Development of Thermodynamic Criteria for Determining the Composition of Duplex Stainless Steels with High Corrosion Resistance.

One of the most popular methods for ranking duplex stainless steels (DSSs) and predicting their corrosion properties is the calculation of the pitting resistance equivalent number (PREN). However, since DSSs are two-phase materials with a significant fraction of secondary phases and precipitates, the application of the PREN can be highly limited. This article attempted to use a new approach to describe the corrosion resistance of these steels. The corrosion resistance of two DSSs of the same class was investigated. Under identical solution heat treatments in the temperature range of 1050-1200 °C, the crevice corrosion resistance of one steel increased, while that of the other decreased. It was demonstrated that the amounts of austenite and ferrite changed similarly in these steels, and the different corrosion resistances were associated with the behaviors of secondary phases: niobium carbonitride and chromium nitride. SEM-EDS analysis was conducted to analyze the redistribution of elements between phases in both cases, showing good agreement with the thermodynamic modeling results. The PREN was calculated for each phase depending on the treatment temperature, and a method for calculating the effective PREN (PRENeff), accounting for phase balance and secondary phases, was proposed. It was shown that this indicator described corrosion properties better than the classical PREN calculated for the average steel composition. This study demonstrated how the calculation of critical temperatures (the temperature of equal amounts of ferrite and austenite, the temperature of the beginning of chromium nitride formation, and the temperature of the beginning of σ-phase formation) could describe the corrosion resistance of DSSs. Maximum possible deviations from these temperatures were defined, allowing the attainment of the required corrosion properties for the steels. Based on the conducted research, an approach for selecting new compositions of DSSs was proposed.

The Effect of the Operation of Downhole Equipment on the Processes of Corrosive Wear (by the Example Inflow Control Devices of Nozzle Type).

Existing approaches to assessing the reliability and safe operation of downhole equipment provide for assessing only the direct impact of erosion and corrosion processes on the degradation of the material. At the same time, the influence of downhole equipment operating modes on the intensity of corrosion and erosion processes is not evaluated. The necessity of using inflow control devices is shown. The necessity of selecting inflow control devices for specific operating modes of the well is highlighted in order to avoid increased erosion wear due to increased sand production. In this article, a series of studies was conducted to assess the influence of the hydrodynamic characteristics of the fluid flow, which vary depending on the operating modes of the nozzle-type inflow control device, on corrosion processes in the well. It was shown that the level of wall shear stress (WSS) of the base pipe, immediately after the flow control device in the direction of fluid flow, affects the intensification of corrosion processes in downhole equipment. Studies of WSS on the pipe wall were carried out using a dynamic autoclave and elements of mathematical modeling. The design factors (the angle of inclination of the main hydraulic channel of the inflow control device relative to the base pipe) affecting the change in tangential stresses on the wall of the base pipe were investigated. The mechanism of corrosion wear of the base pipe was revealed.