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Thread: Copper Corrosion

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    Copper Corrosion

    This thread is a technical continuation of the discussion on failures of copper heat exchangers in gas heaters in pools/spas discussed in this thread starting at this post.

    This link and this link give nice summaries of copper pipe corrosion. This link provides more technical detail when chlorine is present, as is often found in household plumbing (up to 1 ppm FC with no CYA) and is of course present in pools (though with CYA in the water the equivalent amount is about 0.1 ppm FC). The description of the possible corrosion mechanism sounds suspiciously similar to that of stainless steel in terms of disruption of a passivity layer (in this case, of copper oxide). It's interesting to note that Dissolved Inorganic Carbon (DIC) plays a significant role by enhancing the solubility of copper(II). In pools, DIC is carbonates and the 0 to 20 mg C/L where the effect is strongest is equivalent to a TA of 0 to 153 ppm (ratio of roughly 100/12 calcium carbonate to carbon molecular weights and adjustment at pH 7.5 of 0.915 bicarbonate to total carbonate) and that this effect is stronger at higher pH above 7.5.

    This research at the EPA showed that pitting corrosion occurred at low DIC levels of 5 and 10 mg C/L which is a TA of 38 and 76 ppm and high pH water of 9 in the presence of chloride at 20 mg/L (that's VERY low) and was not observed at pH of 7 or 8. This link seems to confirm chloride's role in the mechanism of pitting attack on copper. This link from Health Canada implies that low levels of chloride may reduce copper corrosion, but at high chloride levels it may increase pitting corrosion -- note that "high" in this context won't be very high with respect to pools since this is about drinking water (too bad they don't give more specific levels).

    It looks like the general consensus from the above and other articles is that sulfates can play a significant role in pitting corrosion and that chlorides also increase pitting corrosion. This is very similar to what is said about stainless steel and appears to be consistent with the inhibition of forming a passivity layer. When one combines this with chlorine oxidation and, most importantly, with water flow rates through the tubing, corrosion seems inevitable. This link gives a lot of detail of the various factors and types of corrosion, but unfortunately nothing definitive on specific chloride levels.

    Unfortunately, none of this would specifically determine how much faster corrosion would be at higher salt levels such as found in SWG pools. It only indicates that such corrosion is more likely to occur at faster rates at higher levels, but not specifically by how much. This link from the EPA on stainless steel corrosion of various alloys says the following on page 28:

    Non-halide salts have little effect on stainless steels, but chlorides particularly tend to promote pitting, crevice corrosion, and stress-corrosion cracking. In some cases sulfates seem to aggravate the effects of chlorides. Chlorides present in amounts as little of 0.3% with sulfates present can produce severe corrosion. Even quite low concentrations of chlorides can cause corrosion when concentrated by occlusion in surface films.

    So it may just be that the thinner less expensive heat exchangers combined with high flow rates, U-tube bends, and higher salt levels push corrosion rates higher so that they become more visible. Fixing any of these factors would likely help. The easiest approach is to use more corrosion-resistant materials such as cupro-nickel or titanium. So if you are in the market for a gas heater, it would probably be worth getting one with such materials, independent of your pool type but most especially if you have the higher salt levels in an SWG pool.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    JasonLion's Avatar
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    Re: Copper Corrosion

    The "conventional wisdom" is that the great majority of copper corrosion is because of low PH. Have you seen anything in your reading that supports that conclusion?
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    Re: Copper Corrosion

    Chemgeek, your findings support what I've always believed (I was a rep for both Raypak, and for Teledyne/Laars/Jandy). The combination of flow and chemistry has almost a synergistic effect on the copper, and the availability of cupro-nickel exchangers has given longer exchanger life. The issues with using more resistient materials in gas heaters has to do with the COP and just how much condensation and heat efficiency will become factors.

    Heat pumps, which are not subject to gas flame temperatures, all now use titanium exchangers.
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    Re: Copper Corrosion

    Jason,

    Yes, low pH is a big factor and the cause for much more rapid corrosion, but what I was commenting on were corrosion factors that appeared at more normal pH ranges above 7. Clearly, someone can abuse their pool by using Trichlor and not adjusting pH and find that the low pH in their pool makes their copper heat exchanger fail. That's not what I was looking for, however. SWG pools tend to be higher in pH, not lower, and we already knew about how acidic conditions are very bad for metal corrosion. My water district uses the approach of a higher pH of 8 combined with 300-500 ppb phosphates as a corrosion inhibitor. They used to use chlorine for residual sanitation, but now use monochloramine.

    budster,

    You bring up an interesting point about condensation which isn't something I found through reading published papers, but was something my Pool Builder told me. In his experience, gas heater exchangers fail most quickly when the gas heaters are used in an on/off cyclic mode at cooler temperatures around 50-60ºF. Apparently, this causes condensation to form outside the exchanger where presumably corrosion occurs from the outside in. That is, of course, quite different than what I described in my post above.

    I couldn't find much quantifying the salt effect -- plenty saying there was an effect, but not much giving specific levels and rates. What was consistent was that the corrosion is similar to that of aluminum in the sense that it is about interference with the formation of a passivity layer which is why flow rates are a factor since they physically interfere or remove such layers. The chlorides interfere with formation, though probably less so than with stainless steel. Chlorine is an oxidizer, but it's unclear how much it is a factor vs. dissolved oxygen. We know with stainless steel that chlorine without CYA can be a problem, but it's speculation that the same is true with copper, though it sounds reasonable.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Re: Copper Corrosion

    my findings have been the same, and recall reading something similar however that borates would infact help prevent the corrosion

    to high flow on copper bends i have also seen errosion

    we have a fibreglass pool company here that we will not provide heaters to as their advice to clients is that their pools do not need calcium in them, the salt chlorinator will do the job fine. Even high quality cupro nickel heat exchangers will late a maximum of 6 months before they disintegrate - the result of salt water beinig sprayed throughout the heater does not do them any favours at all.

    The condensate formed by combustion of gas has a pH of 6.8, which is also corrosive to the heat exchanger is formed by either short cycyling, or too high flow through the heat exchanger

    Titanium is fine for heat exchangers in heatpumps, however the extreme high temps and heatup/cooldown times is not suitable for titanium

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