Subsurface Corrosion as the Main Degradation Process of 17GS Pipeline Steel after 50 Years of Operation

The structure and physical and mechanical properties of a 17GS steel specimen cut from a main gas pipeline section under use for over 50 years at a working pressure of 5.5 MPa is investigated in this study. The pipe under study laid in a straight-line segment and covered with sand was in service according to regular parameters. Metallography studies using methods of optical and scanning microscopy for analyzing transformations in the crystal structure of the metal are carried out. Mechanical tests are focused on the impact strength and static tension to determine changes in the physical and mechanical properties. The data obtained are compared with the as-delivered state specified by the manufacturer, currently accepted standards and industry requirements for pipes intended for reuse. The research reveals the basic mechanical characteristics of the steel to be virtually the same as those stated by the manufacturer; no structural transformations in the metal are found. The principal degradation process in the specimen of interest is identified to be sub-surface corrosion occurring no deeper than 100 µm. from the result of this study is that changes in the structure and mechanical properties appear insignificant over 50 years of service provided that the maximal stress in the pipes is below the limit of macroelasticity.