Friday, March 13, 2015

Corrosion tests

Fetched the test sheet that I have had on my boat since the beginning of July. The boat is in salt water and there have been several storms since July last year. Before that it was laying on my veranda through the winter and spring. The veranda has wooden impregnated planks and no roof.

When I put it on my veranda I put some salt on it and water, and let it lay there on the wooden planks through snow and rain. When on the boat, I simply tied it to the light pole. I can see very little change since I took it off my veranda and on the boat. There may be a bit more galvanic corrosion around the rivets which were not dipped in Duralac. I can only conclude 3 things:

  1. Letting the (slightly salted) aluminium lay on wooden planks (basically on the ground) cause corrosion
  2. Letting the aluminium be on a boat in a real marine atmosphere does no visual harm, except maybe some added galvanic corrosion between the stainless rivets not dipped in Duralac and the aluminium.
  3. Duralac and primer prevents all corrosion, also the veranda type :-)
I should have put a fresh set directly on my boat, but I envisioned lots of added corrosion, so I didn't. Anyway, since the salt water marina must be some of the stupidest places to place an aircraft, and no appreciable amount of (added) corrosion is seen, any corrosion problems for me simply do not exist. I will continue using Duralac on the last rivets. It also has the added benefit of making them water tight, and I can be 110% sure absolutely no corrosion will occur. In essence, this result is much better than expected.

From a more scientific point of view, I think what is going on is this: The marina is a salty atmosphere and it is also more or less continuously sprayed with sea water when it is windy. But, it is also windy very often, and it is raining a lot, so the salt will either dry or get washed away rather fast. When laying on the wooden planks (on the ground literally), it will be wet more often, and galvanic and surface corrosion have time to happen. The ground itself is also full of organic stuff (moist rotting leaves etc) that creates a very bad environment for the aluminium.

This also means my unsolvable problem with painting is finally solved. I will simply polish the aircraft. This will mean some extra work each year, but no more than I do when painting my boat beneath the waterline each year. 6061 for small aircraft is indeed the correct material.

Set 1 (veranda + boat , veranda below).

Set 2 (veranda + boat , veranda below).

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