CORROSION SCIENCE SECTION CORROSION—Vol. 68, No. 11 1015 Submitted for publication: January 8, 2012. Revised and accepted: April 10, 2012. Preprint available online: July 11, 2012, http:// dx.doi.org/10.5006/0636. ‡ Corresponding author. E-mail: frankel.10@osu.edu. * Fontana Corrosion Center, The Ohio State University, Columbus, OH 43210. ** DNV Columbus, Inc. *** CNEA Argentina. Hydrogen Permeation and Corrosion Fatigue Crack Growth Rates of X65 Pipeline Steel Exposed to Acid Brines Containing Thiosulfate or Hydrogen Sulfide M. Kappes,* G.S. Frankel, ‡, * R. Thodla,** M. Mueller,** N. Sridhar,** and R.M. Carranza*** ABSTRACT Corrosion fatigue crack growth rates were obtained for X65 pipeline steel in acid brines containing thiosulfate (S 2 O 3 2– ) or hydrogen sulfide (H 2 S). Samples were exposed for 72 h at the open-circuit potential to allow bulk hydrogen charging. The corrosion fatigue crack growth rate increased with par- tial pressure of H 2 S and correlated with the steady-state flux of hydrogen permeation during corrosion in the same solu- tions. The rate of hydrogen absorption increased with increas- ing S 2 O 3 2– concentration to a maximum at 10 –3 M S 2 O 3 2– , owing to a competition between increased surface concentration of H 2 S from S 2 O 3 2– reduction and increased rate of iron sulfide film formation. Corrosion fatigue behavior in S 2 O 3 2– -containing acidified brines is the same as in solutions with low par- tial pressures of H 2 S, in accordance with predictions from the hydrogen permeation results. This suggests that, for corrosion fatigue studies, S 2 O 3 2– solutions are possible candidates for replacement of H 2 S gas, as long as the H 2 S partial pressure of interest is low. Also, the expected concentration of hydrogen inside the metal specimen at the end of the exposure period correlates with the rate of corrosion fatigue crack growth, sug- gesting that hydrogen charged during the pre-charging period controls corrosion fatigue crack growth. KEY WORDS: corrosion fatigue, hydrogen permeation, hydro- gen sulfide, steel INTRODUCTION Corrosion fatigue affects the integrity of carbon steel pipes used in deepwater oil extraction. Corrosion fatigue can be aggravated by the presence of sour (i.e., hydrogen sulfide [H 2 S]-containing) brines, which cause a decrease in fatigue life with respect to air per- formance. 1-3 Therefore, testing of materials for this application usually requires the use of H 2 S. Thiosul- fate (S 2 O 3 2– ) solutions have been studied 4-11 as substi- tutes for H 2 S solutions in sour corrosion and sulfide stress cracking studies due to safety concerns. S 2 O 3 2– is an innocuous anion, in contrast to H 2 S, a flamma- ble and lethal gas. In situ generation of H 2 S during steel corrosion in S 2 O 3 2– solutions was reported in the literature, 4,7,12-13 so caustic traps and H 2 S detec- tors are still required for S 2 O 3 2– testing. However, the main advantage of performing tests with S 2 O 3 2– solu- tions is that high-pressure H 2 S cylinders would not be required. Recent work showed that H 2 S is produced at the steel surface as a result of an electrochemical reduc- tion reaction of S 2 O 3 2– , which would make S 2 O 3 2– solu- tions a candidate for replacing H 2 S-bubbled solutions in corrosion studies. Despite the in situ generation of H 2 S, the degree of embrittlement in a slow strain tensile test of carbon steel specimens fully immersed in S 2 O 3 2– solutions was found to be well below 7 that obtained with the same solution saturated with H 2 S. This lesser embrittlement was correlated with the lower amount of hydrogen permeation in S 2 O 3 2– vs. H 2 S solutions. 7,15 There could be several reasons for the lower hydrogen permeation, including the lower con- ISSN 0010-9312 (print), 1938-159X (online) 12/000185/$5.00+$0.50/0 © 2012, NACE International