What is Stainless Steel?

The stainless mechanism relies on chromium's strong affinity for oxygen. When at least 13% Cr is dissolved in the iron matrix, chromium atoms at the surface react with atmospheric oxygen to form a continuous, transparent chromium oxide (Cr2O3) film approximately 1-5 nanometers thick. This passive layer has three critical properties: 1. **Self-healing:** When scratched or damaged, the exposed chromium immediately reacts with oxygen to reform the protective layer. This happens within milliseconds in air. 2. **Adherent:** The oxide bonds tightly to the underlying metal, unlike iron oxide (rust) which is porous and flakes away, exposing fresh metal to further corrosion. 3. **Electrically insulating:** The passive layer interrupts the electrochemical corrosion process by blocking electron flow between anodic and cathodic sites. **The carbide problem:** In martensitic stainless steels used for scissors, carbon is essential for achieving hardness through martensite formation. But carbon also combines with chromium to form carbides (Cr7C3 and Cr23C6). Each percentage of carbon consumes roughly 12-17% of available chromium in carbide formation (the exact ratio depends on heat treatment). This creates a fundamental tension in scissor steel design. Higher carbon = higher hardness = better edge retention, but also more chromium consumed by carbides = less chromium in solution = reduced corrosion resistance. Steel designers balance this by: - Adding other carbide-forming elements (vanadium, molybdenum) that preferentially form their own carbides, "freeing" chromium to remain in solution. VG-10 uses this strategy with vanadium and molybdenum additions. - Controlling austenitizing temperature during heat treatment. Higher temperatures dissolve more carbides, putting more chromium back into solution, but risk grain growth. This is why heat treatment quality directly affects corrosion performance. - Using nitrogen as a partial carbon substitute. Nitrogen contributes to hardness and corrosion resistance simultaneously without forming chromium-consuming carbides. Sandvik 14C28N exemplifies this approach. The 13% threshold for "stainless" status comes from ISO 15510 and EN 10088, which define stainless steel as containing a minimum of 10.5% Cr. However, in practical knife and scissor metallurgy, 13% in solution is the commonly cited threshold for reliable passivation in real-world conditions. The 10.5% ISO minimum applies under ideal laboratory conditions that do not reflect salon environments. Common stainless scissor steels and their total/in-solution chromium (approximate): - **SUS420J2:** 13% total / ~11% in solution — borderline stainless - **GIN-1:** 15.5% total / ~13-14% in solution — reliably stainless - **440C:** 17% total / ~13-14% in solution — excellent stainless performance - **VG-10:** 15% total / ~11.7% in solution — stainless but with less margin - **ATS-314:** 14.5% total / ~11-12% in solution — similar to VG-10