Since I started this thread and it has grown, I thought I would cite the source for the information I had about "mill scale". Up front, I,m an aluminum
virgin-so I asked the question out of an abundance of caution. Installing a new fuel tank
is not fun-I am hoping this will be my one and only tank install thus all the questions. The following text was posted on aluminumalloyboats.com. I can't judge the accuracy of what this builder
claims, but what he says sounds credible. Understand this is not the normal beneficial oxidation that we see on unpainted aluminum mast
for example. This is a waxy substance on aluminum stock that gives it a shiny surface and comes that way from the mill-perhaps to make it easier to handle or store as part of the manufacturing process. Thought you all might find it thought provoking-the original writer has been building aluminum alloy boats commercially for 30 years and seems very knowledgeable and passionate about his work
"Mill scale or mill finish either term can be used to describe the coating that comes from the mill on aluminum sheet. Its shiny and is not (chemically the same as) the aluminum and if not removed will cause (facilitate/participate/enable/interact) corrosion
of the underlying aluminum.
Look up "galvanic differential" or galvanic corrosion
, then the idea will be more clear. There is a slight galvanic difference between the alloy under the mill scale/finish and the parent/native/final/underlying metal.
Next look up poultice corrosion, this is also facilitated/enabled/created/assisted by the mill scale/finish on marine
aluminum alloy sheets/plates. This is where water
is kept in contact with the sheet in a tight space and the oxygen goes out of the water
so it becomes acidic and then speeds the surface corrosion using the scale/finish to enable the water retention and the reaction. Similar 'cells' in descaled panels
show almost no trace of the level of corrosion.
This scale/finish is most often removed with acid, or (correctly selected) SS wire wheels, or 'buffering' (Scotchbrite TM type) pads or sand blasting.
builders and manufacturers leave the scale on and their boats show the results in a year or two, while 30 year old boats that have been descaled/etched to remove the finish/scale are clean.
This layer also retains atmospheric moisture (dew, condensation
, humidity) so much it has to be removed to weld or the weld will have so many porosity gas bubbles it will be structurally unsound. [Many 'sound' MIG welds have some root porosity] However, since this is only needed in the weld zones, and is not always done there either, the remaining surface may or may not have this 'finish' removed, but it is best practice to remove this manufacturing byproduct, just like it is in steel
sheet plate, prior to painting, or even if the boat
will remain bare metal finish."
He goes on to suggest an acid etching process for tanks
that I am still considering-that and coal tar or some type of epoxy
barrier coat on the exterior. Do you get the sense I never want to do a tank install again?