Heating/cooling coil material for chrome plating

A. Hi Dante. Teflon cooling coils, or teflon or PVDF heat exchanger would be the safest bet. There are several different types of chrome plating baths, some of which contain fluorides. If yours is free of fluorides, titanium might do.

Luck & Regards,

A. If there is no fluoride catalyst in your hard chromium plating bath, then titanium would be a better choice. Otherwise, lead heating coils may be a better choice.

A. Hi Chris,

Initially I, too, thought of titanium ... but did have a lot of success using PVDF heating (sic. cooling!) coils.

The heat transfer isn't as good as metal but the corrosion resistance scintillates.

Go and ask any reputable (thermo)plastics fabricator or try G.F. Fischer.

A. Typical for 316 SS is 1-2 years. Double the wall thickness and you will slightly more than double that time. A thick walled titanium will last 2 - 10 years. If the coil becomes anodic for any reason, even at low millivoltages, the life of any metal coil will be significantly reduced. I am not in love with plates. Tubing needs to be seamless. If you use threaded pipe, the fittings need to be very heavy. If you use welded, it should be Tig welded with lots of backup gas or preferably in a bubble chamber if not electron beam welded. Not a job for a novice or a sloppy welder.

Teflon supercoils are a bit fragile, but if protected are excellent because of the huge surface area. (watch the pressure!) You can use less area if you are willing to start your heat cycle a couple of hours earlier. Hard chrome generates a huge amount of heat, so I would assume that you are running the coil on a cooling cycle more than you are on a heat cycle. Heat is for startup only.

A. Chris,

I'm somewhat confused. You say you're running the fluid through your heating coils up to 160 °C ... I would have thought that you wanted COOLING, just like James Watts infers!

Also, PVDF is good up to 140 °C and I would have thought it to be definitely structurally stronger ... & less expensive .. than teflon coils.

Back in '79 I came across a weird set-up. Rather narrow chrome tanks in which very large diameter paper making rolls were plated. The heat build-up was enormous ... I suggested that they PUMP some of the hot liquid into a horizontal PVC mist eliminator (12 micron, a so-called Chromic Dry Scrubber in those days) where, due to the cooling effect of the ambient air being sucked up by the fume hoods, it could be easily caught and returned, nicely cooled, thank you, back to the chrome tank.

This worked apparently very well. They replaced one of the two PVC blade banks in around 2000... a Vancouver Company called EBCO ... and the system is apparently still going on and ON!

A. I am going to disagree with Jim, which is a rare thing, but I have seen many titanium coils with warning labels and even grounding clamps that tell you that you must connect the coil to the anode rod (perhaps due to belief that titanium oxide rather than titanium metal is resistant to acid) and you must maintain anodic polarity to preserve it and/or on the principle that by connecting to the anode rods the anodes will galvanically protect the coils. But on the other hand, I have heard that, depending on the exact alloy, the oxide has a breakdown voltage which may be less than what you might use in chrome plating. Also in chrome plating there are no soluble anodes to galvanically protect the coil anyway.

While I don't like to parrot the old line "talk to the manufacturer", here's the thing. I was an engineer with three suppliers of plating equipment for many years, so we specified titanium coils and electric heaters hundreds of times, and there were a handful of dramatic failures. In one case we ended up with several pallets full of perforated titanium heaters. So, it's simply the case that if you don't do exactly what the manufacturer says, you end up with the responsibility regardless of what really caused the problem. If they say don't connect to the anodes, and you do, a failure will be your responsibility even if the problem was really a bad run of alloy; if they say do, and you don't, it's your responsibility too. Demand that the manufacturer tell you clearly whether to connect the coil to the anode, earth ground, or nothing--not necessarily because it will make any difference, but because you can't let them shift the responsibility for a failure to you :-)

A. Ted, we do not really disagree. Somewhere between zero volts and the voltage to anodize Ti, there is a voltage that will break down the Ti. I do not have a clue as to what that voltage is. I do agree with the statement of do what the manufacturer says. Further, I have no experience with Ti heat exchangers, only electric heaters.

A. Chris
I've used titanium coils for cooling different types of chrome solutions for 20 years and are still using the original coils. They are not electrically connected to either the anode or cathode, but I'm sure the cooling system will be grounded somewhere. My rectifiers are Floating(neither the anode or cathode is earthed) and the way some of the anodes bend I'm sure they touch the coils, but this doesn't cause any problems, until one of the operators shorts out a job on the coil and blows a hole in it, and our Mr fixit welds it up. For heating the solution I use 3Kw PTFE "bat" heaters controlled by a thermostat in a glass tube filled with water. On a 1600 L vat I would use 2 heaters(or one if using about 400 A continuously)

Regards Steve.

A. David,

Wow, I guess that Venezuela is a rich country for you to use PVDF ...

Personally I like PVDF ... next to teflon but it doesn't like pHs of around l0 but v. good on temperature.

I am not familiar with the Hypalon expansion joints. We used to make our own but that was in Sch. 80 materials which have 'meat' in order to machine in grooves for the O-rings.

EPDM could fail on you. Your usage of Viton seems A.OK. May I suggest you go to the first page of this august site, hit # 3 (or is it #4?... it used to be #6 but they've made changes, I don't know why) for the on-line library and you'll come across an article on materials for chrome plating.

Although I'm pro-plastics, maybe I'd have used Sch. 40 or 80 mild steel instead of PVDF. But there are PVDF heat exchangers around, as you might know. Very good, too.

A. According to the chemical resistance charts and prior experience the only electric heater shells that stand up to chromic acid/sulphate plating solutions are those with an outer shell of Quartz or Teflon (PTFE).

A. Hi Manoj

Wouldn't PVDF be a better material? Superb in acids. OK for max 140 °C ... but doesn't like high pH.

A. As we all know, chromic acid is disgusting stuff that creeps everywhere and attacks just about everything. Gene and Freeman are right in suggesting a polymeric or ceramic coating to protect the heaters - pH will not be an issue because chromic acid has such a low pH that is almost falls off the scale!

A. Use lead bonded heater for chrome solution.

A. Hi Brijesh. PVDF is Polyvinylidene Fluoride. It's a fluorinated plastic with exceptional chemical resistance similar to Teflon, available in sheets, pipe, and some molded or cast parts like pump parts. It's resistant to chromic acid / chrome plating solution, but I'm not really sure that it is a better choice than Teflon for cooling coils because soft flexible Teflon coils are available as commercial products, whereas to my knowledge you would have to have the PVDF custom fabricated into a semi rigid piping grid at pretty fabulous cost.

Regards,

A. Hi Ajaya. I think we need to be careful with the slang phrase "milk chromium" because I don't think it has a universal meaning and chemistry. On this site, the phrase has only appeared once (thread 54204); in that case it described the inner, softer, non-porous, chrome layer in a duplex-chrome plating consisting of that layer followed by traditional hard chrome. I inferred that it used a traditional fluoride-free Sergeant chromium plating bath, but I obviously can't say that it always does, and fluoride baths need different materials of construction.

Anyway, we appended your inquiry to a thread about heaters for chromium plating baths. If you are speaking of electric heaters when you say "cases", teflon coating sounds the most reliable, with titanium as a possible alternative.

Luck & Regards,