8mm Forum: Aluminium Corrosion

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Topic: Aluminium Corrosion

Martyn Stevens
Film Handler

Posts: 36
From: Leighton Buzzard, UK
Registered: Jul 2007

posted July 19, 2007 04:29 PM

Many of the old projectors I collect are made of a lightweight cast/die-cast material that I assume contains at least a fair bit of aluminium. I have come across it in machines ranging from the Pathé Baby (projector and camera) to the Siemens 2000 via the Pathé Vox.
All these machines can suffer from a kind of corrosion, a sort of white rust or effloresence. It seems to be caused at least in part by damp. You get a white, unpleasant-smelling dust which lifts the paint off and pits the surface. It is quite different from the well-known Mazac distortion/splitting problem, and is mostly a cosmetic issue rather then anything more fundamental.
Does anyone know:-
-what the material is
-what causes the "rust"
-what is the chemical process going on here
-how I can cure it
-how I can prepare and repaint the surface to prevent it happeming again

Martyn Stevens

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Paul Adsett
Film God

Posts: 5003
From: USA
Registered: Jun 2003

posted July 19, 2007 05:21 PM

Hi Martyn,
Really good to have you posting on this forum.
Pure Aluminum has very good corrosion resistance as the aluminum naturally forms a microscopic protective surface layer of Aluminum oxide from contact with air. Corrosion resistance of aluminum alloys is variable, depending on the alloy. Alloys can corrode to form Aluminum hydroxide (contact with water) or Aluminum chlorides (contact with sea water), as well as by galvanic corosion (contact with other materials in the presence of water). Magnesium alloys are very susceptible to corrosion.
Present day Aluminum alloys include 6061 wrought alloy for machining of parts (extensively used in commercial and military aircraft) and investment casting alloy A356 which can be heat treated to a high tensile strength. The old alloys used on 9.5mm projector castings are anybody's guess and could even be some kind of aluminum/magnesium alloy
In any case the way to eliminate corrosion of any aluminum or Magnesium alloy is by means of 'Irridite coating'. This is a yellow/brown zinc chromate coating applied by dunking the part into a bath. It is applied to the bare aluminum part, prior to painting, and it not only eliminates corrosion but provides an excellent surface preparation for any subsequent painting operations. The irridite coating, military specification MIL-C-5541, is standard procedure for aluminum in all military equipment, and commercial aviation. It is very inexpensive, and any respectable plating house should be able to do it for you for very little cost. Once applied, aluminum/magnesium alloys will never corrode, unless the surface is deeply scratched. So you will first need to get your parts stripped of all existing paint, then degreased, and then irridited, and finally apply the paint which will now stick really good to the irridite surface and will never ever come off.
Another way to protect Aluminum is by anodizing. This is an electrolytic process which deposits a hard coating of aluminum oxide onto the surface. Color can be introduced into this process to get a gleaming , hard, colored finish that will last forever. Anodizing is expensive, and you certainly don't need it for your projector, the Irridite process is all you need to do.
To eliminate rusting of steel parts, the parts should be 'Passivated' per Federal Specifiaction QQ-P-35C, or equivalent ASTM specifications. This is an acid pickling operation which removes free iron molecules (which combine with oxygen in the air to form ferrous oxide - rust) from the surface of the part. Again , a very inexpensive process for any plating house. You can also get steel parts 'electropolished'. This is a reverse plating operation which yields an extremely smooth and polished surface, which is also corrosion resistant.

[ July 19, 2007, 06:24 PM: Message edited by: Paul Adsett ]

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Kevin Faulkner
Film God

Posts: 4071
From: Essex UK
Registered: Jun 2003

posted July 20, 2007 03:42 AM

I also did a little research on this very topic recently due to the paint on my Kawasaki rear foot peg hangers flaking off. This is a very common problem on these bikes and when you get the paint off you find that aluminium oxide has set in just the same way as on your projector.

I "googled" aluminium oxide and found some very interesting stuff to read. One tip I came across was to remove all the oxide which has formed and get the item smooth and degreased etc ready for priming and painting. Then just prior to priming you soak the item in household vinegar for about an hour. Dry off quickly by placing in a hot oven and then get the primer on. The problem is down to airborne moisture so it needs to be treated then primed as quickly as possible.

I read this after doing one of my foot peg hangers where I just cleaned all the flaking silver paint off, cleaned it and then primed and resprayed.

I used this Vinegar method on the remaining foot peg hanger so it will be interesting to see which one acts up first.

I have to say that I much prefer the vinegar on my Fish & Chips [Big Grin]

Kev.

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Josef Grassmann
Expert Film Handler

Posts: 190
From: Hennef-Sieg, Germany
Registered: Apr 2005

posted July 21, 2007 02:19 PM

we found that the white dust is also quite abrasive and bearings worn out.
Maybe we have not the best and not the final solution for this problem. But we can´t strip down a projector and treat the diecast at a reasonable price for customers.
Therefore we suck off the white dust supported by a hard brush as far as possible.
Next step ist to apply a thin layer of oil (spray) on all affected parts. We do it three times with a couple of weeks between each treatment. It cures the problem for years.
EUMIG projectors also suffer very often and very strong from the white dust problem.
It gets much more worst if people store the projector in cellars or upstairs under the roof. High humidity and frozen temperatures speed up the problem.
So we tell our customer to store projectors in bedroom (in plastic bag) if the is no place in the living room.

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Martyn Stevens
Film Handler

Posts: 36
From: Leighton Buzzard, UK
Registered: Jul 2007

posted July 24, 2007 04:48 AM

Thanks for the input on this. I am interested in the aluminium anodising route, because Frost's (www.frost.co.uk), who supply tools and stuff to people who restore classic cars, do a kit for anodising and dyeing aluminium. Expensive to start with, but looks like it could be a lot of fun. They also do kits for nickel plating, zinc, yellow passivate et al, tho not to the standards of the military, I suspect.
The big problem, as Josef observes, is that there is absolutely no economic case for doing anything to any but the rarest of projectors, because they are simply not worth much, so it's all a labour of love. Take the Pathé Baby. By the time you've fitted a new shutter, fitted a new lamp and power supply and new belts, you've already spent almost as much as it is worth, without taking any account of time input. Often they need a strip-down just to get everything working right. Now I like doing all this stuff; I just feel it's a great pity that such superb pieces of engineering are so little valued, where a tatty old bit of furniture to which I would not give house room may be worth thousands.
Sorry, got a bit carried away there.

Martyn

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