Hardened steel is steel subjected to heat treatments, such as quenching and tempering, to dramatically increase its strength and hardness. This process alters the internal crystal structure, making it more resistant to wear and deformation than untreated steel. Despite this enhanced durability, hardened steel does rust, unless the base material is a rust-resistant alloy like stainless steel. Hardening is a physical change in structure, not a chemical change in composition that prevents corrosion.
The Fundamental Chemistry of Steel Corrosion
Rust formation is a natural electrochemical process where metals deteriorate to return to a stable chemical state. This corrosion occurs when iron atoms in the steel oxidize. Iron reacts with oxygen and water (which acts as an electrolyte) to form hydrated iron(III) oxide, commonly called rust. Corrosion requires the simultaneous presence of both oxygen and water.
The process involves distinct anodic and cathodic sites on the metal’s surface. At the anodic site, iron loses electrons and dissolves as iron ions. These electrons travel to the cathodic site, reacting with water and dissolved oxygen to form hydroxyl ions. The iron and hydroxyl ions combine to form iron hydroxide, which then oxidizes into the final rust product. The presence of salts, acids, or contaminants accelerates this reaction by making the water a more effective electrolyte.
Heat Treatment and Corrosion Vulnerability
Hardening steel is a microstructural change designed to improve mechanical properties, not to enhance corrosion resistance. The process involves heating the steel past a critical temperature and then rapidly cooling it (quenching) to transform the internal structure into martensite. Since hardening does not remove iron content or introduce corrosion-resistant elements like chromium, the steel remains vulnerable to oxidation. The chemical requirement for rust—free iron exposed to oxygen and water—is still met.
In some cases, hardening can increase vulnerability to certain types of corrosion. When hardened steel is tempered (reheated to reduce brittleness), carbon atoms precipitate, forming tiny carbide particles within the metal matrix. These carbides create microscopic differences in electrical potential across the surface, accelerating the electrochemical corrosion reaction. The corrosion rate increases with longer tempering times as more carbides are formed.
Practical Steps to Prevent Rust
Protecting hardened steel items, such as tools and knives, requires physically separating the metal surface from its environment. The most effective strategy is applying protective coatings that create a barrier against moisture and oxygen.
Protective Barriers
A thin layer of mineral oil, wax, or a specialized rust-preventative dry coating should be applied immediately after use and cleaning. For items in storage, a humidity-controlled environment is beneficial; placing them in a dry, well-ventilated area slows corrosion significantly. Vapour Corrosion Inhibitor (VCI) products release rust-preventing chemicals into the air and are effective for long-term storage.
Regular Maintenance
Maintenance is necessary, especially after exposure to corrosive agents like salt water or acidic residues. The item should be cleaned promptly with mild soap and water, dried thoroughly, and immediately re-oiled or re-coated to restore the protective barrier. Avoiding scratches and dings is also important, as surface damage exposes fresh, unprotected metal, creating a new site for corrosion.