304HCu stainless round steel (ASTM A276; GB 07Cr19Ni9Cu3H) has a composition of 18.0–20.0% chromium (Cr), 8.0–10.5% nickel (Ni), 2.50–3.50% copper (Cu), 0.04–0.10% carbon (C), ≤1.00% silicon (Si), ≤2.00% manganese (Mn), ≤0.045% P, ≤0.030% S.
Compared to 304H (no intentional Cu), copper (2.50–3.50%) enhances acid resistance-especially in sulfuric and phosphoric acid-via:
Copper sulfate film formation: In acidic solutions, Cu oxidizes to form a thin, adherent copper sulfate (CuSO₄) film on the steel surface. This film acts as a secondary barrier beneath the Cr₂O₃ passive layer, blocking H⁺ ions from reaching the steel matrix. CuSO₄ is particularly stable in sulfuric acid, where it reduces corrosion rates by 60–70% compared to 304H.
Reduction of matrix dissolution: Cu slows the anodic dissolution of iron (the main corrosion reaction in acids) by altering the steel's electrochemical potential. This makes it harder for acid ions to oxidize iron atoms, further reducing corrosion.
Carbon content remains identical to 304H, ensuring high-temperature strength is retained-copper adds acid resistance without compromising the steel's elevated-temperature performance.