COMPORTAMENTO A TENSO-CORROSIONE DI ACCIAI INOSSIDABILI AUSTENITICI E AUSTENO-FERRITICI IN SOLUZIONI DI NaCl IN PRESENZA DI TIOSOLFATO

Due to the growing energy request and to the decrease of the oil and gas stocks, the materials for the extraction plants have to work at high pressure and temperature conditions, with the presence of H2S, CO2 and chlorides. Such conditions result in both uniform and localized corrosion, with significant stress corrosion cracking (SCC) phenomena. An excellent solution to these problems may be the selection and use of duplex and superduplex stainless steels [1-3]. However, a key- point is constituted by the execution of reliable SCC tests simulating the in-service conditions of the materials. In several works [4-7] the standard or modified NACE (TM-01-77) solution is employed, which requires very expensive equipments due to the use of H2S. An interesting approach was proposed by Tsuyikawa [8], adopting the thiosulfate instead of the hydrogen sulfide and achieving a good correlation between the pitting critical values in terms of thiosulfate concentration and of H2S pressure. In fact a concentration of S2O32-- from 10-3 to 10-2 mol/l in a water-20%NaCl solution at 80 °C corresponds to a H2S pressure from 0.1 to 1 MPa at 200 °C. Tsuyikawa [8] showed the results of SCC tests (constant strain; slow strain rate, SSR; constant load) on austenitic, ferritic and duplex stainless steels in both the above mentioned environments. The present work is aimed at verifying such interesting results, extending the study to a superduplex stainless steel. The tests were carried out on specimens achieved from UNS S32750, UNS S31803, AISI 304 and AISI 316L rolled sheets (thickness = 3 mm, gage length = 30 mm, compositions summarized in table 1). The SSR technique (strain rate = constant = 1x10-6 s-1) was employed, recording the time vs stress diagrams. The testing temperature was 80 °C; four solutions were used: 20%NaCl, and 20%NaCl + Na2S2O3 (10-3, 10-2, 10-1 M). With the same solutions and under the same conditions the pitting potentials were also measured. The SSR tests were followed by metallographic and microstructural investigations of the specimens. The results of the SSR tests, in terms of stress-strain diagrams, are collected in fig. 1. It can be seen that in the 20% NaCl solution no SCC susceptivity was observed; the surfaces of the specimens were without any attacks and the fractures showed a typical ductile morphology. Such solution can be considered as inert for the materials under investigation. On the other hand, the presence of thiosulfate variously affects the behaviour of four stainless steels studied (tables 2-3, fig. 2). For AISI 304, the elongation decreases significantly, even at low thiosulfate concentrations; a similar, but not so strong, effect can be observed on AISI 316. For both the austenitic steels, the lowest elongation was observed in the solution with thiosulfate 10-2 M. On the duplex steel the thiosulfate effect is significant only at 10-2 M and 10-1 M, increasing with the concentration. The superduplex steel is affected by the thiosulfate presence only for a 10-1 M concentration. Fig. 2 also shows the higher mechanical properties of duplex and superduplex with respect to austenitic steels. From metallographic observations, for both the austenitic stainless steels, some fissuring attacks were evident, originating inter- and trans-granular cracks (fig. 3). In the case of duplex and superduplex steels, for the higher thiosulfate concentrations, the failure is associated to a single macroscopic fissure, with a significant amount of corrosion products; from this fissure, some secondary cracks are departing, mainly localized in the ferritic phase (fig. 4). The results concerning AISI 304, AISI 316L and UNS S 31803 showed a very good agreement with those of Tsuyikawa [8]. In other terms, the critical partial pressure of H2S can be considered equal to 0.1 MPa for AISI 304, to 1 MPa for AISI 316L and UNS S 31803, to 10 MPa for UNS S 32750. The "compatibility" between the tests in thiosulfate and in hydrogen sulfide was also confirmed by the morphology of corrosion: the preferential attacks into the ferritic phase are in fact well-documented in literature [9-10] for SCC tests in H2S of duplex and superduplex steels. Table 4 and fig. 5 show the pitting potentials (EP) as functions of thiosulfate concentrations. For AISI 304, a concentration of 10-3 M of S2O3- lowers EP to about -350 mV: after an immersion of 30 min there are localized attacks. A similar value of EP is achieved by AISI 316L for a thiosulfate concentration of 10-2 M. Only for 10-1 M thiosulfate there is a significant decrease in the pitting potential of duplex and superduplex: UNS S 31803 passes from about 0V to - 0.2V, while UNS S 32750 goes from 0.8 V to values around 0.2 V. Summarizing, the ranking in terms of pitting resistance in NaCl 20% solutions with thiosulfate at 80 °C is the following: AISI 304 < AISI 316 < UNS S 31803 ≪ ≪ UNS S 32750. This behaviour is qualitatively in agreement with the values of PRE (pitting resistance equivalent number = Cr + 3.3Mo + 30N) collected in table 1. From a general point of view, there is a good correlation between the pitting resistance and the SCC susceptivity. Only when the material is depassivated, the reduction of thiosulfate to S or H2S is possible; in this case, the repassivation is inhibited and the anodic processes are accelerated, leading to localized corrosion or to SCC. As a final remark, it must be pointed out the fact that NaCl 20% solutions with thiosulfate at 80 °C can be used as a reliable alternative to H2S containing solutions at 200 °C, in order to study localized corrosion.