Intergranular and transgranular stress corrosion cracking of carbon steel in fuel-grade ethanol

Janne Johannes Torkkeli

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

The SCC mechanisms in FGE are studied by making slow-strain rate testing (SSRT) and notched constant tensile load testing (NCTLT) in fuel-grade ethanol (FGE) with carbon steel SA-106 and St35 test specimens machined from pipeline samples. The test specimens with SCC were characterized and compared to failed pipeline samples from FGE service. The effect how impurities or microstructure of the carbon steel affect the SCC mechanisms were studied by making experiments with pure compounds and by comparing the results to the observations from the SSR and NCTL testing or from the failed pipeline samples. The selective dissolution of manganese sulfide (MnS) inclusions was studied by making dissolution tests with pure MnS. The selective dissolution of pearlite nodules was studied by making cathodic polarization measurements with pure iron and cementite. Clear differences were found between the intergranular and transgranular SCC mechanisms. With negligible chloride concentrations the transgranular SCC initiates only at the pearlite nodules while the intergranular SCC initiates only at the ferrite phase. With chloride concentrations of approx. 2 mg/l or more, intergranular SCC does not initiate and transgranular SCC can initiate in ferrite and pearlite phases. Conditions inside intergranular SCC crack are not acidic. Selective dissolution of the pearlite nodules, which can be catalyzed by chlorides, occurs in FGE and this effect stops the propagation of the intergranular SCC crack leading to a localized corrosion at the pearlite nodule. With high enough stress level the intergranular SCC mechanism can change to transgranular SCC cracking at the pearlite nodules. The penetration rate of MnS inclusion dissolution front was measured to vary from 11 µm/year up to 127 µm/year inside the intergranular SCC cracks but it is most likely higher inside the transgranular SCC cracks due to more acidic conditions. The sulfur species formed due to selective dissolution of the MnS inclusions are HS- and S2O32- in acidic conditions with chlorides and SO42- in conditions with negligible chloride concentrations and acetic acid. These results indicate that there can be HS- and S2O32- ions inside the transgranular SCC cracks but only SO42- inside the intergranular SCC crack. There is an apparent microgalvanic coupling between cementite and ferrite of pearlite phase. The cementite is more favourable cathode as the OCP of cementite is higher than for pure iron. The OCP of cementite is approximately 120 mV higher in aerated FGE and approximatly 40 mV higher in unaerated FGE. CO and CO2 can form in the cathodic reactions at cementite surface and react into carbonate. There are a lot of similarities with the intergranular and transgranular SCC mechanisms occurring in FGE compared to carbonate solutions.
Translated title of the contributionRaerajoja pitkin ja rakeiden läpi etenevä jännityskorroosio hiiliteräksellä bensalaatuisessa etanolissa
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Hänninen, Hannu, Supervising Professor
Publisher
Print ISBNs978-952-60-7498-6
Electronic ISBNs978-952-60-7497-9
Publication statusPublished - 2017
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • carbon steel
  • fuel-grade ethanol
  • stress corrosion cracking
  • manganese sulfide
  • pearlite
  • electrochemical polarization

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