Study On The Influence Mechanism Of The Cr-Depleted/C-enriched Area At The Grain Boundaries To The Intergranular Corrosion Behaviour In Austenitic Stainless Steels

With the development of global economy,people’s demand for energy increased rapidly.As a result,the oil and gas industry is becoming more and more important.Depend on the excellent corrosion resistance,austenitic stainless steel is widely used in oil and gas field industry.However,intergranular corrosion is the most important factor limiting the application range of austenitic stainless steels.In general,it is believed that the precipitation of Cr-rich carbides at the grain boundary under the sensitizing temperature is the main cause of intergranular corrosion in stainless steels,and the Cr-depletion mechanism is the most classical theory which explained the intergranular corrosion of stainless steels.However,it is still not possible to completely solve the problem of intergranular corrosion of austenitic stainless steels by reducing the C content to suppress the precipitation of grain boundary carbides or increasing the Cr content to hinder the formation of Cr-depletion zone.In order to further understand the intergranular corrosion behavior of austenitic stainless steel,the effect of sensitization temperature and sensitization time on the intergranular corrosion behavior of austenitic stainless steel was studied by using AISI 310S austenitic stainless steel in this research.The mechanism of intergranular corrosion of 310S stainless steel in high temperature and high pressure（HTHP）H₂S/CO₂ environment was revealed.The influence of sensitization on the stress corrosion cracking behavior of 310S stainless steel under HTHP H₂S/CO₂ environment was also investigated.In this paper,the evolution of Cr-enriched/C-depleted zone with sensitization temperature and sensitization time in 310S stainless steel was studied by TEM.The results showed that,the Cr-depletion level in 310S stainless steel varied little with the increase of sensitization temperature,but the C-enrichment level increased rapidly.After a short period of sensitization treatment,a significant Cr-depleted/C-enriched zone would be formed at the grain boundary area of 310S stainless steel.But with the extension of the sensitization time,the Cr-depletion and C-enrichment level in Cr-depleted/C-enriched zone of 310S stainless steel would be increased at first and then decreased.Serious intergranular corrosion would be induced by the co-work of Cr-depletion and C-enrichment at the grain boundary of 310S stainless steel,in which the effect of C-enrichment was more obvious.Secondly,the Volta potential difference between the intergranular M₂₃C₆ phase and the C-enriched zone was successfully observed by SKPFM.After a short time of sensitization treatment,the Volta potential difference is not large enough to induce severe galvanic corrosion.However,after a long period of sensitization,the Volta potential difference is the main reason for the intergranular corrosion of material.DL-EPR and HTHP H₂S/CO₂ corrosion tests showed that,the susceptibility to intergranular corrosion of 310S stainless steel increased gradually with the increase of sensitizing temperature.But as the sensitization time increased,the susceptibility to intergranular corrosion of 310S stainless steel would be increased at first and then decreased.It was showed that the corrosion products in intergranular corrosion area was enriched in Cr and O elements,but depleted in Fe and Ni elements by the observation of intergranular corrosion section in 310S stainless steels.In addition,the enrichment of Cl and S elements could be observed at the tip of the intergranular corrosion area.And the hydrolysis and acidification of Cl^-and S^2-under the action of the autocatalytic mechanism was the main reason for the expansion of intergranular corrosion.Finally,it could be found that the elongation to failure of 310S austenitic stainless steels was reduced significantly in HTHP H₂S/CO₂ environment as compared to that in air by SSRT tests.With the increase of sensitization temperature,the elongation to failure of 310S austenitic stainless steel decreased gradually.At the same time,the sizes and number of secondary cracks on the surface of SSRT tests samples are gradually increased.At the interface of the metal matrix and solution,the cracks of stress corrosion cracking could be both initiated at the grain boundary and inside the grain,but the expansion of cracks was mainly based on the"slip-dissolution"model,and carried out in transgranular way.