How to distinguish 304 stainless from 316 stainless?

[rszemeti]

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.

True, if you can adequately clean and polish the entire welded area and HAZ, then yes, it will self passivate, in practice, it is often difficult to get into the sharp corners where the weld meets the material and that is where the corrosion starts. In most cases. In a good TIG or MIG weld on stainless there should of course be no sharp crevices .. but it is usually better to get the pickling gel out, is it gets into all the tight spots.

We used to manufacture a lot of assemblies for oil rigs etc, and stanless tanks for breweries ... the electric weld cleaner was very handy for that.

 

[vyv_cox]

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?

I cannot see that an anode would have any effect at all on crevice corrosion. As ever, keeping the water out is the best way to prevent corrosion. I spent several hours last year bedding all the fittings on my transom on Sikaflex (or the cheaper Puraflex) in the hope that this would eliminate the rust stains due to crevice corrosion. That would be my action with your ladder.

Edit: with your permission I will use the photo on the website, please.

 

[vyv_cox]

Just an additional comment. A couple of years ago I met with a man who manufactures stainless valves and fittings, supplying them worldwide. He told me that crevice corrosion occurs in just about every version of stainless steel, including some very expensive super-alloys. This aligns with my own understanding that any alloy that relies on a passive oxide layer for corrosion protection is susceptible. There are many others apart from stainless steel that fall into this category.

The only alloy he had found that was immune is Incoloy 901, a very expensive high nickel alloy. It is used for Rolex watch cases as apparently it has some immunological properties.

 

[pvb]

I cannot see that an anode would have any effect at all on crevice corrosion. As ever, keeping the water out is the best way to prevent corrosion. I spent several hours last year bedding all the fittings on my transom on Sikaflex (or the cheaper Puraflex) in the hope that this would eliminate the rust stains due to crevice corrosion. That would be my action with your ladder.

Edit: with your permission I will use the photo on the website, please.

Thanks, Vyv, I'm currently trying to get either a new ladder or a full refund. When I fit a new ladder I'll try sealing the interfaces with Sikaflex.

Yes, I'm happy for you to use the pic on your website.