Corrosion Of 2304 Duplex Stainless Steel And Medium Low Cr Ferrite Stainless Steel

Stainless steels have been widely used due to their higher mechanical strength, better corrosion resistance and lower weight compared with their counterparts. However, "stainless" is a relative concept. Stainless steels’ applications are limited due to some reasons, one of which is its localized corrosion resistance, such as pitting corrosion resistance and intergranular corrosion resistance, is not very high. Moreover, its localized corrosion often develops in the coexisted corrosion environment with anion and oxidant. The recognition that this kind of corrosion is destructive and hard to discover has led to extensive research. As for stainless steels, the research partly focuses on materials composition、corrosion mechanism and heat treatment to improve its anti-corrosion resistance and search the optimal conditions for intergranular corrosion.The present work is focus on the duplex stainless steel 2304, ferrite stainless steel B436L, B429 and B439L. As to duplex stainless steel 2304, the research is focus on the influence of cooling rates between 1350℃ and 800℃ on the microstructure and pitting corrosion resistance; As to ferrite stainless steel B436L, B429 and B439L, the research is focus on searching the optimal conditions for intergranular corrosion. The main results are as follows:(1) In the first part of our investigation, pitting corrosion resistance of 2304 duplex stainless steel heat-affected zone with different cooling rates has been studied by potentiostatic critical pitting temperature(CPT) in 1.0M NaCI. The results showed that, as cooling rate decreased from 100 to 10℃/s in the temperature range of 1350-800℃, the austenite fraction increased from 27.6% to 35.0%, and the CPT value increased from 15 to 19℃. The morphologies after the CPT tests showed pitting occurred preferentially in the ferrite phase for all specimens. Moreover, relationship between pitting corrosion resistance and microstructure evolution was further discussed.(2) The second part is focus on searching the optimal DL-EPR(Double-loop electrochemical potentiokinetic reactivation) conditions for intergranular corrosion of ferrite stainless steel with medium Cr. As to B436L ferrite stainless steel with medium Cr, the optimal conditions are as follows:solution concentration include 0.8mol/LH2SO4+0.05mol/L KSCN, solution temperature is 30℃, scanning rate is 1.66mV/s; As to B429 ferrite stainless steel with low Cr, the optimal conditions are as follows:solution concentration include 0.5mol/LH2SO4+0.0001mol/L KSCN; As to B439L ferrite stainless steel with medium Cr, the optimal conditions are as follows: solution concentration include 0.8mol/LH2SO4+0.02mol/L KSCN, solution temperature is 30℃, scanning rate is 1.66mV/s. The morphologies after the DL-EPR tests verified the reliability of DL-EPR methods.(3) The third conclusion is focus on the research of intergranular corrosion sensibility of ferrite stainless steel B436L and B429. TTS curves of FSS B436L and B429 have been drawn by aging at different temperature for different time. Both of B436L and B429’s nose temperature are about 550℃.