A　gathering　pipeline　in　a　gas　field　in　Northeast　China　transporting　mainly　natural　gas　and　also　some　water　and　sands　was　welded　using　two　20G　pipes　with　different　wall　thicknesses.　The　internal　corrosion　of　this　pipeline　was　inhomogeneous.　The　unequal　wall　thickness　welded　joint　suffered　more　serious　corrosion　damage　than　did　other　parts　of　the　pipe　and　was　found　to　fail　during　routine　maintenance.　In　this　paper,　the　corrosion　damage　at　this　joint　was　studied　based　on　inhomogeneity.　Internal　and　external　factors　affecting　corrosion,　such　as　the　mechanical　and　electrochemical　properties　of　materials,　stress　distribution　at　the　joint,　and　the　flow　condition　in　the　pipe,　and　their　combined　action　were　investigated.　Failure　attribution　was　performed　on　this　basis.　Specifically,　a　hardness　test　was　conducted　on　different　zones　of　the　joint,　including　the　base　metal　(BM),　heat-affected　zone　(HAZ)　and　weld　metal　(WM),　to　investigate　the　abrasive　wear　resistance　at　the　joint.　Microstructures　of　the　three　zones　were　characterized　by　optical　microscopy.　The　morphology　and　composition　of　the　corrosion　film　covering　the　inner　surface　of　the　welded　joint　were　characterized　by　scanning　electron　microscopy　(SEM),　energy-dispersive　spectroscopy　(EDS)　and　X-ray　powder　diffraction　(XRD).　Potentiodynamic　polarization　experiments　and　electrochemical　impedance　spectroscopy　(EIS)　were　conducted　on　samples　from　the　three　zones　to　investigate　their　electrochemical　performances.　Moreover,　a　computational　fluid　dynamics　(CFD)　analysis　was　performed　to　obtain　the　flow　condition　near　the　joint,　and　a　finite　element　method　(FEM)　was　used　to　calculate　the　stress　distribution　at　the　joint.　The　results　showed　that　the　internal　corrosion　mechanism　of　the　pipeline　was　CO2　corrosion　accelerated　by　detrimental　Cl-.　The　geometric　discontinuity　of　the　welded　joint　was　the　main　cause　of　the　accelerated　corrosion　damage　at　the　joint.　The　change　in　wall　thickness　of　the　welded　pipe　not　only　produced　gas　vortices　but　also　led　to　stress　concentration.　Both　of　these　conditions　can　further　accelerate　corrosion　by　destroying　the　protectiveness　of　the　corrosion　film.　It　is　suggested　that　the　corrosion　can　be　prevented　by　eliminating　the　inhomogeneities　of　fluid　flow　and　stress　distribution.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.