Leakage Problem in PVC Lined Chrome Plating Tank

"Plastics and Composites Welding Handbook" on AbeBooks or Amazon (affil links) A. The problem may be due to cracking of the plastic at welded joints or bends. In my humble opinion polypropylene would be a better bet especially bearing in mind temperature. 12 mm sheet should be OK but reinforced externally with a steel cradle to prevent sagging. Over the top polypro air line will be OK to avoid anything penetrating the walls of the tank. The alternative is a stirrer fitted in the corner (air driven motor driving a stainless steel impeller blade) or you could fit recirc/filtration unit. Roger Bridger - Croydon, UK A. You should not be experiencing these types of problems with a new tank. PVC is a correct material for chrome tank construction according to a chart I have. I am assuming that the tank is make of rigid welded PVC Are the leaks near welds? If so this may have more to do with welding technique (on which I do not pretend to be an expert) than on the material the tank is constructed of. Typically, long term wear to the tank usually occurs at the solution line and the bottom corners. Many fabricators will double the thickness of the PVC at these points. When checking on the chemical compatibility of the specific PVC resin used for the tank material remember that proprietary chrome baths contain a fluoride catalyst! You might consider building the tank of CPVC. A circulating pump would be a simple matter. Again, select one to be compatible with the bath chemistry. Gene Packman process supplier - Great Neck, New York

A. 1. Chromic acid is a powerful oxidizing agent, so I think I would disagree with the idea of using Polypropylene.

Chromic acid attacks PVC welds such that an attempt to make repairs can indeed make things worse.

I have never seen a chrome plating tank constructed of rigid PVC or rigid polypropylene, so I'm not sure what problems it brings. The traditional method is to use plasticized PVC liners -- either B.F. Goodrich Koroseal or equal, glued in place, or a prefabricated drop-in liner -- in a steel or FRP tank.

The most resistant lining material is Kynar, but this would cost far more than PVC, so it is often impractical.

Sight unseen, the most practical fix may be to glue in a Koroseal liner, which should have a double layer from several inches below the normal solution level to several inches above it, for the reason Gene pointed to.

2. As mentioned, I would build the tank of FRP or steel. Because I would include reinforcement girths (belly bands), the thickness of the wall would be far less than the 20 mm you mention. It would be more like 1/4 to 3/8" (say 8 mm) for steel or FRP.

3. Air agitation, through a perforated PVC pipe, is not a problem.

4. Circulation pumps are not a problem, but they probably ought to be Kynar.

A. I also disagree with the polypropylene idea. I know for a fact that chromic acid, even in low concentrations will attack and crack the welds. It would leak within a month.

I have seen molded polypropylene tanks hold up for extended periods however.

A common practice for us is to build a 1/2" polypro tank with a 1/4" fabricated PVC liner. This would not be suitable where heavy parts might be dropped.

A. Temperature and concentration will determine the speed of attack on a molded PP tank. A polyethylene molded tank will last for several years. It will actually last a little longer than most vinyl liners. The chrome will rapidly attack the plasticizers in the PP. Since the weld operation has destroyed some of the plasticisers, welded PP tanks are short lived!

My guess is that your PVC tank failed in the welds. First because of the loss of the plasticizers due to welding, especially if it was a rather hot weld. If it was cold weld, it does not take long for the heating and cooling to cause a problem to go along with the oxidation of the weld.

Re-weld is a nightmare! You must scrape every bit of the chrome affected PVC away or you will BURN the weld and it will fail in days.

Unless your tank had massive reinforcement, on the outside, it stood little or no chance of not failing. 1/2" is way too thin for a tank of this length to not have steel reinforcements. Many people forget that plastics' coefficient of expansion is about 10 times that of steel.

A tank of this length is going to have mega stress put on the corners as it expands and contracts, not to mention the bowing as it heats up and softens. In short, PVC was a poor choice.

A. Hi everyone. Why everyone? Because a lot of good people are trying to help someone out. I was directly approached on this problem about a week ago and had sent some copious data to Chris.

Yes. PP is NOT recommended. However, a moulded one piece PP tank, were one available of that size which is not so, would last some years. But then so would a medium density moulded PE tank ... and with Polyethylene tanks (the largest in the USA or Canada being rectangular and having an open top is a paltry 500 gallon Imp.) could easily have the wall thickness increased to 1/2". Ask any reputable rotational moulder of the Engel process. Then the corrosion/chemical attack is vastly reduced. But you must never, ever weld on outlets. They will fail at the weld. Use tank adaptors instead where oxidizers are concerned .... I learned the hard way !

Years and years ago I.C.I. (England) proved that ordinary polyethylene is resistant to chlorine. Ah. Think about it. Of course it will be attacked! Of course it will fail. But what I.C.I. proved was that if the wall thickness were heavy enough, the chemical attack would peter out and a passive layer would be formed. This would apply to thermoplastics but never to fibreglass products as they do not have a homogenous structure.

Then there's the matter of the PVC sheet. Whose? There are some crappy PVC sheets and some damn good ones like SIMONA or, my favourite, TROVIDUR which costs an arm and a leg.

As pointed out to Chris, you MUST have a good thermoplastic welder. A so-called 'union' welder may hit 30% of tensile, a very well trained PVC welder should easily hit 85% of tensile.

Then these leaks probably occurred at welds because of stress distortion. All, YES, ALL the bottom and side corners have to have a decent radius. A radius not, never, a sharp bend!

Because of the size of this tank, the ONLY successful way to make it using type l uPVC is to armour it with fibreglass.......and Y E S that has been done most successfully, I did it for Weiser Lock years and years ago. The bond being the critical factor to prevent deflection.

Because of its size, the most practical $-wise solution is to use a steel tank with a Koroseal type liner with the ubiquitous 1/4" or overlay at the air/liquid interface.

Sure, KYNAR (A trade name for PVDF) would be OK but very expensive. But again only as a liner suitably FRP reinforced. For another 30% more in cost you could go to a proper fluorocarbon such as Halar. PVDF is super for the pump! Consider also a CPVC pump, too, but not a PVC pump as CPVC takes a higher heat and is better in oxidizing areas. Cheers from White Rock,(where it never rains, h'm)

A. Hi Srinivas. PVDF should do it, but at much much higher cost. Is long life critical enough to justify the cost? Most people think not and continue to use PVC.

A. Hi ! Sometimes I think that people approach finishing.com and say they have a problem ... but don't say WHERE the problem is. Can you imagine going to a Doctor and saying "I hurt" and not telling him where the pain is from? Re your PVC:FRP dual laminate tanks ... my question is WHERE are they leaking? At a corner? Which corner? The bottom perhaps? Or is the leakage around the liquid:air interface? Maybe you could glance in the archives at 8042, 7132 and perhaps at 9872. Certainly PVDF:FRP is an ideal material but expensive ... and in a plating tank, one must consider or try to consider protecting the walls, let alone the base ... and for a thin PVDF laminate, Ow! Your wall thickness of the PVC seems to be adequate, maybe I'd opt for 1/4" but the difference is marginal. Lastly, dear Sir, if you have a properly made dual laminate using PVC, heck, you should easily be able to go to 80 °C .... the emphasis is upon 'properly made'. Freeman Newton [dec.] R.I.P. old friend (It is our sad duty to advise that Freeman passed away 4/21/12) A. Why in the world do you want 80 degree resistance for a 55 degree process? If you can control your temperature more closely, you will open up options for more tank materials. You should also be aware that additives can be used in fiberglass that make them resistant to chromium oxidation. Not all fiberglass is the same. Paul Morkovsky - Shiner, Texas, USA

A. Yes, there is fiberglass that can stand the chemicals in a chrome tank, but they do not like temperature changes IE: they crack and eventually leak.
Freeman is right on on this one.

A. PVDF is the most optimum material. Having been using it for about ten years on one chrome line, excellent: little to no problems. But you should at least go to CPVC over PVC for higher temperature safety. Then run dual temp controllers to insure temp. does not get away from you. It will warp crack and do all kinds of not so good stuff. Dual controls are important. Just one time you lose control you're toast.

• Topic #35118 "Polypropylene Compatibility With Chromic Acid Solutions"