CARNAC SAYS
"John,
I'll give my two cents on anodizing and corrosion.
1. Anodizing provides a very thick saphire (Al2O3) layer that provides abrasion resistance. I doubt it really does much for corrosion enhancement in low wear applications as it's a porous layer. Just ask the makers of wear resistant cookware why they anodize. It makes things easy to clean and allows cleaning/cooking with things not made of soft plastic.
2. Corrosion resistance is a fairly complicated thing. As described in the link, it would sound like SS is corrosion proof because of the Cr2O3 passivation layer. But put it in a marine environment without oxygen, such as under a bolt or encased in epoxy, and it will corrode just fine. The passivation layer makes things corrosion resistant, not corrosion proof. They key for passivating layers to work is that there is oxygen around to keep corroding any freshly exposed layer, and that the oxidized molecule is smaller than the metal molecule it replaces so it doesn't flake off. The small molecule is why both Cr and Al can form passivating oxide layers while large Fe oxides will flake away.
The second half of passivation requires oxidation in the chemical sense meaning the oxidizing atom steals electrons from the metal atom. In that sense, turning Ca into CaO or CaF2 is an oxidation reaction in both cases. This broader type of oxidation reaction provides both another avenue for protection and an avenue for attack. To prevent flaking of a thin film layer, one can pre-oxidize the bottom of the layer with an adhesion layer. For example, Ni has a high work function which allows it to steal electrons (ie oxidize) other metal atoms which makes it a good candidate for a glue layer for metals. Once it steals the electrons, that top layer is both electrostatically bonded to it giving good adhesion and it makes the other metal inert to further oxidation, so if one tries to attack the boundary with oxygen, oxygen has to steal electrons from nickel which is both hard and makes a fairly small oxide. The point of this being that protection of the grain boundaries from attack by things like chlorine and sulfur is also a key part of corrosion prevention. I suspect an anodization layer will help here because it may be that one doesn't clean all the microscopic crevices of things like salt. Anodizing might help here by protecting the underlying metal grain boundaries from full exposure to all the nasty stuff in sea water.
I'm not readily seeing anything that says anodizing greatly improves aluminum corrosion resistance in sea water, but I'm also not seeing much talking about repeated dip then dry cycles. This review article does say that salt facilitates the galvanic corrosion of aluminum structures near the ocean, so one does need to be a little careful if the spear has parts made from other metals http://www.sheetpileeurope.com…s)/aluminum_corrosion.pdf
By the way, the superhydrophilic coating people advertise marine protection against corrosion because their coating keeps water away from the surface. I know they advertise marine paint for hulls, but I've yet to find any place selling it, nor can I think of anyone providing a durable coating. "