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.