In this application theres loads of dissolved oxygen in the (sea)water, so apart from the normal corrosion issues there's no potential being generated from the big difference in oxygen concentration like in crevice corrosion situations. The water in this case also gets constantly renewed/replenished so the various (by)products and parts of the reaction get washed away and can't keep the cycle up.MarkL wrote: If this occurs, how can aluminum be used in marine applications where part of the aluminum is submerged and part is above water?
Key point here is that for crevice corrosion like this to start the trapped water needs to get into an stagnant and oxygen depleted state to start the reaction as (combined with naturally dissolved salts) it's what generates the galvanic cell. The lack of oxygen also prevents the alu from (re)forming it's oxide skin so the reactive base metal remains exposed all the time to keep on reacting.
The water itself is not the issue and as long as its free to 'breathe' so the alu can oxidise and skin-over then it's also fine.
You will see the same effect on boats or other setups where for instance 2 alu parts are bolted together and the mating surface is not sealing well so a film of water can get between them. This stagnant water film will start the same reaction and when the parts are taken apart the surface between the parts and around it will have siginificant chunks eaten away.
Bolted pipe flanges can show this effect as a progressing corrosion 'wedge' between the mating sufaces that starts on the outer edge and then begins working it's way into the join.
In many cases it's often not an issue but (also on many other metals) when there are spots in a design where water can pool/collect and get trapped in confined spaces where it has little to to exposure to the oxygen in the air anymore, trouble may be ahead..
Water seems so harmless, but it REALLY wants to be friends with almost everyone and steal their ions if you leave it alone for a while
Bye, Arno.