[LMB] [OT] AKICOTL: metal cutting

Joel Polowin jpolowin at hotmail.com
Sun Oct 14 17:55:35 BST 2018


Pat Mathews <mathews55 at msn.com> wrote:

> If stainless steel is so problematical, how is it that my tableware,
> which had had some scratches etc from ordinary use, cleaning, and even
> use for things it was not designed for, has held up for 20 years?

Your stainless steel is doing exactly what it should.

Chromium and aluminum are very reactive with oxygen.  But solid chromium
and aluminum, when they contact room-temperature normal-pressure air,
don't continue to react.  Instead, they form an oxide layer only a
few atoms thick, which physically blocks the metal from the oxygen.
This layer is fairly tough, and so thin that for all usual intents
and purposes, it's invisible.  If it's scratched, the exposed metal
instantly reacts with oxygen and forms a new oxide layer.  The same
effect happens if the metal is scratched under water.  The oxide layer
doesn't react with plain water at normal pH values.

Iron is different.  It forms an oxide layer like that, but that layer
isn't as effective a barrier.  Moisture breaks down the barrier, and
allows oxygen to contact the metal, letting the reaction continue.
Result: iron oxide (specifically ferric oxide) AKA rust.  And rust is
relatively porous and crumbly, so even if there's a thick layer of it,
moisture and oxygen can still get in.

Stainless steel is mostly iron, with some amounts of chromium and
other metals dissolved in.  When it contacts water, the outermost iron
atoms dissolve away, until chromium atoms are exposed.  When enough
chromium atoms are exposed, you get that chromium oxide layer, and
the reaction stops.

That's in plain water.  If you've got a lot of stuff dissolved in
the water, that changes things.  If it's salt, the water becomes
electrically conductive, which opens the possibility of electrochemical
effects if several different metals are in contact, or even just if the
metal is under stress.  Chloride also surrounds metal atoms after they
have been dissolved, which promotes the tendency for the metal to react.
Sea water is very hard on many metals.

At high or low pH (that is, alkaline or acidic solutions), aluminum
oxide dissolves, wihch exposes the bare metal to its environment.
So: aluminum reacts "vigorously" with acidic or alkaline solutions.
For example, bleach is quite alkaline, as well as being strongly
oxidizing, and it releases chloride when it reacts.  So bleach chews
up aluminum rather quickly.

And some things that are labelled as "stainless", or even "stainless
steel", aren't.  For example, the shower-curtain rings that my sweetie
bought at a very low price are rusting rather rapidly.  They haven't
been abraded by anything tougher than fabric and plastic, gently, a
couple of times.  The environment they're exposed to is plain air, with
pure water condensed on them a couple of times a day.  The package said
"stainless", but they obviously aren't.  It's remotely possible that
they're stainless steel under a plating of some other kind of metal,
which would open the possibility of an electrochemical effect... but
why would someone do that?  And I've got a cheap set of "stainless"
cutlery in which a couple of the knives have developed deep pits at one
of the serrations.  They're almost certainly really just chrome-plated
regular steel.

Joel


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