Just a personal/metallurgical comment, but I get a kick out of all the claims that one steel or another rusts faster. They all rust!
I have used this analogy to describe it:
High School students come in types from very smart to pretty dull. Most all will graduate from High School. The sharper ones will get better grades, and the duller students will struggle to pass the tests.... but they all graduate in about the same four years. Graduating from high school is not an indication of smartness, just an indication of having passed all the tests for those four years.
Carbon steel comes in many varieties, some gets sharper and some will be duller ... but they all rust about the same amount. Rust resistance is not an indication of how good a knife steel is , it is an indication of how much it was exposed to water and air.
I know it is more complicated than that, so lets look at it more metallurgically. The following is concerning the normal carbon steel alloys used for knifemaking.:
The carbon in knife steel is around 1%. That does not change more than a few tenths of a percent in carbon steels. Manganese is around 1%, and doesn't change much more than +/- half a percent. Chromium is normally less than 1%, if present at all. Small amounts of nickel and other alloys are present (V, W, Mo, etc.) in a some knifemaking carbon steels, minute traces of other elements are vestiges of the smelting and ores, and are insignificant. - the rest is iron.
What rusts is iron. Carbon steels are generally between 97% to 98% iron. The amount of rust that would change over 1% of alloy content isn't really much.
Rust is iron oxide. Merely combining iron and atmospheric oxygen at room temperature will make very little rust and may take centuries to get any significant amount. Adding moisture that is present as humidity will accelerate this - Ask anyone who lives along the coastal regions. Adding liquid water to the steel surface ( but not submersing it) will greatly increase the oxidation as the water evaporates. Steel exposed to rain is in this class. Adding ions to the film of water on the steel surface (especially chlorides), will accelerate the rust even faster - things that go in and out of salt water are in this class.
The small amount of non-reactive alloying in carbon steels will not slow down any of these processes significantly. The alloys are there for grain refinement, toughness, and hardening rates and depth ... not specifically for rust control. You have to get to over 10% chromium to gain significant resistance, and above 12% to be considered stainless. Also, non-hardened stainless steel will rust even with the high chromium content. Only after HT does the significant anti-rust property develop.
Someone with a lot of time on their hands should do a rust comparison test and grind 3X1" coupons of O-1, 1095, 5160, 80CrV, 1084, D-2, and 440C. This gives a chromium content ranging form near zero to over 12%. They should be washed in dish soap cleaned with alcohol, and dried with a clean paper towel, set on a labeled board, ... and not toughed after that. The board should be set in the workshop near the bench for a week and observed if any rust forms.
Then they should be set outside, but not in the open (Florida room or covered patio), where they are exposed to atmospheric air for a week. Last, they should be left outside in an open place for a week . Photos should be taken as the rust patterns form.
What does make steel rust faster of slower is surface condition. Steel that is ground at 120 grit will rust much faster than steel at 400 grit. 1000 grit is even slower. Polished steel rusts the slowest. Exposed pores that trap moisture, and the amount of microscopic hills and valleys to trap moisture are the main drivers of rust. The amount of surface area exposed to that moisture combining with the atmospheric oxygen determines the rust rate almost exclusively.
Good HT and finishing to a finer finish will do more to reduce rust than picking one steel over the other. Forcing a patina with FC or other method will also slow down rust, as the blade surface is already covered with an oxide.
BTW, I know that there are already charts and photos of rust comparison tests. Just remember that you have to be comparing apples and oranges to apply them in the case of a knife blade. Most of these tests are for corrosion resistant steel allows, and do not equate to normal carbon steels. They are mainly designed to determine how many years a certain alloy will survive in various outdoor environments, not if the steel will rust.
Stainless steels are a very different thing, and it gets a lot more complex.
