothing makes it likely future problems will arise.

 

[m4rmite]

Copper grease is better because you are looking for anti sieze and insulating properties not lubrication. There are other compounds that would do the same job. The stuff Ford used on my Focus is white but I don't know what its chemical composition is.

I read somewhere that copper grease was not advisable but to use another grease type.

As is usual i can't find that info again.
There is aluminium grease out there that would be as suitable.

 

[Alb]

I've been using copper grease for this purpose for the last 15 years or so and it has performed perfectly but as I said above, there will be other compounds that can do the job.

 

[Lovemyev]

The key point is doing nothing makes it likely future problems will arise.

There is no denying that fact !.
Just ask any tyre fitter who has tried to remove a alloy road wheel from a car after about two years, when this is the first time the wheel has left the car !.
Road wheel rarely get removed at the dealership anymore.
In most cases, the brake pad wear is checked from the back by visual inspection.

 

[Lovemyev]

I've been using copper grease for this purpose for the last 15 years or so and it has performed perfectly but as I said above, there will be other compounds that can do the job.

Same here plus another 15+ years on top of that !.
Dealerships don’t use copper grease anymore.
It helps to bring in brake work.
You will know what I mean if you have any mechanical knowledge!.

 

[Kithmo]

Is Marine white grease suitable for this application ? (it's more resistant to washing off)

 

[Lovemyev]

Is Marine white grease suitable for this application ? (it's more resistant to washing off)

Could well be I guess ?.
But I have always used copper slip for TOO many years now.
It is tried and tested in my book, proper “Old School” I know.
A grease that can stand heat as well as being moisture resistant is a important factor as well.
The likes of Vaseline is no good at all.
No thermal properties.
Okay for babies bums, but that’s it .

 

[Fokmg]

Hi Interesting problem
I am not sure it is "Galvanic Corrosion"? "Probably Differential Air Corrosion"? If you look at the heavy rust patterns on the disc they are concentrated in the areas where there is no alloy wheel metal contacting the steel hub, It looks like moister trapped in the casting cavities causing corrosion.
looking at the design of the wheel casting it does look as if there is water escape port for the 5 pockets but this will only work when the appropriate pocket is pointing down to the road.
I note you say the problem is reduced on the front wheels which will get hotter than the rear and therefore cause the water to dry off before the corrosion starts.
Some of this problem may well have occurred in transportation from China with the added advantage of sea air!
The important point as everyone else has said remove the wheels and apply a very light smear of copper grease to the hub of the disc. This will stop the problem and also make it possible to remove the wheel at a later date!
As I use Kers set at 3 for a lot of the time my brakes/wheels never get warm so the corrosion will be bad on all 4 wheels!
Thanks for the tip I shall get the Copper Grease out before the winter
I have to say that other than silly problems like this the MG5 is a great car

 

[Alb]

The front hub is different to the rear insofar as the faying surface is coated:

The corrosion here is concentrated around the (uncoated) collar plus there are traces in the bolt holes and these aren't natural cavities.
I do like your alternative theory though but I have to say that I've never encountered this on any other car I've owned or worked on. It will be interesting to see what feedback arises from other owners and whether this was a one-off situation or something that will recur again and again if not treated.

 

[Lovemyev]

The front hub is different to the rear insofar as the faying surface is coated:

View attachment 4746

The corrosion here is concentrated around the (uncoated) collar plus there are traces in the bolt holes and these aren't natural cavities.
I do like your alternative theory though but I have to say that I've never encountered this on any other car I've owned or worked on. It will be interesting to see what feedback arises from other owners and whether this was a one-off situation or something that will recur again and again if not treated.

@Alb Absolutely bang on, great image of the evidence that causes the problem !.
The rust around that mild steel hub collar, displays exactly where the issue is triggered.
But you already know this of course, and know how to deal with it my friend .
Not a "One Off" - It will be on every car OBTW.
It will continue to get worse if not teated and the rears will be more likely to be affected more.
Not just on MG's it has to be clearly said, I have seen this condition on many VAG vehicles.
( ....... Nobody is listening, but do you notice the lack of C.G. in the pad department ).
I will say no more than that !.

 

[Barney]

@Alb Absolutely bang on, great image of the evidence that causes the problem !.
The rust around that mild steel hub collar, displays exactly where the issue is triggered.
But you already know this of course, and know how to deal with it my friend .
Not a "One Off" - It will be on every car OBTW.
It will continue to get worse if not teated and the rears will be more likely to be affected more.
Not just on MG's it has to be clearly said, I have seen this condition on many VAG vehicles.
( ....... Nobody is listening, but do you notice the lack of C.G. in the pad department ).
I will say no more than that !.

OBTW ? C.G. ?

 

[Lovemyev]

OBTW ? C.G. ?

C.G. - "Copper Grease".
If you have had the experience of working on a LOT of cars in the past, then you will totally understand where this is intended to be applied in relation to brake pads.
I am not getting into this debate folks, if you know, you know.
If not, leave well alone for safety sake.

 

[Alb]

On the subject of Copper Grease and brakes, I came across a curiosity on my other car where the Ford service manual advised using it on the contact points for the front brakes but gave contrary advice for the rears. Never did figure that out.