Electrolysis of Stainless Steel?

[TreeFiter]

I've come across a pool that has issues with stainless steel wall panels rusting. Its a very old pool with a concrete bottom and stainless steel walls. At the joints between the walls, there is almost always an accumulation of rust. Someone suggested that it is happening because of the salt water. So I'm looking for someone to fill in the blanks for me on this one. I don't know much about how salt water reacts with stainless steel, but I have a feeling that someone on this forum might be able to help me out.

The only thing I've seen in my experience that might combat any corrosion to metals in the pool associated with salt water would be to add a zinc anode. I only slightly understand how a zinc anode works, so if someone wants to explain that to me as well, it would be greatly appreciated.

This isn't a pool that I have been taking care of regularly, so I really don't know anything about the water chemistry. Typically the owner had the company I work for open and close the pool, and I would usually do the initial vacuuming. So other than one visit per year, I have no idea what is being done to it.

 

[JamesW]

Since the rust is at the joints, the rust might be from the bolts that hold the panels together. Can you see actual damage to the stainless steel panels? Are the panels painted?

 

[JasonLion]

There are various grades of stainless steel. Lower grades will rust with or without salt. Higher grades will not rust regardless of salt. In the middle there exist grades that will rust eventually without salt, and noticeably more quickly with salt.

If you are only observing problems at joints, than the problem almost certainly has something to do with the method of attachment. For example, lower grade steel is sometimes used when welding, which could cause welds to rust even though the main panels do not.

 

[PADPOOL]

When the stainless sheets are cut to size with a ferrous blade or wire wheels this often happens. Check your grounds also, a stray current accelerate corrosion, especially when there are dissimilar metals mated together. Look up galvanic corrosion.

 

[chem geek]

As described in this link in the EPA "Report on the Corrosion of Certain Alloys" in section VII "General Corrosion of Stainless Steels":

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. Oxidizing chlorides such as ferric or cupric chloride are specific for severe pitting, although halide salts can cause severe pitting and stress corrosion cracking. The austenitic stainless steels are, however, the most susceptible of all the stainless steels to “chloride” stress corrosion cracking.

The chloride from salt interferes with reformation of the passivity layer in stainless steel that protects the iron in such steel from corroding. Also, increased salt levels increase the electrical conductivity of the water which also increases the rate of metal corrosion.

As others noted, having the corrosion specifically at the joints may be slightly dissimilar metals. The stainless steel shell, as with any metal touching pool water, should be connected to a bond wire and you can reduce corrosion by electrically connecting a zinc anode to that bond wire (or directly to the stainless steel shell, if more convenient) and bury the anode in moist soil. The way that works is that the zinc is able to give up electrons more readily than iron so imparts a small negative voltage to the stainless steel shell that makes it harder for the iron to give up its electrons to corrode. Basically, the zinc corrodes into the soil and sacrifices itself so that the iron doesn't corrode (or corrodes more slowly).

 

[jumpinjackieflash]

I have never seen or heard of a stainless-steel lined pool. Is this more commonly done in certain parts of the US? Is it as costly, say, as tiling the entire pool with 1x1 tiles? I looked for images on the internet but I can't really find any that make it plain that I'm seeing SS. Plaster over gunnite is what is normally done here, and if you have money to burn, you can tile the entire floor and walls.

 

[Tepelus]

The anode zinc is called a sacrificial anode. It is corroded in the water instead of the steel that is in contact with it. It is widely used in boats. Zinc is also used in modern cars as a supplemental layer providing rust protection on the same principle. Stainless steel is adversely affected by welding. High temperature and/or incorrect welding material results in a small but important area at the welding site where you have an alloy closer to iron than to stainless steel.

 

[JayBauman]

The anode zinc is called a sacrificial anode. It is corroded in the water instead of the steel that is in contact with it. It is widely used in boats. Zinc is also used in modern cars as a supplemental layer providing rust protection on the same principle. Stainless steel is adversely affected by welding.

+1

Zinc is great for these purposed. In fact, galvanized steel/iron is simply metal that has had a thin layer of zinc plated onto the outer surface.

 

[JasonLion]

jumpinjackieflash, SS pools are very very rare and quite expensive.

 

[Tepelus]

Zinc is available as a spay panint as well. Clean the rusted area and spay a zinc layer before any other protective paint.

 

[JasonLion]

Zinc is available as a spay panint as well. Clean the rusted area and spay a zinc layer before any other protective paint.

That isn't a good idea. The zinc will corrode first, causing the paint to fail. You want the zinc electrically connected to the other metals but placed somewhere where it's corrosion will not cause secondary problems.

 

[swimcmp]

The stainless steel pool is usually coupled with a concrete floor done in a commercial application. We put a salt chlorine generator on one and had to remove it as there wasn't enough grounding points and we had a corrosion problem. It has to do with stray current off of the generator, a sacrificial anode will help but most likely will not cure it. Likely you will have to remove the salt system.