After 3 weeks.... corrosion!!!

[BIC]

So how is the SS screw going? Any signs of corrosion?

Not yet :-D, I looked this afternnon (only after 2 days) and they look good.

Also, as I said in a previous post, I took a SS screw that I bought myself and put it in a panty hose and attached it to my ladder. This one has been in the pool for a while. I will take a look soon to see how it goes.

BIC

I took out yesterday the screw at had put in a panty hose (attached to the ladder). It is clean, no rust.

The new SS crews for the ladder are still looking good, no rust.

Question:

The skimmer screws are still showing a little sign of rust, but they don't rust quickly. It sounds like a very slow process. Can I replace those screws around the skimmer without any problem or should I not touch them? I know those screws are screwed throught plastic (the skimmer), the liner and finally the pool wall, so if I replace them one by one is it going to damage the skimmer (plastic) or the vinyl liner?

BIC

 

[JasonLion]

You want to replace them before the rust makes them unsound and difficult to remove. Also, as they rust they will release iron into the water, though that happens quite slowly. As long as you replace them one at a time everything should be fine. If you were to remove all of them at once things could move around and there could be problems.

 

[chem geek]

As was stated in a previous post, stainless steel is a mixture of metals (i.e. it's an alloy) and not a coating, but the way it works is that the chromium content of the stainless steel gets oxidized instead of the iron and the resulting (possibly hydrated) chromium oxide forms a very thin passivity layer. This is similar to what happens with aluminum which would normally be highly reactive -- aluminum oxide forms a thin layer that protects the aluminum underneath.

The chromium oxide is resistant to oxidation, but not impervious to it. So some of it is broken up, but then more underlying chromium is exposed and forms a new layer. So stainless steel does in fact corrode, but slowly over time under normal conditions and does not exhibit rust (it just gets very slightly thinner). At higher chlorine levels (say, without CYA in the water in indoor pools) the corrosion rate is accelerated. The other factor that comes in is the chloride level. At higher levels of chloride (i.e. salt) in the water, the chlorides interfere with the formation of the passivity layer, possibly by forming chromium chloride compounds instead of chromium oxide (the former are soluble in water so do not form a passivity layer). So the way to think about it is that there is a race between chlorine (and dissolved oxygen) oxidizing the stainless steel and the stainless steel "healing" itself through formation of a passivity layer. Higher chloride levels slow down the latter process so the net corrosion rate increases. If things get too far out of balance, then the iron will oxidize and form rust which will become visible. The other effect that occurs is that the corrosion becomes more vertical into the metal in what is known as "pitting" corrosion. So instead of corroding evenly on the surface, deep pits can form instead. This isn't always serious for a bolt, but is a real problem for a stainless steel tank (which is why you don't see those anymore in filters) and it's a problem if the stainless steel is under stress since the pitting forms a "crack" and is called stress cracking corrosion.

You can read more about stainless steel and corrosion in this EPA report. Read section VII "General Corrosion of Stainless Steels" in particular.

The main difference between 304 and 316 stainless steel is that the latter contains Molybdenum which apparently is more resistant to the chloride interference of passivity layer formation and is why such stainless steel (316 and 317) is often called "marine grade".

Richard

 

[BIC]

Just to give you an idea of the kind of rust I'm talking about regarding my skimmer screws. Here are some pictures of them:

http://img527.imageshack.us/img527/5093/img3302qu4.jpg

http://img508.imageshack.us/img508/6433/img3293hq8.jpg

http://img508.imageshack.us/img508/2447/img3287dh5.jpg

I'm afraid that if I screw new SS screws into the same holes, this will damage the plastic of the skimmer and maybe the vinyl liner. The new screws will create a new thread through plastic of the skimmer and will not screw correctly (slack).

Would you change them?

BIC

 

[Strannik]

it shouldn't be a problem to change them if you do it carefully.
Just change them one by one, and you can also put some silicon grease on the screws to help them slide into old holes

otherwise you will end up with rusted screws in a few years time, and it will be much harder to get them out

 

[The Mermaid Queen]

could you take one screw out, take it to the hardware store, and get the exact size in SS? I know at HD, they have a little thing* to measure screws and bolts. It looks like you have enough screws there that you could safely remove one from the middle of the top (above the water line) for an hour or so... Could be wrong, tho...

*ETA gage, the word is gage!!

 

[BIC]

I wil have a close look at the skimmer screws and will change them if they get worst. I don't want to wait until they get to rusty. I have now got my stabilizer to 70 ppm as recommended by waterbear . I also got my Calcium higher than 200 ppm (I was at 180ppm and Goldline recommends 200 - 400 ppm).

By the way, the NEW SS ladder bolts still look perfect, no rust 8)

The water is very clear and looks very nice. I'm still waiting for my TF pool Kit. For the moment I use a cheaper Kit that I bought. the kit always give me more FC than the two brand of strips I use ouince in a while (just to compare with Kit).

BIC

 

[BIC]

I changed my skimmer screws and will see if the new ones get rusty next summer. I can say that the old ones were not that rusty. I was able to remove almost all the rust on it (by rubbing them) after I removed them, so that was not that bad. The fact is that they were a little bit rusty, not as much as it could appear before I changed them.

The new ladder screws have not rusted after I put new SS ones. The old ladder screws were just a very bad quality and probably not SS at all.

BIC