Based on visual, metallographic, mechanic and model tests of X60(L415MB) grade pipes 1220mm in outer diameter, 16 mm nominal wall thickness selected from second line of transmission gas pipeline in Slovakia after service since the year 1974 the following conclusions can be drawn: 1. The results of structural and phase analysis of pipe steel and fabricated longitudinal welded joint are adequate to conventional quality of X60 grade pipes. 2. Root bead overlap as well as presence of lack of fusion in the circumferential welded joint proves lower quality of assembly welding. 3. Replica method for non-destructive study of microstructure proved comparable results with conventional destructive preparation of metallographic specimen. Moreover, this method was proved suitable also for the identification of surface micro-defects of welded joints namely in service. 4. Corrosion damage of outer pipeline surface is of local planar and pitting corrosion character. It occurs prevailingly in the bottom pipeline part especially in the areas of damaged insulation. 5. Mechanical properties of parent metal of pipes and welded joints still fulfil the specified yield strength, tensile strength, elongation and impact toughness values. 6. Model tests of pipes proved their high load capacity also in such high wall thickness weakening as was revealed in pipe I (h _min = 4,8 mm). The pipe I failed at 12 MPa, pipe II at 15,2 MPa and pipe III at 16,5 MPa pressure. Strain gauge measurements did not reveal any stress increase in corrosion weakened areas either during or after pulsating pressure test. In pipe II strain gauge measurements proved hardening effect of 'clock spring' in the area of its application but already in plastic strain region of the pipe.