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Metal compatability problems?

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Cornish Jack04/07/2010 17:45:58
1219 forum posts
171 photos
Hi
I have to repair a roller blind shutter which has fractured the mounting lugs. The headbox ends and the  mounting lugs APPEAR to be made of Mazak or similar and I cannot see any way of repairing the fractures. However I COULD pop rivet metal brackets (Dexion offcuts) onto the ends and screw mount these to the wall.
Question 1 - does Mazak react to close contact with steel?
Question 2 - Would the above combination be more likely to corrosion in a VERY salt laden atmosphere? This last is prompted by the state of the headbox ends after a year or so in situ - powdery surface deterioration. 
Any suggestions/comments welcome.
TIA
Rgds
Bill
Jeff Dayman04/07/2010 20:20:46
2237 forum posts
47 photos
Hi Jack,
 
The steel will likely cause galvanic corrosion in contact with Mazak unless you use silicone grease or a piece of PTFE between them. It may corrode at the rivets though.
 
Aluminum sheet and rivets would corrode less than steel but still will corrode eventually.
 
A better if not easier idea would be to make new brackets out of Delrin or other acetal. Just as strong or stronger than Mazak and will not corrode at all, ever.
 
Or, use your existing Mazak parts as patterns to sand cast marine bronze fittings (probably way over the top, but definitely high quality!)
 
Just my 2 cents.
 
JD
Cornish Jack05/07/2010 15:02:47
1219 forum posts
171 photos
Thank you, Jeff - the Delrin method sounds ideal. The only problem I can see is identifying the right stuff! I have two or three pieces of "man-made" material but I'm not sufficiently 'au fait' to tell one from t'other   Presumably there will be considerable differences in strengths to make one sort better than another. Probably a web search for suppliers of the real Delrin would be the best bet. Thanks again
Bill
wotsit05/07/2010 21:22:24
188 forum posts
1 photos
Jack,
 
Jeff Daymans remark about casting replacement parts is a good idea - I had a similar problem some years ago,  but I made my castings from aluminium using the 'lost foam' process - I was really surprised by how easy it was (usual remarks about safety when handlng molten metals) - I was so happy with the results, I now frequently cast parts I need, rather than fabricating - I have made things like mounting feet for electric motors, spindle supports for polishing wheels and so on. Corrosion will of course be worse with ali than with bronze, but it isn't going to corrode that fast (unless you live in the sea).

There is lots of info available on the Internet, and you don't need any special equipment for small amounts of casting. - if you are making your own, you also have the possibility to redsign and strengthen to cope with heavier loads.
Cornish Jack06/07/2010 16:38:00
1219 forum posts
171 photos
Thank you Keith.
Casting parts is undoubtedly a good idea, unless, like me, your manual skills are a liability rather than an asset!! Re. living in the sea - well the next best (worst) thing - we live directly (10' approx) above the beach exposed to the standard NE'lies off the North Sea. Salt there is aplenty.
It would seem that Delrin is produced in various forms but NOT in rectangular blocks such as would most easily suit my needs. I have ordered some round offcuts and will machine them to a right angle bracket form. Having no idea of stress levels or how to calculate same, I hope they will be effective rather than elegant!!
Again, thanks for the input.
Rgds
Bill
Sam Stones06/07/2010 23:21:22
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880 forum posts
326 photos

Hello Jack,

For what it’s worth, if you are keen to determine the `nature’ of a plastics (polymer) material, I know from experience that raw material manufacturers normally have standard testing laboratory facilities, and routinely test their product for quality. Identification tests can also be carried out in their more extensive laboratories to determine more closely, the type, grade and therefore application suitability.

However, finding a raw material company who will carry out basic tests and at what cost could be the stumbling block.

Another possibility is to locate a technical college who teach polymer science, and who are equipped with polymer identification facilities.

Generally speaking, homopolymers will be hard and brittle while copolymers are softer but tougher.

Good luck with your repairs.

Sam
Cornish Jack07/07/2010 11:20:03
1219 forum posts
171 photos
Sam, thank you for that but see the following.
As Confucius say, "When all else fails, read the %£**&! instructions!!" Having at last turned up the brochure, it states that the end plates of the headbox are, in fact, diecast aluminium rather than Mazak. So, presumably, although the same caveat re. dexion applies, ali fixing brackets would be OK? Does anyone know if differing types of aluminium would be problematical in contact or are there any precautions I should take in attaching the new brackets?
Any comments appreciated
Rgds
Bill
V8Eng07/07/2010 18:08:26
1730 forum posts
6 photos
If you have the brochure does that mean the manufacturers are still about? If so it might be cheaper and faster to get the parts from them.
 
I was working out how to repair our 40+ year old garage doors, an internet search found the makers, and they supplied parts very reasonably.
 
At least that way I could get on with my hobbies.
 
 

 

Edited By V8Eng on 07/07/2010 18:09:19

Cornish Jack07/07/2010 22:39:07
1219 forum posts
171 photos
Thank you, V8Eng
That would be a possibility if the brochure carried any contact info... it doesn't and the 'net doesn't offer any help either. The other problem is that window shutters are bespoke so the spares (if available) are likely to be as expensive as the original I would (obviously) prefer to re-attach the old shutter if it's possible and if it is likely to remain attached for a year or three. I would still like some reassurance from someone who understands metals that the aluminium brackets are feasible.
Rgds
Bill
Gordon W08/07/2010 09:07:55
2011 forum posts
There are lots of different types of aluminium, some good in seawater, some hopeless, I don't know of-hand which is best, but the chances of getting the correct one is remote, except maybe from the makers. Why not give them several coats of good paint?
Martin W08/07/2010 10:54:11
921 forum posts
30 photos
Hi
 
As we neither know the size of the brackets nor the weight they are expected to support I don't see how any sensible advice can be offered as to suitable materials to replace the existing failed brackets . I assume that the failure of the existing brackets is due to stress fracture at a load point rather than corrosion.
 
One alternative solution to the problem would be to get the existing brackets, assuming the brochure is correct and they are aluminium, welded and if space allows an extra piece/fillet of aluminium added where the failure occurred. I would expect that this could be done locally and should not prove to be too expensive.
 
Another point to be aware of is that mixing other metals  with aluminium based materials can be disastrous. Metals like stainless steel when in contact with aluminium in a marine environment will cause the aluminium to corrode rapidly yet both on their own will survive in this type of environment.
 
Hope this is of some help.
 
Martin (Ex Kernow)
Cornish Jack08/07/2010 12:27:13
1219 forum posts
171 photos
Thank you Gordon and Martin.
Some progress made The manufacturers on the brochure are not, apparently,  the makers of these particular shutters although the design is similar. Spare parts may be unavailable. 
Re. the size/weight issue, the headbox was supported on two square section 'spigots' (for want of a better word) approximately 1" sides and 2 - 3 " long, which were part of the endplate castings. These were inserted into the tops of the two side guide rails for the shutters, these guide rails being attached to the wall with 4 off 8 gauge screws each. These have broken off. The whole lot, including roller shutter, motor, drive tube, headbox etc. was a fairly easy lift/carry - so maybe 30-40 lbs?
The intention, at the moment, would be to make up aluminium angle brackets, say 1/4" thick and 2" 'legs', 1" wide pop rivetted to the remains of the existing end plates and then attach these to the wall. The guide rails would no longer support the headbox. The end plates are about 6" square but the rivets would have to be positioned to avoid the roller blind when it's back in the headbox. Perhaps two brackets on each plate (top and bottom) for insurance?
If that works, several coats of quality paint, as Gordon suggests, might increase the life-span? Apologies for the ramble but I'm trying to test the possibilities on a knowlegeable audience, working from a personal knowledge base close to zero
Rgds
Bill
Ian S C08/07/2010 12:30:25
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7468 forum posts
230 photos
Hi Cornish Jack, I would suggest looking for some off cuts of stainless steel angle of a suitable size, and fabricating the brackets out of that. To join it either use stainless bolts, or get it welded. If you have a choice, go for 316 stainless. Ian S C
Cornish Jack08/07/2010 12:48:21
1219 forum posts
171 photos
Thank you Ian.
There would appear to be a difference of opinion 'tween yourself and Martin (see his last para above.) DOES the combination provoke corrosion or is it dependent on the method of joining?
Rgds
"Confused of Norfolk"
Bill
Martin W09/07/2010 00:24:55
921 forum posts
30 photos
Bill
 
The reason I raised the problem re stainless and aluminium was that prior to my retirement I was working with a group that manufactured and serviced specialist ocean buoys. The superstructure was, for obvious reasons, constructed from stainless steel. However it was necessary to attach items that were made from aluminium/aluminium alloy to this superstructure and if these units were allowed to contact the steel they corroded very quickly. Therefore wherever there were potential mating surfaces between these materials they were isolated from each other either by application of specialist coatings/paints.
 
The problem arises as dis-similar metals can form a battery and when salt water is applied the battery is effectively activated and starts dissolving one electrode, often referred to as electrochemical corrosion. This process is used to advantage in shipping and other areas by reducing the corrosion of the steel hull of the ship by bolting a 'sacrificial anode' to the hull below the water line. These sacrificial anodes are frequently made from zinc or aluminium and slowly corrode away thereby preserving the steel hull.
 
While your environment is not this severe there will be a degree of this type of corrosion in the environment you describe, hence the reason I highlighted the potential problem. If you do decide to use stainless or any other steel in conjunction with aluminium then I suggest you paint each component thoroughly and assemble when dry in an attempt to minimize any potential problem. The other consideration is how long do you want the repair to last? If only a few years then bolt it all together and coat liberally with grease or an oil based corrosion inhibitor then forget it till next time.
 
Sorry for the ramble but have witnessed the extreme effects of this type of corrosion.
 
All that said I would prefer to use stainless steel as a support primarily as it is so much stronger and more durable that aluminium . Its just the mix that causes the problems!!
 
All the best.
 
 
Martin

Edited By Martin W on 09/07/2010 00:31:13

Edited By Martin W on 09/07/2010 00:36:34

Ian S C09/07/2010 11:08:04
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7468 forum posts
230 photos
Martin, I was thinking of making the bracket of stainless, but I was not taking into account the rest of the unit which I imagine is aluminium, although this proberbly has steel pivot pins or something. If the two are insulated should be OK, if not perhaps jack can hook up a light to his new battery. Ian S C
Cornish Jack09/07/2010 17:40:30
1219 forum posts
171 photos
Martin and Ian - Thank you for the explanation AND the possibilities of a potential new power source
I did a bit more searching and finally found a contact number for a manufacturer of these blinds. Apparently not exactly the same as mine but similar. Long story shortened - maybe a local rep will combine a beach afternoon with a looksee at the damage and advise - fingers tightly crossed. Even if that is successful, the foregoing  hints and tips have been  very helpful and illuminating.
Thank you all
Rgds
Bill

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