Inhibitive effectiveness of hydrazine oxygen scavenger on low-Alloy steel in near-neutral ph underground conditions

Buried pipeline steels are susceptible to external corrosion from corrosive soil agents present in the trapped electrolyte in contact with exposed surfaces beneath disbonded coatings. Corrosion of these surfaces is often simulated with electrolytes categorized based on pH. A representative near-neutral pH dilute bicarbonate electrolyte with chlorides and sulphate represents one such environment. Although normally deaerated with a 5% carbon dioxide/ 95% nitrogen gas mixture, in practice soil aeration can support oxygen presence in this environment, accelerating corrosion mechanisms. Temperature influences and shielding of the cathodic protection (CP) current further exacerbate this problem. This paper investigates the effectiveness of oxygen scavenger injections on corrosion inhibition of a modern pipeline steel alloy (API-X100) in near-neutral pH conditions which may simulate trapped water environments. The influence of the pipeline's operating temperature on inhibition is studied at 50 and 25 °C. Polarization and Electrochemical Impedance Spectroscopy (EIS) results are highly dependent on temperature, dissolved oxygen, hydrazine, and CP levels. Current shielding effects on corrosion and inhibition process are analysed at field-relevant cathodic potentials (around the-0.85 VSCE NACE CP criterion), yielding unique insights into the applicability of this established standard.