I want to refinish a set of Cromodora wheels with1985-86 casting dates. Has anyone determined the casting method and alloy used? There are a lot of issues with refinishing magniesium alloys and I want to do it right before I mount new tires. One wheel has a nick on the rim edge, likely do to handling and may need a small weld to rectify. Knowing the actual alloy and manufacturing method would be helpful. Powder coating seems to require temperatures of at least 360 F to outgas and melt the resins. This is getting close to the aging temperatures of AZ (Aluminum and Zinc alloying metals - AZ91 is 9% aluminum, 1% zinc) magnesium casting alloys which start at 420 F. I'm not sure what this means for 40-year old castings but it may lead to hardening. I was surprised there was no anealing temperature shown but apparently anealing is not relavent for these alloys. I did find out that a particular alloy can be identified by X-Ray Fluorescence (XRF), a non destructive test that requires at least 1 square inch of exposed metal. I was quoted 150 to do the test and 200 to interpret the results by a metallurgical test house in Texas, a long way from CT. $350 plus shipping seems reasonable to settle the question. Anyone want to share the cost? Then there is the adhesion and corrosion protection issue. Historically, magnesium alloys required pickeling in a chromic acid bath as defined by the DOW7 process in MIL-C-3171, Type VI and now by SAE AMS-M-3171, Type VI. I was quoted $200/wheel by a local outfit. Flight line repairs for exposed metal used a brush on or spray on process defined by a DOW19 process. DIY kits are available starting at about $180 but I'm not sure how much product would be necessary to treat 4-wheels if stripped bare. It may be better to just spot coat exposed metal prior to spot overcaoating. It is not clear if these chromate conversion coatings can tolerate the 360 F+ temperatures of powder coating. I'm told a proprietary DOW19 replacement called Tagnite is good to 425 F by its manufacturer. Again, the purpose of these coatings is to protect the base alloy from corrosion and promote over coat adhesion. I did find this kit that looks pretty comprehensive: https://www.chemical-supermarket.com/Magnesium-Preparation-Treatment---Coating-Kit-p703.html I visited a local magnesium foundry and asked if they could help. They told me they only deal with the bare metal and ship aircraft wheels and other products with just an oil coating. So no real help.
There's a few things to read that might be of help, one is this thread: https://www.ferrarichat.com/forum/threads/wheel-paint.301091/ Another is this from Steve Kouracos: https://stevekouracos.com/restoration/tips-and-tricks/aluminum-magnesium-wheel-restoration/ And maybe this too: http://dino246blog.blogspot.com/2013/12/go-biggs-or-go-home-achieving-exact.html?m=1
Thanks for the links dyerhaus. I'm trying to get a handle on the underlying metallurgy and its suseptibility to high temperature due to powder coating and its long term adhesion and corrosion protection. I'm less concerned with color.
I've heard of people who've got them powder coated, wouldn't surprise me if I know someone who's done it, so I'm guessing they can handle that. Although it's not something I would do (unless I never drove my car). With painting, when the inevitable scuffs, rash, and chips occur, you can simply touch them up. Can't do that with powder coating (or at least not easily and quickly). Maybe this will help, this is a response from Steve Kouracos to a comment asking about powder coating from one of the links I sent earlier: "Powder coating is fine if your wheels are in perfect shape with no dings or nicks in them. If you fill these dings and nicks with filler and then send them to the powdercoater the heat will bubble the filler. If your wheels need repair in any way it’s much better to coat them wet. The paints and clears now a days are very durable. Another tip: all your prep on your wheels can be done with the tires off – including primer. But before you paint them mount the tire just don’t air them up, mask off the rubber, base coat and clear coat the wheels, pull the paper off the tires and air them up."
Find a mag wheel refinisher .Normal powder coating process as you infer leads to gassing out .This manifest itself in tiny star shaped creations appearing under the paint a few weeks later . They need that special aircraft grade green primer ( see the links ^^ ) applying correctly initially before the final surface finish . It’s the only way there’s no quick ( AKA cheapo ) fix to his .
Pic paint a 1000 words . Image Unavailable, Please Login Image Unavailable, Please Login He tried 2 x 2nd time diff temps to first ^ . The degassing occured a few months later .On the day / week of collection they looked fine . Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Then paid to send them to a U.K. mag wheel specialist….^^^ They have not changed over the yrs .
Thanks for the input guys. Portofino, your photos show a particularly bad outcome. I talked to a couple of powder coaters, including one that just did a set of Lamborghini wheels for Wayne Carini. They both pointed out the necessity of prebaking the wheels. This would have the effect of heating trapped gas, expanding it and driving it out of the casting porosity, if maintained for sufficient time and temperature. This would likely also evaporate any liquid cleaning fluid trapped in the pores and drive it out too. If the the wheels were then immediately coated with powder and returned to the oven while still hot, residual trapped gas should should shrink upon cooling, resulting in a low pressure cavity that is less than atmospheric pressure, resulting and no outgassing. So it could work if done properly. What is not addressed adaquately IMHO, is the adhesion and corrosion issue. I found this simplified advice on the vintage bonanza web site for painting magnesium alloy flight surfaces: https://www.vintagebonanza.com/prep_magnesium_for_painting.htm I wonder what process the Navy would use to repair exposed metal on say a carrier based F-18 or F-35B/C wheel?
Bill - this may provide some helpful additional perspectives, ideas and techniques: https://pantera.infopop.cc/topic/refinishing-campi-wheels
Thanks Joe. I found this post by Tom for reference: The uploaded files are from Materials Engineering, annual Materials Selectors from 1991 and 1993, my go to source when we didn't have internally generated data. The pages show information on magnesium casting alloys of the day. Note that there is no anealing temperature shown but there is a Stress Relief Temp shown of 500 F. Aging temps start at 420 F and I'm still not sure what that means. My current understanding is that aging is a hardening process but I'm yet sure what it means for our case. You can see that weldability depends on the specific alloy so if dings need welding, it's good to know the actual alloy. I found this post by TonyT4 in the Vintage (thru 365 GTC4) forum concerning 275 GTB Campagnola Wheel Alloy. He suggests the ally used may be a variation of AZ91. I wonder where he got this information from.
The key component to a successful outcome here is the chromating process prior to paint . Very technique sensitive, surface prep , oven temps at various stages ( already touched upon ) timings and of course the actual “ chromate “ substance itself . Nothing is un DIY er able first time without research but this is my view is better left to guys who do this for a day job , day in day out . The problem is issues like gassing out don’t necessarily manifest early .So your wheels could be sent back by a regular wheel guy who’s done his 2 cents of research prompted by you …..looking fine , or indeed you could post “ looks at this incredible job guys “ pics a week later of a job done in your back yard yourself . Additionally as has been said ^ I understand there are issues of embrittlement inherent with aged , heated gassed out 70 s mag alloys . If I remember correctly ( going back to 2018 ) mine were x rayed looking for hidden cracks as part of the quality assurance process after painting .
Another option, talk to these guys, https://queencityplating.com/ I reached out to work with them on an F40 restoration that required all the magnesium cast parts to be stripped, neutralized and conversion coated prior to final finish coat. However in the end the owner elected to not take up the project. I did ship them all the parts for inspection, etc and they looked at all the options to undertake the project. And yes, it's expensive to properly deal with cast magnesium alloys.
Portofino, I believe you are correct in identifying the chromate conversion coating as being the key process prior to paint. Have you done the process before? Edward96GTS, It seems to me that chemical stripping, abrasive blasting, or both would be sufficient to remove powder coating. I think it's just an organic (plastic) resin coating isn't it? smg2, What did you eventually do to refinish the F40 magnesium parts? In addition to X-Ray Fluorescence (XRF) testing there is also the more sophisticated X-Ray Diffraction (XRD) testing which can reportedly identify alloy phase changes in addition the alloying element fractions. I'm looking for someone locally to do the test otherwise I may bite the bullet and ship a wheel to Texas. In a phone call some time ago, BigTex advocated a conservative approach of just touching up the bad spots and leaving the bulk of the finish intact, IIRC. Not bad advice.
Nothing, the owner elected to store everything until a later time. Happens more often then one would think. Interestingly I reached out to and spoke with a few restoration shops in Europe that specialize in the F40, nobody that I could contact has ever undertaken striping and recoating the cast magnesium parts. It's less expensive and troublesome to source the few parts out there. And by now, those are pretty much NLA. Hands will be forced to deal with magnesium corrosion... Not fun.
No not personally.I took them initially to my regular wheel guy .( who I have used a lot prior and since and so does all the hight end traders around ) He immediately knew the real mag issues .Spoke to a colleague ( rival competitor) .As I posted , those pics of the “ creations “ are 6 months after his second ( FOC ) attempt. The first attempt the “ crenations “ came out , or started to come out after about a week very slowly one or two per day randomly scattered . In the end he funded sending them away in the trade to a London based mag wheel team ,with x ray facilities btw .Which turned out a very important step earlier omitted for obvious reasons . So cost wise I paid £90 per wheel .Done twice by my guy .The second team charged £340 per wheel + carriage etc .My first guy stood this .I did offer to help with this but he insisted due to the aggro and car off the road time plus I guess a measure of reputational embarrassment ……….there should be no extra cost to the customer etc etc . A lot of lesions were learnt along the way to getting it right .Financial and practicality. All back in 2018 ….FWIW vaguely from memory his chromate job ( second attempt) failed because the wrong chemical was used to remove the first jobs new paint .Soaked into the porous mag alloy despite oven etc was retained and came out . His first attempt was done without chromate ( researched like you ) …..some supposedly super primer technique that reckons to hold the gas in ……the oven temp thingy you have touched upon ^ . Not exactly sure of the paint strip + oven regime or chromate procedure or paint and filler build up or indeed the final surface finish of the £340 per wheel guys . Good luck what ever route you choose .
Dow 17 per Mil M-45202. This was the original greenish coating. This process uses strong acids, which are horrendously expensive to dispose of. The aircraft parts processing shop I used for the DOW 17 coating stopped for that reason. TAGnite (Technolgy Applications Group) in ND has a proprietary Mg coating that satisfies and /or excels DOW 17. I have used TAGnite services with excellent results. They are great to work with. I have not had the need for TAGnite on wheels (years ago, as the source for the DOW 17 was available), but many other Ferrari Mg engine parts. The best approach imho from experience, is to not disturb the original coating with mechanical stripping such as blasting. The delicate cast features like "CROMODORA" & wheel size get worn down. The last wheel re-finishing I had done was to use chemical stripping. It was brush-touched with an aircraft supply Mg touchup for nicks where bare Mg was seen. IIRC, it was made by Henkel Loctite, and I purchased it from Skygeek.com. It was essential to convince the painter to NOT sand or otherwise abrade that greenish coating! Important to not take this venture lightly. Mg needs special treatment.
2dinos, DOW7, the dipping process, is the OE level process of choice and normally requires mixing and handling of toxic chemicals. Locally, I was quoted $200/wheel as DOW19 is the brush on treatment meant spot repairs, but can also be used to treat the entire wheel. Please reference post#1. DOW17 is different. It is an anodize process, not a chromate conversion coating. The Dow Chemical Company invented Dow 17, the first anodized magnesium coating, in the mid-1940’s. At that time, Dow Chemical Company was the world’s largest producer of magnesium. Dow 17 can be applied with either alternating or direct current. The electrolyte used to apply the coating is comprised of sodium dichromate, ammonium acid fluoride and phosphoric acid. The electrolyte has a pH of approximately 5 and should be operated at or above 160° F. The coating has little or no inherent salt spray resistance. On test plates made from alloys such as ZE41A, corrosion sites can be seen in as little as 48 hours. Dow 17 does provide good paint adhesion with the exception of migration from a scribe in salt spray. Corrosion can easily migrate in the very porous Dow 17 structure, even tunneling under the paint. A suitable DOW19 kit can be found here: https://www.chemical-supermarket.com/product.php?printable=Y&productid=703 I don't yet know how much surface area this $587 all inclusive kit can treat. A bare bones kit can be had for $179 and can be found here: https://www.chemical-supermarket.com/product.php?productid=421&cat=165&page= I sent out one of my wheels, with a date code of 1984, for XRF testing. This will identify the magnesium alloy used.
These are the results of XRF testing of one Cromodora 7x16 wheel with a casting date of 1984, compared to 3-candidate alloys: The Avg Weight % was measured by XRF. The test house concluded the alloy was AZ81A, but with diminished aluminum content.
I inadvertantly double posted the chart. Would a moderator kindly delete the large font version as I am unable to delete it.
I did some follow up research on AZ81A. From the ASM Metals Handbook, Volume 2, Tenth Edition, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials AZ81A chemical composition 7.0-8.1 Al 0.4-1.0 Zn 0.13 Mn max 0.10 Cu max 0.01 Ni max 0.30 max other (total) Balance Mg Consequence of exceeding impurity limits. Excessive Si causes brittleness. Excessive Cu degrades mechanical properties and corrosion resistance. Excessive Ni degrades corrosion resistance. In any of the magnesium-aluminum-zinc alloys, an increase in aluminum content raises yield strength but reduces ductility for comparable heat treatment. Weldability. Gas shielded arc welding with AZ92A rod, very good relative to other magnesium alloys. AZ81A is also known as: ASTM. Sand castings: B 80. Ingot: B 93. Permanent mold castings: B 199. Investment castings: B 403 SAE. J465. Former SAE alloy number: 505 UNS number. M11810 Government. Permanent mold castings: QQ-M-55 Foreign. Elektron A8 British: BS 2970 MAG1 (superseded by BS EN1753) German: DIN 1729 3.5812 French: AIR 3380 G-A9 From the ASM Metals Handbook, Volume 4, Tenth Edition, Heat Treating. Heat treating of nonferrous alloys. AZ81A as cast results in a T4 temper (AZ81A-T4) via solution heat treating. This consists of loading the casting into a thermal chamber at 260C (500 F) and then linearly bringing the temperature up to 413 C (775 F) over a 2-hour period. A carbon dioxide or sulfur dioxide protective atmosphere should be used above 400 C (750 F). To restore T4 temper after weld repairs using AZ92A or AZ101 rod, the following heat treatment is specified: ½ hour at 413 +/- 6 C (775 +/-10 F) in a carbon dioxide or sulfur dioxide atmosphere. Heat treatment of all Mg-Al-Zn alloys will provide stress relief without significantly affecting mechanical properties: 1 hour at 260 C (500 F). Based upon the above process temperatures, it seems likely that an hour or two at a powder coat temperature of about 400 F will not significantly affect the mechanical properties of the weld alloy.
Very interesting thread!!! But the discussion starting to get way beyond my technical understanding (re: metallurgy). Nonetheless, I’m intrigued by the discussion because I had to repair one of my Cromodora wheels on my 85 QV about 4 years ago after a brake caliper leaked. Repair was concentrated to inner barrel only and I generally followed Steve Kouracos method to repair. 4 years later now (with street use!) and no blemishes…worked like a charm. My thinking was if it didn’t work, I’d probably have to send all my wheels out to get redone professionally (for thousands). So it was worth a DIY try I thought…and it wasn’t in very visible location. Turned out better than I hoped. I guess my question is whether it’s worth going through all the trouble with powder coating effort/problems, etc if the car is going to be driven. Rock chips, etc aren’t easily repairable…so paint and clear just make more sense to me. If the car is a show car….alright…I understand the added effort and expense. Again, I appreciate the discussion, I’m not trying to throw flames on it. Just trying to understand the fuss if there are other tried and proven methods out there. Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
I just found out one of my rears has what is likely a crack or at the very least a casting defect. I viewed the area on the back side with operating microscope and at about 50X could see what appeared to be a crack running through a line of very small surface bubbles. Zyglo testing seems to confirm a crack. I need to investigate welding and heat treating now. Here is some more info that will be useful for repair work like welding. I'll want to restore T4 properties so I'll need to find someone who can heat the wheel to 775 F for an half hour in a CO2 atmosphere. This will also stress releive the metal which occurs at 500 F. Alternatively, anybody have an 8-inch wide Cromodora for sale? . Image Unavailable, Please Login
Tom Shaughnesssy who was quoted earlier in the thread would be my 1st call. Though he does less of it now his nickname used to be The Wheelman.
I'd advise against commercial paint strippers. The chemical gets into the porus metal and isn't easily removed prior to coating. My body shop made the mistake of stripping my rims prior to painting, even though I specifically told them not to and asked them to just sand them. I found out after they had already sent them to the guy who does their stripping & powdercoating. Well, as I feared, they came back, were prepped and painted, and there were issues. Fortunately not too bad, but they definitely turned an excellent set of original rims into driver-quality ones. Anyone have a lead on a really nice set of '79 14" rims?
Hard to say it is worth it yet. I have 4 fronts and 2 rears. All wheels have their issues. One is a process sample because there is a large weld repair that is porous and badly finished. So far, that wheel has been chemically stripped which only removed the silver paint-not underlying primer. The remaining wheels were just bead blasted. The sample stripped wheel went through a Sanchem process that consists of: 1. apply wash/surface activator and rub with scotchbrite. Rinse with distilled water. 2. apply conversion coat 1 and maintain wetness for 15 minutes. Rinse with distilled water. 3. apply conversion coat 2 and maintain wetness for 15 minutes. Allow to air dry These are the Sanchem chemicals to conversion coat magnesium Image Unavailable, Please Login The container on the left contains the waste for one wheel. I used less than 1/2 gallon of distilled water as shown in the middle container. Image Unavailable, Please Login Apply J-B Weld original epoxy (which is good to 550 F) to fill and contour defects. Smooth as necessary. Let cure at room temperature for 24 hours. Proceed to powder coating or painting. If powder coating: Prebake for 30 minutes at 360 F to dry and drive out any vapor/gases. Powder coat at about 360 F. I think powder coating is a less labor intensive process than multiple paint coats. I would use the same Sanchem process as a base for painting. The conversion coat provides corrosion protection and promotes overcoat adhesion. I would think Dings and scratches on a powder coated wheel could be touched up with converntional paint. Thanks for the lead.
