I have been using Wickes version of aquapanel screws (all 40mm), ceramic coated. I like them, and having been told that stainless steel
*can* still rust, I would like to use ceramic coated screws where they will be embedded in concrete. But I need some 50mm long. Where can I get them ? TIA, Simon.
Just been looking on US sites for decking screws. They rate them as zinc coated = good, ceramic coated = better, stainless steel = best. Maybe I should use stainless steel after all. What d'y'all think now ? Simon.
Because you can get stainless virtually everywhere and in any size I've been using them almost exclusively for everything but concealed work where theres no chance of damp or water. Never had any problems for what must be at least
3-4 years now on showers, wet rooms, kitchens or external work.
I guess the stainless steel corrosion is under some very particular or harsh environment. My screws will be embedded in concrete as part of a composite panel I am making. I'm sure SS will be OK. Cheers, Simon.
Stainless steel will rust if is immersed in stagnant water - that is water with very little dissolved oxygen. AIU this is because the rust resistance of stainless is provided by a surface coating of some kind of oxide which needs to be renewed continually.
A well known problem with keel bolts on yachts, small amounts of moisture (stagnant seawater/bilgewater) and no oxygen can mean hidden but sometimes huge cross sectional area losses. Sometimes this is picked up on survey but sometimes the boat just looses its keel, and is never seen again. Stainless *needs* oxygen to stay rust free.
ISTR also that there are several "grades" of stainless, to suit different purposes.
The message from "dcbwhaley" contains these words:
Chromium. The trick is that chromium oxide isn't more bulky than chromium 'cos of the way the oxide forms. This means that it doesn't flake off and reveal more fresh metal underneath. Iron oxide is much bulkier than iron so it puffs up and reveals more metal to rust some more.
There are literally hundreds of grades, basically falling into three basic groups. But no matter what grade is used It won't stop them severely corroding in the conditions described.
...
Yes, but even double vacuum annealed implant quality stainless will still corrode under the conditions described.
Colin Bignell
Indeed, but my point was really that the term "stainless" on its own is insufficient. A great many people think it describes the properties of a single material.
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