Coat/Paint aluminum alloy rims - or not?

Question

Coat/Paint aluminum alloy rims - or not?

MBGraham (Chemical)

(OP)

25 Apr 21 15:35

The alloy wheels on our '98 Mercedes had rims leaks. Cause was corrosion in a few spots in bead area. Originally rims were coated with some sort of paint that sanded easily but was well adhered. My first effort was to sand out the corrosion, roughen the surface and apply a 1-part polyurethane moisture cure coating. This worked to some extent, but adhesion was not uniform.
I decided to strip the rims from outer edge down through the bead area to the inner rim. (about 2" wide area). I tried various methods of stripping, but in the end rotary wire brush was only thing that worked. But it left a rough surface that I still need to sand smooth.

So now I should have bare aluminum with hopefully a relatively smooth matt surface finish.

Environment:
- outer edge of the rim beyond the tire is exposed and could see salt and water.
- surface that tire rubber bead seats on may have crevices and subsequent corrosion (as occurred on original rims)
- surface inside tire bead could see moisture that condenses from air.
- the bead surface needs to be hard enough to not be damaged by tire bead pressure or movement.

Finally my questions! How do I best finish the job?
- Leave rims bare and allow them to oxidize?
- apply a phosphate primer/coating and leave it at that? (e.g. https://www.aircraftspruce.ca/catalog/cspages/zinc...)
- apply phosphate primer and apply thin finish coating?
(chromate primers appear to have been outlawed)

https://files.engineering.com/getfile.aspx?folder=83ddecde-9217-4ac2-a8ba-2

TugboatEng (Marine/Ocean) · Answer 1 · 2021-04-25 16:50:48

Coatings cause crevice corrosion on aluminum. If you aren't going to go through the extent of using a chromate conversion coating, you're better off leaving it bare.

swall (Materials) · Answer 2 · 2021-04-27 15:21:18

RE: Coat/Paint aluminum alloy rims - or not?

swall (Materials) 27 Apr 21 15:21

Will you be doing the work yourself?

MBGraham (Chemical) · Answer 3 · 2021-04-27 19:06:14

Yes, doing it myself.

Any coating would need to be available retail in Canada. Marine (and other) stores have zinc phosphate primer for use on underwater aluminum stern-drives and other aluminum surfaces. including the Temp one I mentioned earlier. This one I can get from local store: https://www.wholesalemarine.com/moeller-zinc-phosp... It likely doesn't have much corrosion resistance on it's own, so would likely need an overcoat.

We used to use Alodine before painting aluminum sailboat masts with 2-part polyurethane. But chromate treatments are no longer sold retail, it seems.

I like the idea of leaving the aluminum unprotected, but I suspect corrosion would soon develop on outside edge of rim. The inner and outer exterior surfaces of the wheels/rims were factory painted and are still perfect after 22 years.

swall (Materials) · Answer 4 · 2021-04-29 15:43:56

I once worked on the aluminum wheels on my Jeep Cherokee. They had a painted (gray) center spoke section and the rim was aluminum with a clear finish which was peeling. I stripped the edges of the rims and buffed them to a shiny finish. This sounds similar to your situation and as you have found out, it is a lot of work. I used an air tool, an assortment of buffs and finished off with hand polishing with something like Simichrome polish. I did not go over the polished edge with any sort of clear finish; just left them bare. I got them looking presentable, then sold the vehicle a few months later.

MBGraham (Chemical) · Answer 5 · 2021-04-29 16:04:07

Thanks Swall,
The work I am doing is on the inside of the rim through the bead area. There had been some corrosion and rim leakage. The outside of the rims are still cosmetically excellent, so a shame to abandon them. I did buy aftermarket rims, so no urgent need for the originals unless the new rims develop leaks. But if I ever sell the car, buyer may prefer the originals.

Today, I am going to continue with the sanding and polishing of the bead areas. I will put rims on one at a time on a car in garage and spin the wheels while sanding with several grades of paper until I get an acceptable finish. That will get me back to making a decision on what coating, if any, to apply. Seeing rims won't be used immediately, maybe I will use a zinc phosphate primer followed by a light coating of the marine paint as used on boat outdrives. The rims should then at least look good while in storage :) Paint will have a long time to fully cure and dry!

swall (Materials) · Answer 6 · 2021-04-30 11:19:11

I see. In that case, I would inclined to coat the bead area with your zinc phosphate coating and let it go at that. When I worked in the aircraft wheel and brake business, procedure was to strip the paint during overhaul in order to facilitate penetrant inspection, then re-paint, which obviously included the tire bead area. Of course, the minute movement of the tire during service would chafe the paint over time, so it was a losing battle.

MBGraham (Chemical) · Answer 7 · 2021-04-30 14:00:55

Thanks swall,
I will likely do just that or a start. I do have 6 rims, two of which were spare tires and have never been used. I will coat the other 4 with zinc phosphate. If I do overcoat the primer, it will just be a very thin coat, more for appearance than anything. If I had access to a chromate primer like Alodine, I would probably just use that. Apparently zinc phosphate doesn't offer much in the way of corrosion protection. Just a tie coat for overcoating.

TugboatEng (Marine/Ocean) · Answer 8 · 2021-04-30 14:41:19

It's available in small quantities. Will they ship to you? https://pilotshq.com/products/henkel-alodine-1201-...

MBGraham (Chemical) · Answer 9 · 2021-04-30 16:14:16

They will ship - at a cost! Same is available from aircraftspruce in Canada. Needs an etching solution first. Adds to cost! Then, based on this, it would still need to be overcoated. I only need to cover about 5sq.ft, so even although quantities are small, they are also too big :)

Q: Can Alodine 1001/1201 be used without a top coat and provide long term corrosion protection? Example would be to treat the wing ribs and other aluminum small parts.
This is not recommended for long term protection as the rain or any water/liquid sprayed onto this will wash it off.

I found I can get a Dupli-Color zinc chromate primer in spray cans locally from Canadian Tire. Must be old stock because Dupli-Color website doesn't mention it. Trying to find out more, but it does say for use on aluminum and galvanized surfaces. Otherwise, I can use the zinc phosphate primer mentioned before. Will also need a topcoat. THis is what most vendors suggest now for aluminum.

Jughandle (Materials) · Answer 10 · 2021-05-14 16:33:01

If it was my 1998 and my daily driver, I would smooth out the wheel bead seats as you have already done, paint any exposed metal, and use a tire bead sealant, e.g.,
https://www.amazon.com/XTRA-Sealer-Black-Brush-Qua...

I have also used successfully used RTV in local spots where my VW wheels leaked.

MBGraham (Chemical) · Answer 11 · 2021-05-14 17:09:37

I have completed 2nd to last step. Coated interior of rims with zinc phosphate primer.
I chose a Moeller marine product that is made to be used on outdrives and outboards. There is a similar product by Tempo, but it was not available locally. I compared the Moeller with the Rust-Oleum self etching primer they recommend for aluminum. The Moeller SDS says it has 3-7% zinc phosphate. Rust-Oleum 1-1.25%. So I went with Moeller.

They say to put just a thin coating on. It sprays well and does not run. Masked first one, but found that was not needed. I will apply a Moeller top coat that is also used for underwater drive units. Have to wait at least 24hrs before overcoating, it seems.

MBGraham (Chemical) · Answer 12 · 2021-05-15 19:02:00

Just to conclude. Painted the rims today with Moeller Honda Oyster silver engine paint.

I have new aftermarket rims on car, so these will go into storage until needed.

https://files.engineering.com/getfile.aspx?folder=b95d3ea3-2f0e-42de-b7c5-2

WKTaylor (Aeronautics) · Answer 13 · 2021-05-28 22:22:00

I just asked GOOGLE what 'metal alloy' Mercedes 'alloy-wheels' were made from.

I got one perplexing article:https://www.mbscottsdale.com/blog/why-are-alloy-wh... … which stated the following...

What Makes Alloy Wheels Better Than Steel Wheels?

While alloy wheels are not standard equipment on all new vehicles, you’ll notice many Mercedes-Benz models come equipped standard with Alloy Wheels. Alloy wheels are made with a blend of aluminum, magnesium, and nickel. These metals are superior to steel wheels which are made of – you guessed it: steel! and offer the driver performance, as well as style benefits.

Because alloy wheels are composed of a blend of aluminum, magnesium, and nickel, they are much lighter than steel wheels. A lighter wheel will offer performance benefits including faster acceleration and deceleration, as well as increased fuel efficiency. The mainly-aluminum makeup of Alloy wheels also improves your wheel’s ability to dissipate heat away from the brakes. During harsh winter conditions, keeping heat away from the brakes will reduce the risk of some sort of brake failure.

Hmmm... in aerospace alloys, nickel is considered a 'tramp' contaminate... aggravating corrosion... and providing limited value-added. Also... magnesium even in solution/alloy with aluminum... is a difficult material to protect from corrosion. The stated alloys seem incompatible with the function. What am I missing with what appears to be a proprietary/trade-secret wheel-forging alloy?

NOTE.
I had a 1978 Mercedes-Benz 450SEL [gasoline powered] car for almost 24-years... rims were always dirty from braking dust, but never corroded and never leaking air. I assume these were probably a more conventional aluminum alloy and finish??

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
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MBGraham (Chemical) · Answer 14 · 2021-05-28 23:21:39

Will,
One of the cars I have is an 85 Mercedes. It has what are commonly called Bundt wheels (looks like a German cake!) These were first introduced in 1969. They are bullet proof. Never a problem with corrosion. However, these were forged aluminum alloy. Other later alloy wheels were often castings and some were not that good.

By the way, the very early racing wheels, also used on some classic cars, were magnesium. Often called Mags! Very light. Later as you mentioned, they were aluminum/magnesium alloys and more than likely castings. According to this link, not much magnesium is present in the alloys used these days

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Coat/Paint aluminum alloy rims - or not?