Material of construction for anodising tanks

A. Polypropylene and PVC, the most common plastic tank materials tend to be somewhat brittle at 32 °F. They will not stand anywhere near the impact abuse that they will take at room temperature. The welding process takes out some of the plasticizers in the welded joints which makes them even more fragile.
Plastic has a coefficient of expansion that is around 10 times that of metal, so reinforcing very large tanks becomes a problem.
EN has a far larger temp change, but tanks are normally smaller and the plastic gets pliable at the higher temperature.
Drop in liners offer a considerable amount of insulation value (R value) for their thickness and insulate the tank from the anodes.
Rotationally molded tanks can be made of a linear polyethylene which will stand a great amount of physical abuse. They tend to be thinner so will need more support.
Poly Cor rotationally molds some large tanks of crosslinked PE with thick wall sections (no welds) that would require a minimum of external reinforcement.

If you do not already have it, get Charlie Grubbs article on aluminum cathodes. Clariant will have it and I believe it was published in Products Finishing or one of the other trade pubs.

Since you are a large anodizer, you might want to talk to Metalast. Their process appears to be cost effective for your application, uses a proprietary additive and their developed pulse parameters. They anodize at a lot higher currents, so are faster. Also do not have anywhere near conventional aluminum dissolution in the acid.

A. Hi !

It seems that James Watt's comments are pretty accurate . However, I'd like to add some comments.

If the tanks are not too large (rectangular, I assume) I, too, would consider the regular rotationally moulded Polythylene material which is normally a medium density PE. BUT, BUT you can add to the wall thickness up to a max. of 1/2". The largest open top tanks in the USA & Canada are 500 gal. For a price of l0% or so more, you can get the wall thickness increased somewhat. As James Watt said, the expansion of plastics is considerably more than metal... PE & PP being the worst at 8.5 at 10^-5/°F, uPVC at 3-1/2 at 10^-5 °F. BUT a dual laminate (e.g., PVC:FRP etc.) would be down to around 1-1/2 at 10-5.

Another BIG advantage in increasing the wall thickness of a moulded PE tank is that in 'nasty' circumstances (e.g., nitrics!), a shortish life expectancy might well double or triple. But beware of any welding where strong oxidizers are used with PE or PP ! You'll get stress cracking.

For large tanks consider, as mentioned by James, a flexible PVC liner. Pretty long life if well made & installed and not 'hit/scratched' too much.

One point I'd like to disagree on is the statement that welding takes out some of the plasticizers at the joints.

The manufacturers of high quality, say, PVC will only want their welding rod used. These should never contain plasticizers. If they did, any tensile tests would definitely show the 'weakened' result. Having run tests in the past, excellent PVC (hand) welding will attain over 84% of tensile, one exceptional German test result showed 99% ... and in l962 or '63 at Alcore Fabricating near Toronto, one welder actually DID HIT 99% of tensile. 99%!

Ergo, no plasicizer was used!

Freeman Newton, also retired (recently)

A. Hi Jon. A proper grade of fiberglass (vinyl ester) should be fine. And the polypropylene should also have been fine. So if your tanks are "falling to pieces" after 6 years, it sounds to me like you may need more focus on construction technique than material selection. The polypropylene should be at least a 1/2 inch thick, and the fiberglass casing 1/4"-3/8" thick depending on size. The steel bracing should be completely encapsulated in fiberglass.

Although plain polypropylene is fine up to a certain tank size, a fiberglass tank with polypropylene liner is my favorite construction for large tanks and I find it to be exceptionally robust and safe with regards to emergency containment, because the fiberglass can safely hold the solution even if the polypropylene cracks. It might be fancier than you need, but if foil tapes are run along the four vertical seams and the four tank floor seams, both the liner and the shell can be periodically spark tested to insure double containment. Good luck.

A. Hi John !

Re your PP tanks, I'm somewhat surprised that they didn't last too long. You didn't give a size nor indicate if the fibreglass was bonded to the PP ... a must, I feel, for larger tanks as the FRP would restrain thermal movement quite a bit and, if properly engineered, no additional metal supports should be necessary.

To save a lot of time, (my time!) may I suggest you have a gander at letters 7132, 6775, 8042 and 8l69.

Some might say, quite rightly, why not use fibreglass? There being some very good resins around ... caveat emptor! FRP doesn't take kindly, as you know, to abrasion hence a homogenous thermoplastic often is much better for plating tanks.

As your temperature is low and the sulfuric is a very weak 12%, why didn't you opt for PVC:FRP? Should cost less, too, quite apart from being more sulfuric resistant (up to 93%).

There used to be a terrific Company in Birmingham called Plastic Constructions ... and in the late sixties were just getting into dual laminates. You might give them a try if they are still around.

I wish you success!

Cheers!

A. In my opinion, James, Polypropylene is the best overall choice for Type II anodizing or Polypropylene Copolymer for Type III.

A. Hi Beau. To my knowledge stainless steel is not an acceptable material of construction for an anodizing tank (unless it is PVC lined).

I would go with polypropylene, or vinylester fiberglass, or a polypropylene liner in a vinylester fiberglass shell.

Regards,

Hi Andrew. There are corrosion resistance charts at www.professionalplastics.com/professionalplastics/ChemicalResistanceABS-PP-PVC-HDPE.pdf
www.plasticsintl.com/plastics_chemical_resistence_chart.html
and www.polymerplastics.com/images/charts/corrosionResPropChart.pdf

If you can describe your situation rather than just requesting abstract data, I think the readers can be helpful. Lots of us have been involved in the design, fabrication, and maintenance of anodizing lines for decades. Good luck.

Regards,

A. Hi Andrew. There is nothing in the plating/anodizing field as nasty as aluminum bright dipping installations. I put in only three in my 45 year career and detested each one. The tanks are usually double-walled type 316L stainless steel. The exhaust systems are usually "garages" open at one end rather than lip-type exhaust. Standard fume scrubbers tend to be only partially effective.

I can't say whether polypropylene tiles will last 2 years or not but I'd suggest that you keep an open mind toward some kind of cover that doesn't actually have to be immersed in this noxious stuff. And remember that the first rinse may fume almost as bad as the bright dip tank itself :-)

Search the site for "aluminum brite dip" for additional discussions on the subject which may or may not help identify whether polypro will last.

Regards,

A. Andrew,
I had a career in metal finishing of similar length to Ted with chemical polishing very much at the centre. I loved it. Beware false prophets!

The brightener tends to slowly degrade polypropylene causing it to become brittle and break up. This may not be too important as the PP is not being used contain the brightener. But the decomposition products tend to act as surfactants and degrade the polishing action giving an etched appearance. There is no cure, apart for dumping. The usual way of dealing with unwanted organic matter is to cook at 130 °C with plenty of nitric acid: here it makes matters worse.

It may be that there are better and worse grades of PP for this application. I know that this is the case for polyethylenes. I am talking here storing these brews. PE is too close to it's melting point at the working temperature to be of any use.

Best wishes
Harry

Thanks Harry,

I think I will have to look into teflon as an alternative for this application

Regards

Andrew

A. Hi Chris. Polypro should be fine for an anodizing line. I'd look at the welds and look for evidence of re-work, which might indicate ongoing trouble. And I would not put old fashioned electric heaters in polypropylene tanks.

I've had the best luck with fiberglass tanks, but they must be vinyl ester fiberglass, not other resins, and they have to be built right with no exposure of glass fibers.

Regards,

A. Hi everybody.

Nowadays people are using polypropylene tanks for corrosive chemicals which gives best solution in ambient as well as heated condition. It's also easy to maintain and repair within short time without spending more time and amount.