Rust in pool from heaters cast iron headers

Stains on the pool surfaces, pool equipment
or on the swimmers, or off-color swimming pool
water. Discolored but clear pool water.
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Rust in pool from heaters cast iron headers

Postby Guest » Sun 13 Apr, 2008 11:50

I discovered rust in my pool heaters cast iron headers. There were small globules of rust built up around the inlets and outlets. I knocked/wire-Brush/scraped off and all the rust and soaked the headers in vinegar. The interior of the heaters are porcelain coated and most of the coating seems intact.

New headers will cost almost $500 and I am trying to get a few more years out of the heater and I was wondering if there is anything I could use to coat the inside of the header to minimize the rust, maybe so sort of epoxy paint. There is a product call “Navel Jelly” which will arrest rust(turns it black) and prepare it for paint, not sure if that is a good idea either.

Any suggestions?


chem geek
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Postby chem geek » Sun 13 Apr, 2008 20:37

Assuming the header is electrically connected to the bonding wire, you can electrically connect a zinc block to the bonding wire and bury the block in moist soil (i.e. effectively ground the block directly to Earth). This will put a small negative charge on all metal components touching the water and will lessen the rate of corrosion. You'll have to replace the zinc block every few years (the zinc is a sacrificial anode). If you have any aluminum metal connected to the bonding wire (say, with a pool cover header), you'll need to use a magnesium block instead of a zinc block.

Richard
Guest

Postby Guest » Sun 13 Apr, 2008 21:03

Hi Richard, Thanks for the great idea! Funny thing is I just recently saw an episode of "Dirty Jobs" on TV where they were replacing the zinc blocks deep inside a bridge or levey to slow the rusting of structure.

New Questions:
1.In between my two cast iron-headers are the copper heating tubes. Will the copper be affected by the zinc?
2. Will the zinc affect my pool water chemistry at all?
3. Just curious, What affect would the zinc have on any aluminum that was electricaly connected to the zinc?

There is an aluminum tube that supplies gas to the pilot, I am 99% sure it is electricly isolated from the headers, but I will use a ohm meter to make 100% sure. If it is connected, I will use magnesium then.

chem geek wrote:Assuming the header is electrically connected to the bonding wire, you can electrically connect a zinc block to the bonding wire and bury the block in moist soil (i.e. effectively ground the block directly to Earth). This will put a small negative charge on all metal components touching the water and will lessen the rate of corrosion. You'll have to replace the zinc block every few years (the zinc is a sacrificial anode). If you have any aluminum metal connected to the bonding wire (say, with a pool cover header), you'll need to use a magnesium block instead of a zinc block.

Richard
chem geek
Pool Industry Leader
Pool Industry Leader
Posts: 2382
Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Postby chem geek » Sun 13 Apr, 2008 22:04

The copper will be protected by the zinc as well. Basically, all metals typically used in pools except aluminum will be protected by the zinc block. If you used a zinc block with aluminum in the pool, then this can increase the rate of corrosion of the aluminum -- essentially the aluminum becomes a sacrificial anode for the zinc. You can see the electrochemical series here . For practical purposes in terms of metals used in pools or used as sacrificial anodes, the series from most reactive to least reactive is:

Magnesium, Aluminum, Zinc, Iron (steel), Copper

So you can protect any metal that is on the right by using a sacrificial anode using material to the left of it. Stainless steel is an exception and is somewhat like aluminum in that there is a passivity layer (that is, a partially corroded layer) that protects the metal underneath from further corrosion. In stainless steel, it is thought that this layer is hydrated chromium oxide (the "stainless" part of stainless steel is the added chromium). With aluminum, it's aluminum oxide and protects aluminum from corroding rapidly in air which it would if it were not for this effect.

Chlorides interfere with the formation of the passivity layer which is why salt pools can be more corrosive depending on the level of other corrosive factors (dissolved oxygen, chlorine levels which are especially high if CYA is not used, presence of sulfates, etc.).

There will be no effect on the pool water chemistry whatsoever from using a sacrificial anode -- except that you should get less rust (or metal ions) from entering the water!

You should probably figure out why you got corrosion in the first place. Normally such corrosion won't occur (or at least not very quickly) unless your pH got too low or chlorine levels extremely high (or no CYA) or TDS (mostly salt) way too high or some combination. Perhaps someone put Trichlor pucks/tabs in the skimmer since it's very acidic and when the pump is off this acidic water builds up. Either that or someone may have added acid via the skimmer -- an absolute definite no-no (it should be added very slowly over a return flow in the deep end with the pump running).

Richard

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