Non-corrosive plating over copper

We are developing a product that requires the utmost in water flavor even after the tap-water is held in a 195-199F "boiler". Our pre-production boiler case was made from passivated 304 (we will switch to 316 next time) and the internal heater plates were copper with the phosphorous type of electroless nickel plating applied.

We assumed that we would have only nickel and chromium surfaces and no corrosion.

After nine months we found that the stainless was fine but the nickel showed considerable darkening. Furthermore, where the nickel plated plates were joined there were areas where the plating was missing allowing the copper to corrode.

What to do? This is a very expensive product and can probably run to certainly silver if not gold plating. The product needs the best solution. We can't very well substitute silver for the copper, but anything else may be okay.

My concern about gold plating is that I remember (in a previous lifetime) that gold plating can be a bit porous (one electrical connectors) and allow the material underneath to erupt through.

 

[mcguire]

You haven't given us enough details to answer accurately, but consider that mixing metals and using plating introduces a lot of potentially uncontrollable variables in terms of quality control. Galvanic issues add to the problem.
Is there not a single material or close family of materials from which your product can be made? Consider 316 tank, nichrome heating elements, etc.

 

I am not sure what further information would be useful?

I am using two different metals to accomplish the temperature control that I desire. Copper for heat transfer, stainless for insulation. That part works.

 

[kenvlach]

Agree with mcquire that an all SS system might be best.
Copper’s no good if it contaminates your water. Gold plating will not help much since seams between the plates will prevent good plating and open and close due to thermal cycling. Also, a Ni barrier layer is needed to keep the gold plating from diffusing into the Cu. Alternatives:

a) Coat present HXer with Teflon®. Increase area 20%. Flexibility will handle the seam problem.
b) Use an SS HXer, with thinner wall than Cu, or
c) Use a PVDF HXer (Geo. Fischer sells), only if low pressure steam.

While these alternatives might require a larger surface area than Cu, I don’t believe much problem maintaining a 4^oF range. Millions of SS HXers in use.

I have presumed heating with steam. If electric heating, there are Teflon®-, quartz- and SS-coated heaters.

 

[JimMetalsCeramics]

Stainless steel wire is certainly available and can be used, provided that you don't require high electrical cconductivity (low resistivity). The resistivity of copper, in micro-ohm-cm, is 1.67. Stainless steels range from 60 to over 300.

The prediction of corrosion of metals is tricky, not knowing all the facts of the running environment. Electrical potential, chemistry of the water and the mating of different materials, will influence corrosion. If you have a means by which to run accelerated testing, you may want to get a sample of wire of various metals and compare their performance. You can purchase very small amounts of a wide variety from ESPI:

http://www.espi-metals.com

Here are some candidate metals and their resistivity values:

Metal micro-ohm-cm
Gold 2.35
Molybdenum 5.34
Titanium 42.0
Tungsten 5.40
Monel 48
Niobium 16
Zirconium 41

I would not use silver. Again, a good stainless might be fine. As kenvlach points out, polymer-coating the wire is a possible option. For direct wire material substitution, since you want good thermal transfer, perhaps molybdenum (or the molybdenum alloy TZM) is the best bet.

 

[mcguire]

A suggestion on materials. Sometimes in this environment stress corrossion cracking is a problem,if you have residual stress. Some producers of water heaters have used grade 444 to address this potential problem. It sounds like you don't have a problem, maybe very low chloride or very low stress.
444 is a potentially good material for your heating element, since it has better thermal conductivity than austenitics and equivalent corrosion resistance.

 

I seem to have been unclear about the basic design. I use stainless for external appearance, and the basic insulation of such a rotten thermal conductor. The walls of the tank are a minimum of .25 thick and in some cases .5". In through holes in the back are two cartridge heaters that are clamped to .5" recesses in the bottom of a .75 thick copper plate. Fastened to the sides of this plate are .25 thick copper plates vertically forming walls. The copper is used to conduct heat from place to place in the water, and be a thermal mass. All the copper was plated electroless nickel.

The performance of the boiler is to be able to supply water at roughly +/- .2 F, which it does.

Does this clarify the idea? I do need the performance of both materials.

To address some suggestions...I imagine that teflon coating would be sufficiently thick that it would hamper the performance of the copper as a thermal conductor?

 

[mcguire]

I think plating is the correct solution, but I'm going to defer to an expert on plating as to how to improve upon what you have already.

 

[boo1]

Consider coating, we coat our heat exchangers (copper/ni fins on Nickle tubes) with ElectoFin coating. Check the site at AST:

http://www.electrofin.com/