Creep rate in Cu-OFP under applied electrochemical potential and presence of sulphide

Tiina Ikäläinen, Timo Saario, Zaiqing Que

Research output: Book/ReportReport

Abstract

The main objectives of this work were to find if the creep rate of Cu-OFP is influenced by the presence of sulphide and either anodic or cathodic polarisation.
In the first part of the study performed in sodium nitrite (1M NaNO2) without sulphide, the creep rate was found to be accelerated by anodic polarisation to +1 mA/cm. However, the reason for the acceleration remains somewhat unresolved since the specimen failed by SCC, and the contribution of SCC micro-cracks (forming early on during the test) onto the creep rate could not be determined.
Without polarisation (i.e. at the corrosion potential), addition of sulphide (up to 10−3 M, equivalent to 25 mg/l) had no measurable effect on the creep rate. However, stopping the sulphide feed resulted, after some incubation time, in a transient increase of creep rate by about an order of magnitude, lasting
for about 40 hours. Anodic polarisation to +1 mA/cmresulted in a clear (transient) acceleration of creep, although smaller than in the case of 1M NaNO2. Cathodic polarisation resulted (in some cases) in a similar transient increase of creep rate. These results may possibly be due to reductive dissolution of films resulting in injection of vacancies into Cu-OFP (in case of anodic polarisation),
or to hydrogen entering the Cu-OFP material and interacting with the dislocation structure (in case of cathodic polarisation), which again accelerate creep. Confirming either of these hypotheses would need further investigation.
Surface and cross-sections studied with SEM revealed crack-like surface defects extending a few µm into the Cu-OFP material in specimens tested in air and in water without sulphide, but not in specimens tested in water with sulphides. This was explained by the rather thick sulphide film (about 10 µm) having consumed the Cu-OFP surface material in which the crack-like surface defects appeared
in the specimens tested in air and water without sulphide. This may be taken as an indication that pre-existing µm-sized cracks are not able to initiate SCC in Cu-OFP in the presence of sulphides. The main conclusion of the work is that the creep rate of Cu-OFP is affected by all three variables studied, i.e. anodic polarization, cathodic polarisation and sulphide exposure. However, the effects
were found to be of transient type, lasting at maximum a few tens of hours.
Original languageEnglish
PublisherSvensk Kärnbränslehantering AB (SKB)
Number of pages85
Publication statusPublished - Sep 2022
MoE publication typeD4 Published development or research report or study

Publication series

SeriesSvensk kärnbränslehantering AB. Technical Report
NumberSKB TR-22-05
ISSN1404-0344

Keywords

  • Cu-OFP
  • creep rate
  • sulphide
  • polarisation

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