[TheForge] stainless rust

[ries]

Bruce, you are a scientist, and I am not, but it has not been my  
empirical experience that stainless steel will spontaneously rust.

I have seen, and made, a lot of stainless objects that sit outside,  
for years, and sometimes decades, and do not rust.
On the other hand, I have seen rust appear in just a few days, where I  
have scraped a piece with a forklift fork, or imbedded a bit of mild  
steel mill scale in a piece due to sloppy cleanup practices.
So I know that contamination from mild steel will cause small rust  
spots.
However, I have pieces where I have used something like a 3M pad on a  
4 1/2" grinder, and physically removed the rust spot, and then come  
back 2 years later, and still no rust.

So its my story, and I am sticking to it, that most rust on stainless  
is caused by polluting the surface with mild steel inclusions.

I know that under normal circumstances, stainless forms a chromium  
oxide layer, which does not rust.

In Seattle, near where I live, there is a large outdoor stainless  
sculpture that was installed in the mid 70's. As far as I know, no  
passivation, no electropolishing, just sanding with a large sanding  
disc on a big 9" grinder. 30 odd years later, in Seattle weather,  
which is wet, to say the least, no rust.

Now if you forge stainless, and do not either take it up to yellow and  
quench it, or physically clean the surface, you can get some rust, as  
there has been some kind of phase shift in the state of the metal- you  
can probably explain this better than I- but under normal  
circumstances, even wire brushing (with a stainless wire brush, of  
course) can remedy this.
I have a set of large, heavily forged towel hooks outside by my hot  
tub, a mile from the Salt Water- and after 5 years or so, with just a  
fresh forged surface that was wire brushed, still no rust.

All my experience tells me that stainless wont rust on its own- you  
have to give it a starting point.

ries


On May 12, 2009, at 8:59 PM, Bruce Freeman wrote:

I may be wrong, but I expect the citric acid will remove existing rust
but not prevent further rusting.

Stainless steel is largely iron with typically >25% chromium and
nickel.  The iron at the surface will rust.  Passivation is the
removal of the surface iron to prevent the rust.  Note that SS will
not rust AWAY.  Nitric is a good choice to remove surface iron because
it is an oxidizing acid.  It seems to me that any passivation
treatment would require an oxidizer and either an acid or possibly an
alkali.  If someone knows a way of passivating without using nitric
acid, I'd be quite interested.

On Tue, May 12, 2009 at 11:25 PM, Steve howell <ballardforge at msn.com>  
wrote:
> Thanks Jim,
> I think I'll go with the Citrisurf. I try not to handle any more  
> muriatic than necessary.
>
> Steve
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-- 
Bruce
NJ

The total lack of evidence is the surest sign that the conspiracy is  
working.
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Ries Niemi
Industrial Artist
http://www.riesniemi.com/