Corrosion Testing of 9 Stainless Knife Steels

Why rust show only in spots , why not on all surface ?
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Natlek said:
Why rust show only in spots , why not on all surface ?
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Probably because of the way droplets form when you spray liquids on surfaces. It seems that once the first rust spots formed further rust generally came from growth of those original spots. So even though I cleaned and re-sprayed specimens (new distribution of droplets) the result was spots. There may also be some metallurgical mechanisms such as areas of weak/missing chromium oxide layer but that is another layer deeper.
 
This is very informative. Thanks for taking the time to put this together.
 
Larrin said:
Probably because of the way droplets form when you spray liquids on surfaces. It seems that once the first rust spots formed further rust generally came from growth of those original spots. So even though I cleaned and re-sprayed specimens (new distribution of droplets) the result was spots. There may also be some metallurgical mechanisms such as areas of weak/missing chromium oxide layer but that is another layer deeper.
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How fast protective layer form on surface on fresh grinded steel ?
I found this ..............to remove free iron from surface ? That is only relevant on to say only for NEW steel or ?
When stainless steel is newly made, it is cleaned of oils and greases used in the fabrication process. Acid, usually nitric, is used to remove free iron from the surface. Slowly and naturally, a passive layer develops on the surface as the chromium reacts with oxygen in the air. This layer is invisible and only a few molecules thick, but it provides a barrier to prevent oxygen and moisture from reaching the iron underneath.
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And this .............
After stainless steel equipment has been installed and is in operation, the existing passive layer can be damaged or removed by physical abrasion (brushing, grinding, scraping) or by chemical reactions. It can also be weakened by physical damage due to expansion and contraction caused by heating and cooling. If this damage happens faster than the passive layer can heal itself naturally, rusting will result.
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That is called “passivation” which is not typically performed on knives. However, “passivation” is not necessary to form the passive oxide layer on the surface, which is nearly instantaneous. The terminology is confusing. I will go back in time and get them to change it.
 
Great stuff as usual!

It obviously doesn't really matter, but I'm just curious as to where 15n20 might fall among the other carbon steels at the bottom of the list with the little bit of nickel it contains.

Thanks for sharing. :)


~Paul
My Youtube Channel
... (Just some older videos of some knives I've made in the past)
 
coldsteelburns said:
Great stuff as usual!

It obviously doesn't really matter, but I'm just curious as to where 15n20 might fall among the other carbon steels at the bottom of the list with the little bit of nickel it contains.

Thanks for sharing. :)


~Paul
My Youtube Channel
... (Just some older videos of some knives I've made in the past)
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Nickel can have a small effect on corrosion resistance, but it isn't part of my predictive equation. The rating would still be less than 1.0 regardless.
 
Very informative. I saltwater kayak fish a few times a week. My H1 rescue knife has been on my vest for a couple years. Rinse it every now and then... still zero rust, even on serrations. Think I will try some Z-Finite for my next boat knife. I had one in 390 that held up well, it discolored but spots cleaned up easily. Davy Jones has it now....
 
WC53 said:
Very informative. I saltwater kayak fish a few times a week. My H1 rescue knife has been on my vest for a couple years. Rinse it every now and then... still zero rust, even on serrations. Think I will try some Z-Finite for my next boat knife. I had one in 390 that held up well, it discolored but spots cleaned up easily. Davy Jones has it now....
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I really like z-finit and vanax. Comparable to AEB-l and s35vn respectively in performance. The nitrogen steels seem to top out at Rc60, but I have one that tests Rc63. Only one, and same heat treat as the charpy samples, which all topped out at Rc60. No idea why.
 
Willie71 said:
I really like z-finit and vanax. Comparable to AEB-l and s35vn respectively in performance. The nitrogen steels seem to top out at Rc60, but I have one that tests Rc63. Only one, and same heat treat as the charpy samples, which all topped out at Rc60. No idea why.
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The high chromium in solution increases the retained austenite limiting potential hardness. I wrote about it here: https://knifesteelnerds.com/2019/03/25/heat-treating-vanax/
There is usually a tradeoff between hardness and corrosion resistance. You can get the PM tool steels to 66-67 Rc, the standard stainless knife steels to 63-64 Rc, and the "super stainless" steels to 60-61 Rc. That's ignoring high speed steels, of course.
 
Larrin said:
The high chromium in solution increases the retained austenite limiting potential hardness. I wrote about it here: https://knifesteelnerds.com/2019/03/25/heat-treating-vanax/
There is usually a tradeoff between hardness and corrosion resistance. You can get the PM tool steels to 66-67 Rc, the standard stainless knife steels to 63-64 Rc, and the "super stainless" steels to 60-61 Rc. That's ignoring high speed steels, of course.
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I have no idea why one got Rc63. The Rc60 limit makes sense. I broke a z-fi it blade straightening it in temper. It’s a few Rc points higher at the break than the rest of the blade. I should send it to you for analysis. It broke in a water quench after the 6th temper. The warp was straightening a bit each cycle. I suspect some form of work hardening creating the break.
 
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