Which metal will best withstand the elements?

Question

I am looking to buy a piece of equipment. It is to be used in the north sea where corrosion from the elements mean we usually use stainless steel. The equipment is only available in aluminium or Zamak (mazak in the UK). Which would be better at resisting the weather?

Answer 1

I think it's Mazak.

Answer 2

Aluminum.

The purity on the Zamak has to be very high to even stand a chance - I'd say forget it.

You didn't say what kind of aluminum, whether wrought or cast, what alloy, what heat treatment or temper, and if it had any surface treatments.  In general, the more pure the aluminum, the better the corrosion resistance, but the softer it will be.  6061 (or equivalent) performs OK, but 5000 series is better.

MARINE ALLOYS

These alloys are used for boat building and shipbuilding, and other marine and salt-water sensitive shore applications.

5052 aluminium

5083 aluminium

5086 aluminium

6061 aluminium

6063 aluminium

Surface treatments include anodizing, but the coating must be thick and throughly 'sealed'.   Anodizing is brittle and vulnerable to cracking if the part is bent or bendable.  (Not all alloys series respond well to anodizing)

Organic coatings (such as epoxy or polyester powder-coat) can also be applied to the part to prevent corrosion and are more forgiving to bending/flexing.

Aluminum is a very reactive metal, so anything that causes the breakdown of the natural thin-oxide layer is going to be a problem.  You really do run the risk of pitting corrosion and stress-corrosion cracking (especially if the aluminum is of the 7000 series aluminum and there is any residual stress - just a matter of hours).

You mention "resisting the weather".... does this mean the part will be in service outside?   I'd say neither of these materials is going to hold up on their own outside for any length of time in severe service without some further surface treatment.  

If the part is exposed to UV sunlight too, you would be well-served to get a polyester powder-coat paint coating applied to the aluminum parts.  It doesn't cost much and will dramatically extend the life of the aluminum parts.  

If not exposed to UV light, then epoxy powder-coat would be another good corrosion-resistant coating, and there are low-temperature coatings (~300°F) available.  

Powder-coated parts will have to be heated between 300°F and 400°F to apply the coating, so disassembly is required.  You can usually obtain adequate adhesion with a bead blast before coating.

Spray-coated parts using solvent-based paints usually have pin-holes, and unless multiple coats are used, will fail.  Catalyzed epoxies are commonly used (if not exposed to sunlight), multiple coats, chromate coating on the substrate is mandatory (also MAY or may not require disassembly).

If you're working in the North Sea, surely there are corrosion specialists available to consult?  If not, contact your powder-coating shop or corrosion paint shop for recommendations.  If you have a choice, it would be better to specify the coatings ahead of time and have the manufacturer to the disassembly/reassembly for you.

Answer 3

Aluminum, never heard of the other stuff...Ideally the AL will be plated with a Hard Black or Hard Clear Anodize to prevent corrosion which is standard industry procedure, or at the very least Alodined, Primed, and Painted.

Answer 4

Sea water contains salt which can cause stress corrosion cracking in stainless steel.  Titanium will work.  High nickel alloys such as Inconel will also work.  Zamak is a zinc based and would be better than Al.

Answer 5

I really can't help you with Zamak sorry, but i can shed some light on an interesting property of aluminum.

It corrodes VERY easy, and thats a GOOD thing. Almost every peice of aluminum you see will be covered with a thin layer of corrosion, "oxidised aluminum". This thin layer actually acts like a seal, preventing air and moisture from reaching the untarnished aluminum under the corroded layer, preventing further corrosion. If the layer is scratched, the scratch will corrode and form a new layer.

The only thing you have watch out for is certain other corrosive metals coming in contact with the aluminum for periods of time, or build ups of undisturbed fluid on the product, as this can facilitate continuous corrosion.

Hope this helps

Answer 6

either steel or titanium.

Answer 7

Metals are divided into two categories. Stuff that corrodes, and titanium. I recommend the latter.

Stainless steel is called that because it stains less.