Aluminum Oxide

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Aluminum Oxide

Post by welder » Sat Aug 24, 2013 10:15 am

Aluminum Oxide

Post Number:#11 Unread post by kmorin » Sat Aug 24, 2013 12:27 am
Wannaboat13, almost any aluminum alloy boat, including the rivets canoes and press formed Lunds, will last a lifetime if they are treated well. If you have one of Glen's guys boat, and take care of it, it will last your children's children's lifetimes; but so often we don't treat aluminum too well and it gets corrosion due to our poor or maybe ill-informed treatment.

Aluminum oxide forms on pure elemental (not alloyed) aluminum in three seconds. It may get thicker over time and go from a couple of mills to four or so, but the first film is formed in seconds. Marine alloys are 50 or 60 series alloys (5052, 5086, 5083 and 6061 mainly) and oxides form on them, when cleaned with acid or by mechanical means, in a few seconds- like pure aluminum.

99.99% of all corrosion happens because people don't understand aluminum oxide, and that includes more than 80% of most builders; large or small scale.

The pictures showed mill scale corrosion.

Mill scale is porous aluminum is not.

Mill scale's pores hold water vapor, 'dry' mill scale is 'wet'; if the temp of the metal is below the dew point of the room. Only hot mill scale is dry mill scale- but as the day cools off, water vapor will condensate onto the metal and collect in the thin film on the surface of the metal if this material is not removed.

Water vapor loses oxygen to the aluminum oxide THEN becomes acidic (changes pH due to oxygen imbalance) NEXT the liquid (now acidic) 'retrieves' its missing oxygen molecules from the adjacent oxide and take some metal too. This point is not clearly understood by too many aluminum people.

Next the wet spot dries out UNLESS its covered with mill scale... then the mill scale retain vapor so the process becomes micro scaled and you see 'white flowers'.

Then the spot (dried acid) is wetted, perhaps with some salt to make the electrolyte more conductive? and the cycle repeats.

What to do? get the mill scale off, leave a bare aluminum to the air so it can get all the oxygen is needs to continue to keep an oxide layer intact.

If there are dissimilar metals involved- and they're wetted they form a battery. To stop the battery working; put plastic between them and make sure the site (of contact) can dry out to avoid trapping water and making a slower version of the crevice corrosion/poultice corrosion cell described above.

If you're just getting the boat learn to use Zep-a-Lume or another etching acid, and wash the entire boat, especially inside if the bilges are bare, to insure you have nothing 'eating' the hull- it can happen from leaves blowing in and sitting in pile to compost into acid.

Paint and almost all coverings have to be done pretty well, molecular level bonded to the bare metal by acid etching and careful rinsing and adding the Allodyne while wet the conversion coatings will be on top of aluminum oxide instead of having a chromium oxide film to paint to.

It almost all been reviewed here before, and I'm totally the site fascist regarding mill scale and surface maintenance so you'll have to take my remarks with a bag of salt. :deadhorse:

cheers,
Kevin Morin
Lester,
PacificV2325, Honda BF225
2386

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