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Author Topic: fixings for aluminium  (Read 1126 times)
djh
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« on: June 19, 2017, 10:39:25 PM »

I'm planning to build a pergola (which is supposed to act as a brise soleil). The main structure will be aluminium, for rigidity over long spans, and I can source that. But I'd welcome any advice of fittings and fixings for connecting aluminium parts together and for connecting timber components to the aluminium.

My main worry is galvanic corrosion but I don't know how worried to be about that. Are stainless fixings acceptable, or do they need to be aluminium, or what?
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Cheers, Dave
rogeriko
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« Reply #1 on: June 20, 2017, 12:01:31 AM »

All PV ground mounts are aluminium struts bolted together with stainless nuts and bolts, 10,000 PV installers can't be wrong.
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djh
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« Reply #2 on: June 20, 2017, 12:19:35 AM »

All PV ground mounts are aluminium struts bolted together with stainless nuts and bolts, 10,000 PV installers can't be wrong.

Do they use anything like nylon washers and bushes?
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brackwell
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« Reply #3 on: June 20, 2017, 07:40:10 AM »

Electrolytic corrosion takes place between s.s. amd ali. The result is the eating away of the aluminium.  Water is an essential ingredient in order to create what is in effect a battery and is made worse in a salty enviroment eg you live within 5ml of the sea, and temp.

The effect can be much reduced by placing a insulator between the 2 materials and at DIY standard, sealant or a strip of plastic would do.  Remembering that the fastner itself is a conductor so never use self tappers and steel rivets. Use ali pop rivets and arrange for these to be in shear if poss.  I know people use s.s. bolts but they should not technically and usually get away with it because the atmospheric conditions are not particularly aggressive or sheltered. I have a lot of experience of this and i am sure that the long term weakness of present PV fitting systems is the ss/ali connection.  How long is long term is a bit of string- my guess is 10yrs near the sea and 25yrs inland.

Remembering that the above is in addition to any residual leccy currents flowing around but that is outside my experience.

The problem of wood to ali connections is a little more technical as the wood is always wet. Ideally use SS bolts with non threaded portion thus reducing contact area SS/Ali.

Pv uses Ali rails because it can be extruded in intricate shapes but if this quality is not required then galvanised steel is better, being stiffer (for a given weight) and cheaper.

Dont get over stressed by this as life is too short but just take some small precautions.

Ken
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djh
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« Reply #4 on: June 21, 2017, 01:41:59 PM »

Thanks for that, I'd forgotten about alumium pop rivets, which I've already got. I'd been thinking about getting some alumium nuts and screws.

Galvanized steel will corrode won't it? I know it'll take a long time before it's structurally unsound but I don't think it will look good for very long?
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« Reply #5 on: June 21, 2017, 05:30:34 PM »

Dave,

If the galvanising is done properly it should last decades. The stuff on some my Land-Rovers is over 60 years old and still intact and the tracker steelwork has a nice 'patina' on it now but not a trace of rust.

On the subject of Land-Rovers you could always use sherardised nut and bolts to secure aluminium with almost no galvanic issues.

Regards

Richard
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Philip R
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« Reply #6 on: June 22, 2017, 01:28:44 AM »

I had experience of what happens when incorrect materials used on the Aluminium phase isolated busbars, used to connected the turbine generator to the generator & unit transformers had its removable access panels fitted with aluminium fasteners, not stainless steel.

Large surface currents flow in the aluminium skin in antiphase to the internal three phase busbars. Therefore it represented an accelerated life rig with respect to galvanic corrosion. Non use of aluminium fasteners resulted in quick corrosion of either the fastener or the surrounding aluminium.

The other issue was that the alu bolts galled readily with their aluminium counterpart, just like st steel does.

OH, Your pergola installion is not a PIB with large currents flowing in it. I would recomend that you visit your local specialist fastener shop or go online and buy some aluminium fasteners.

Philip R
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