rossnzwpi posted:

… It talks about first pretreating the magnesium casting with a chromate conversion or hard anodic coating ...

... either strontium chromate or strontium chromate containing up to 10% of barium chromate ...

They don't mention zinc chromate or etch priming though.

The document "Surface Treatments for Magnesium Alloys in Aerospace & Defense" is a great document, thanks for providing the link. It seems that information regarding anodizing the magnesium part has some merit.

Strontium, barium, lead, and mercury are all quite poisonous. Those old primer recommendations are no longer viable (at least not in my world). If a chromate primer is needed to best adhere to a chromate conversion coating then perhaps anodizing would be the better "Modern" process to follow. By better I mean it would be less toxic and easier to acquire compatible paints for it. But can a part that has been treated with a chromate conversion be re-treated via anodizing? Probably not.

One thing that has not been explained well in this topic, but is clarified by that document is the simple fact that a zinc chromate primer does not repair the chromate conversion coating when it is scraped-off by curb rash, clip-on weights, etc. That's what Dow 19 was good for. After repairing damaged areas with Dow 19 then a "compatible primer" such as zinc chromate could be applied. I guess Dow 1 was the product used when the entire wheel was being re-coated with a replacement for the conversion coating.

By the way, members have referred to the yellow and green primers sold by "Tempo" as zinc chromate primers but they are not zinc chromate, they are zinc phosphate primers.

That still leaves us with the questions (1) how to repair a chromate conversion coating in the absence of Dow 19, and (2) what modern primers are best compatible with that conversion coating? Marvic is one possible business that may offer advice or guidance. Nate's approach has merit too (i.e. let an expert do it). Problem with that one is finding an expert.

Another possible solution would be a set of replica wheels made from mono-block aluminum billets.  Although I write that jokingly, that is kinda where my head is at.

Hi Nate, thanks for the reply. Yes wheels are are real danger area so maybe you are right to find a suitably qualified refinisher (maybe near an airfield?) to crack test, anneal, refinish.

You can buy touch up kits of DOW chemicals. And yes they are recipes that you could also make up if you were a very competent DIY chemist. I have read that Dow 17 is an anodizing process -- Dow 7 is the more typical chromate process for magnesium. 

 (here's the link to recipes : https://www.finishing.com/faqs/magnesium.shtml)

 Henkel's touch up kit https://www.chemical-supermark...mp;cat=165&page=   or via.  https://www.skygeek.com/henkel...m-treatment-kit.html  ). In the aircraft industry they use mag parts, including wheels and helicopter gear casings  and then do touchups and repairs. 

As for the metal valve stems - I guess they are isolated by rubber seals. There are lots of advisories about how to minimise inserts of other metals since metal to metal corrosion is a major problem with magnesium.

Cheers

Ross

Hi George, thanks for the reply. There are touch up kits for Dow chemicals - Henkel make one and you can get it from aircraft suppliers like Skygeek. One guy I read about here in New Zealand tried in vain to find the right treatment for his motorbike mag wheels and ended up going to an airfield where a helicopter engineer mixed up and gave him some in exchange for a few beers.

The more I read, the more I want to use a modern non-toxic treatment! I've ordered a quart from the USA and will try it on the non-critical parts of my FIAT Dino engine (casings, covers, sump) and see how it goes. At least they don't spin at 100MPH and take all sorts of shocks.

Cheers

Ross

rossnzwpi posted:
I'm considering using a non-toxic pretreatment instead from Pantheon chemicals called PreKote instead.

Have you determined the method you will treat the wheels.

I wonder if the original chromate conversion layer is not removed, will the PreKote fill cracks and voids.

I just had the thought of using the oven heating to 400F to char the paint and thus make it easy to blow off and hopefully leave the conversion layer.   I did find a paper that showed while conversion coatings for alumiun deteriate with heat, magnisium actually improves

(https://apps.dtic.mil/dtic/tr/fulltext/u2/685089.pdf)

Hi David. The aluminum valve stems are Gr-3 parts for a bit lighter weight especially with heavier 10", 13" and 14" wide rear wheels. Some vendors (Hall, Larry Stock) have them on the shelf. They are not expensive but can be damaged by flying road debris. Safer to use slightly heavier chrome plated brass stems if you drive such roads. A few motorcycle shops also stock aluminum stems for lightness, and they're popular with rice-boy wheels in pretty anodized colors like red, blue and green.  But with those you have to be careful on the sizes to fit in Campy's machined recess. And since the nut that retains the valve stem in a wheel is also aluminum, use anti-sieze or you'll be cutting the nuts away on the next tire change. Same goes with aluminum caps on aluminum stems.

Add Reply

Likes (0)
Post
×
×
×
×