Rust on stainless: Homogeneity of steel

[shecky]

Threads pop up where folks complain about rust on their stainless knife, and replies from others stating no rust at all on their knife made with the same steel.

In these anecdotes, it's impossible to account for environments, usage, and so on. Still, I find it striking how some describe significant rust even with knives facing no significant usage, while others have no problems using the same model.

Yesterday, I noticed a couple stainless bookshelves from Ikea in my house. They are one piece, fabricated from sheet metal, welded and joints ground smooth. Both identical, except one has rust. Not only that, one has rust only on a single panel. Both shelves hang on the wall next to each other and receive no significant difference in treatment.

It occurs to me that it's totally possible the one panel on the one bookshelf isn't the same kind of stainless as the rest of the material (though it still appears to be stainless), and would corrode differently. However, it also occurs to me that it may be the same grade of stainless, but from a different batch or source, and behaves differently because of variances in manufacturing when the metal itself was manufactured.

If the latter scenario is the case, it would seem to throw a monkey wrench in blade evaluations. If the corrosion resistance of a particular stainless can vary significantly from one batch of steel to another, is it not reasonable to assume that other characteristics can also vary? The question is, how much can a particular grade of steel be counted on to be homogeneous from year to year, factory to factory, knife to knife?

 

[Cliff Stamp]

The characteristics would be expected to vary significantly because the steel composions themselves can vary. Look at the range allowed for the 440 series stainless for example. The grades actually overlap with each other.

-Cliff

 

[knarfeng]

Rust is a fairly complex chemical reaction. Because there are so many factors that can affect the reaction process, the phenomenon isn't reliably reproduced. (Sometimes it happens, sometimes it don't.) Even in Salt Spray cabinet testing, it is common to run multiple samples because sometimes you see rust and sometimes you do not. This is on coupons cut from the same sheet of material and put in an environment that is as exactly uniform as possible.

The best corrosion folks can do is to tell you whether an alloy is prone to rusting in a specific environment, not whether a specific piece actually will or how much it will.

 

[Cougar Allen]

Once rust starts it grows fast, because it's spongy and collects condensation. I would not expect to see one with no rust and the other with only a tiny bit of rust, not unless you happened to observe it when the rust had just begun to start. What you're likely to see is either no rust at all or a lot of rust.

 

[guyfalks]

Surface finish also affects rust formation. (At least in my limited experience with structural steels and austenitic stainless steels.) Bead blasted will start rust faster and more evenly than a bright finish. Also, brushing the surface of stainless steel with carbon steel brushes can cause a good place for rust bloom. Slight scratches, rough finish, or imperfections in spots may be a possible cause of the differences.

 

[me2]

Bead blasted will start rust faster and more evenly than a bright finish. Also, brushing the surface of stainless steel with carbon steel brushes can cause a good place for rust bloom. Slight scratches, rough finish, or imperfections in spots may be a possible cause of the differences.

It was a heartbreaker, but I saw 6 hours worth of blasting ruined by the humidity increase during a passing storm. We were under a bridge being repaired, and 4 girders were blasted, and let me tell you, flowers arent the only thing that blooms when a storm passes. You could almost watch the rust grow it happened so fast. This was before any rain started. I've often wondered why knives are the only objects I know of where a bead/sand blasted finish is considered a final finish. I remember reading that sulfur was sometimes blamed for tiny sulfide inclusions in stainless steels that lead to pitting. Thats all the detail I remember, and I have no idea where the article went.