Mill scale removal?

[gandrfab]

I need to learn more about it.
I have made hundreds of small fuel tanks (more working for someone else) now self employed I have someone asking for an 8' long tank that will hold about 150 gallons.
If I get the job I will ask more about wall thickness and baffles.

[gandrfab]

I'm not in a location I can sand blast, running a DA seems a bit time consuming. Unless I can run it over with the 120 grit in one fast pass?

[kmorin]

g&r, not sure what there is to learn that is not posted here in plenty of locations? If a fast pass with 120 would do it, then; it might get done more often?

What about just chucking up some tooling and seeing what it takes to get to white metal?

(in the 1970's & 80's and as late as the late '90's) A pretty well known manufacturer of tanks in the Puget Sound built "CG approved tanks" for an also well known boat manufacturer. This far from the builders' shop (Alaska versus P.Sound) any tank repair falls to the job shop or local builders- shipping a tank to Washington state from AK and back is worth much more than the tank. After getting a dozen repair tanks a year; for many years I was in business building skiffs, I have the clear impression that all those mill scale lined tanks rotted/corroded/pitted/holed from the inside of the tank to the outside of the tank.

So, I made a few tanks that way in 1978 and never did again, because I was offering life time warrant on my work. The welds held in the tanks I repaired, the fittings were 99% tight, the baffles and mounts worked (the foamed in tanks corroded faster than any tanks I ever saw) all the features worked fine, but the mill scale inside seemed to foster internal crevice corrosion where water bottoms could stand. Once the tanks were repaired, pressure tested and then washed out with acid to clean them I didn't get any back to re-repair in the remaining years I kept a shop.

I think it's worth the time, inside and out, and I don't see it as a low priority item in my QA check list, so my reply is "I'd do whatever it takes?" to get the mill scale off any of my products.

I've written elsewhere, here on tank waterbottoms and related topics, so I won't beat that horse again. But the key decisions on how to manage a tank are just as important as good construction, and ignorant owners are the first to blame their fab shop for their own refusal to accept chemistry and physics- so a little paragraph or two as a hand out- "The Care and Feeding of Welded Aluminum Tanks" by G&R Fabricators is a good business idea. Such a pamphlet could educate clients, and help you stand for the best practices in your market place.

Handling acid wash is not that big a deal in the warmer climates. I'm not you could avoid full PPE. A single plastic eaves trough w/ends glued on could be set on the shop floor and the cut/sheared/tanks' pieces stood on edge in the trough. Then with weed sprayer an acid solution can be applied, letting it foam for 5 minutes; then pieces would be taken outside and rinsed with a hose, when you're done- white metal. Disposing of used/diluted/reacted acid solution is in 5gal bucket with snap on top, Haz waste. OR in many locations it can be diluted to a near neutral ph and emptied into the drains- completely legal if the ph is tested. PH testing strips are usually available at the local drug store counter.

Might be a good idea to consider a sloped top or arced top to a tank that large as water, even condensate from humid climates, could leave a puddle on the top of large flat tanks?

my few cents as the AAB.com self-appointed mill scale gnat-zee so you're welcome to add all the salt you need to these remarks.

Cheers,
Kevin Morin
Kenai, AK

[gandrfab]

acid brand/kind recommendation

I go over board on my pressure testing almost all have left with a rounded top.

Thanks, kmorin

[kmorin]

g & r, we use Zep-a-Lume http://www.zep.com/ZepSearch/default1.a ... &country=U which is a strong mixture of several acids but...

A year or mote past Captn'Dave of the JettyWolf, whom I think is based in Florida found getting this product was a big hassle in Florida so contacting chemical distributors and going through the supply chain with Zep Chemical is going to be required- probably?

My only problem with your air test overboard is the potential to 'stress crack' weld roots. If a pressure test leaves a permanent bow in a tank panel, the elastic (stretch and return to shape) and plastic (stretch and stay deformed) limits of the metal has been reached. This means that for a fact the weld's inside root has been deformed.

I'd never personally allow any work out of my shop that had suffered deformation during a pressure test, my concerns are based on initiating a stress crack failure to something that would never have had that expose without the overpressure deformation. My purpose in testing welds with pressure is to insure that there are no leaks- weld integrity; either in pressure or in vacuum (often the latter are much more damaging) and not to deform my tanks or boats.

Cheers,
Kevin Morin

[gandrfab]

When I say overboard I'm talking 8 10 psi, it's enough to make a tank bulge and stay that way.
I had one slip on me and made it to 50+ psi I scraped it and started over.