ABSTRACT
Effects of pH and H2S on the corrosion resistance of the UNS S42000 steel (13%: Cr martensitic stainless steel), the UNS S41426 steel (Super 13%Cr martensitic stainless steel) and the UNS S31803 steel (2205 duplex stainless steels) were studied. The test solutions contain 500 ppm of NaCl with different pH saturated with 3.5% H2S in CO2. Electrochemical impedance spectroscopy (EIS) is used to study the properties of the passive film and the potentiodynamic test is used to determine the corrosion resistance behavior of the specimens. The pH of the test solutions after saturation with 3.5% H2S were 1.6 to 3.9. At pH 1.6 all of the specimens corrode significantly especially the UNS S42000 steel with a corrosion rate of 10.6 mm/y; the corrosion rate of the UNS S41426 steel and the UNS S31803 steel are 5.4 and 4.6 mm/y respectively. At pH 2.6, the UNS S31803 steel has high film resistance with a corrosion rate of 0.0063 mm/y and the critical current is 0.011 mA/cm2. On the other hand the UNS S42000 steel and the UNS S41426 steel have low film resistances and corrosion rates of 0.66 mm/y and 0.35 mm/y, respectively. At pH higher than 3.8, corrosion resistance of all specimens increases significantly. Corrosion rate of the UNS S42000 steel is about 4 times of the UNS S41426 steel and 6 times of the UNS S31803 steel. The film resistance of the UNS S31803 steel remains un-changed (110,300 ohm.cm) and is twice of the resistance at pH 2.6 (65,650 ohm.cm).