Negative CSI and metal corrosion

[Guest]

WaterBear, it actually works quite well. Anyone using bromine should keep it to an absolute minimum, perhaps 10% to 15%. I adjust the amount of each chemical based on the chemical readings. The pH stays very stable. I personally like the high alkalinity.

Now, back to the original topic; Calcium carbonate provides multiple protections for metal including insulating it from chlorine, dissolved oxygen and electrical currents. It also neutralizes acids that come into contact. In fact, Tums, the antacid, is made out of calcium carbonate.

[Edited by SeanB - see first rule]

Antacids are also made from magnesium hydroxide, sodium bicarbonate, bismuth subsalicylate, and other alkaline materials that are non toxic. What they all have in common is that they are alkaline (can raise pH and will neutralize acid).
Calcium carbonate is simply raising the pH. It has NO effect on chlorine, stray currents, or the dissolved oxygen. If you are talking about a scale coating on metal parts, I believe we have already discussed that but you may have missed it.. As far as your example pool, bromine is a much less active oxidizer than chlorine. which is why your pool does not contain chlorine, only bromine. In an established bromine pool there is a bromine reserve so only a small amount of bromine needs to be introduced to keep it up. The chlorine that you are intoducing is simply oxidizing the bromide bank into hypobroumous acid. Chloriine, per se, does not exit in your example pool in solution.

 

[PoolOwnerNumber9]

WaterBear and Richard, even though we may disagree at times, I do think that you two are some of the most knowledgeable people about pool and hot tub chemistry I have ever known. I just wanted to be clear that just because I disagree with you on a few things doesn't mean that I don't think that you know what you're doing.

 

[SingingSailor]

Interesting thread!
I seems like the common advice, at least in Sweden, is that to low calcium hardness can cause corrosion, of both metals and pool surfaces. See for example these links:
http://www.pahlen.com/eng/poolguiden.ph ... lkalinitet
http://www.havuz.org/pool_pool/pool_mai ... rdness.htm

I don´t have the knowledge to add anything to your debate, though after reading it it seems reasonable to me that calciumhardness by itself has a negligible impact on corrosion.
My question is: Is there a logical explanation to why some parts of the industry say that calciumhardness is important for corrosion?

Second question:
Following common advice, I added CaCl2 to raise my water hardness from 80 to 250, to stop corrosion of my stainless heat exchanger (see story below). By doing that I raised the amount of Cl- ions, that have a negative impact on stainless steel. Is it possible that raising the calciumhardness in my case actually makes the water more aggressive??

Heat exchanger story:
I have a stainless steel heat exchanger, taken from an old furnace where it was used to heat tap water. The first year I let the pool water stay in the exchanger over winter, resulting in a rusty mess coming out of if at spring startup, still working fine though. This first year I didn´t now anything about pool chemistry, I kept pH correctly and used chlorine daytabs (dichlor) and that was it ( a very small pool, less then 1gal). Learned later that we have a very soft water of about 80ppm CaCO3.
The second year I balanced my water to 100ppm alk, 250ppm CaCO3, and at winter I fill the exchanger with car cooler fluid. No noticeable problems now.

 

[chem geek]

The amount of chloride added by calcium chloride isn't in the range where chloride is seen to start being an issue. It's salt pools at 3000 ppm that are more of a concern though usually they are OK with regard to stainless steel unless the chlorine levels get very high, such as indoor pools with no CYA where we've seen stainless steel corrosion within one year.

I have stainless steel rods in my pool where some of the mounts (which might be steel and not stainless steel or might be inferior) rusted when a Trichlor feeder parked itself nearby (in my first year of pool ownership before I learned pool water chemistry). The affected mounts (but not others further away from the initial problem) still produce a small amount of rust even today when the pool water chemistry is in balance and no Trichlor is used anymore and the calcium carbonate level is saturated (I have a plaster pool). So once some corrosion gets started, it seems that even calcium carbonate saturation won't stop it though it does appear to be a slow corrosion.

In any event, whatever works in a given situation is what works so if having saturation of calcium carbonate helps in your situation then that's great, though you also aren't having water in your heat exchanger over the winter anymore (you are using car cooler fluid) so the comparison isn't fair since two things have changed. Anyway, I wouldn't be concerned about having the water be saturated with calcium carbonate in your vinyl pool. It's not going to cause harm so long as you keep the water balanced. If we find something consistent with metal corrosion in vinyl pools at lower calcium and that such corrosion is stopped or reduced at higher calcium levels, then our recommendation will change.

Richard

 

[dschlic1]

Perhaps the reason you are still experiencing "rust from the SS metal mounts is that the surface passivation of those has been removed by the trichlor. Stainless steel per say does not really "rust" but suffers what is called brown staining. SS is subject to crevice corrosion which is more insidiuos. There are commercially available compounds to restore surface passiviation, however you will have to drain the to a level below the mounnts :-D

 

[SingingSailor]

Thanks for your replies.
I have no reason to believe that increased CH has helped, I lean more towards that the car cooler fluid during winter is doing the trick, plus better water balance in terms of alkalinity and pH now.
It is though good to here that CaCl2 at least won´t cause any harm. Maybe I´ll put in some next year as well.

 

[chem geek]

Perhaps the reason you are still experiencing "rust from the SS metal mounts is that the surface passivation of those has been removed by the trichlor. Stainless steel per say does not really "rust" but suffers what is called brown staining. SS is subject to crevice corrosion which is more insidiuos. There are commercially available compounds to restore surface passiviation, however you will have to drain the to a level below the mounnts :-D

I'm not really clear that the metal mounts are actually stainless steel in the first place, or certainly not of the same quality. The actual stainless steel bars didn't pit or show signs of corrosion from the Trichor, but the mounts did and it looks to me like normal reddish-brown rust (and these mounts don't have the shiny appearance of most stainless steel). Of course, once the passivity layer is removed by strong acid, the steel that is underneath can rust. Normally the chromium in stainless steel will reform a passivity layer, but with strong acid from Trichlor it will keep rusting. What I found surprising is that with the Trichlor no longer around the rust kept reforming. So maybe the mounts weren't really stainless steel, but either plain steel or perhaps some sort of coated steel where one would need to recoat it in order to prevent further corrosion.

I learned a lot about stainless steel when researching SWG pool corrosion and found this EPA document.

 

[SPP]

Interesting guys....

I am not good with chemistry but I use alot of stainless steel on yachts where I work on.

I know that stainless steel can have two situation where it can be passive or active. So it can be in rust mode or no-rust mode.

We on boats use sacrificial zinc anode to prevent galvanic corrosion because each metal has difference voltage potential, stick one to another and the less noble metal will corrode in sea water, the battery effect. So we tie all underwater metals with a bonding cable ( copper )and run that cable to a zinc anode for it to be a sacrificed because zinc is one of the lowest in the galvanic scale. I believe some water heater has sacrificial zinc anode too.

Now back to stainless steel. We know that 316L ( L = low carbon ) has the best corossion resistance, 304 is averagely good.
However stainless steel actually will corrode faster when it does not come in contact with oxygen ( active ) and when oxygen is sufficient it will become passive. I don't know if in a situation above 21% oxygen.

In a boat fitting where we use stainless steel, where airflow or waterflow can not go nicely around the stainless steel, there's where the rust will be. For example a base of a a stainless steel ladder where it touches the boat fiberglass and being screwed on and sealed with polyurethane sealant for water proofing. Stainless steel pipe fittings where the threads are partially covered by teflon tape. All screws on a boat swim platform where grime and dirt from the water sometimes create a film making oxygen flow to screws hampered. All the the threads of stainless steel screws that sank inside the fiberglass swimplatform, guaranteed corossion.

Swimplatform often has stainless steel supporting frames made out of SS pipes, part of it submerged in water attached to the boat hull and part of it hang out of the water ( to the swimplatform actually ) because when boat is empty, a swimplatform is usually 5-6 inches above water. The part of the supporting pipe that gets water all the time is always much less rusty than those in open air. The slime and dirt from the water does give the same slime that prevents good air exchange for the SS pipes that are in air.

Propeller shaft for boats are from stainless steel family but with different mix for strength, the part where we have the big bolts on the shaft to secure the propellers, crevice corrosion sometimes do occur on the threads. Stainless steel is not the best underwater metal actually but it is the most common.

Take a SS bolt and a nut, throw it in salty water or sea water. Where the threads of the bolt gets "covered" by the nut. that's where we will see the corrossion. As far as for PH effect on stainless steel, I don't know but if I am not wrong 316L can withstand high acidic condition.

For marine engine heat exchanger, we use cupro-nickel ( 30% nickle and 70% copper ). For high end German engines, they use titanium heat exchanger. Heat exchanger is like a pool heater, its just opposite application. One remove heat, the other add heat. Stainless steel is never used as heat exchanger on marine engines, most probably due to the fact that it does not do well in conducting heat as copper would and probably that poor poor property when corrosive waterflow is stagnant. Temperature accelerate corrosion of metal in salty water to a great deal, much worse than water velocity. If there is a cupro-nickel type pool heater, I am sure it will do well in salt water pool. I don't need a pool heater so I never pay attention to what the market offer.

That's all I can contribute for metal corrosion in water...pool, saltwater pool..whatever..