Is anodized aluminum suitable for indefinite salt-water exposure or will it rust? Also note that it would be used for the hull of a boat (more like a platform), and if it is not suitable could you suggest another metal?
Aluminum would be an ideal material stainless steel a better one, aluminum does not rust it deteriorates to electrolite when in contact with salt water and a magnetic or ferrus metal. Therefore as long as your boat is fitted with salt water anodes it should create no problems if not it will deteriorate rapidly in salt water. All powered salt water craft/vessels carry anodes ie there is an anode fitted to the aluminum drive leg of an outboard or Z drive if intended for salt water use, On larger ships the anodes are normally fixed to the rudder blade so equip your platform the same. Zink anodes are cheap and available at every salt water chandlers. You should also use non ferrus bolts and screws to screw your construction together with, or to fix it to the hull, or better make a set of jigs to suit shape size and then get it TIG welded never mix ferrus and non ferrus metals in the presence of salt water without a zink anode, the damage is rapid and attacks the structure of the non ferrus metal creating an electric field and destroying the weakest current catalyst ie the aluminum itself. The non ferrus metals that in general maritime use that are are not so much effected by salt water are phosphor bronze, brass,and copper.Non of these would be suitable for making your platform, therefore aluminum. If weight is not a problem though i would recommend a middle grade stainless steel such as se 80 or around that grade, And again it would be better to be TIG welded than bolted together, bolts are subject to movement (especially at sea) and therefore hole enlargement. As are safety standard pop rivets.
