Super90

I'm either going to have this new one Ceramic coated or get it treated with black chromate by a fellow that knows our Porsche mag parts well and does great work. He does everything from our car fan housings to 917 engine cases. He did my intake manifolds on the 1970 coupe and they are picture perfect.

R.

gccch

Are you able to inspect for this while the part is still in the car? I have the top fan shroud removed for replacement. Wondering if it is possible to get your hand in there to feel for damage. Or can you see this from the outside looking in? I can't go out and check. My car is a couple hundred miles away.

Metal Guru

I think that if your fan or housing is as heavily oxidized as the one in the picture, then disassemble and inspect, just like you would if you car was an aircraft. If it's ok, then that would be the time to blast and coat.

cobalt

I never had luck with black chromate on mag. My friend had his 935 & 962 wheels done in black chromate and it did not hold up. My friend who does my yellow cadmium for me recommend against it on mag. Essentially it is zinc chromate in black. It doesn’t hold remotely as well as the yellow cad on steel so not sure how it will do much on mag.

Super90

I thought that black chromate was the factory coating? Now Im confused. And the brown coating on older engine cases? Is that not a chromate too?

Thanks

cobalt

AFAIK the black chromate process is the same as Zinc Chromate just in black. It is a chemical conversion that leaves a very small zinc plating that is a sacrificial barrier to protect hardware like steel. The zinc will protect the steel until the coating is eaten through. The chemical conversions or Dow treatments used on mag is a Sodium Dichromate treatment also known as chromium and is an acid based bath. My grandfather worked with Dow chemical pioneering their Dow Metal AZ92AT6 magnesium and the protective coatings used on mag alloys by the industry. I have all of his hand written notes on the subject but his handwriting is a bit rough to read and the papers go back to the 1930's. I never was able to figure out what Porsche used back in the day but I am thinking as with many of these processes it has been outlawed in many countries and can only be performed by grandfathered business that do military work here in the US. My friend does all my Yellow Cadmium plating for me he also does Zinc/black/blue/clear colored Chromate coatings. He recommends the Yellow cad for hardware as it is more durable and will act as a sacrificial barrier but take longer than zinc to break down. Zinc is higher on the list of galvanic series for metals with Magnesium being the highest followed by Zinc while Gold and Platinum are on the extreme opposite end. I am not sure what coating the surface with Zinc will gain you on mag. It will break down quickly and leave the mag exposed. I am not a chemist so this is as far as I can go with it. I did process mag parts through a sodium dichromate chemical conversion for far longer than I should have. When I started they did not list it as a carcinogen and the early MSDS forms said it was safer that it was unfortunately.

If You figure out what Porsche used on the timing chain housings and covers along with the fan housing please let us know. I do find it strange that they did some special conversion for these parts yet painted the fan and valve covers without any chemical conversion first. My friend was working with Kevin Jeannette of Gunnar racing on his 962 wheels they had them black chromate finished and after a year of sitting inside a garage started to break down. You can see the white spots forming. If these were exposed to the elements it would be far worse.

Metal Guru

I don't regard myself as an expert on corrosion, but I did study under a guy who was one of the leading experts in the field when I was in E school (Mars Fontana, PhD. He owned a cool Corvette too . His textbook is still regarded as the seminal source for corrosion engineering and he's been dead for over 30 years!)
He boiled it down to this: there are two types of anti-corrosion coating, sacrificial coatings and barrier coatings. Zinc is used as a sacrificial metal because it is very reactive. It sits near the bottom of the galvanic series chart (gold is near the top or the chart, so it is known as being among the most noble, or stable, metals. Gold is very corrosion resistant). Magnesium sits below zinc in the series in dead last, so if you put magnesium and zinc together, the magnesium ends up protecting the zinc. The picture that Anthony posted of the 935 wheel shows the white Magnesium oxide corrosion that you would expect to see because Magnesium can't really be protected with any kind of galvanic coating. This leaves barrier coatings as the only alternative for corrosion protection. Barrier coatings are not perfect, though. Any kind of coating breach, be it a scratch or pinhole defect will cause accelerated corrosion due to what's called the small anode to large cathode effect. A small exposed patch of Magnesium will be attacked by oxygen in the atmosphere. Water vapor makes it worse.
Currently, Magnesium is being coated with electroless Nickel and plasma-electrolytic oxydation and organic powder coating. Those coatings may be beyond the reach of hobbyists like us but maybe someone could check this out. E-coating was mention too, but I know even less about this.
I actually went the powder coat route seven years ago. I had the fan bead blasted, followed by immediately powder coating. The coating is still holding up, with no evidence of any corrosion occurring under the coating.

dhc905

But to summarize: If you have cracks in your fan (I noticed some on the bases of the fan blades and some delamination at the tips) replace your fan. Keep in mind mine lasted almost 30 years without any special coating ex. what came originally and probably would have gone longer had I not been overly cautious on this front.

I replaced mine with a new part and expect it will last for the remainder of the functional life of the car.

Right?

Metal Guru

Yes. Cracks are bad juju. Things that crack eventually come apart. Having the fan come apart when revving to the redline would be tragic for the engine and maybe even the decklid.

Vandit

There's threads on here and the 993 forum where folks fans have let go, either blades or alternator shafts sheering. It does tear up things pretty bad.

Vandit

Example 1
https://rennlist.com/forums/964-foru...an-blades.html

Example 2
https://rennlist.com/forums/964-foru...tergrated.html

Metal Guru

Yikes.
Thanks for posting, Vandit.
Those failures remind me of jet engine turbine blade failures I used to see when I worked in that business. One blade with a crack fractures and proceeds to wipe out all adjacent blades and vanes. Turbine blades are replaced on a fixed schedule, so it's rare for this to happen in service.

Barty964rst

Fantastic explanation on Mag parts, very concise. When I had my engine rebuilt the fan housing looked perfect, but had the mildest media burst to clean before powder coating. Item opened up and the internal mad was dust. Porsche still sell new ones, had it on order a while but for $1000.00 3 years ago I got a new part. looks great.

almico

It's not that great. Here is the fan from my '96 993. Rotted to hell.

jeff33702

Crap.. I just did the fan in my car and I didn't look really close at the housing, to be honest. I had my hands all over it for the alt wiring and rear cone...nothing stood out but now I wish I had inspected it, specifically.