Does PVC Sleeve Corrode Stainless Steel Tube

[briand2]

A supplier of stainless steel tube states the following with regard to sleeves used to route that tube (the underline emphasis is mine):

Sleeves shall be of a material capable of containing or distributing gas, e.g. copper, steel, polyethylene, or other suitable plastic material.
Care should be taken to ensure that PVC does not come into contact with stainless steel owing to the risk of corrosion.

I've not come across this issue before (although I have seen reports of stainless steel wire rope corroding inside a PVC sheath, but this just because sea water got into the gap between the wire rope and the sheath).

Is it really likely that if a stainless steel tube is run inside a PVC tube, contact between the two materials may lead to corrosion (more than if any other plastic had been used for the sleeve)?

Thanks, Brian

 

[DiplingAndre]

The problem is the chloride in pvc. Chloride-ions destroy the passive layer and block the repassivation of stainless steels so pitting corrosion can occur.
But I think chloride-ions are only emitted under special conditions (temperature, pH).

 

[briand2]

Whilst I realise that chlorides have a detrimental effect on stainless steels, I'm not convinced that the chlorides can somehow leach out of PVC tubes to attack adjacent stainless steel tubes.

Also, PVC Coated Stainless Steel Wire Rope is available for yachting, although some people don't use it because of reported ingress of sea water causing corrosion in the space between the coating and the wire rope. Another instance of stainless steel products incorporating PVC is stainless steel braided brake hoses for motorcycles, for which the marketing material states "If stainless steel braiding seems a bit pass you can have it with a coloured PVC coating". Also available is PVC backed Stainless Steel flooring for outdoor usage.

If the manufacturers of these, and other, products are happy to incorporate PVC in close contact with Stainless Steel, and all in outdoor / arduous conditions, why should another manufacturers make the global statement to enscure "PVC does not come into contact with stainless steel owing to the risk of corrosion"?

Thanks, Brian

 

[gr2vessels]

Brian,
You should stick to what you know;- any PVC will be a very stable polymer, to the limit of its temperature rating. Over that, who cares, it should not be used. My bet is that a young over-zealous salesman with limited understanding of the modern PVC manufacturing processes wrote that advice from his granpa.
However, it is true that PVC pipes for water mains manufactured prior to 1977 contained elevated levels of vinyl chloride which they were prone to leaching (Flournoy et al., 1999). But that was then...
cheers,
gr2vessels

 

[briand2]

gr2vessels,

Thank you for adding your experience / knowledge!! I was in any event going to question the manufacturer's statement regarding contact of his product with PVC tubing (as that's how the product has already been installed); I'll now feel much more comfortable doing so, and I suspect he'll have difficulty showing how, in normal circumstances, there will be any adverse effect on his product.

Brian

 

[EdStainless]

I have take recent samples of commercial PVC drain line and tossed it into hot DI water. The elevated Cl levels are measurable, but much lower than with older PVC products.
The coated cable that I have seen is Nylon coated.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[stanweld]

Plastic sleeves can provide crevices at the sleeve to stainless steel surface interface suitable for crevice corrosion if chlorides in solution concentrate. Have experienced such and once chlorides were appropriately concetrated, pitting corrosion was especially rapid.

 

[briand2]

stanweld,

I guess the question is not so much "can such pitting corrosion occur in the presence of chlorides" as "what are the chances of such chloride concentrations occurring in the circumstances described". These circumstances comprise a semirigid (bendable) corrugated stainless steel pipe jacketed with a yellow polyethylene cover, say 25mm / 1" outside diameter, routed through a PVC pipe / conduit, say 38mm / 1.5" inside diameter.

Even allowing for the polyethylene cover being torn in a few places, resulting in the stainless steel pipe touching the inside of the PVC pipe / conduit, how likely is it that such contact may result in (localised) corrosion? If it is likely, then how is it that other manufacturers as I've mentioned previously happily combine stainless steel with PVC in their products, even in fairly arduous conditions? Other examples of such a combination include PVC protective (!!!) coating on stainless steel architectural elements during transport / construction, stainless steel valve stems on PVC-body valves, tap / faucet filters made of (T304) stainless steel wire cloth moulded into PVC rings, Type 304 (18-8) stainless steel hot tap saddles for PVC pipes, PVC-clad stainless steel panels used to replace existing concrete pool walls, etc.

I'm not a materials engineer / corrosion engineer, but I'd really like to understand how such a conservative statement can be made (ensure that PVC does not come into contact with stainless steel owing to the risk of corrosion) given these many apparently worry-free combinations of stainless steel with PVC?

I appreciate that ingress of water containing chlorides into the annular space would be a problem, but this would occur regardless of the outer tube material.

Regards, Brian

 

[stanweld]

briand2,
I was not referring only to PVC. We had stainless steel (316) risers installed at gas lines to the meter. The sleeve between the PE pipe was PE. In certain locations, chlorides concentrated to initiate crevice corrosion under the sleeve as well as crevices induced through contact with stones. It all depends on the nature of the crevice and the chloride concentration.

 

[rconner]

Temperature might indeed be a critical factor, and it might not even be necessary for the two materials to be in intended contact. If (when?) there happens e.g. to be an accidental etc. fire involving pvc construction materials or for that matter trash in the vicinity (generating hydrochloric acid and worse), corrosion among other problems could be the result. [See e.g.

http://jfs.sagepub.com/cgi/content/abstract/7/3/145

and I suspect other references.]