Rust on stainless steel knife?

[Nemrod]

Very good.. not a common knowledge piece of information...

Thanks but a couple of others also mentioned it. It is fairly well known, just not well known enough.

From this web page: How to Brew - By John Palmer - Passivating Stainless Steel

Quote:

"A situation that often comes up is, "Hey, my stainless steel is rusting! Why? What can I do to fix it?"

Stainless steel is stainless because of the protective chromium oxides on the surface. If those oxides are removed by scouring, or by reaction with bleach, then the iron in the steel is exposed and can be rusted. Stainless steel is also vulnerable to contamination by plain carbon steel, the kind found in tools, food cans, and steel wool. This non-stainless steel tends to rub off on the surface (due to iron-to-iron affinity), and readily rusts. Once rust has breached the chromium oxides, the iron in the stainless steel can also rust. Fixing this condition calls for re-passivation.

Passivating stainless steel is normally accomplished in industry by dipping the part in a bath of nitric acid. Nitric acid dissolves any free iron or other contaminants from the surface, which cleans the metal, and it re-oxidizes the chromium; all in about 20 minutes. But you don't need a nitric acid bath to passivate. The key is to clean the stainless steel to bare metal. Once the metal is clean (and dry), the oxygen in the atmosphere will form the protective chromium oxides. The steel will be every bit as passivated as that which was dipped in acid. The only catch is that it takes longer-- about a week or two.

To passivate stainless steel at home without using a nitric acid bath, you need to clean the surface of all dirt, oils and oxides. The best way to do this is to use an oxalic acid based cleanser like those mentioned above, and a non-metallic green scrubby pad. Don't use steel wool, or any metal pad, even stainless steel, because this will actually promote rust. Scour the surface thoroughly and then rinse and dry it with a towel. Leave it alone for a week or two and it will re-passivate itself. You should not have to do this procedure more than once, but it can be repeated as often as necessary."

Let's not play with nitric acid kiddos:

Nitric Acid Acts Upon A Copper Penny Experiment - Video


N

 

[Gombessa]

Or you could soak it in coca-cola for a day or two.

 

[Stu S.]

Or you could soak it in coca-cola for a day or two.

That might be worth an experiment. I have a degree in metallurgy and materials, which means I don't know a thing about... Coca Cola. What kind of acid is in that stuff? We have seen what happens when Mentos candy goes into Diet Coke. The Gusher!

If Coke would work, we could sell it in a four ounce bottle with a dive flag on it as knife cleaner. Six dollars.

Anyhow, acids used in metal finishing are quite strong. If you try making an acid bath at home, neutralize the acid when you are done, don't dump it down the drain as acid, that's not legal.

 

[4sak3n]

Phosporic acid is the passivating agent in Coca-cola.

I used it on my broken-off steak knife DIR-style knife and it was rust free after that. The nice thing though was that it broke down the existing rust so it just wiped straight off. No scrubbing!

 

[Rudebob]

All materials have their trade-offs whether it be weight, stength, ductility, hardness, temperature resistance, corrosion resistance or cost, etc. It seems like most the dive knives made from "stainless steel" (really corrosion resistant as previously stated) are either 300 series austenitic or 400 series martensitic steel.

300 series is the most corrosion resistant but has the lowest tensile and hardness properties of all cres materials so it does not hold an edge well (usually around 80 HRB). Typically, most 300 series alloys contain approximately 18% chrome and 8 % nickel (these percentages vary somewhat by specific alloy). Carbon is highly undesirable (<0.08%) as its presence can results in the formation of carbides which reduce corrosion resistance and make the material susceptible to embrittlement. Contrary to some statements made 300 series cannot be hardened by heat treat. The only way the strength can be increased is by work hardening. 316 is the most corrosion resistant of this family of materials in that 2 - 3 % molybdenum is added for added protection-usually for marine applications.

400 series steels usually have similar or a higher percentage of chrome than 300 series but without nickel. Carbon content in these alloys is a needed critical function of the alloy as these materials can be austenitized by heat treat (quench and temper). Some 400 series alloys are actually ferritic but the alloys used for knife applications are martensitic. I have seen some knives made with 420 but 440C seems to be most common. The latter has about 1.0% carbon and can be heat treated to a hardness of HRC 60. It will hold and edge better than most materials (including titanium). However, due to the variations in chemistry and microstructure this alloy will stain and superficially rust if not cared for.

Incidentally, there are ferritic stainless steels (200 series and a few 400 series alloys) that are used in very few applications other than exhausts systems. The are also precipitation hardening stainless steels such as 17-4, 15-5 and 17-7. These steels have some of the most versatility in application but you don't usually see them used in knife making applications.

So the tradeoffs for dive knife materials are having 1. The best corrosion resistance but low hardness at a reasonable cost (300 series steels), 2. High hardness but reduced corrosion resistance at a reasonable cost (400 series). Or 3, you can forgo stainless altogether for titanium with a fairly high hardness, excellant corrosion resistance, but at a significant cost increase. Personally, I would go with 440C and live with a little corrosion for cost and performance (and a pretty likely chance I will end up loosing it anyway).

mg

 

[dbulmer]

Another way to clean your knife is to heat up some water so that it is warm (to the touch not boiling) and then add a little white wine vinegar to the water. Stick your knife in the solution for a couple of hours. Take your knife out rinse it in water and then dry thoroughly with a cloth/tea towel.

Before a dive, very lightly apply silicon grease to the knife.

 

[sschlesi]

Another solution is to dry it very well, and then oil the knife. Some may say that the oil is bad for the environment, and they are right. OTOH, a light oil will help the knife when not in use, and if you want to be greener, than rinse the knife in a sink well before you start a dive trip.

If you are really concerned about being green remember that your sink runs into the local ocean/river/lake eventually too. You could use a vegetable oil, it's not as effective as products designed to protect metal like Boeshield, but it's earth friendly and more effective than nothing. If you pick the right oil it smells good too.