Acid Wash, vs. Caustic Etching?

General boating discussion
Karl in NY
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Acid Wash, vs. Caustic Etching?

#1

Post by Karl in NY »

I only see references to acid washing here, but strong alkalis (e.g., sodium hydroxide) do a very nice job, resulting in a uniform matte finish. I used it regularly on a job where I built electronic prototypes, housed in both sheet aluminum and die-cast aluminum enclosures, to give them a more finished look, or, to enhance paint adhesion. These were small enough to do by total immersion in vats, rather than by spraying, which would obviously be needed for a large item (although, we did do a huge enclosure using string mops rather than spray).

I'm curious why the process has not been used for aluminum boat hulls and accessories, and why acid-washing seems to be universal instead.

Other industries use the caustic, alkaline approach, but it seems to be unheard of in the marine business.
kmorin
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Re: Acid Wash, vs. Caustic Etching?

#2

Post by kmorin »

Karl, I think everyone else is busy with Christmas today? so I'll offer my understanding of the question's answer.

It is my understanding and I say that with you knowing I'm not a chemist or chemical engineer- that caustic (base) etching takes off about 10x as much metal as acidic (acid) etching does? I'm not sure about this, but its what I recall when I asked the question decades ago.

Zep Chemical and other provides aluminum etching products that offer 'do it yourself' products; they sell acidic compounds to the wider market but caustic etching is still used in many applications that required a 'deeper' metal removal to obtain the level of surface preparation -anodizing, for example- for the final coatings. Also castings, of various forms, have vastly more 'spare' material so a few mills of extra metal removal in surface prep are not considered a 'big deal'.

In sheet products or extrusions formed in post billet 'worked' forming processes- every fraction of the material is considered an engineering factor in the design - so removing less metal to get to a 'chemically cleaned' surface is highly desirable.

Last, there is 1/10th the metal ' trapped '/treated/handled in the dirt/waste water treatment/waste disposal process to deal with if the surface treatment removes a thinner layer of metal in the etching process?

All answers; pretty speculative on my part.

Cheers,
Kevin Morin
Kenai, AK
kmorin
Karl in NY
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Joined: Fri Dec 17, 2010 2:30 pm
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Re: Acid Wash, vs. Caustic Etching?

#3

Post by Karl in NY »

My experience, over 20 years old now, was the aggressiveness of caustic-etching was entirely dependent on three factors: concentration of the lye solution, exposure time, and finally, temperature. The process produces off-gassing (hydrogen?) and noxious fumes, and we worked outdoors whenever possible.

A hot, concentrated solution would produce bubbling to the degree it resembled water boiling, yet a mild concentration produced just tiny bubbles and needed much longer exposure time to get similar results, but was more controllable...as I recall, the process left a thin black residue on the aluminum, easily removed with just a bristle brush or even a sponge with no real effort. We would rinse twice in vats with just water, then a neutralizing dip in very mild acetic acid, followed by a final water rinse.

We called it "poor man's anodizing"..., and you could have it in any color you wanted, as long as it was silver...no opportunity to add dyes. Much of the work was done on 0.125" thick 19" rack panels, and there was no measurable loss in thickness, just texture change.
kmorin
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Re: Acid Wash, vs. Caustic Etching?

#4

Post by kmorin »

Karl, I agree that temperature of the reactive agent- acid or base; concentration; and to some degree soak time all contribute to the degree of etching action from the 'extreme ph' materials on aluminum alloys.

The last, soak time, is limited in sprayed-on etching since there is a limited volume of the acid so soak time is set by the volume sprayed on... meaning that an immersion tank is not comparable to spraying on a coating to react. But Temp. and Concentration are two big factors- we can etch with vinegar but it's not usually sold as a very concentrated solution- so, by heating it - vinegar's etching action is increased.

We usually encourage owners to heat up the boat- not if they live in Arizona and are etching in the afternoon in August- to help conserve the expensive etching solution. Also its been widely accepted that a test panel with different concentrations of acid is worth the time to set up and test-reducing rework due to an unsatisfactory surface coloration with weaker concentrations of acid.

Acid, even hot, concentrated acid will not leave any residue on aluminum if it is rinsed before the temp 'dries out' the foam. The rinse is usually done on a boat in waves- vertical 3' to 4' wide stations of the boat are etched keel up and rinsed before going on to the next few feet. I think there is one skiff builder (Work Boats??? not sure on this?) who has a tower or shower that each skiff is pulled up into- bow first and hung- then drenched, etched and rinsed all in one operation? I've read about it but not seen that and have never been able to afford a 'tank' large enough to handle a skiff.

Cheers,
Kevin Morin
Kenai, AK
kmorin
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