Letter 5843 Headache in Chrome tank  . Dear Sir, We are facting a serious leakage problem in our PCV chrome plating tank which has been used for two months in our customer. Our customer use the machine to have Chrome plated on Al wheel in automobile industry. The Al wheel is firstly plated with Cu , Ni and then final with chrome Cr. The chrome tank is made of 20mm pvc. The operating themperature is 46 degrees C. There are four 6Kw Teflon L-shaped heaters put into the tank for heat gereration and TE cooling coil. There is no any air pipe and filter or circulation pump inside the tank. We have tried to repaire the leaked tank and the situation become worse than before. Now, we have to make a new chrome tank for our customer as a replacement. Before doing this, we have to solve the following questions. 1. What is the material should be used for chrome tank fabrication? 2. What is the thickness of the tank wall with tank size as 36"x110"x65"? 3. Is it possible to add air pipe into the chrome tank with refer to the chemical property aspect? 4. Is it possible to add ciruclation pump to the chrmoe tank to prevent localised heat develop inside the tank that may demage the welding of the tank? We hope you would let me to have your professional comment on this issue, which is much helpful to us. Thanks & Best Regards, Chris L [name deleted for privacy] - Hong Kong , China 1 of 2 simultaneous responses - Sep 11, 2000 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. 12mm sheet should be OK but reinforced externally with a steel cradle to prevent sagging. Over the top polyprop 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 impellor blade) or you could fit recirc/filtration unit - try Serfilco for this. Roger Bridger - Croydon, UK 2 of 2 simultaneous responses - Sep 11, 2000 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 occurrs 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 proprietery chrome baths contain a fluoride catalyst! You might consider buiding 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 . 1. Chromic acid is a powerful oxidizing agent, so I would disagree with the idea of using Polypropylene; I don't think it will hold up. 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, so I'm not sure what problems it brings. The traditional method is to use plasticized PVC--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 practical lining material is Kynar, but this would cost about 10 times as much as PVC. 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. 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" 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 ought to be Kynar. These are not "professional comments". You get professional comments from a qualified expert who has seen the installation and who is compensated to make sure s/he is right. You don't get professional comments for free over the Internet :-) I am sorry to criticize the wording of your request, but people must never rely on anything they see on these pages as being a professional opinion. Ted Mooney, P.E.  finishing.com Brick, New Jersey   . 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. Bruce O'Brien - chandler, az, usa . 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 plasticisers in the PP. Sinse 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 plasticisers 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. Reweld 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' oefficient 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. James Watts - FL

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 l/2". Ask any reputable rotational moulder of the Engel process. Then the corrosion/sic. 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 learnt 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 cost 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 occured 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 ubitquituous l/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 flurocarbon 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)

Freeman Newton - White Rock, B.C. Canada

 

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