Effects of Inclusions on Pitting Corrosion on Line Pipe Steel

Objectives

  • Qualitative and quantitative estimation of various non-metallic inclusions,
  • Establish deterministic corrosion parameter as an index of corrosion damage;
  • Identifying the extent of corrosion;
  • Identify the most susceptible inclusions and develop a ranking system;
  • Develop a damage potential index;
  • Based on above results, determine a bench mark.

Technical Details

  • Collection of API 5L X 52-60 grade steel from different sources (different heats / manufacturers) and sampling from them to have substantial difference in quantified microstructural features;
  • Qualitative and quantitative estimation of various non-metallic inclusions, identifying their morphology, determining their class and their distribution;
  • SEM examination using EDS to cross check as well as to confirm the nature of inclusions (need based);
  • Establish the deterministic corrosion parameter as an index of corrosion damage using pitting corrosion studies;
  • Identifying the extent of corrosion using the deterministic parameter as above;
  • Develop a damage potential index through correlation of the statistical distribution of inclusions and the corresponding corrosion damage.

Achievement (Output/outcome)

The variations in microstructure of line pipe steel (Grade X60) lead to significant changes in its corrosion behavior. Use of any particular steel for a given environment needs a robust data bank through which life predictions of components can be made confidently to avoid catastrophic failure. In the present endeavor, an adaptive neuro fuzzy based model is proposed to study the effect of microstructural variation on the performance of the candidate steel. This model has been verified with the experimental findings. The developed model will be a useful tool to justify the acceptance/rejection of the alloy on a commercial scale.

Impact

It is always desirable to have steel as clean as possible, preferably with no inclusions. Complete removal is not possible. Lowering of inclusions below a level adds to the huge cost of steel. A compromise has to be arrived between cost and cleanliness level. Looking at the inclusion rating level commonly achievable, it is recommended that the reported specifications with respect to inclusion severity level in steel used for line pipe manufacturing may be followed. If the limits are not specified, the line pipe supplier / manufacturer may supply steel with high severity inclusions, as has been noticed in the present investigation in some of the steels with inclusion severity rating as high as 3.0.

Prediction of Pitting Potential using Fuzzy based model with higher (HCI) and lower (LCI) 99% confidence interval for a API grade steel.Variation in pitting potential with complex inclusion at various area fraction of cementite for a API grade steel
Prediction of Pitting Potential using Fuzzy based model with higher (HCI) and lower (LCI) 99% confidence interval for a API grade steel.Variation in pitting potential with complex inclusion at various area fraction of cementite for a API grade steel.