Magnesium anodizing processes & coatings, HAE, Dow 17

A. Hi Michael, I think you've created a pretty good list of anodizing methods, and letter 685 addresses the "Microarc Oxidation" method that you mentioned.

Tom Pullizzi's faqs: "Early Pretreatment of Magnesium". Good luck.

A. Dow 17 and HAE anodizing processes have been used on magnesium for some time. Their process parameters are widely available from a variety of non-proprietary sources; including the United States Military Specification MIL-M-45202 [Ed. note: now AMSM45202]. A new class of "spark anodizing"coatings have come on to the market in the last several years. It appears that they are all superior in both corrosion and abrasion resistance to the earlier commercialized coatings and are similarly priced. These new coatings are not, however, covered by military specifications. Commercial specifications, such as ASTM B893, have been written to cover these new hard anodic coatings.

adv.
Based on our own and other independent testing, I believe Magoxid-Coat® is the best of these new coatings, particularly in abrasive wear resistance. Also, unlike some other anodizes for magnesium, it contains no heavy metals. Of course, I am somewhat biased as Luke Engineering is a jobshop applicator of the patented Magoxid-Coat.

A. Dow 17 is an anodizing process -- Dow 7 is the more typical chromate process for magnesium. Powder coat is the typical finish used in the auto industry to protect mag; it has to be totally encapsulated for any pit or void in the coating will initiate instant corrosion. There are a couple of firms offering a colored anodized magnesium process that comes from New Zealand: International Hardcoat in Detroit being one that offers it and I believe Metal Cladding of Lockport, NY as well. To be brutally honest, there isn't a lot of magnesium being used for the simple reason that there is no good means of coating it to prevent the onslaught of corrosion.

A. Considerable efforts are being made to enable greater use of magnesium in automotive. Searching the SAE website gives 7458 results for magnesium corrosion. Mostly Technical Papers, e.g.,

SAE 2000-01-1124 Corrosion Fatigue of Anodized Am50a Magnesium Die Castings

SAE 2001-01-0421 Corrosion Prevention for External Magnesium Automotive Components

SAE 2002-01-0081 Evaluation of Protective Coatings on Magnesium for Phosphate Process Compatibility and Galvanic Corrosion Prevention

SAE 2004-01-0133 Protection of Mg Alloys for Structural Applications in Automobiles

SAE 2005-01-0732 Corrosion Protection of Structural Magnesium Alloys: Recent Developments

Also, the newest update of the old MIL-M-3171 standard:
AMSM3171(October 2005) Magnesium Alloy, Processes for Pretreatment and Prevention of Corrosion On

Purchase and download at www.sae.org

Thank you for the great references, Ken!
Denise, I suggest you order a printed copy and wait for the postman though as the SAE downloads are fouled with DRM technology.

A. Hi, Omran. You might need to do a literature search for authoritative answers to those questions, but in addition to the literature already discussed, you might try www.ecm.auckland.ac.nz/course/cm423/423%20Teaching%20Chapter-2b%20Processing.pdf [Ed. note: Sorry, that link is now broken].

And, if you can possibly find a copy, the old Dow Magnesium Finishing handbook should help.

Good luck.

Regards,

A. Hi Dan,

I have never used aluminum for jigging in a Dow 17 solution. I would expect the solution to attack the aluminum.

In a previous company we used to have copper flight bars with magnesium jigs. The jigs were masked off with PTFE coating and only the contact points left exposed. The magnesium was attacked, and probably lasted only a few weeks before needing changing. The one thing magnesium jigs did for us was not to introduce tramp metals into the solution.

The one favour you could do yourself is to mask off as much of the jig as possible to reduce the area that can be attacked by the solution. It still isn't ideal, but will reduce the attack.

A. Hi Gary. Although there is such a thing as real magnesium wheels, people use the phrase "mag wheels" today similarly to the way they use the phrase "tin foil". Virtually all of them today are actually aluminum alloy because it proved so impractical to keep magnesium wheels shiny. Please see Topic 03, "Coating/restoring Halibrand wheels; history of magnesium wheels".

If they are really magnesium, you probably won't keep them shiny. If they are in fact aluminum alloy, clear coating will probably prove more satisfactory than anodizing. Good luck.

Regards,