| For the brittle fracture and corrosion problems of key equipment to purify phosphoric acid, super duplex stainless steel SAF2507 with the superior performance has been chosen as the main research object. It is expected to get the best material of equipment under high temperature and high concentration phosphoric acid. The corrosion behavior of SAF2507 in high temperature and high concentration phosphoric acid solution is studied by coupon testing in lab and a new electrochemical measuring technology such as EVT, LP and EIS. Some analyzing methods, such as metallographic microscope, software of Image J and scanning electron microscope(SEM), are combined with calculation of PREN equivalent,potentiodynamic scanning and slow strain rate test to study microstructure evolution and corresponding corrosion law of SAF2507 by high temperature(1020℃~1150℃)solution treatment.The results can be obtained as follows:(1) Uniform corrosion behavior of SAF2507 in flowing concentrated phosphoric acid at 85 ℃ is studied by coupon testing. The results show that SAF2507 has a good corrosion resistance. Temperature is closely related with corrosion resistance of super duplex stainless steel(SAF2507). The corrosion resistance at 85 ℃ is 3 times than25℃. It still had good corrosion resistance. The corrosion resistance at 126 ℃ is 40 times than 25℃. Then, the corrosion resistance is poor at 126℃.(2)Electrochemical corrosion behavior of SAF2507 in different concentration phosphoric acid is studied by electrochemical test. And Self-passivation of SAF2507 is appeared in phosphoric acid solution( 85℃). It shows that SAF2507 has a good corrosion resistance.The reaction process is controlled by discharge step of the charged particles passing through the surface electric double layer electrode.(3) It has a good corrosion resistance due to the broad range of stable passivation area of SAF2507 with the temperature range from 25℃ to 70℃. The impedance spectra at 25℃ is composed of capacitive reactance. The reaction process is controlled by electrochemical process. When it is at 55℃~ 70℃,EIS is consisted of the capacitive reactance arc at high frequency and the Warburg impedance at low frequency, which shows that corrosion of SAF2507 is controlled by the electrochemical reaction and then it turns to be controlled by electrochemical reaction and the material diffusion. With the temperature increasing, the capacitive arc radius,polarization resistance and corrosion resistance decreases gradually.(4) Experimental result shows that,with temperature increasing, the ferrite content increase from 45.54%(1020℃) to 61.58%(1150℃). PREN value of the sample with the temperature of 1080℃(holding an hour), water quenching reached a maximum. And its corresponding corrosion current density is small, the corrosion rate is 0.00793mm/a. It had a good corrosion resistance. The stress corrosion sensitivity coefficient of SAF2507 in phosphoric acid(55℃,76%) is very small. Therefore, it is nearly no tendency to stress corrosion. |
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