How to distinguish 304 stainless from 316 stainless?

[pvb]

This is a non-boaty question. I recently bought a new stainless steel ladder for a swimming pool. It was supposed to be made in 316 stainless. After only a couple of months' use, a lot of rust appeared where one plastic tread joined the stainless tubing. On dismantling the ladder, I found a significant corrosion cavity in the tubing. I suspect that the ladder was not made in 316 stainless as claimed. Is there a quick and cheap means of finding out whether or not it's 316?

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[Graham_Wright]

This is a non-boaty question. I recently bought a new stainless steel ladder for a swimming pool. It was supposed to be made in 316 stainless. After only a couple of months' use, a lot of rust appeared where one plastic tread joined the stainless tubing. On dismantling the ladder, I found a significant corrosion cavity in the tubing. I suspect that the ladder was not made in 316 stainless as claimed. Is there a quick and cheap means of finding out whether or not it's 316?

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Quick reply is no but the magnet is a good start. Unless the product has been hammered, 316 should show no magnetic properties. 304 may show some magnetic effect but it is not conclusive.

Rust spots can be the product of ferrous swarf deposited on the stainless but that would not penetrate.

 

[VicS]

This is a non-boaty question. I recently bought a new stainless steel ladder for a swimming pool. It was supposed to be made in 316 stainless. After only a couple of months' use, a lot of rust appeared where one plastic tread joined the stainless tubing. On dismantling the ladder, I found a significant corrosion cavity in the tubing. I suspect that the ladder was not made in 316 stainless as claimed. Is there a quick and cheap means of finding out whether or not it's 316?

Analyse for molybdenum!

30 years ago I could have done that ( or given it to the colleague who did all the metal analysis )

Its odd that its affecting the ladder where one particular tread is fastened. If the entire thing was made of the wrong alloy then you expect to see the same elsewhere.....

 

[pvb]

Analyse for molybdenum!

30 years ago I could have done that ( or given it to the colleague who did all the metal analysis )

Its odd that its affecting the ladder where one particular tread is fastened. If the entire thing was made of the wrong alloy then you expect to see the same elsewhere.....

How easy is it to test for molybdenum?

I agree that the isolated corrosion is odd, although the other end of the same tread was starting to corrode too.

 

[Mistroma]

How easy is it to test for molybdenum?

I agree that the isolated corrosion is odd, although the other end of the same tread was starting to corrode too.

Shouldn't be that difficult if you can get your hands on some hydrochloric acid, some potassium thiocyanate and perhaps sodium thiosulfate. I could probably have rustled up the stuff I needed when I was about 10 as I had a Chemistry set. I wonder if Chemistry sets even exist these days? You can have fun with iodine crystals and ammonia solution, err probably not.

 

[VicS]

How easy is it to test for molybdenum?

I agree that the isolated corrosion is odd, although the other end of the same tread was starting to corrode too.

Easy with access to an ICP- OES set up for metal analysis, and can use it. Even easier if you can say, "Hey Barry, can you look at that please and tell me if it's 316 or not? "

 

[penberth3]

...Its odd that its affecting the ladder where one particular tread is fastened. If the entire thing was made of the wrong alloy then you expect to see the same elsewhere.....

Can't tell from the photo, but if it's at or near water level - more oxygen available?

 

[daveyw]

How are the steps attached to the stainless frame? Since it only appears to be affecting that one area I would suspect the bolt/screw is corroding and you are seeing the staining from that

 

[vyv_cox]

What you have is a beautiful example of crevice corrosion. Makes little difference if it is 304 or 316 or even higher grades. Lots more examples on the website.

 

[VicS]

How easy is it to test for molybdenum?

I agree that the isolated corrosion is odd, although the other end of the same tread was starting to corrode too.

I reckon its due to "stray currents" .... not got an electro-chlorination plant by any chance ?

 

[VicS]

What you have is a beautiful example of crevice corrosion. Makes little difference if it is 304 or 316 or even higher grades. Lots more examples on the website.

You are saying that the damage to the tube in the picture below is due to crevice corrosion ? In a couple of months ?..... I thought it was due to pvb being clumsy with an angle grinder

 

[rszemeti]

Looks to me like corrosion due to the item not being passivated after it was welded.

Stainless MUST be passivated after welding, or you will get corrosion, even on 316. When you weld, the iron comes to the surface and unless you etch it away, it will begin to rust, in a process that will eat away at the material deeper and deeper. Passivation removes the iron, leaving a chrome-rich surface.

You can passivate the weld either with "pickling paste", which is a mix of Hydroflouric and Nitric acid, or with an elecronic weld cleaner, which uses a citric/phosphoric mix ... I have a Fronius one, they are about a grand, but well worth it if you do a lot of this stuff.

 

[rszemeti]

https://www.thewelderswarehouse.com/Welding-Supplies/Pickling-Paste.html

Works a treat, but DO NOT get it on your hands, or any other part of the body. Or on any body else, or your pets. Really nasty stuff!

 

[VicS]

Looks to me like corrosion due to the item not being passivated after it was welded. .

but there is no weld !

 

[PaulRainbow]

You are saying that the damage to the tube in the picture below is due to crevice corrosion ? In a couple of months ?..... I thought it was due to pvb being clumsy with an angle grinder

It does look a bit like grinder damage, but the rust stains suggest that the damage was under the plastic tread, not convinced PVB would destroy the ladder to see why it was rusting.

Perhaps he could clarify ?

 

[PaulRainbow]

Looks to me like corrosion due to the item not being passivated after it was welded.

Stainless MUST be passivated after welding, or you will get corrosion, even on 316. When you weld, the iron comes to the surface and unless you etch it away, it will begin to rust, in a process that will eat away at the material deeper and deeper. Passivation removes the iron, leaving a chrome-rich surface.

You can passivate the weld either with "pickling paste", which is a mix of Hydroflouric and Nitric acid, or with an elecronic weld cleaner, which uses a citric/phosphoric mix ... I have a Fronius one, they are about a grand, but well worth it if you do a lot of this stuff.

Whilst generally correct, it isn't entirely true that you "must" passivate stainless using either of the methods you describe. It's certainly true that doing so will cause the surface to be immediately passivated, but provided the surface is clean it will self passivate. If freshly welded stainless is cleaned and polished there is no need to passivate, it will self passivate. If it isn't cleaned it could be rusty the next day.

 

[pvb]

What you have is a beautiful example of crevice corrosion. Makes little difference if it is 304 or 316 or even higher grades. Lots more examples on the website.

Thanks, Vyv, I did wonder if that was the cause. The plastic steps are a tight fit to the tubing, and are then clamped by a bolt through the tube and a nut under the step.

If I were to re-assemble with Sikaflex or similar sealant between the step and the tube, might this prevent crevice corrosion?

EDIT: The ladder supplier has said that connecting an anode to the ladder underwater would stop the corrosion. Any thoughts on that?

 

[pvb]

I reckon its due to "stray currents" .... not got an electro-chlorination plant by any chance ?

Yes, I have a salt-water chlorinator. But the ladder is mounted in plastic fittings sunk into the floor, which would insulate any current to earth.

 

[pvb]

It does look a bit like grinder damage, but the rust stains suggest that the damage was under the plastic tread, not convinced PVB would destroy the ladder to see why it was rusting.

Perhaps he could clarify ?

Yes, the corrosion was where the plastic step fits tightly against the stainless steel tube.

 

[pvb]

How are the steps attached to the stainless frame? Since it only appears to be affecting that one area I would suspect the bolt/screw is corroding and you are seeing the staining from that

No, the bolt/nut were fine.