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O/T Anodizing


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At the weekend was talking about anodizing.

Yes it can be easily done to aluminium.

Can it be done to steel?

It is as far as I am aware basically an electroplating technique so why isn't it used on steel as a decrotaive/protective finish?

Regards

Brendan

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Thanks for your replies folks. This enquiry was the result of an idle conversation.

I never realised that an anodised coating was porous so unsuitable for steel.

The idea of zinc passivation sounds interesting and might be an alternative to powder coatings. Powder coatings are not so good on sharp edges and once the coating starts to lift rust can quickly spread underneath the coating.

Whilst aluminium does not corrode/rust par se it is suseptible to bi metallic or electrolytic corrosion. I used to have ali dropped forged carabiners in my caving days. These 'corroded' due to the pitting from general knocks received underground and presumably from the slightly acidic nature of cave water. It was possible to peel the outer skin off these karabiners which exposed the 'granular' texture of the core of the carabiner. Fortunately when they looked horrible they were still above the rated strength as I had access to tensile test machines.

Regards

Brendan

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passivation is usuall done on zinc plate, I say usally as it's onnly done on plate as far as I knowbut someone is bound to know

better than me. Look under the bonnet of a car, see the gold nuts and bolts, or sometimes black or olive green. Generaly all

the steel bits that aren't painted[unless it's a roller, I think they had some plated and passivated prior to painting]are plated

and passivated.

You can anodise Titanium as well as ali, handy to know when you have a landy.

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Anodising aluminium was first used in the aircraft industry mainly because anodising increases the skin hardness of aluminium. It also forms a layer to prevent corrosion which the underwater users want to prevent corrosion in a salt water environment. Aluminium is fairly soft and anodising makes it that bit better to resist abrasion.

With powder coating it is very important to radius edges to prevent chipping of the paint. Structural steel can be hot zinc sprayed or hot dipped galvanised and then powder coated. I did this on a lot of parts for the nas a few years ago and although some of the powder coating has chipped-off the galvanising is still protecting the steel. Hot zinc spray is especially good for parts which may distort when dipped in hot tank for galvanising.

Zinc and passivate is an electrolytic process and has a few microns thickness, hot zinc spray coats the surface much like a paint spray gun and galvanising is dipped into a tank and you can get 'runs' of zinc with galvanisers not taking too much care. Both hot zinc and galvanising add a fairly thick coat and the cost of both processes is a factor of the amount of (expensive) zinc used.

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