Magnesium parts. Factory finish, refinishing and corrosion protection

[Richard Casto]

There are a handful of magnesium parts in Porsches with the most comment being engine cases, transmission cases and cooling fans. I am interested in…

1. What was the process that the factory used?
2. How to refinish but with a focus on replication of the factory finish?
3. How to refinish but with a focus on low cost and longevity at the potential expense of replicating the factory finish?

I know that there has been quite a bit of discussion on the forum about anti-corrosion protection being used by the factory with Tectyl being the main topic. There have also been discussions about other items to use to protect magnesium parts with the use of the Gibbs brand lubricant being one. Others will polish and then paint with a clear coat or even powdercoat. I am looking to expand the discussion beyond those items. One thing that I am currently curious about is if the magnesium parts had some type of process applied prior to the application of the Tectyl product.

I have a theory that after the parts were cast and machined that a chromate process was used to provide a level of anti-corrosion. This process may have been to help long term or maybe just for the short term (between casting and assembly). I believe there is some evidence that some type of process other than Tectyl was used. I have seen mention of a “pickle” being used on the transmission cases (chromic acid pickle bath?). That if you clean the case to be careful that you don’t remove this coating via aggressive chemicals and/or mechanical means.

I have a spare differential cover plate from a 914 transmission and on the interior there appears to be a classic slightly iridescent gold/tan color to the metal. This is the type of color you would see when something has been plated. This color extends outside of the area that would be bathed in transmission oil so it is most likely not oil stain. The parts of the plate that have been exposed to the elements does not show any evidence of this. My theory is that this part had a chromate process applied to it’s entire surface. It was then assembled as a working transmission. The entire transmission was then coated with Tectyl. Over the years the Tectyl has worn off. This left the very thin chromate layer. This was then quickly worn or corroded away as it provided even less protection than the Tectyl to harsh elements. This left bare magnesium which either formed a thin oxidation layer and/or was covered/protected by caked on oil and grease.

Related to this is “how to clean” a magnesium part. I have heard everything from acids (yes it will clean it by eating away part of it) to soap and water. It seems that “Simple Green” is popular, but I think that even it works as well as it does due to some level of acid. So while many methods will “clean”, it sounds like most of those also work by eating away part of the magnesium and this would include any remaining chromate conversion layer as well as affect mating surfaces.

My selfish reason is that I would like to clean and refinish my magnesium transmission case using the factory method as closely as I can. If it turns out that I am correct about the chromate conversion process, I am afraid that I probably can’t use that option, but it would be nice to know from a historic perspective what was done. Chromate conversion for magnesium commonly uses hexavalent chromium. This is the chemical that was feature in the movie “Erin Brockovich”. While I believe you can still have this done, most industries that use magnesium have looked to move beyond this and to newer more EPA friendly methods. My understanding is that many (all?) of these methods would result in a coating that has a different color than you would get with the chromate process. Even then these processes are usually just a precursor for some other coating with paint being a common one and for my transmission Tectyl or something similar.

So with all of that being said, does anyone know anything about this beyond the Tectyl coating? How do you clean your parts? What do you do (if anything) prior to reapplying Tectyl when you are done?

[jluetjen]

Here's a few threads on the "Engine Rebuilding" forum regarding this subject.

Welcome aboard Richard!

[911s55]

Here's some inflamation on magnesium re-finishing. Henry Schmidt- help please

David

[Grady Clay]

Welcome.
I see you have done some 'homework'. Good job.


Yes, there was a surface treatment after all the machining. I used to know the technical name and I know how to find it again.

The current spare part magnesium 7R crankcase does not have that surface treatment and is ‘shiny silver’ color.

The ‘proper’ color is a dull gray with a hint of brown.

I’ll find the name of the treatment in the next few days.

As to cleaning, I like regular Stoddard solvent and a bristle brush.
It is time consuming but doesn't damage the finish or surface treatment.

Best,
Grady

[Richard Casto]

Jon and David,

Thanks for the links. Some of those threads I had not seen before. If I was to summarize what I learned from those is that for fans and valve covers most people use an acid based cleaner. This eats away some of the magnesium, but it does bright it them! If the ultimate goal is to make it nice and shiny (silver) then I can understand doing this. I would suspect that if any of these did have a chromate process applied that the acid would remove it. But I don’t know if fans or valve covers ever had this done or not. It seems that Mineral Spirits and Diesel fuel are also popular.

At least one of those threads mention of using “Alodine” as well as “Iridite” which are both brand names for conversion processes. Alodine is produced by Henkel and there are formulations for different purposes. If I am reading the info on Henkel’s site correctly Alodine 1201 is a chromium based product for use on aluminum while Alodine 5200 is a chromium free product for use on aluminum (I am guessing the Alodine 5200 is newer and was created to specifically avoid the hazards of working with the chromium based formulas). I “think” that Alodine 5900 would be the chromium based product for use on magnesium, but I am not sure. It seems that the DIY aircraft people do enough work with aluminum that Alodine 1201 is packaged and available small quantities (such as at http://www.aircraftspruce.com ). But the major consumer of these types of products are not DIY types, but for industrial use. So I have yet to find a source for Alodine 5900 (which I am not even sure if that is what I need). I have not yet called or emailed Henkel to see if they have any suggestions for a source willing to sell in small quantity.

Richard

[Richard Casto]

Grady,

Thanks for the info. Your posts regarding Tectyl have been a great help! While checking your records/sources, see if you can determine if Porsche used different processes/methods for magnesium vs. aluminum. I think that some conversion processes may work on both, but that it is likely that they used one for magnesium and another for aluminum. The dull gray/brown on magnesium comment makes me wonder if it was Dow #1, Dow #7 or Iridite #15. All of which are chromate processes.

Here are two links that describe some of the processes used on Aluminum and Magnesium, what the resulting finish looks like, level of production and affects on the base metal…

http://www.finishing.com/faqs/magnesium.html

http://www.cygnetstamping.com/coating.html

Two extra questions…

First, you mentioned Stoddard Solvent. Is this is the same thing as Mineral Spirits?

Second, I know the Pelican no longer caries Tectyl due to shipping hassles (if I remember correctly). Do you know of a good source these days?

Richard

[djpateman]

http://www.temarry.com/chemicals/stoddard_solvent.htm
Stoddard solvent is a petroleum mixture that is also known as dry cleaning safety solvent, petroleum solvent, and varnoline; its registered trade names are Texsolve S® and Varsol 1®. It is a chemical mixture that is similar to white spirits.

[Wavey]

Richard, there are a couple of Ditzler/PPG automotive products you might check out. Alumiprep 33 is an acid based cleaner wash for aluminum. DX501 Aluminum Conditioner is a conversion wash to be used after the acid wash, and it leaves a light orange-ish finish, at least on aluminum. I tried both on a 911 valve cover. The wash left it a medium gray, and the conversion didn't change the color enough to be noticeable. The conversion is definitely a chromium product. I believe the piece can be left unpainted after the conversion.

[304065]

Richard, here's the thread I started about my mag manifolds, you may have seen it already.

What finish on these magnesium manifolds?

As far as the mag manifolds, I am coming around to the idea that it was NOT a ferric nitrate bright pickle, but rather cadmium with a yellow chromate conversion coating. If you look closely at my early manifolds and the solex ones, there's the telltale red-green-blue twinge to the yellow coating. That suggests to me the coating was the same as certain of the engine bolts.

As far as race engines, the ones I have seen are probably NOT Alodine or Iridite, but rather Dow 7-- this gives a brown finish very similar to what you see on 917 engine cases.

[Richard Casto]

Originally Posted by djpateman

http://www.temarry.com/chemicals/stoddard_solvent.htm
Stoddard solvent is a petroleum mixture that is also known as dry cleaning safety solvent, petroleum solvent, and varnoline; its registered trade names are Texsolve S® and Varsol 1®. It is a chemical mixture that is similar to white spirits.

Thanks for the info. I had seen mention of Varsol somewhere, but didn't realize what it was. I initially asked about Mineral Spirit vs. Stoddard solvent because when Grady mentioned Stoddard Solvent, I Googled it and I was seeing a lot of interchangeable use of Mineral Spirits/White Spirits/Stoddard Solvent. I am wondering if maybe Mineral Spirits is a somewhat broad term and something like Stoddard Solvent is a "type" of Mineral Spirit??

[Richard Casto]

Originally Posted by john_cramer

Richard, here's the thread I started about my mag manifolds, you may have seen it already.

What finish on these magnesium manifolds?

As far as the mag manifolds, I am coming around to the idea that it was NOT a ferric nitrate bright pickle, but rather cadmium with a yellow chromate conversion coating. If you look closely at my early manifolds and the solex ones, there's the telltale red-green-blue twinge to the yellow coating. That suggests to me the coating was the same as certain of the engine bolts.

As far as race engines, the ones I have seen are probably NOT Alodine or Iridite, but rather Dow 7-- this gives a brown finish very similar to what you see on 917 engine cases.

John,

Thanks for your reply. I had forgotten about the magnesium manifolds. I need to post a photo of my differential cover plate so you can see the color. It is similar, but yet different than what is shown in your photos. But it is clear that you are not seeing bare magnesium. That some type of conversion process and/or plating was done.

Richard

[304065]

Here is a picture taken by Carl Thompson of a 917 engine



Here is one of Armando Serrano Jr's pictures of a 906 engine



Looks like Dow 7 to me.

[Grady Clay]

Guys,

Bruce A and I discussed this a few years ago, prompted by the Factory delivering new spare part mag cases without any treatment that we could tell. They were ‘silver’ color and close to the freshly machined surfaces.

His recollection (today) is it was a Dow process, now illegal. I suspect there was a worker exposure issue or the difficulty of properly disposing of the waste.

I suspect it could be duplicated on an individual basis.

Best,
Grady

[jcge]

Summary of some magnesium surface finishes

Note - Appendix III - chromate finishes (like DOW 7) are not protective in their own right - they serve as a base for resin sealers and paints. (Tectyl will behave as a wax sealer on the chromated surface)

www.magnesium-elektron.com/data/downloads/DS256SU0.PDF

Regards,
John

[Richard Casto]

Here are some photos of a diff cover plate I pulled from my parts stash. The cover seemed pretty corroded, so I am going to use it as my refinishing experiment…









In the close up photo of the outside, you can see just a bit of the gold iridescence deep in the recess where it is likely oil and grease has allowed some remnants of the coating to remain. In the close up photo of the inside, you can see much more including some outside of the o-ring seal.

I am trying to find some photos online (via Google image search) of examples of Dow 1, 7, 9 and 19 in an effort to determine what was used on the transmissions.

I have found that Henkel does produce a Dow 19 product known as “Magnesium Treatment Kit”. Dow 19 is designed mostly to be a touch up process and this kit is packaged as such. If I was to attempt a DIY reproduction of the chromate finish, this might be the easy way to do it. However it uses Chromic Acid (contains hexavalent chromium) and I need to determine how I would dispose of material left over (rinse water, left over chromic acid, etc.). I just don’t know if it will be worth the hassle. It’s also over $100 for a small kit.

http://www.aircraftspruce.com/catalog/cspages/magtreatkit.php

[Richard Casto]

I forgot to say thanks in my post above for the great info!

Also, does anyone have any comments regarding media (glass bead) blasting magnesium parts? I know it would remove any remaining chromate conversion, but other than that are there any issues?

Richard

[jcge]

Originally Posted by jcge

Summary of some magnesium surface finishes

Note - Appendix III - chromate finishes (like DOW 7) are not protective in their own right - they serve as a base for resin sealers and paints. (Tectyl will behave as a wax sealer on the chromated surface)

www.magnesium-elektron.com/data/downloads/DS256SU0.PDF

Regards,
John

Richard - page 3 of the PDF covers blasting media

Regards

John

[Grady Clay]

Guys,

I’m going to experiment with media blasting some magnesium transmission cases. One thing I am NOT going to do is let any media get inside the casting.

I found rubber-with-steel-washers plugs designed as emergency replacement for the ‘freeze plugs’ in water cooled engines. They work perfectly sealing the openings for the stub axle, input shaft and shifter. I’ll just use an old shift pivot and gasket on that opening.

Solid CO2 (dry ice) might be the media of choice as it doesn’t leave any residue other than the metal. I don’t care how careful and through a cleaning is, glass beads and chards inside a transmission or engine are not something I am even remotely comfortable with.

Back to the issue of surface treatment. The original treatment (Dow something?) was with a fresh surface just out of the pressure casting mold. I suspect the process was multi-step involving cleaning the surface first. I’ll bet there was no media blasting.

This brings us to the issue of applying some surface treatment after some media blasting. I’m not convinced this is a good idea.

Convince me.

Remember, the Factory Workshop Manual (Vol. I, page SR 140) specifically specifies “Tectyl”.

The section says:

“Warning:
Pressure cast parts must not be cleaned with acid, since acids attack the magnesium alloy. After any transmission repairs during which the one-season underseal is washed off the gearbox housing, or when the underneath of the vehicle is washed, we recommend that corrosion protection be improved on the pressure-cast gearbox housing by applying a bitumen or wax based one-season underseal, e. g. Tectyl, which should be sprayed or brushed on.”
© Dr. Ing. h.c. F. Porsche Workshop Manual, Vol. I, Page SR 140.

It is noteworthy that Porsche chose the word “Warning”.
It is also noteworthy that Porsche has used Tectyl (wax based) for almost 40 years and continues to this day.

Best,
Grady

[Richard Casto]

Originally Posted by Grady Clay

Remember, the Factory Workshop Manual (Vol. I, page SR 140) specifically specifies “Tectyl”.

The section says:

“Warning:
Pressure cast parts must not be cleaned with acid, since acids attack the magnesium alloy. After any transmission repairs during which the one-season underseal is washed off the gearbox housing, or when the underneath of the vehicle is washed, we recommend that corrosion protection be improved on the pressure-cast gearbox housing by applying a bitumen or wax based one-season underseal, e. g. Tectyl, which should be sprayed or brushed on.”
© Dr. Ing. h.c. F. Porsche Workshop Manual, Vol. I, Page SR 140.

It is noteworthy that Porsche chose the word “Warning”.
It is also noteworthy that Porsche has used Tectyl (wax based) for almost 40 years and continues to this day.

Ok, so it sounds like any chromate was most likely just to protect the parts during manufacturing and the Porsche really only counts on the Tectyl for long term protection. And that unless you were trying to recreate "new from the factory" there probably is no reason to try to re-chromate a transmission or engine case. So I am pretty sure I am going to give up on trying to re-chromate my transmission case.

When I get the time, I am going to do some tests on my diff cover plate. I probably will clean different sections with (1) soap/water (2) simple green (3) stoddard solvent and (4) media blasting (with fresh/clean glass beads). Then coat different sections with (1) Nothing (2) Gibbs brand lubricant and (3) Tectyl. Then let it sit outside in the weather for a few months.

The only thing holding me back is that I don't know of a good source for small quantity of Tectyl. Anyone know of a source?

[tijnes]

From my research "chemical film" (dow 7 for magnesium, macdermid 14-2iridite for aluminum) treatment does provide protection against corrosion.

"Most chromate films are soft and gelatinous when freshly formed. Once dried, they slowly harden or “set” with age and become hydrophobic, less soluble, and more abrasion resistant. Although heating below 150°F (66°C) is of benefit in hastening this aging process, prolonged heating above 150°F may produce excessive dehydration of the film, with consequent reduction of its protective value. Coating thickness rarely exceeds 0.00005 in., and often is on the order of several micro inches. The amount of metal removed in forming the chromate film will vary with different processes.

Chromate conversion coatings can provide exceptionally good corrosion resistance, depending upon the basis metal, the treatment used, and the film thickness. Protection is due both to the corrosion-inhibiting effect of hexavalent chromium contained in the film and to the physical barrier presented by the film itself. Even scratched or abraded films retain a great deal of their protective value because the hexavalent chromium content is slowly leachable in contact with moisture, providing a self-healing effect"

I have a pair of "new" Webers that have the original "chemical film" treatment and provided they have not seen much exposure they sure look good after 30+ years. I would certainly consider getting the dow 7 treatment done on magnesium parts.

The attached link might be a lead to find a company that can still do Dow 7. http://www.finishing.com/194/52.shtml



Martijn