رسائل ماجيستير
English
2014
Investigation of Oxidation and Hot Corrosion Resistance of Austenitic Stainless Steel (316L) Codeposited by Cr and Si
حيدر حسن جابر, رؤى حاتم هاتف, علي هوبي حليم
كلية هندسة المواد-جامعة بابل · العراق
Diffusion coating is carried out by a simultaneous chromizing-siliconizing process to deposit chromium and silicon on the surface of austenitic stainless steel 316L alloy, using a pack mixture containing (67 wt.% Cr, 8 wt.% Si, 2 wt.% NH4Cl, 23 wt.% Al2O3). It was found that a diffusion coating time of 6 hrs at 950°C produces a coating thickness of 80–100 μm and consists mainly of CrSi2 and Cr3Si phases.
Cyclic oxidation tests were conducted on the coated and uncoated austenitic stainless steel 316L alloy at a temperature range between 700–900°C for 52 hrs with 4-hr cycles in air. The cyclic hot corrosion tests of coated and uncoated austenitic stainless steel 316L alloy, deposited with 2 mg/cm² NaCl:Na2SO4 (100:0, 50:50, and 0:100 wt.%) were conducted at 850°C for 52 hrs with 4-hr cycles and with 5 mg/cm² (67 wt.% V2O5 + 33 wt.% Na2SO4) deposits at test temperatures (550, 650, 750, 850, and 950°C) for 4 hrs at each temperature.
The oxidation kinetics for uncoated austenitic stainless steel 316L alloy were found to be parabolic. The oxidation rate constant (kP) values obtained at 700, 800, and 900°C in air are (1.77 × 10⁻¹⁰, 2.25 × 10⁻¹⁰, 3.36 × 10⁻¹⁰ (mg².cm⁻⁴)/sec), respectively, whereas the oxidation rate constant (kP) values obtained at 700, 800, and 900°C for coated austenitic stainless steel 316L alloy with chromized-siliconized diffusion coating are: (6.944 × 10⁻¹¹, 1 × 10⁻¹⁰, 2.25 × 10⁻¹⁰ (mg².cm⁻⁴)/sec), respectively.
In cyclic hot corrosion tests, the oxidation kinetics of uncoated austenitic stainless steel 316L alloy follow a “Linear-Rate-Law” when oxidized with (100% Na2SO4) deposits (kL = -3.055 × 10⁻⁷ mg.cm⁻²/sec), 100% NaCl deposits (kL = -4.861 × 10⁻⁶ mg.cm⁻²/sec), and with (50% NaCl + 50% Na2SO4) deposits (kL = -5.833 × 10⁻⁷ mg.cm⁻²/sec).
Whereas coated austenitic stainless steel 316L alloy with chromized-siliconized diffusion coating follows the parabolic law when oxidized with (100% Na2SO4, and 50 wt.% NaCl + 50 wt.% Na2SO4 deposits (kP = 5.136 × 10⁻⁹, 1.078 × 10⁻⁷ mg².cm⁻⁴/sec), respectively) and “Linear Rate-Law” when oxidized with (100 wt.% NaCl) deposits (kL = 1.305 × 10⁻⁶ mg.cm⁻²/sec).
In cyclic hot corrosion tests, the oxidation kinetics of uncoated austenitic stainless steel 316L alloy exhibit loss in specific weight when oxidized with (67 wt.% V2O5 + 33 wt.% Na2SO4) deposits, whereas the oxidation kinetics of coated austenitic stainless steel 316L alloy with chromized-siliconized diffusion coating exhibit gain in specific weight under the same test conditions.
The phases present on the cyclic oxidation and hot corrosion of uncoated austenitic stainless steel 316L surface under most test conditions, as revealed by XRD analysis, are Cr2O3, Ni-Cr-Fe, and NiO, whereas the phases that formed on coated austenitic stainless steel 316L alloy with chromized-siliconized diffusion coating during most of the oxidation exposure conditions are SiO2, Cr3Si, and Cr2O3.