Corrosion Behavior and Microstructural Characterization of Friction Stir Welded API X70 Steel

Abstract

Friction stir welding is an excellent alternative process used in pipelines circumferential welds due to fusion welding low corrosion resistance. This study describes the corrosion resistance as a function of the microstructural features resulting from an API X70 FSW weld joint. The microstructural features were examined by optical microscopy, scanning electron microscopy, and X-ray diffraction. The corrosion analysis was conducted employing potentiodynamic polarization tests using an electrochemical microcell, which allowed to test small circular areas delimited by a 2mm in diameter O-ring gasket. The base metal and heataffected zone were the most susceptible region to corrosion due to the banded microstructures, and the re-stirred-zone and stir zone was the most resistant region to corrosion because of grain refinement.

The effect of hydrogen on the fracture toughness of friction‐stir welded API 5L X70 pipeline steels

“…When the stress concentration exceeds the critical stress of the M/A constituent, a microcrack originates at the matrix‐constituent interface and gradually propagates through the matrix 86–90 . BM and SZ are reported to have a percentage of second phases of 10.2% and 2.83%, respectively 55 . The high percentage of secondary phases observed in BM came from degenerate perlite and was reported in previous work 55 .…”

“…BM and SZ are reported to have a percentage of second phases of 10.2% and 2.83%, respectively 55 . The high percentage of secondary phases observed in BM came from degenerate perlite and was reported in previous work 55 . In the SZ, it is assumed that the percentage of the second phase is composed mainly of M/A and cementite because this region is formed by laths and grains of bainite, with the bainite laths composed of ferrite disposed of in parallel and uniform strips containing the constituents M/A intertwined as well as cementite 91 .…”

“…Verifying other previous microstructural studies on FSW in API steels, a complex microstructure was observed within all microstructural zones. 44,55,56 The microstructures of the base metal (BM) and stir zone (SZ) are shown in Figure 3. The BM comprises equiaxed and polygonal ferritic grains and degenerated pearlite bands, shown in Figure 3A.…”

Friction Stir Processing Influence on Microstructure, Mechanical, and Corrosion Behavior of Steels: A Review

“…The results of these tests showed improvement in corrosion resistance of FSPed SDSS by about 86%, owing to the presence of an oxide layer containing Cr, which is formed by an increase in grain boundary density in the FSPed zone. For pipeline applications, Giarola et al [ 59 ] studied the corrosion performance of friction stir welded API X70 steel using microcell and PDP to locally investigate the corrosion behavior of SZ, thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ), and BM in 3.5% NaCl. The authors used a composite tool of PCBN-WRe at a traveling speed of 100 mm/min and a rotational speed of 300 rpm.…”

“…For pipeline applications, Giarola et al [59] studied the corrosion performance of friction stir welded API X70 steel using microcell and PDP to locally investigate the corrosion behavior of SZ, thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ), and BM in 3.5% NaCl. The authors used a composite tool of PCBN-WRe at a traveling speed of 100 mm/min and a rotational speed of 300 rpm.…”