Under-deposit corrosion commonly called cell corrosion represents one of the most damaging forms of corrosion to the piping system. It is one of the most critical phenomenon leading to pipeline failures. It is largely found in sub-sea water injection pipelines well-fluid pipelines and at times in large diameter long distance trunk lines. Under-deposit corrosion is typically very aggressive and localized causing deep penetration of the metal surface with lesser general corrosion in the surrounding areas due to surface deposits electrical imbalances or some other initiating mechanisms. Normally all corrosion factors attract a select number of individual sites. In some cases pitting is extended throughout the entire metal surface giving it an irregular or very rough surface profile. In other instances pits are concentrated in specific areas leaving the majority or the metal surface in new like condition. Microbial induced corrosion is also associated with under-deposit corrosion or engaged in development by the existing internal deposits within the piping system. Presence of microbial bacterial population produces an acidic environment helping to dissolve the metal piping at highly accelerated rates exceeding 75 mils/year.This paper pertains to a study of an API 5L X-60 Carbon steel injection water pipeline which failed prematurely in 4 years after commissioning due to under-deposit corrosion. A detail study was carried out related to pipeline material constitution injection water parameters(suspended solids turbidity microbial counts ( SRB) dissolved oxygen levels iron contents pigging operations chemical addition/ doses (ammonium bi-sulphite – oxygen scavenger) water injection corrosion Inhibitor (WCI) were evaluated to assess the existing corrosion control measures and to investigate the reasons which lead to the failure of the pipeline.In depth study revealed that multiple factors were responsible for initiating pipeline corrosion which finally became very aggressive with time. This corrosion phenomenon gradually enhanced to such an extent that it became UDC which ultimately lead to the failure of the pipeline. Cationic and anionic analysis of sea water (source of injection water) revealed presence of chloride to the tune of 20590 mg/ltr. Largely it has been observed that chloride count over 10.000 to 100000 ppm immensely contribute in enhancing pitting corrosion. Chloride ion in solution can penetrate into surface corrosion product film and lead to increased corrosion in the system. High chloride can also reduce the effectiveness of the chemical corrosion Inhibitors. Chloride ion also known to promote pitting and other forms of localised as corrosion as it breaks down protective films. Chloride is suspected as a potential species contributing to UDC.Presence of sulphate reducing bacteria (SRB) in the system leading to microbial induced corrosion (MIC) was another component possibly responsible for the failure of the pipeline. Scanning Electron Microscope (SEM) analysis of the corrosion product at various magnifications depicted: round edge pits along the internal pipeline surface rod shaped bacterial cells (SRB) in vertical condition with its bottom end linked to each other. SRB are known to form colonies on the pipe surface at 6 o clock was observed on the metal surfaceEDS analysis also depicted the presence of chloride oxides of iron in the point and shoot mode. SEM analysis also revealed presence of scale deposits sulphide iron oxide and chloride. Another SEM micrograph revealed different layers of the corrosion scale in the cross sectional view with the external layers indicating higher concentration of calcium scales. The presence of sulphide iron oxide and chloride is also observed in these different layers depicting the combined effect of under deposit corrosion and microbial corrosion. Another micrograph revealed presence of stringer under higher magnification of the sample which make the material weak and vulnerable to corrosion. All above factors i.e. presence of sulphide iron oxide chloride and calcium scale with presence of stringer are sufficient enough for the pipeline to develop leak/failure.Calculation of flow velocity carried out in the injection water pipeline showed the range of 0.53-0.93 m/s. This is less than the desired optimum velocity of 1m/s to present deposits. This lower than the desired velocity lead to accumulation of deposits along the pipeline bottom and thereby facilitated the under deposit crevice corrosion.Considering above factors: presence of chloride (20590 mg/ltr) sulphate reducing bacteria (SRB) rod shaped their bottom linked with each other at 6O clock position of the pipeline identification of sulphide carbonate (CO3 -- ) iron (Fe+2 & Fe+3 ) iron oxide and traces of chloride high dissolved oxygen (> 20 ppb ) on a number of occasions low flow velocity (< 1mt/sec) presence of stringer formation of channel/ groove in the pipeline confirm that under-deposit corrosion as the most likely reason which lead to the failure of the API 5L X-60 carbon steel pipeline. This paper will provide details of this case study.

Keywords: Under-deposit corrosion, water injection line, corrosion inhibitors