MECHANISTIC STUDIES OF SOUR UNDERDEPOSIT CORROSION IN THE PRESENCE OF CHEMICAL INHIBITION

摘要

Low dosage hydrate inhibitors (LDHIs) have been used as alternatives to methanol or ethylene glycol to control hydrate problems. The major advantage is much lower dosage, typically 1-5%, compared to methanol or ethylene glycol. Anti-agglomerants (AAs), as one type of LDHI chemicals, functions to terminate the growth and agglomeration of small hydrate particles. Recently, there have been concerns of potential pitting corrosion in offshore umbilical applications by an incumbent Anti-agglomerants LDHI. To address these concerns, electrochemical method of cyclic potentiodynamic polarization (CPP), a modified ASTM G-61 method at ambient temperature, and immersion corrosion testing at elevated temperatures were carried on to evaluate the pitting tendency of AA formulations with corrosion inhibitors on UNS S31600 steel and UNS S32001 steel. From CPP test, two types of corrosion inhibitors were identified to reduce pitting tendency. One type increases the pitting potential of steels. The other one increases the re-passivation potential, the difference between pitting potential and corrosion potential. The low pitting tendency was also confirmed with the immersion test, conducted at elevated temperatures and long testing period. The temperature tolerance of the AA formulations with corrosion inhibitors were also identified based on immersion testing result. The hydrate performance testing (blockage test) showed that the addition of corrosion inhibitors in AA formulation have no effect on the hydrate inhibition performance.