Metal finishing Q&As since 1989

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5 year outdoor corrosion protection for a variety of mild steel and aluminium parts

2006

I would be grateful for advice on the finishing of the following items which will make up a range of birdtables which I am intending to mass produce and sell to the public.

They will be permanently outside and need to resist corrosion for 5 years minimum.

The iron phosphate and powder coating direct on the mild steel, which I had done originally, didn't last long and I have now had all the parts chemically stripped. I would like the final finish to be a shiny colour, as in powder or nylon coating; the parts include:

Mild steel
1. Laser cut parts
2. Parts with threads, some internal, some external
3. A part spun from perforated sheet with a rolled edge
4. A tube dia 25mm x 1 m long
Aluminium
1. A bowl with a rolled edge which will contain water and will be bolted, with a rubber washer sealing the hole, to a mild steel surface (I believe that aluminium should not touch mild steel or they will corrode, but I am hoping the coating will prevent this)

My current plan is:
To dip in acid to remove surface rust, zinc electroplate, de-grease, iron phosphate then polyester powder coat.

Will the parts need bead blasting before commencing, and will the iron phosphate ensure that the polyester adheres to the zinc plating, or will it need to be zinc phosphate?

I am finding it difficult find impartial advice, have also tried zinc undercoat powder coating which seemed ok, but I simply don't know what will last and don't want to sell products that will go rusty, and obviously want to keep costs down as much as possible too.

Adam Vines
Manufacturer - Wallingford, Oxfordshire, UK

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2007

Dear Mr Vines;

For all your Mild Steel and Aluminium Parts, thick Zinc plating of over 12 microns followed up with topcoat of outdoor use U/V proof Car Paint should easily work for you for your 5 year intended use.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India