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In article <1144337281.238097.292960@t31g2000cwb.googlegroups.com>,
"larry" <larry.a.schurr@boeing.com> wrote:
> >Can you explain what you mean by Clad coating?
>
> Well... Aluminum is a whitish-gray material, not slick and shiny like
> the sheet metal you buy. The sheetmetal product, and much plate stock,
> is almost always 'clad-coated' and to my knowledge, not available any
> other way. That shiny stuff is the 'clad' which is simply more
> aluminum that has been pressed tightly to eliminate natural porosity.
> Of course, the *process* has changed over the years but the result is
> the same: shiny and slick sheetmetal.
>
> Beneath the clad coating is "raw" aluminum. This has been the bugaboo
> of corrosion problems among many. Raw aluminum is quite reactive to
> air and water and protects itself with an oxide layer of white powder
> (that also turns mysteriously black when you handle it -- kinda weird).
> Depending on alloy, once the raw surface is exposed, the oxide layer
> can go quite deep -- often deeper than the sheetstock IS. The clad
> coating, while still aluminum, keeps corrsion at bay to a much greater
> extent because it reacts far, far, slower than 'raw' aluminum. Once
> this clad is gone, all bets are off.
Sorry -- but the cladding is *pure* aluminum! The stuff under the
cladding is the alloy, which is usually somewhat prone to corrosion. The
cladding acts as a self-sacrificial protectant and sealant for the
alloy. Materials tables show a slight reduction in allowable stress on
clad vs unclad parts, as the cladding furnishes virtually no matreial
strength.
> Sanding or etching removes this coating pretty much every time. Once
> removed, you're gonna hafta treat that surface pretty quickly with
> aluminum specific coating (like alodine). Another poster quite rightly
> pointed out that color isn't supposed to be very 'deep'. "Well-done
> fried chicken" brown is too deep. "Light Honey" brown might be more
> appropriate. Gotta go, getting hungry alla sudden :-)
This part is true. You should not sand clad aluminum, except to remove
existing corrosion.
BTW, to the previous poster, it is the sodium hydroxide etch that should
be warmed to 120 F. The alodine works fine at room temperature.
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Re: Question about Alodine 1201
