Protect Metals submerged in swimming pool

[mrelet]

I am going to build and submerge test hardware in a swimming pool. What type of stainless and what type of coating for alumium is recommended for long-term submersion?
Should sacrificial anodes be considered?

 

[TVP]

Can you more precisely define long-term? One month? One year? One decade? Is the external structure/housing going to be stressed during immersion, or merely holding the guts of the test instrument? Will the hardware be attached to something underwater, or just immersed?

 

[mrelet]

The open tank (i.e swimming pool) will be stainless steel, frames will attach to inside of tank, polcarbonate sheets will attach to the frames. The frames will see about 100 lbs deadweight, and 100 lbs live load under test conditiotns that last less than 1 minute. The open tank and frames will be in constant contact with the pool water.
None of this is built yet, so it's all adjustable.

 

[CoryPad]

Constant contact for how long? If the structure is to last one month, unpainted steel. If it is for a year, galvanized steel. If it is for a decade, 6 % Mo stainless. More details (e.g. Cl^- concentration) allow us to give you a better answer.

 

[gr2vessels]

mrelet,
The water in the swimming pool can be contaminated and chlorinated to the extent that even the ordinary stainless steel may corrode in relative short time. The temperature is also important, as it can accelerate the corrosion rate. Sometimes, the small kids in the pool might have an 'accident', contributing to the corrosive contaminants. Birds, algae and other contaminants in an open pool tend to aggravate the corrosion rate. You should also visit a public heated swimming pool and look at the roof supporting structure how rusted it is, perhaps have a look at the stainless steel fittings around the pool, some of them are quite rusted, particularly if they were in contact with some carbon steel components. Most of that rust is due to the vapour condensing on metallic surfaces or worst, submerged in the pool water. The chlorine corrodes everything at the given temperature of this heated pool. So try to be a bit more specific with the pool water chemical composition, particularly in chlorides are present, the water temperature and any other contaminant which could be in the pool and is not normally present in, say potable water. Then you might get in this forum a professional advice on your project.

Cheers,
gr2vessels

 

[mrelet]

gr2vessels and CoryPads,

The environment is heated (80 to 100 F)and treated water but does not otherwise see the normal pool loads, i.e. no people, few biologics. So, it probably does not require strong oxidizers to keep the water clear. As a test tank, it will be in use for 20 years or more. The fixture frames can be stainless steel or anodized aluminum but can be easily replaced. The important thing is that the tank envelope last 20 years. I am reading here and elsewhere that I should limit stresses and wear on submerged stainless steel, especially around chlorine/bromine. I can use a non-chlorine treatment that is a less effective treatment because I have no bathers, I'm inside a building, and I will cover the tank at the end of each day. To limit metal stress I can reduce bolted fittings on the inside walls of the tank. And to reduce condensation on other metal surfaces surrounding tank I can have good ventilation followed by covering the tank.
What non-chlorine treatments are recommended?
Is 6 % Mo stainless the recommendation as from CoryPad?
Can I get a 20 year tank in the end?

 

[EdStainless]

Many swimming pools built in 304 have survived for 20 years or longer. The biggest problems are along the water line.
You also have to avoid dissimilar metals. A steel nut or bolt dropped in can start localized pitting that can get serious.
If I was doing this today I would look at using 2205. This material is very strong and much more corrosion resistant than 316. It also has a reasonable cost.

For any stainless make sure that the welds are pickled before it is put into service. This includes the weted side of where braces are welded to the outside. If there is any discoloration pickle it.

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Plymouth Tube

 

[racookpe1978]

Sounds more like your making a specialized test pit for testing products under controlled conditions that will resemble a commercial pool for your company
- so it will be the sole and immediate user that will maintain it under your test facility's control, rather than be making a "dunk kit" that gets dropped into a commercial/government open-water swimming pool.

Right?

 

[gr2vessels]

Under the conditions described, the water will be pretty clean, the temperature very mild, so I cannot see reason for using stainless steel better than 304 or 316, whichever cheaper. As long as you keep the contaminants out, the 'swimming pool' should be OK. The chlorides tend to induce a stress corrosion cracking of the stainless steel, in the area of poor design (high stress in operation) or areas of poor manufacture (bad welding procedure, bad workmanship, loss of corrosion resistant properties due to carbide precipitation, senstitization left untreated, etc...). Make sure to have it fabricated by the manufacturer specialised in stainless steel, with appropriate tools, welding procedures and a reasonable QA system.
Cheers,
gr2vessels

 

[CoryPad]

I think it is not likely to avoid high stress concentrations and crevices, and it is likely that some areas are not pickled and passivated perfectly, so I don't think 304 or 316 would be best. Duplex 2205 could work, and 6 % superaustenitics definitely provide long life in submerged applications.

 

[gr2vessels]

I always believed that for stress corrosion cracking you need something to provide the corrosion bit (chlorides for eg.). I might be wrong, so how is the SS 304 going to be affected (and its crevices) by the clean water with no chlorides, temperature up to 100F ? I assume that the design and fabrication will be completed by reasonably professional people, including correct welding, avoiding Fe contamination, pickling / passivation.
Cheers,
gr2vessels

 

[CoryPad]

The thread originator stated he can use a non-chlorine water treatment, but didn't say what that would be. I suspect that is a wish, not a reality.

 

[mrelet]

CoryPad,
The non-chlorine treatment I found is ECOsmarte. They describe it as a "copper ionization technology" and "Oxidation is achieved when water
passes through the titatnium electrode plate with a
low voltage electrical current".

I don't know, but the copper may be corrosive to the stainless steel.

http://www.ecosmartepool.com/spa-treatment.html

 

[CoryPad]

The Ecosmarte site mentions oxidation in addition to the copper ions, specifically the generation of oxygen. This can affect the corrosion rate of metals in the system. Without Cl^-, I would wager that any stainless steel alloy would work, but you should get a detailed water survey with the Ecosmarte system to ensure the integrity of your test structures and equipment.

 

[EdStainless]

There are units that dissolve Cu into the water in order to kill microbes (yes Cu is toxic). The Cu bars need to be replaced periodically since they are consumed in the process.
This little unit look fine. Since you won't have a high organic load in the tank this should be plenty. Don't worry about too much Cu, it would take a heck of a lot to cause problems.

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