Underwater hose clips

Attack by what, precisely? Not oxygen....

The problem was long, close fitting, bolt holes. Plenty of oxygen in the water around the boat, but not enough getting to the surface of the stainless steel.

So the bolt was rusting from the inside out?

If the bolt is in a hole where the oxygen cannot get, then the bolt cannot 'rust'.

One of the first chemistry experiments I did, consisted of pitting a nail in a sealed bottle of distilled and boiled water. It did not rust. Another one in an unsealed bottle of half water and half air, did.

No, of course not. it always starts at the surface of the steel. The problem is that the oxygen level drops to a value where the passivation layer cannot be maintained but there is enough for rusting to take place.

But it isn't. The oxygen can't get there quickly enough to maintain the passivation layer.

Yes, I did that one too, but not with stainless steel nails where one end was in an oxygen-rich environment and the other in a low oxygen environment.

John

If you still don't believe, how about the British Stainless Steel Association as a reliable source of information?

John

Right, so what is happening is NOT rusting. It is reactions with chlorides - not oxygen.

Like salt on aluminium if water is present. Guaranteed to rot in the absence of oxygen.

because it cuts through the Al2 O3 layer

And has absolutely no bearing on corrosion in a fresh water environment and does not cause 'rusting' as such.

Chloride ions are a powerful catalyst of the corrosion of iron. That doesn't stop the final result being mostly iron oxide. Whether that takes the form of "rust" or a slimy black sludge doesn't make much difference to whether the object falls apart or not. Fresh water is certainly much more benign than sea water, but it still contains some dissolved salts in most situations. John

Actually it does,

Ferrous chloride is green in color. The 'sticky black sludge' is probably ferric chloride. Rust is red. And brittle

Whether that takes the form

But not chlorides. Mainly carbonates. Possible a bit of nitrates.

Ferric chloride was brown the last time I looked at it. The black material in stainless steel corrosion products may well be carbon from the steel. (Even the 316L alloy used in medical implants has some carbon and some other alloys have significant amounts.)

Well, that is debatable. A pond filled from tap water is likely to contain some chlorides. Any pond within a few miles of the sea certainly will. Fresh water in rivers also contains chlorides. After all, how else did the sea get salty in the first place? Whether those concentrations matter is a different question. The OP expressed concern about whether a hose clip would "rust". We both know perfectly well that not all corrosion of steel is "rust" and it doesn't matter anyway. The outcome of interest is whether the hose clip falls apart on not. A low oxygen concentration will not guarantee that it survives for a long time and will sometimes make it more likely to fail.

John

Well without carbon it aint steel, but yes, ther will be some carbon in there.

as Colin mentioned 316 grade SS is good ..... 304 is the cheaper grade and not as good on corrosion resistance.