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Influence of AC on Hydrogen Permeation at Steel Under Cathodic Polarization

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Product Number: 51315-5995-SG
ISBN: 5995 2015 CP
Author: Guenter Schmitt
Publication Date: 2015
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~~The paper reports on the influence of alternating current (AC) on the hydrogen uptake of X80 pipeline steel cathodically protected with a constant current density of -100 A/m2 in different media (0.2 M H2SO4 2M and 0.8 M NaOH 0.2 M Na2SO4 0.2 M NaCl/MgCl2/CaCl2 artificial soil solution Type NS4) at room temperature. Under these conditions AC current densities of 30 100 and 500 A/m2 were applied.While the Devanathan-Stachurski method was not successful to indicate AC effects on hydrogen permeation the volumetric measurement of permeated hydrogen could be readily applied.In the absence of promoters (e.g. arsenite selenite) no hydrogen permeation was measured in all media regardless of the presence of AC (30 – 500 A/m2). Hydrogen permeation started only after addition of promotors. In H2SO4 solution containing selenite as promotor and NaOH solution containing arsenite as promotor an AC effect could be detected. Surprisingly after shut-off of the AC polarisation during cathodic polarisation (-100 A/m2) the promoter containing sulphuric acid yielded a decrease of the hydrogen permeation while in case of the promoter containing NaOH solution an increase of the H permeation rate was observed.As a side effect intergranular ‘cathodic’ iron dissolution with formation of pits was found in promotor-free NaCl/ MgCl2/CaCl2 solution with a total salt concentration of 0.2 mol/L and a molar ration of Na:Mg:Ca=1:3:10. Within 24 h pit depths of 50 µm (1-2 grain layers) could be formed in combination with Ca- and Mg-containing deposits exhibiting filigree structures. In the presence of AC the formation of such deposits and scales was retarded. The mechanism of this ‘cathodic’ iron dissolution is discussed.During the experimental investigations no indications were found that AC favours hydrogen related failure mechanisms at high strength pipelines steels under corrosion protection.
~~The paper reports on the influence of alternating current (AC) on the hydrogen uptake of X80 pipeline steel cathodically protected with a constant current density of -100 A/m2 in different media (0.2 M H2SO4 2M and 0.8 M NaOH 0.2 M Na2SO4 0.2 M NaCl/MgCl2/CaCl2 artificial soil solution Type NS4) at room temperature. Under these conditions AC current densities of 30 100 and 500 A/m2 were applied.While the Devanathan-Stachurski method was not successful to indicate AC effects on hydrogen permeation the volumetric measurement of permeated hydrogen could be readily applied.In the absence of promoters (e.g. arsenite selenite) no hydrogen permeation was measured in all media regardless of the presence of AC (30 – 500 A/m2). Hydrogen permeation started only after addition of promotors. In H2SO4 solution containing selenite as promotor and NaOH solution containing arsenite as promotor an AC effect could be detected. Surprisingly after shut-off of the AC polarisation during cathodic polarisation (-100 A/m2) the promoter containing sulphuric acid yielded a decrease of the hydrogen permeation while in case of the promoter containing NaOH solution an increase of the H permeation rate was observed.As a side effect intergranular ‘cathodic’ iron dissolution with formation of pits was found in promotor-free NaCl/ MgCl2/CaCl2 solution with a total salt concentration of 0.2 mol/L and a molar ration of Na:Mg:Ca=1:3:10. Within 24 h pit depths of 50 µm (1-2 grain layers) could be formed in combination with Ca- and Mg-containing deposits exhibiting filigree structures. In the presence of AC the formation of such deposits and scales was retarded. The mechanism of this ‘cathodic’ iron dissolution is discussed.During the experimental investigations no indications were found that AC favours hydrogen related failure mechanisms at high strength pipelines steels under corrosion protection.
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