Stainless steel: · A form of steel containing chromium (and other metals) · Notable for its corrosion resistance, and it is widely used for food handling and cutlery among many other applications · Not readily corrode, rust or stain with water as ordinary steel does · It is not fully stainproof in lowoxygen, highsalinity, or poor aircirculation environments · There are various grades
SUS Passivation process What is Stainless Steel? Stainless steel: · A form of steel containing chromium (and other metals) · Notable for its corrosion resistance, and it is widely used for food handling and cutlery among many other applications · Not readily corrode, rust or stain with water as ordinary steel does · It is not fully stain-proof in low-oxygen, high-salinity, or poor air-circulation environments · There are various grades Grades of Stainless Steel Stainless Steel Application Why we need Passivation? Passivation: Usually occurs naturally in the surface of stainless steel, but we can improve quality and thickness of passive film layer by oxidizing acid treatment To provide more resistance to corrosion, oxidation, rusting and mild chemical attack Mechanism of Stainless Steel The chromium at the surface reacts with O2 in the air to produce chromium oxide The term passive refers to steel now being ‘unreactive’ 4Cr + 3O2 → 2Cr2O3 Passivation layer : Chromium(III) oxide (Cr2O3) Mechanism of Stainless Steel SUS Passivation Process General process: Degreasing Alkaline cleaner Activation Acid salt Passivation Nitric acid, sodium dichromate Drying Hot air Water rinse *2 between each steps This diagram does not represent the process of SUS passivation Pretreatment: Degreasing Degreasing: It is process of oil, grease or visible soil removal Normally used detergent or alkaline cleaner, operate at temp 60-70 oC, time 10-20 Pretreatment: Activation Activation: To remove contaminants , thin layer of metal and promote the formation of the passive film on a freshly created surface Often with acid solution HNO3: is oxidizing in character, tends to promote and preserve the corrosion resistant quality (passivity) but cannot remove oxide scale from SUS HF, H2SO4, HCl: are reducing agent in character Descaling by reducing the oxides and also reduce the protective oxide film and thus lay the underlying metal open to corrosion attack Because of inherent characteristics of oxidizing and reducing acids Mixtures of two acid types are used for activation 12-15 v% HNO3 + 1% HF at RT, 1-2 is usually satisfactory 10 SUS Passivation Chemicals Nitric Acid: Widely used in commercially available for stainless steel passivation because of its strong oxidizing property, so it can generate the chromium oxide layer at the same time Sodium dichromate: To help the formation of the passive layer and chromic oxide film Citric acid: Weaker oxidizing acids, help in the formation of the passive layer 11 Passivation types QQ-P-35C/AMS-QQ-P-35A Specification 12 Type II SUS Passivation Make up solution: 20-25v% Nitric, 2-2.5 w% Sodium dichromate Temp.: 49-54 oC (120-130 oF) Time: 20 For passivation series: 303, 410, 416 and 17-4PH 13 Type VI SUS Passivation Make up solution: 25-45 v% Nitric acid Temp.: 21-32 oC (70-90 oF) Time: 30 For passivation series: 302, 304, 305, 316, 900 14 Quality Test Method Water Immersion Test High Humidity Test Salt Spray Test Copper Sulfate Test 15 Thank you for your attention 16