It’s happened to you before. You go into the drawer and pick up a knife you haven’t carried in a while and the moment you open it up… RUST! But how?! This is stainless steel! Well ladies and gentlemen corrosion comes in many forms for us fans of the edged tool. I’ll be discussing today the different types of corrosion knife collectors and users generally encounter and how to make that rust work for you or banish it all together with some proper maintenance.
First things first. Stainless steel doesn’t rust. Right…? Wrong. There are hundreds of “stainless” alloys of steel. Of those hundreds a few dozen are suitable for a blade. It is generally accepted that to be considered “stainless” a steel alloy must contain 13% chromium. The more chromium the more corrosion resistant or rust resistant the alloy is. The chromium forms a passivation layer of chromium oxide (Cr2O3) when exposed to oxygen. It is a super thin barrier that resists water and in turn prevents the formation of iron oxides (rust).
So all steel regardless of alloy wants to rust. Why? Well the moment any piece of steel is formed, ground, poured or pressed into shape it begins to break down microscopically. No matter the alloy if the steel is in water or the presence of air moisture it will slowly begin to corrode into iron oxide. It may take a week or a decade but the result is always the same. Steel rusts.
Now here is the kicker. Not all rust was created equal! You can use that rust to protect your blade. In fact many common metal finishes harness the power of rusting to coat and protect the metal surface. Bluing is a very common technique in the firearms world for providing limited rust resistance to carbon steel alloy parts. Bluing is basically forcing steel to rust through electro chemical means to form magnetite (black iron oxide) and then stabilizing the rust with boiling water. The bluing process is very caustic and a total pain to do on a small scale. Add in the fact that a blued surface is very susceptible to scratches and you have a recipe for disappointing performance on a knife blade that sees some real use.
The most common forced rust process for knives is acid etching and patina. A patina is a thin layer that forms on the surface of carbon steel with age and use. Patinas are a kind of corrosion that can contain many chemical compounds such as oxides, carbonates, sulfides, and sulfates. Because the chemical composition of each patina is unique to the alloy and the exposure/use of that alloy many different hues, shades, and colors can form in a natural patina. A naturally formed well-seasoned patina a beautiful thing on a blade and it will provide decent rust prevention if kept oiled and well maintained. If you want to form a patina on your carbon blade then get in the kitchen and start chopping vegetables. Wipe the blade clean after each use and over weeks/months thin layers of patina will begin to form on the steel.
But Will, I have a carbon blade and I just can’t wait a month or two for a natural patina to form. Have no fear! You can force a patina quite easily with any number of household acids. In my shop I use Ferric Chloride to acid etch both stainless and carbon blades. It is extremely fast and powerful and gives a very even dark grey/black etch. It’s also toxic, highly corrosive and acidic. Not fun stuff to just keep around the house. Grapefruit, onions, mustard, coffee grounds and vinegar are common examples of food safe items anyone can use to force a patina on a carbon steel blade. Stick the knife in the acidic material and start to experiment with soak times. Patina activity can start as quickly as an hour or as long as a few days. It all depends on the air/acid exposure level of the metal surface. Many beautiful patterns mimicking Damascus/pattern welded steel can be achieved with layers of masking. Grab a bottle of mustard and get creative! Just remember to strop that edge when you’re done to realign any burs created from the acid erosion.
Here in the Georgia wetlands it is hot and humid 9 months of the year and in the winter the oppressive dew point means every unprotected metal outdoor surface is coated in water every night. The perfect breeding grounds for rust. Over the years I have seen several contributing factors to the success of a knife resisting corrosion.
We have already discussed steel (ferrous) alloys but I would like to add there are many interesting (exotic and super damn expensive) nonferrous alternatives in the market now. SM-100 is a nickel titanium alloy that is 18% lighter than steel, nonmagnetic, almost completely corrosion resistant, and can be hardened to RC 60. Since SM-100 is titanium based the oxide formations from the heat treat process leave brilliant orange, brown and blue colors on the surface of the blade. Stellite 6K is a cobalt alloy with a unique carbide matrix formed from the addition of chromium, molybdenum and tungsten. Traditional hardness testing yield a low RC rating however the cutting edge of a honed Stellite 6K blade will do food prep on vegetables and fruits for damn near an eternity.
Heat treatment is an important consideration when evaluating corrosion resistance. Modern CPM or powdered metal alloys are chosen for their ability to develop very fine grained carbide microstructures. These microstructures are refined during the repeated quenching of the steel as a part of the heat treating process unique to every alloy. Typically the corrosion rate increases for repeated quenches. This implies that corrosion rate increases as grain refinement increases. The repeated quenching refines the grains, which renders greater anodic areas than coarser grained structure and thus activates the corrosion of steel. Simply put too little grain refinement and your blade won’t hone to a fine edge. Too much quenching and refinement means you open yourself up to potential corrosion issues.
Surface finish is a critical and fairly obvious contributing factor to rust formation. Finer scratch patterns and mirror polished surfaces have been slightly work hardened and leave very few places for moisture to hide and fester into rust pitting. But mirror polished blades are absolute hell to keep looking good so I have always adopted a strategy of preemptive strike on rust oxides. Acid etched stonewashed blades are excellent at hiding damage from repeated use and the acid etch forms a protective patina inside the scratch pattern retarding any corrosion progress.
The last and most hotly contested method of fighting rust is external protectants. Many petroleum based lubricants offer varying levels of corrosion protection. Remember though that the best lubricants and best rust inhibitors aren’t necessarily found in the same product. Frog Lube, Eezox, Tri-Flow, Slip2000, and HDCi are all top quality corrosion inhibitors. Oil based protectants are designed to displace water and lubricate moving parts. Waxes are designed to seal the exterior metal surface from moisture and oxygen. Unfortunately waxes can wear away unevenly and are best utilized in conservation work or display pieces that rarely see hard use.
As a knife enthusiast you will forever be locked in mortal combat with corrosion. It is a never ending chemical process that wishes to reduce your collection to oxidized dust. With a little knowledge and some preparation any blade can be properly stored, protected, and seasoned to last the user many lifetimes.
If you have more questions on the subject, I’ll be more than happy to answer you in the comments.