Abstract
Duplex Stainless Steels (DSSs) are widely used in the oil and gas industry. When the steel is subjected to a thermal gradient, as in welding may occur precipitation of intermetallic phases. Among these phases, there is the sigma phase, which even in small quantities reduces drastically the mechanical and corrosion resistance properties. According to some studies in the literature, the limit amount of sigma phase present in steel in welded joints must be in a maximum volume fraction of 2,5%. In search to improve the detection sensitivity of the sigma phase, this paper presents results obtained by Linear Sweep Voltammetry (LSV) in Duplex Stainless Steel UNS S31803 as a Nondestructive Testing (NDT) model. The innovation in the application of this test is to use a microcell-based system that allows a reduced area of analysis obtaining density of currents in the order of microamperes and in the use of selective electrolytes to the elements of the oxides present in the intermetallic phases. With the use of these systems and through the optimization of control parameters it was possible to detect very low amounts of deleterious phases such as the sigma and chi phases.
Highlights
The effect of deleterious phases on the mechanical and corrosion properties on the Duplex Stainless Steels (DSSs) is well known and studied by many researchers. 1-6 These phases may be formed during hot rolling process, components operation at certain temperatures and welding, which constitute a broad application of DSSs. 7,8 the knowledge of the effect of the intermetallic phases on the DSSs properties is necessary
Many electrolytic etchings are found in the literature to selectively reveal the presence of deleterious phases in the structure of stainless steels
21 NaOH solutions have been considered by possibility of revealing the contour between the austenitic and ferritic phases as well as the possibility of distinguishing between sigma and chi phases using the correct choice of attack parameters
Summary
The effect of deleterious phases on the mechanical and corrosion properties on the Duplex Stainless Steels (DSSs) is well known and studied by many researchers. 1-6 These phases may be formed during hot rolling process, components operation at certain temperatures and welding, which constitute a broad application of DSSs. 7,8 the knowledge of the effect of the intermetallic phases on the DSSs properties is necessary. 1-6 These phases may be formed during hot rolling process, components operation at certain temperatures and welding, which constitute a broad application of DSSs. 7,8 the knowledge of the effect of the intermetallic phases on the DSSs properties is necessary. 13 Another important concern is the technique applicability in regions such as the Heat-Affected Zone (HAZ) in welding process, as the HAZ may presents small dimensions whereas the contact area commonly used in the DL-EPR technique is considerably larger, which could generate problems for applications of the technique to such approach. As the film formed on the deleterious phases in DSSs is composed by oxides of its forming components (chromium and molybdenum), the application of anodic potential in hydroxide solution generates the selective dissolution of these oxides, ensuring the selectivity of the method. As the film formed on the deleterious phases in DSSs is composed by oxides of its forming components (chromium and molybdenum), the application of anodic potential in hydroxide solution generates the selective dissolution of these oxides, ensuring the selectivity of the method. 14,16 In a similar way, it is reported in the literature the use of microcells for the study of localized corrosion in metals and their alloys, the reduction of the area of analysis allows the comparison between microstructural characteristics and local electrochemical processes that could hardly be obtained in the study of global corrosion. 17,18,19 In addition, there is a gain in the quality of the signal obtained from the possibility of high rates of mass transport from the working electrode, reduction of capacitive currents and reduction of Ôhmic drop 20
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