Carbon steel is a combination of iron and carbon, having only traces of a few other elements. In general, carbon steel can be made to be very hard, and sharpen keenly, but it tends to dull quickly because simple carbon steel lacks alloys that increase wear resistance.
Simple carbon steel can be extremely brittle and it corrodes easily, especially while in storage through oxidation.
Great-grandpa's farm tools were made from carbon steel. But what people think of, as carbon steel today, is frequently a more complex alloy that just happens to have low corrosion resistance.
Despite its drawbacks, simple carbon steel was the primary material used in knives for years for lack of anything better. In the 1940's stainless steels were introduced, but the initial formulas were not suitable for cutlery.
Still a lot of stainless steel was tried in knives, and unfortunately this caused a misunderstanding between manufacturers and consumers which lasts to this day.

A BIG MISUNDERSTANDING
There are still many people that believe the simple carbon steels of yesterday are superior to the perfected high-carbon stainless steels of today.

Because this market exists some popular brands of pocketknives are still made with alloy tool steels (still popularly called carbon steel) that are subject to corrosion.

While these knives come from the factory with shiny, bright blades, over time oxidation will turn them dark. This affects appearance but not cutting performance.

Many custom knife makers use alloy steels that are subject to corrosion. It so happens that in the efforts to make the "perfect" knife, custom knife makers' tasks frequently lead them to something other than the stainless steels. This doesn't mean stainless steel is bad or carbon steel is good. All is a matter of judgment and consideration of the application of the product.
Homemakers do not like rusty kitchen utensils. That's why it's hard to buy a "carbon steel" kitchen knife.

ALLOY STEELS: MORE THAN YOU THINK
Alloy steels take basic carbon steel and add other elements to obtain more durable qualities. Perhaps the best well known is stainless steel. There are many different qualities of stainless steel. Please remember that the materials used to make soup cans and kitchen sinks are nothing like that used in good cutlery.

Low-carbon stainless steels are used for making soda fountains, flatware, and cheap knives that disappoint their buyers.
Quality manufacturers select blades of steel that have a much higher carbon content than kitchen sinks, hence the term "high-carbon stainless steel." This has become a buzz word used to identify good cutlery, but the word carbon sure causes a lot of confusion, as well as undue emphasis on this element, as if it were the only controlling factor of quality.

Chromium is an important alloy used to form high-carbon stainless steel. It gives the steel greater wear-resistance, allowing it to hold an edge longer, and increases toughness, making it less brittle than carbon steel. Best of all, chromium increases corrosion resistance, greatly reducing the likelihood of rust.

IT'S STAIN-"LESS" NOT STAIN-PROOF
Note that "stainless" means "stain less" not "stain proof." Some people expect their "stainless" cutlery to stay as bright as a car bumper. With proper care this is possible, but given the chemical action of food acids and harsh detergents plus indifferent care, disappointments are likely to result.

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