Chromium (18.0–20.0% in SUS304) is the primary element responsible for corrosion resistance, acting through a two-step mechanism:
Passive Layer Formation: When exposed to oxygen (air, water), chromium reacts to form a thin (2–3 nm), adherent chromium oxide (Cr₂O₃) layer on the bar's surface. This layer is impermeable to water and most chemicals, preventing underlying steel from oxidizing (rusting).
Self-Healing: If the passive layer is scratched or damaged (e.g., during fabrication), chromium in the steel reacts with oxygen to rapidly reform the layer, restoring corrosion protection.This mechanism makes SUS304 resistant to most atmospheric conditions, freshwater, and mild acids. However, in chloride-rich environments (e.g., seawater, salt spray), the passive layer can break down, causing pitting corrosion. To address this, high-chromium variants like SUS316 (16–18% Cr) are used, but SUS304 remains sufficient for most non-marine applications. Chromium content is verified via spectroscopic analysis, with results ensuring the passive layer forms reliably.