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51317--9745-Corrosion-Fatigue of Steels in Sour Environments: Effects of Strain on the Electrochemical Behavior

The effects of cyclic stress and strain on corrosion-fatigue in sweet and sour corrosion were studied with an electrochemical approach. Cold-drawn and cold rolled carbon steel samples were mechanically cycled in a confined and CO2-saturated synthetic seawater solution, and their electrochemical response was acquired with a classic 3-electrode apparatus.

 

Product Number: 51317--9745-SG
ISBN: 9745 2017 CP
Author: Diego Leyser
Publication Date: 2017
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Corrosion-fatigue in sweet and sour corrosion was experimentally analyzed with a focus on the mutual interactions between mechanical solicitations and electrochemical reactions.The cyclic changes of corrosion current density were measured during corrosion-fatigue tests on a cold-worked grade of carbon steel in an artificial seawater solution (ASTM D1141) saturated with CO2 with and without the presence of H2S.The experiments were realized in potentiostatic conditions with an “imposed open circuit potential” approach e.g. the steady-state electrochemical potential value is acquired from the sample at unloaded condition and kept constant throughout the corrosion-fatigue test. Trapezoidal load waves were used in order to emulate the stress transitions with constant strain rate and the stabilization decays during constant levels of stress (in traction compression and unloaded conditions).The results showed that the electrochemical responses to load transitions followed a linear proportionality to the applied stress amplitude inside a purely elastic range (< 75 % of conventional yield stress). Beyond this value the electrochemical activity patterns are changed by the emergence of anodic peaks related to effect of localized plasticity. The anodic dissolution rate is locally enhanced in the regions were microplasticity is developed such as slip bands and stress concentrators. This brings instabilities to the flat surfaces allowing the generation and growth of grooves that were found to have a close relationship with the corrosion-fatigue crack nucleation process as seen in scanning electron microscopy observations at post-mortem samples and at interrupted tests.Throughout the corrosion-fatigue life of the samples at constant stress amplitude relevant changes in the corrosion current density signature were observed as a result of the cumulated plastic deformation. In the end of the corrosion-fatigue life some new patterns of electrochemical response raised and were correlated to the nucleation and growth of corrosion-fatigue cracks.The comparison between the sweet and sour solutions shows two different effects of the increased intake of hydrogen to the alloy promoted by the H2S partial pressure. At the groove creation step the localized anodic dissolution’s activity is interpreted as being an effect of the adsorption of hydrogen. This provokes a decrease on the surface energy known to induce instabilities on superficial wavinesses turning them into grooves. At the corrosion-fatigue short-crack nucleation and propagation step hydrogen embrittlement takes place and accelerates the process. The possibilities of using this method as an assessment of the plastic or cracking condition are also discussed.

Key words: Corrosion-Fatigue, Stress Enhanced Corrosion, Time-Domain Modelling, Electrochemistry

Corrosion-fatigue in sweet and sour corrosion was experimentally analyzed with a focus on the mutual interactions between mechanical solicitations and electrochemical reactions.The cyclic changes of corrosion current density were measured during corrosion-fatigue tests on a cold-worked grade of carbon steel in an artificial seawater solution (ASTM D1141) saturated with CO2 with and without the presence of H2S.The experiments were realized in potentiostatic conditions with an “imposed open circuit potential” approach e.g. the steady-state electrochemical potential value is acquired from the sample at unloaded condition and kept constant throughout the corrosion-fatigue test. Trapezoidal load waves were used in order to emulate the stress transitions with constant strain rate and the stabilization decays during constant levels of stress (in traction compression and unloaded conditions).The results showed that the electrochemical responses to load transitions followed a linear proportionality to the applied stress amplitude inside a purely elastic range (< 75 % of conventional yield stress). Beyond this value the electrochemical activity patterns are changed by the emergence of anodic peaks related to effect of localized plasticity. The anodic dissolution rate is locally enhanced in the regions were microplasticity is developed such as slip bands and stress concentrators. This brings instabilities to the flat surfaces allowing the generation and growth of grooves that were found to have a close relationship with the corrosion-fatigue crack nucleation process as seen in scanning electron microscopy observations at post-mortem samples and at interrupted tests.Throughout the corrosion-fatigue life of the samples at constant stress amplitude relevant changes in the corrosion current density signature were observed as a result of the cumulated plastic deformation. In the end of the corrosion-fatigue life some new patterns of electrochemical response raised and were correlated to the nucleation and growth of corrosion-fatigue cracks.The comparison between the sweet and sour solutions shows two different effects of the increased intake of hydrogen to the alloy promoted by the H2S partial pressure. At the groove creation step the localized anodic dissolution’s activity is interpreted as being an effect of the adsorption of hydrogen. This provokes a decrease on the surface energy known to induce instabilities on superficial wavinesses turning them into grooves. At the corrosion-fatigue short-crack nucleation and propagation step hydrogen embrittlement takes place and accelerates the process. The possibilities of using this method as an assessment of the plastic or cracking condition are also discussed.

Key words: Corrosion-Fatigue, Stress Enhanced Corrosion, Time-Domain Modelling, Electrochemistry

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