|Abstract or Summary
- Oil drilling requires that miles of steel drill pipe, called the drill string, be exposed to fatigue loading and corrosive environments. Furthermore, it has been reported that pitting corrosion is responsible for initiating catastrophic drillstring failure. Pitting corrosion is responsible for initiating fatigue cracks, with pits acting as local stress raisers. This project investigates the pitting corrosion susceptibility and influence on fatigue life of high strength low alloy drillstring steels S-135 and UD-165 in a simulated drilling completion fluid, 6.7 M CaCl₂.
Immersion test results show that both alloys are susceptible to pitting corrosion and a non-protective film of hydrated rust (FeOOH) was found. Non-metallic inclusions are well known to initiate pits in carbon steels. Preliminary testing of S-135 shows inclusions rich in Mn and S, while UD-165 shows inclusions rich in either Fe, Al and O, or Nb and C.
Potentiodynamic scans of both alloys show active dissolution. Cyclic voltrammetry results shows a repassivation potential in the cathodic region. Through potentiostatic testing pit growth was found to be independent of anodic polarization time, meaning pit growth was unachievable through anodic polarization. Shallow, wide pits (3-5μm deep, 10-50μm wide) were only observed after the potentiostatic anodic polarization.
Fatigue testing was performed on three surface treatments: 1) virgin 2) pitted by galvanostatic methods, and 3) pitted by galvanostatic methods, dried and left in the laboratory (20°C, 20-40% humidity) for 14 days. Statistical testing by Student's t-tests showed that UD-165 had a statistically significant drop in fatigue life for 1) virgin samples against 2) pitted samples while no significant drop was found for S-135. Both UD-165 and S-135 showed no statistically significant drop in fatigue life for 1) virgin samples against 3) samples pitted and left in the laboratory for 14 days.