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1528 posts

Uber Geek


  # 2102895 7-Oct-2018 13:48
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Gilco2:

Myself and 4 neighbours have just replaced our hot water cylinders. All houses built by same builder in 1993.  When plumber came to ours he said "builders name" I said yes.  Whoever put cylinders in uses galvanised steel fittings in copper cylinders. So setup for electrolysis  there plus chlorine in water didnt help. Just surprised they lasted that long.



This is the reason. Dissimilar metals in the plumbing. While it was untreated water there would have been insufficient free ions to propogate the electrolysis. With the recent introduction of chlorine, there will be excess chloride ions ready to stimulate the reaction.

It's simply poor plumbing practice, masked for years by untreated water.

8700 posts

Uber Geek


  # 2103167 8-Oct-2018 09:26
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It's not really "electrolysis" if copper is corroding preferentially to iron, zinc etc.  Copper has lower electrolytic potential than iron/steel or zinc, so would be protected as a cathode by the other metal acting as the anode.

 

The article in stuff suggested that iron in sediments may be corroding the copper, exacerbated by presence of chloride ions.  If you've made PCBs for electronic projects, you may have used ferric chloride as etching solution for the copper layer.

 

Some of the Chch bores are a bit low pH.  They're supposed to be injecting NaOH solution to standardise pH.  I wonder if there's some complication with NaOH and chlorine injection.  There are a lot of existing old iron and A/C water mains, protecting those from corrosion was the main reason for standardising pH.

 

Chlorine content in our tap water seems to be all over the place, sometimes unnoticeable - sometimes it seems like swimming pool water.  That's only using the nose test - so purely subjective.  I think the carbon filter on our fridge supply seems to get most of the chlorine out, but even then it's chilled which may mean it's just less noticeable, and I think there's a tank in the fridge door, so perhaps the chlorine dissipates.


 
 
 
 


809 posts

Ultimate Geek
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  # 2103459 8-Oct-2018 17:08
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Evidence of chlorine was identified in the corrosion products of all pitting sites but was not detected on surfaces other than pits.

 

Christchurch City Council head of three waters and waste John Mackie said

 

"The issue of hot water cylinders pitting and leaking is not new and was an issue that was investigated in the early part of 2016, when there was no chlorine in the water supply,"

 

 


1848 posts

Uber Geek

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  # 2103519 8-Oct-2018 19:11
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3 year old hot water cylinder..... element 'blown'.  Chorline the culprit they all say... plumber has seen plenty ! 

 

(Christchurch). 

 

Click to see full size

 

 

 

Replaced this aluminium one with a titanium type..... 

 

Apparently many have replaced with standard aluminium and they blew again....

 

 

 

Edit: This photo was taken a few hours after the element was removed/replaced.


2994 posts

Uber Geek


  # 2103629 9-Oct-2018 06:49
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"Aluminium element"?? I thought it was the sacrificial anode in mains pressure tanks that was aluminium.

8700 posts

Uber Geek


  # 2103769 9-Oct-2018 10:37
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Bung: "Aluminium element"?? I thought it was the sacrificial anode in mains pressure tanks that was aluminium.

 

I thought the same about "aluminium element" - I thought the tube was either copper or a nickel alloy.  The latter might look a bit like aluminium.  Titanium sounds expensive.

 

Not sure if the anodes in the tanks are aluminium (alloy). They used to be magnesium/zinc alloy.  It gets a bit complicated, IIRC there's an issue with (pure) zinc anodes in hot water.  Aluminum anodes work fine (if made properly) in cold water. - on boats etc.


2994 posts

Uber Geek


  # 2103811 9-Oct-2018 10:54
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I have seen some comment that magnesium is too active in some water and the anode is changed to aluminium. I'd assume that local plumbing suppliers would know. However they all seem to deny knowing anything.
The anodes are supposed to be checked 5 yearly?? I've never known that to happen.

 
 
 
 


8700 posts

Uber Geek


  # 2103820 9-Oct-2018 11:04
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Bung: I have seen some comment that magnesium is too active in some water and the anode is changed to aluminium. I'd assume that local plumbing suppliers would know. However they all seem to deny knowing anything.
The anodes are supposed to be checked 5 yearly?? I've never known that to happen.

 

Off topic - but this reminds me somewhat of comments I heard in the past about "good" anodes lasting forever.  I had one in a Yamaha outboard like that - stupid me - I should have known.  The anode had almost no erosion, then one day I went to start it and the cylinder filled with water from a hole which had been eaten straight through the cylinder head.  Was an OEM Yamaha anode too.

 

I've also seen people paint anodes on boats - they last longer that way of course.


2994 posts

Uber Geek


  # 2103845 9-Oct-2018 11:23
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Staying ot I had shares in an old Mercury outboard that was pretty moth eaten, apparently a lot of the US market is fresh water use rather than salt.

361 posts

Ultimate Geek


  # 2103937 9-Oct-2018 15:07
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Also Christchurch based. HWC heating element failed - less than 18 months old. Plumber had seen plenty recently and blamed the chlorination.


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