Corrosion analysis of API X70 steel in seawater environment

Abstract

This study is designed to understand the corrosion and the mechanical properties of API X70 steel in a simulation of seawater. A special machine was built to simulate the corrosion of marine pipelines. The elemental structure and microscopic properties of the steel were identified with the help of spectroscopy and microscopy. Tests were conducted on welded samples for tensile strength, impact, compression, and hardness, first in air and again after being in seawater for twelve months. The equipment used for testing was a scanning electron microscope (SEM) to observe the fracture surfaces. It was found that the mechanical properties of API X70 steel became less resistant after being exposed to seawater. When tested in seawater, parent and weld materials had a yield strength of around 428 MPa and 416 MPa, which was a decrease from 560 MPa and 578 MPa in air. For the parent metal and the weld joint, tensile strengths were 634 MPa and 674 MPa, as compared to 512 MPa and 529 MPa afterward. Tests conducted on parent metal produced higher Charpy impact properties than HAZ and weld, and the lowest were found in seawater. Being in saltwater lowered the rigidity and toughness of the bones. Tests exposed brittle as well as ductile types of failure in SEM. The reductions in tensile strength due to the environment were 1.2-1.3, and those for impact were higher at 2.0. This information shows API X70 steel can be applied offshore and helps researchers understand how to handle corrosion better.