Copper piping and cathodic protection?

[Ken Fabian]

We rely on collected rainwater for household water. We get some copper staining via the cold water taps but not the hot water. The hot water system has a sacrificial (cathodic?) inclusion - I think it is zinc.

I was thinking of adding a piece of zinc into the rainwater tank to stop what is slow corrosion of the copper pipework - and prevent that staining. Changing pH by periodically adding chemicals does not appeal - and would require regular water testing and treatment. Anyone here that could advise on whether a piece of zinc (perhaps hanging inside the water tank) would work? Or offer suggestions?

 

[Baluncore]

Or offer suggestions?

The lowest cost and longest lasting solution is to replace metal pipes with black poly pipe, made from high density polyethylene, HDPE.

 

[Ken Fabian]

The lowest cost and longest lasting solution is to replace metal pipes with black poly pipe, made from high density polyethylene, HDPE.

I am a big fan of avoiding unnecessary expenses and extending the working life of the existing infrastructure, which still works fine, seems like a very reasonable approach. It seems likely to be a lot easier and cheaper.

I think my question is whether the sacrificial cathode needs to be in contact with the copper pipework to work? Some more reading suggest that it does and hanging zinc in the water tank won't do it; probably lazy of me to put the question here.

I need some way to do so without constricting the flow - I am thinking of adding a dead end tee-join with a removable end and put a piece of zinc in there.

 

[Baluncore]

I am a big fan of avoiding unnecessary expenses and extending the working life of the existing infrastructure, which still works fine, seems like a very reasonable approach. It seems likely to be a lot easier and cheaper.

Less work, but the scrap value of the copper would pay for the poly pipe and fittings needed.

What is the roof made from?
What are the rainwater tanks made from?
Does your rainwater contain sulphur from acid rain?
Does your electrical system use the copper pipes as a ground?

You might install plastic couplings or plastic sections, between sections of copper pipe, so that corrosion currents cannot circulate between the different metals at different ends of the system.

You might use a short copper pipe section, with larger diameter, to house a zinc sacrificial electrode.

 

[tech99]

I would be concerned about the toxicity of zinc.

 

[Baluncore]

I would be concerned about the toxicity of zinc.

Zinc is an essential trace element for humans. Zinc is not a problem where houses have zinc roofs and zinc water tanks. There is however, a concern in that people taking 100–300 mg of zinc daily may suffer induced copper deficiency.

 

[Ken Fabian]

Thanks.

Not used as an electrical Earth - there is a copper stake for that. Using both a garden pH tester and a pool test strip it looks very close to neutral but it hasn't rained lately and I suspect pH varies following rain; dissolved CO2 in rainfall? Nitrates after thunderstorms? Something off the roof, eg ash residues from wood heater - water off that part of the roof is not collected but ash could spread wider... but I'd think that would send it alkaline.

Roof is steel 'zincalume' - a variant of galvanised, cathodic protection for roof sheets; aluminium/magnesium/zinc. The water tank is similar material but has a drinking water safe polymer coating inside. Some rivets and outlet fittings are exposed to the water. It is a bit of a mystery why the corrosion given neutral pH.

I don't think a piece of zinc will present a health problem; it is washing water, not for drinking. There is a separate tank, pipe, outlet used specifically for drinking water - in years past we relied more on water pumped from an earth dam for washing water. With a bigger water tank that has not been necessary. So far.

It is not uncommon for zinc galvanised steel tanks to be used for drinking water. Some contamination from zinc coating for tanks and pipes from the zinc itself being contaminated, eg from some lead content was an 'in the olden days' risk - I'll be using high purity zinc.

My trying some zinc for this seems a low risk, low cost experiment. I found a blind tee already in the pipework. I am just not sure yet how I will attatch the zinc - it may end up being soldered into an end cap, silver solder if that will work with zinc and copper. I have some checking to do for suitable solders, suitable fluxes and cleaning before installation.

 

[Baluncore]

We get some copper staining via the cold water taps but not the hot water.

Rainwater contains dissolved CO₂ which forms carbonic acid, H₂CO₃. Carbonic acid will react on the zinc surfaces to form a white insoluble coating of zinc carbonate, ZnCO₃. Once the metallic zinc surfaces have been reacted, the carbonic acid will react with the copper pipe to form a soluble copper carbonate. I expect a zinc sacrificial anode will also become passivated with a coating of zinc carbonate, but with a metallic connection to the copper it may offer some cathodic protection. I have to ask why the zinc galvanised tank, connected to the copper pipe, is not providing that cathodic protection now.

Where groundwater contains copper in solution, adding burned aluminium cans to the solution, precipitates out a greater weight of copper. Unless you can get a good refund on the can, the scrap price of aluminium cans is significantly less than that of copper. Maybe you could use a sacrificial Al anode ahead of the copper pipe.

What colour is the stain ?
Green Cu₂CO₃(OH)₂; or
Blue Cu₃(CO₃)₂(OH)₂.

 

[Ken Fabian]

@Baluncore - pH is 7 - neutral - measured to the limits of pool test strip and garden pH meter, ie not very precise. I expected some acidity.

The hot water system does have a sacrificial zinc element in it (which is probably due for checking); given the lack of staining from the hot taps that suggest to me zinc cathodic protection works to prevent copper corrosion. And if the zinc were considered unsafe I doubt it would be permitted.

Some compression fittings are used as well as silver soldered. Compression fittings do have contact between pipe ends and brass body - touching - but not necessarily a reliable electrical connection. Distance between the zinc in the hot water tank and the length of the cold water pipes may be a factor.

The water tank is metal but it has a polymer lining. There is a plastic pipe connection between tank and pump, so the water tank should be electrochemically isolated from the copper pipework.

A bit unhelpfully, I would call the staining blue-green.

The other alternative - short of replumbing - is a 'water conditioner' filter cartridge, with a requirement periodic replacement.

I will treat it as an experiment and let you know.

(Attaching the zinc with a regular tin/lead solder - going by memory - the flux would be zinc chloride and just some hydrochloric acid should suffice - but even though there would not be much of it I would prefer a lead free solder. It sounds like zinc won't handle the high temperature silver solders need - and gives off toxic fumes)

 

[tech99]

I suspect that providing cathodic protection to the interior of pipes is problematic, because the current will tend to flow locally, close to the zinc electrode. I am wondering if the discolouration of the copper you are seeing is dead algae and bacteria rather than a chemical action. It might also be a reaction with dissolved salts, such as chlorides, and will probably self-protect after some time.

 

[Baluncore]

Rainwater is usually pH 5.0 to 5.6. If you measured pH 7.0, then it has been neutralised before you sampled it.
If you sampled the water in the tank, then there is something about the roof or the tank that is reacting to neutralise the water, but that does not explain the hot/cold difference inside the house.
If you measured the pH from a cold water outlet inside the house, then the copper pipes are probably neutralising the water by corroding, which explains the blue-green colour. The zinc anode is doing a good job sacrificing itself to protect the hot water pipes.

The traditional way to neutralise rainwater, to protect all the copper pipes, is to feed the water first through a canister of calcite, limestone, CaCO₃. That will exchange calcium for the acid in the water, venting CO₂, buffering the pH to near 7. A concrete tank can also neutralise the rainwater because the cement contains lime.

Another advantage of calcite in the tank water, is that it will to some extent, protect your hot water sacrificial zinc anode, so you won’t need to replace it as often.