Australian 316 stainless - the good, bad & ugly

[Jim Cate]

Originally Posted by Guy

Steel tools damage the oxide coating and then contaminate the surface so it can't form an oxide coating again. Then the rust grows.

So, the oxide coating is "damaged", and some transferred material then prevents the oxide from reforming? Seems that the transferred Fe would itself oxidize. We're talking very thin bits of transfer here... how does that keep the Cr from oxidizing (I believe that it is the Cr oxide that actually makes it "stainless"). I'm having a hard time visualizing this process... and, having been using steel wrenches on s/s fasteners of various grades for many years, I haven't seen the heads or nuts being compromised in this way. There are dozens, perhaps hundreds of s/s fasteners on our boat. All have been tightened with steel tools. The only areas that are observed to corrode are where seawater gets trapped and becomes anoxic... like under a poorly bedded washer or such, and that is from a different form of corrosion I believe.

So, help me out some more on this subject, I'd like to understand better.

Jim

[Jim Cate]

Originally Posted by Kenomac

Jim,

By "work hardening" stainless 316, the molecular structure of the metal becomes altered, making it magnetic. Work hardening is the mechanical manipulation of the metal examples such as Hammering or cold working the metal to make it thinner etc.

Ken, I don't think we are talking about the same sort of issues here. I understand that cold working can change the paramagnetism and the metallurgy of 300 series s/s. I'm questioning the idea that use of steel tools in itself is damaging. Use of stainless, bronze or monel based spanners would not alter the deformation of the fastener from that inflicted by a steel tool as far as I can see.

Perhaps I am missing something...

Jim

[Kenomac]

Jim,

Stainless steel is usually sold in the annealed condition. It just means that the material is in the soft or annealed condition. The 300, 316 series of stainless steel cannot be hardened by heat treatment (like carbon steel) but can be hardened by cold working. This cold work can be eliminated by a heating treatment (annealing) that will restore the original soft condition.

It's possible and probable that the cheap rusty stainless was not annealed (heat treated) properly leaving it in a magnetic state. As I stated earlier, the internal molecular structure of the stainless is altered when it's work hardened. Think in terms of the crystalline structure of the metal where all the atoms are aligned in a specific way being banged, worked and pushed out of position, resulting in the metal now having some molecules normally on the inside, now on the outside. Annealing the metal will then return it to the soft, non-metallic state where the molecules line up in their original crystalline state.

Also, Guy is correct in post #29 regarding the scratching of the surface of stainless. Some of the metal from the tool is transferred onto the surface of the stainless, then surface rust will develop on the transferred metal.

[KASHMIR]

The point I was trying to make is that you cannot ALWAYS blame cheap stainless steel.
Stainless steel is still steel, and steel rusts.

[Wotname]

I'm waiting along with Jim for a better explanation of the claim that tooling is causing 316 fasteners to rust. I haven't experienced any such problems and I have been around SS almost as long as Jim - well maybe a decade less .

In addition, really good spanners don't (or rarely) rust so I'm assuming there isn't any ferrous material on their surfaces.

[Cheechako]

Tools like power driver tips etc do leave steel on the stainless. .... and it does rust some and rather quickly. From what I've seen, whether it degrades the SS may depend a lot on if the SS is nicely passivated to start with.
Flat bar , sheet and the like are often give a "Cold Pass" at the final rolling at the mill. This makes for more accurate size and stronger strength. (cold work) also, making screws on a screw machine cold works the screws as the material is heavily worked in the cutting process. I doubt if any screws are solution heat treated after cutting as the strength is desired.

[Cadence]

To be crass . I think all the talk about a steel tool causing rust on good quality 316 is nonsense.

[Cottontop]

Perhaps a few comments that will lead to more authoritative information available online (as this is from my memory when I worked with this stuff, and may be completely wrong).

A magnet is a good test, but not perfect. Some 304 will not exhibit magnetic reaction, and some 316 will. Both are unusual, and depend on how the material was initially produced and subsequently worked.

There is a better test, however. 316 has better corrosion properties than 304 because of the presence of molybdenum in 316 which is absent in 304. There is a test kit available; a drop of solution is placed on the piece to be tested and the reaction indicates the presence or absence of molybdenum.

Passivation is a chemical process (a nitric acid bath is common) that results in a chromium-enriched surface layer in 316 (now, 316L), that further enhances corrosion resistance and also provides some welding benefits.

[chala]

Originally Posted by Alii

Whew, how can a full-time cruiser stay so organized as to retain and find this kind of paperwork years on?

Prudence.

Originally Posted by Jim Cate

So, the oxide coating is "damaged", and some transferred material then prevents the oxide from reforming?

The transferred material does not prevent the oxide from reforming. To the contrary a passive film of chromium oxide will try to form on the transferred material. If there is not enough chromium available to create a passive film then a pit will start and the SS will keep corroding (pitting, crevice,)

Originally Posted by Guy

Steel tools damage the oxide coating and then contaminate the surface so it can't form an oxide coating again. Then the rust grows.

Originally Posted by Jim Cate

The only areas that are observed to corrode are where seawater gets trapped and becomes anoxic... like under a poorly bedded washer or such, and that is from a different form of corrosion I believe.

Good observation. In the same condition I do observe staining but rarely corrosion.

[model 10]

Originally Posted by chala

Prudence.
The transferred material does not prevent the oxide from reforming. To the contrary a passive film of chromium oxide will try to form on the transferred material. If there is not enough chromium available to create a passive film then a pit will start and the SS will keep corroding (pitting, cr

That is a bit pedantic. I said the tool damages the oxide coating, it does not re-form correctly and then it rusts.
The note about chrome makes me think you should use nice chrome tools