Materials Performance

AUG 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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52 AUGUST 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 8 CHEMICAL TREATMENT examined. The deepest cracks found in f low loops initiated from linear welding defects at the circumferential welds that joined elbows to straight sections. Approxi- mately nine months after the first leak, a second pipeline leaked at a girth weld. The recovered spool displayed similar features to those described previously, which are typical of IGSCC (Figure 2). Stress Corrosion Cracking SCC requires a tensile stress and a cor- rosive environment. The tensile stress is usually residual stress from welding but also can be caused by mechanical damage or cold forming. SCC can be transgranular or intergranular and is found over a lim- ited potential range in environments that promote passive behavior of the material. Cracking occurs in the active-to-passive transition and in the transpassive zones (Figure 3). Only a few environments are know n to cause IGS C C in C S; amin e s (MEA, diethanolamine [DEA] and methyl- diethanolamine [MDEA]), carbonate/bi- carbonate, caustic, nitrates, phosphates, gas liquor, and anhydrous methanol. Hydrogen Sulfide Scavenging Reaction H 2 S is removed from the gas by scaveng- ing with MEA triazine. The chemical treat- ment package consists of MEA triazine plus 6% MEA and an organophosphate scale inhibitor. The triazine reacts with two mol- ecules of H 2 S to form dithiazine and 2 MEA. Un d e r c e r t a i n c o n d i t i o n s , d i t h i a z i n e polymerizes to amorphous dithiazine and ejects additional MEA. The pH of spent triazine is 12.5. The scavenger is directly injected into f low loops to improve contact time and spent chemical scavenger freely enters the f lowline. The injection rate is 2 to 3 gal/d (7.6 to 11.3 L/d) for a standard well. Short flowlines are not pigable. Amine and Carbonate/ Bicarbonate Stress Corrosion Cracking Amine SCC was first recognized at refin- eries in 1951 in amine units used for CO 2 and H 2 S removal. Cracking was slow, and could take from 5 to 15 years. It was more severe when using MEA vs. DEA or MDEA as a scavenger. Following a refinery disaster in the 1980s that resulted in several fatali- ties, amine SCC was studied extensively. Ironically, that failure was caused by sulfide stress cracking (SSC) of a weld repair due to a leak caused by amine SCC. Amine cracking is always intergranular and is most prevalent at ~130 °F (54 °C). Gutzeit and Johnson 4 repor t ed crack s between 100 and 140 °F (60 °C) in MEA units. Hughes 5 reported cracks in ME A storage tanks at ambient temperatures. API 945 6 advises mitigation in solutions con- taining >2% MEA and operating at ambient temperatures and upward. Cracking does not occur in pure MEA solutions; it requires the presence of CO 2 and/or heat stable acid salts (formates, acetates). Cracking is more prevalent in lean amine but also occurs in rich amine. Cracking is slightly affected by the concen- tration of MEA, although most refineries operate with ~25%. SCC occurs at poten- tials between –0.5 and –0.8 V at pH 8 to 11. Amine cracking is unaffected by the hard- ness of the steel but low-carbon, higher- yield-strength steels appear to be more susceptible. MEA SCC has been successfully pre- v ent ed by PWH T. D et e ction m eth o d s include WFMT and shear wave ultrasonic testing, but cracks are very fine and diffi- cult to find. Radiography can detect severe cracks but early cracking may be missed. Parkins and Foster 7 proposed that MEA SCC is a form of carbonate/bicarbonate cracking, and that amine is just a means of producing pH values in a range where CO 2 w i l l f or m a s o luti on of bi c a rb on at e / carbonate. This dependence is shown in Figure 4. FIGURE 3 The narrow ranges of potentials that cause SCC at the active-to-passive transition and transpassive regions. FIGURE 4 CO 2 is in solution at a low pH, but mixtures of bicarbonate and carbonate ions are formed at a pH above eight.

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