The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Nitric Solution Tank

We are looking for a 24 inch wide by 24 inch long by 12 inch deep tank/basin for leaching of glass and plastic parts in 20 percent nitric acid solution. Have been unable to find anything that can withstand this solution thus far.

Rick Trask
- Omaha, Nebraska, USA
2001

---

2001

Rick, Perhaps you haven't given us the whole story ... leaching glass with ONLY nitric! Um! What, no HF?

There are or should be oodles of plastics that will take this solution, assuming temperatures are not highish.

l. uPVC 2. CPVC 3. PVDF 4. ECTFE (i.e., HALAR or any of the weldable fluorocarbons such as FEP, TEFZEL).

Materials I wouldn't use would be Pe, PP, ABS & fibreglass.

Structurally you have a piddly sized tank, hence # l, 2 & 3 by themselves should be OK using 1/4" thick material but # 4 should be made of a dual laminate (max, available thicknesses is, I believe, 0.090") to achieve sufficient structural strength.

I hope that that helps. Mind you, the 'higher' the above number, the higher the price!

Currently fumehoods are being made for Assay service for aqua regia, nitric, sulfuric, HCL, HF and Perchloric (my favourite)... with the sulfuric @ 93% concentration @ boiling away @ over 200 °C --- all PVC:FRP construction. And I have these in this service, still working, since l980. Terrible repeat business as they never seem to 'rust' away. Hence I feel that your chemicals are pretty paltry and can be easily contained by any competent fabricator.

Cheers !

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).

PP will last less than a year. Most welded plastics will eventually fail at the weld.

A very safe option for your tank is a low linear density or medium density polyethylene ROTATIONALLY MOLDED tank. These happen to be very inexpensive and are available from a large number of MFG's and thousands of vendors. Nalgene and united states plastics come to mind as very good.

Use common sense, when the tank starts showing hints of stress cracking in the corners or bottom corner, replace it. Normally several years. Also put it into an adequate tray for secondary containment.

James Watts
- Navarre, Florida
2001

---

2001

Hi again,

What James Watt says about Polypropylene is probably true as in an oxidizing environment, both PP and, yes, and, AND Polyethylene are not terribly good ESPECIALLY at welded areas where you can encounter stress cracking.

If you can get a moulded (Engel Process, i.e., rotationally moulded) Pe tank, go with it. But DON'T weld on any outlets. Use tank adaptors!

Where James may err is to state that 'most plastics will fail at the weld'. NOT so! But plastic welding is an ART and poor welding will not last! Having run tensile tests and getting in the 'union trained (hah!)' samples which were all done by an independent Company, the best, BEST union weld was 30%. The best PVC weld (a real exception, nevertheless) came to 99% or l00% but most highly trained thermoplastic welders easily hit 85% of tensile. These tests were done on 1/2" uPVC for electrolytic cell tanks ...Further, butt fusion (sheet) welding machines should achieve l00% weld strength.

One more thing about these rotationally moulded tanks. If the manufacturer is local, ask them to make a tank but with a heavier wall thickness. The max. wall they can do is 3/8", I believe, but a small tank would only be around 3/l6".

This is NOT an expensive matter! All they have to do is to add additional 'powder', But if you could increase the wall thickness for an oxidizing environment, the life expectancy will zoom! I often did this.

Cheers !

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).

Most tanks will fail - eventually--at the welds because welds normally occur at the corners or edges. This is where the greatest amount of stress is and is the place where virtually any tank will fail. (short of being impaled by a fork lift)

Welds remove some of the plasticizers which will accelerate the failure in that area.

Not all welders are good, few are very good, but they still weld. PP is a bad choice for any oxidizing agent as it reacts with the plasticizers and stress cracking results. About 3 to 6 months for a welded tank of any significant size.

James Watts
- Navarre, Florida
2001

---

2001

Regarding James Watts latest comments, Yes, PP (and I said that right in the beginning!) is not too suitable for oxidizing environments and problems occur at the welds, even when done by experts.

However, the comment that most welded tanks will fail ain't (to use colloquial English) right! When Robert Shaw Fulton Controls had a plating plant in Toronto in the late 60's, they bought a number of tanks, all 4-1/2 feet high x 2 ft square (I forget the exact size). Unfortunately they ceased their operations l4 years later. I was HIGHLY curious about the condition of those tanks as they were the first ever true dual laminate tanks ever made in the world, i.e., PVC with fibreglass bonded to them.

They were never repaired. The welds lasted. But the top had distorted due to excessive heat shortly after being installed. Yet l4 years later that distortion had not worsened nor had the welds failed.

Moreover, the PVC used was somewhat experimental as it came from a Company in Coburg, Ontario, that extruded sheet for (snow) fencing, it was not a high quality pressed sheet like most of them are today. Further, the colour was a royal blue (I had the rare choice as I'm partially colour blind but love the deep blue) and everyone in the shop cursed me as they were getting blinding headaches due to the colour and had to weld using sun glasses! (Alcore Fabricating, Richmond Hill, Ontario, Manfred Junkert (now the owner but then was an assistant foreman to Canada's first German trained plastic welder).

Lastly the comment about plasticizers being leached out.

The manufacturers of a high quality thermoplastic sheet will produce/should produce a weld rod with identical properties and certainly not impart undesirable effects by using any plasticizer. I think that this really applies l000% to polyolefins like PP an Pe. In some cases (Trovidur) had a superb PVC but welding it with their Trovidur rod was really difficult so I cheated and used their high impact weld rod which DID have a plasticizer ... but allowed for easy and good welding!

Lastly the comment about welds failing at corners and edges.

For a good job, a well designed tank will NOT have any welds right in the corners. These are high stress areas. Hence the bottom corners must be heat formed and then welded onto the already heat formed (i.e., bent) sides and the bases well rounded corners. This applies particularly to larger tanks all duly FRP reinforced of PVC, PVDF or fluorocarbon construction.

Finally, Pe can work with low concentrations of oxidizers such as nitric .... IF .... moulded one-piece (Engel rotational stress free method) tanks are used and if the wall thickness is OK ... AND no welding is done!

Lastly, way back in the dark ages of l960, I.C.I. (the big U.K. chemical giant) proved that whereas chlorine will easily attack Polyethylene (and most plastics!) if a massive wall is used, the attack mechanism peters out and an impervious boundary layer forms. Hence, I always liked to think, the idea of a rotationally moulded Poly tank with an extra wall thickness is an excellent, inexpensive and a reasonably long lasting solution to minor oxidizers.

Here endeth the lesson.

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).