Can SS rust?

[Steve Jenkins]

Learned something

[Malcolm McLeod]

The die used to stamp the words is not stainless steel and some of it scuffs off on the knife during the process. This is typical with many stainless items.

Learned something

I have heard the same thing about many SS fittings & fasteners used in industry. The corrosion resistance of most SS types can be compromised by the use of improper tools.

A high carbon tool (steel), when used to install/remove a SS part, can actually embed carbon into the SS - especially in high torque applications. I hear the term "carbon transfer" used. The carbon rich area becomes a hot-spot for corrosion to initiate - and I believe it will spread (caveat - been a long time since Metallurgy class). I haven't looked in many years, but some companies used to offer dedicated tool sets for use on SS.

Nothing like buying a new spendiferous-manufactiriffic-machitech-ROI-earning- ... thing, and have the resident wrench bender hurry over to fine tune it with the wrong tool box. Oops.

[Malcolm Schweizer]

"Stainless Steel" is a misnomer. All steel will rust. The addition of Nickle, Chromium, and Molybendum, depending on the alloy, causes the metal to react with oxygen in the air and form an oxidizing layer around the metal that causes it to inhibit rust. Here is a big catch: It requires exposure to air to work. Have you ever seen rust streaks coming from under the heads of stainless steel screws? That's because they didn't have sufficient exposure to oxygen to maintain the oxidizing layer.

There are too many alloys to discuss each one. I used 316 Stainless for my centerboard on the boat I'm building, which is very resistant to corrosion. Just below that would be 304, which is still resistant to corrosion, but has less resistance than 316. The difference would be that 316 has more nickel and also adds molybendum. One downside of stainless is that although it's tougher than carbon steel (believe me, I know, as I'm grinding a massive chunk of it for my centerboard) it can be more brittle. Many offshore sailors will not use stainless chain on their boats because although it's strong, it doesn't give like galvanized carbon steel will and will therefore snap before galvanized will.

[Malcolm McLeod]

... and form an oxidizing layer around the metal that causes it to inhibit rust...

I stand corrected, or at least reminded. For anyone who can't sleep...

I pulled out the handy research books and the oxide layer is the actual inhibitor (just like aluminum). I'd forgotten this detail.

And it also seems that my use of the term 'carbon transfer' is induced by my spending too much time in my wayward youth with evil and decadent machinists. In defense of them (OK - me), the use of 'carbon transfer' is due to most people seeing the effects of improper tool use on SS, rather than a manufacturing process error - like sandblasting SS with recycled iron-contaminated sand. So presumably to most, this IS a carbon steel transfer.

It is actually iron transfer that does the deed to SS. Iron particles from tools (i.e. dies) or other processes and fixtures damage the oxide layer, embed themselves, and provide the starting point for corrosion. Most quality suppliers of SS components should be able to eliminate the contamination during manufacture.

...Hello? Anybody still awake?

And hope the keel grind is going well, Mr Schweizer; still following your boat build. And don't use any grinding media that was used on carbon steels!!

[Lee Schierer]

Stainless steel should be passivated at the end of the manufacturing process to remove iron contamination for the welding, machining and bending processes. Iron contamination gives rust and corrosion a place to start.

[Malcolm McLeod]

I thought I might put everyone in a coma if I started in on pickling & passivation.... Nice to have another analytical around!

[lowell holmes]

Maybe you should wipe the steel to remove residue and hit it with Johnson floor wax. It worked for me on my Cal 34 sailboat.

[Bruce Page]

Any SS will corrode under the right conditions but some SS’s are much better resisting corrosion than others. 316L & 321 SS are often used in nuclear reactors and reactor test hardware.
I made 4 of these tie-down strap thingy’s for my gleaming new ‘95 Dodge Ram out of 17-4ph h900 stainless. The Ram has spent the last 23 years outdoors and isn’t very shiny anymore but aside from some dirt, the 17-4 tie-downs are as shiny as the day I made them.

[andrew whicker]

It's a chromium oxide layer that develops. The Moly and Nickel (and etc) are there for other properties. The chrome needs to be around 11% -12% minimum to create the oxide layer. 410 is the bottom of that limit, for example. But, as said above, you can heat treat it to strengthen it. This makes it a very useful type of stainless steel even if it does have less corrosion resistance. As someone else said above, "the more stainless, the softer". This is obviously way over simplified, but makes sense for the common stuff you'll find. The 300 series is softer because you can't heat treat it which means you can't strengthen it. It's a trade off between strength and corrosion resistance.

It gets all very nerdy, but the magnetism is basically used for everyday problems to let a person know if it is 400 series or 300 series (which is a huge oversimplification, but not everyone has to nerd about this stuff for work). My kitchen sink holds a magnet, so it is probably 400 series. I don't know what type exactly. I'm sure it's a common type (409 or 410). (The series number is actually a bit misleading if you know more, but it's a good simplification)

316 and generic structural steel have basically the same 'strength' (tensile), but 316 is harder (hardness). 316 also work hardens, which is why it is such a pain to machine.

Anyway, I have to nerd out about this stuff for work. Stainless steels and corrosion is SUPER interesting and I totally suggest going deeper into metals in general. It really is fascinating stuff. Lots of smart people figuring out difficult problems over a long period of time. Metals are very cool.

I personally don't think pickling is necessary unless you plan on using the stainless in an anaerobic (no oxygen) environment. You're not doing this if you put it in regular water. Pickling is basically a safety check to ensure that you have this magical Chromium Oxide layer before you put the thing into service. However, the layer self heals if loose oxygen is around. Lakes are aerobic, so you're fine.