| Stainless steel has been widely used in different fields due to its excellent corrosion resistance, such as petrochemical industry, metallurgy, nuclear energy engineering etc. However, corrosion resistance of stainless steel is not a state of absolute immunity but rather a slow rate of corrosion. For lots of stainless steel equipment, corrosion failure still occurs during the service because of the action of special temperature, pressure and medium. Pitting maybe appears for stainless steel in medium containing chloride or bromide. Pitting has enormous damage to chemical equipment due to a very high penetration rate though it results in a little weight loss, and it is the source of localized corrosion cracking in some cases, such as intergranular corrosion, stress corrosion, corrosion fatigue and so on. In order to assure the long periodic safety operation of the whole equipment, it is very significant to study mechanism and rule of pitting, and to take effective measures against pitting. In the present paper, uniform corrosion and pitting behavior of AISI316L and SAF2205 stainless steels in acetic acid solution containing bromide, which is a mainly corrosion medium in the production of PTA, were studied by measuring the corrosion weight loss and electrochemical analysis. At same time, mechanism of resulting in pitting was analyzed. Effects of sensitized and plastic deformation on the corrosion resistance of AISI316L and SAF2205 material were investigated since they often happened in the course of actual manufacturing. From above experimental research, failure analysis of pitting was carried out, and feasibility of repair with attachment was ascertained by the changing suitable material. These studies can guide manufacture, repairing, prolonging lifetime or refabrication of stainless steel equipment in the production of PTA. Finally, safety evaluation of a Px wash tower containing pitting defects in the state of working was done according to API579 based on fit-for-service, and its lifetime is predicted by corrosion rate. These investigations can offer technology support for safety operation of the equipment in the future. According to above studies, main conclusions can be obtained as follows: (1) Whether original or sensitized AISI316L and SAF2205 stainless steel, the corrosion depth per year and the pitting susceptibility were increased significantly, while the pitting potential was decreased rapidly with increasing of temperature and bromide concentration. The corrosion resistance of SAF2205 stainless steels was superior to AISI316L. (2) Chloride was more aggressive than bromide in acetic acid solution. Small amount of bromide has a remarkable inhibiting effect on corrosion behavior in acetic acid solution containing chloride ion, whereas addition of small amount of chloride to acetic acid solution containing bromide does not have significant harmful effects. (3) The corrosion depth per year and the pitting susceptibility were increased significantly, while the pitting potential was decreased rapidly, when the stainless steel was sensitized. Bromide induced pitting preferentially occurred at austenitic boundaries for sensitized AISI316L stainless steels, whereas pitting preferentially occurred at austenitic boundaries, ferrite-austenite boundaries and ferrite boundaries for sensitized SAF2205 duplex stainless steels. (4) The α′-martensite phase induced due to deformation was hardly found in austenitic AISI316L stainless steel under uniaxial tension at 25℃, whereas a lot of glide lines were found with increasing degree of elongation. Slight plastic deformation results in corrosion resistance remarkable decrease, pitting susceptibility increase in acetic acid solution containing bromide. The corrosion resistance of AISI316L stainless steel is related to distribution of the dislocation. (5) The fit-for-service (FFS) assessment results indicate that the working of Px wash tower containing pitting defect is safety. The MAWP approach provides a systematic way of determining the remaining life. But this method has some limitations in the application of composite plate. (6) Short service life of AISI316L stainless steel is attributed to severe corrosion medium and worse material due to machining and manufacturing. Under the samecondition, SAF2205 stainless steel has longer service life in contrast to AISI316L. From the above analysis, pitting failure of Px wash tower is caused by several factors: increase of temperature and bromide concentration due to fluctuating of operation, sensitization of welding, rather large plastic deformation induced by cold working. Repairing by local attachment of SAF2205 stainless steel is suitable. |
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