A bit confused about aluminum corrosion

It is my understanding that bare aluminum will corrode, but will then form a very tight layer of aluminum oxide which will protect against further corrosion. If you clean off the aluminum oxide, the aluminum will corrode a bit more but will then form another layer of protective aluminum oxide.

I was talking today at work about wanting to put a false floor on my boat. A guy said that saltwater would get trapped in the small gaps and cause corrosion. I have also seen on this forum about how some people are concerned about the same thing by putting a layer of foam between the hull and fase floor, trapping a film of water.

What is there about this that wil result in extra corrosion? Why wouldn't it just form a layer of aluminum oxide and then become passivated?

aluminum will not corrode unless it comes in contact with certain chemicals. to my knowledge salt water will not harm it.

A few months ago I removed the 7x5 ft 1/8 alu floor there was foam flotation under it (water logged), the boat 1993/16X60 Only water its ever been in is Salt Water and rain..Other than dirty it looked like New.I only removed it due to Galvanic Corrosion on the outside (bottem) where the carpeted treated bunks touched the bottem and had corroded in patches almost through in some areas.And a few areas that i had poked holes in too.Welder need floor removed so he did'nt start something on Fire..As for any other corrosion the only was every SS bolt /screw that was used to mount something there was a small area( Quarter size).Of corrosion /paint blistering /with white powder under it. some came out some twisted off..I got them all out was able to sand those areas without any repairs to them.The welder did his thing.The floor had been alu/alu pop rivited from factory so thats what I put back in .Prepped the entire inside and put 2 gal Tuff Coat(bedliner for boats I call it)..As for mounting thing So far I have alu/alu pop rivited in everything, but there are some things that I will have to use SS bolts (plan use nylon washers to keep metals seperated.Wife got me on another mission for now.In closeing the only area that there was SS bolts that did not have any corrosion was the Motor mounting bolts.I am guessing that the Zinc on motor protected them..Hope this helps ..cva34

When I started my mod, one of the first things I did was to remove the flooring. Below the carpet/aluminum was a layer of 1.5" foam. It was so badly water logged that there was no way it could ever dry out. So the hull was constantly exposed to having a layer of water trapped in there. Pitting was non existent on the hull floor. The transom wood was also saturated and that was keeping the inside of the transom constantly wet. The inside of the hull at the transom was pitted so badly that it had eaten all the way through in some areas. So just having water trapped against the aluminum is not the problem. I'm not sure why my transom corroded so badly when the floor didn't but they were both exposed to prolonged periods of water being trapped so there is some other factor causing the problem. I don't have many details about my boats history but as far as I know this was all based on exposure to fresh water.