Abstract
This work examined the pitting corrosion resistance of two types of duplex stainless steels: UNS S31803, with molybdenum, and UNS S32304, without molybdenum. Cyclic potentiodynamic polarization and critical pitting temperature (CPT) tests were performed in 3.5 wt% NaCl solution. Scanning electron microscopy (SEM) was used for analysis of the pits on the surface of samples. The two steels studied presented susceptible to pitting corrosion in 3.5 wt% NaCl solution. UNS S31803 steel showed a higher corrosion potential (Ecorr), pitting potential (Epit), and polarization resistance (Rp) compared to UNS S32304 steel. It was also observed that the UNS S31803 steel showed higher capacity to repassivation and CPT than the UNS S32304 steel. Keywords: Duplex stainless steels; Pitting potential; Electrochemical impedance spectroscopy; Cyclic potentiodynamic polarization; CPT.
Highlights
Pitting corrosion is a highly localized form of corrosion attack and it is characterized for presenting a very small attacked area compared to the total area exposed to the corrosive environment [1]
When the protective film breaks, it forms an electrochemical cell consisting of the anode, a small metal area exposed by the rupture of the film, and the cathode, the largest area protected by the film
The stainless steel anode active region is bounded by a potential value, above which, even with the increase of the potential, the current remains more or less constant
Summary
Pitting corrosion is a highly localized form of corrosion attack and it is characterized for presenting a very small attacked area compared to the total area exposed to the corrosive environment [1] This type of corrosion occurs in stainless steels in most cases in the presence of environments containing chlorides or when there is incomplete passivation [2]. The breaking of the passive layer results in the formation of an electrochemical cell, where the anode is the active area and the cathode is the passive area This type of corrosion is typical of metals/alloys that form a thin protective film of oxide or hydroxide of about 30 to 80 Å of thickness, called the passive layer. Once the formation of the pit is initiated, it will continue as a self-catalytic process
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