Galvanize and Potable Water

[MrProjectEng]

Hello,

I would like to ask that potable water affects to galvanized piping negatively.

If yes, what is the reason. Which material in galvanized pipe is affected by potable water?

Thanks

 

[PEDARRIN2]

Galvanized piping should not be used for potable water, due to the fact the galvanic coating/layer often gets impinged and then the galvanic corrosion that occurs is worse than if there was no galvanized layer.

It is typically at the threaded/mechanical fittings where this issue occurs because the galvanic layer gets scratched/abraded/etc. during installation.

 

[bimr]

Galvanized pipes are actually steel pipes that are covered with a protective layer of zinc. Galvanized pipes were installed in many homes that were built before the 1960s. Over many years, zinc erodes from galvanized pipes. Corrosion can build-up on the inside walls of the pipes. Corrosion in galvanized pipes can lead to lower water pressure and water quality issues. In-home galvanized pipes can release iron and cause discoloration.

 

[rconner]

You didn't mention if you were dealing with new or existing pipe, nor whether the pipe is/was galvanized specifically on the outside, inside or both surfaces. In any case, I would recommend a reading of prior threads e.g.

http://www.eng-tips.com/viewthread.cfm?qid=272534.

I believe it is well-known zinc can extend the life of otherwise unprotected piping and other products. While it has not been/is not highly publicized however, some amount of lead metal was used for many years in specifically in conventional or traditional "hot-dip" galvanizing processes as explained in e.g.

https://en.wikipedia.org/wiki/Hot-dip_galvanization

and

https://en.wikipedia.org/wiki/Galvanization

etc. The latter site and prior thread also indicates some normally quite small diameter "black" iron and steel pipes were/are? even galvanized on the "inside" of the pipe. The prior history patent e.g. document download from the portal

http://www.google.com/patents/US20100291404

some elaborates on such aspects of some hot-dip application products, and therein alleged alternative.

 

[SandCounter]

I have seen galvanized for potable used as a standard specification. I am not 100% on board with it because like with an anode, the galvanization dissipates over time. One thing to look out for is that some water purification equipment introduces ozone into the water as a feature (or gimmick), this can react with the galvanization and result in premature corrosion and brown water.

I used to count sand. Now I don't count at all.

 

[moltenmetal]

Galvanized piping should not be used for potable water, due to the fact the galvanic coating/layer often gets impinged and then the galvanic corrosion that occurs is worse than if there was no galvanized layer.

Dead wrong on that. In fact the opposite is true: a galvanizing coating is superior BECAUSE it provides at least some anodic protection to the underlying steel pipe in areas where the coating is damaged, like it is at every cut thread or rolled Victaulic groove in the pipe.

That said, galvanized pipe isn't a long term solution in potable water because there is a continual source of fresh oxygen in that water. Eventually the zinc will corrode away and leave the steel underneath unprotected. But it will STILL be superior to ungalvanized steel in terms of survival.

In treated cooling water or closed loop cooling systems, galvanized will last a very long time, and the galvanic joints matter much less.

Where galvanized pipe fails prematurely is typically at unmanaged galvanic joints. When people thread brass or bronze valves into galvanized pipe, or make threaded transitions to copper lines using brass fittings, galvanized pipe fails rapidly. Any attempt at galvanic isolation at these joints needs to avoid being short-circuited by pipe supports and ground wires. And no, teflon tape provides no useful galvanic isolation in a typical NPT joint!

Copper and plastics (PEX, CPVC) are better choices, though copper can corrode quite quickly in some potable waters- it depends on the pH and mineralization of the water.

 

[SJones]

...it provides at least some anodic protection

The zinc is an anode and provides cathodic protection.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[hacksaw]

Given the standards of quality in most water systems, ranging from chlorinated water, brackish water (high saline content), and the range of mineral content, depending on your actual service, the upstream piping and storage MOC you would be better off without zinc lined piping.

Remember even distilled water can be corrosive!

 

[weldstan]

My first two homes had galv pipe for the water system. The one that was over 40 years old needed some repair adjacent to the brass spigots.

 

[PEDARRIN2]

Moltenmetal,

I would tend to agree but I have seen studies which indicate localized corrosion of galvanized pipe. While these studies are for fire suppression dry systems, the underlying theme is similar, I.e. impinged zinc coating and replenished oxygen in the water.

http://ecscorrosion.com/wp-content/...-Dry-and-Preaction-Fire-Sprinkler-Systems.pdf

http://corrview.com/rokdownloads/cfp_01.pdf

Some have even stated that galvanized pipe is worse than black steel for these systems. I haven't accepted this yet, but it is of interest.

I haven't yet stopped specifying galvanized pipe in fire suppression, but would not specify it for domestic uses. On remodels, I typically have it removed and replaced.

 

[moltenmetal]

I would not use galvanized for domestic potable water, as there are far better options available. I personally prefer PEX greatly, as corrosion immunity trumps corrosion resistance in every service.

I can believe that the zinc in some services may not be worth its cost- but have great difficulty believing that the zinc is of no use at all or worse than using bare steel pipe. That makes no sense to me at all. If nothing else, the zinc provides external corrosion protection at a cost far lower than painting small-bore pipe, which is the primary reason we use it for most of our Category D piping.

Thanks for the correction SJones- I'm forever messing up anodes and cathodes, anodic and cathodic, anions and cations...