M. Jerome

Necessity of a palladium coating to ensure hydrogen oxidation during electrochemical permeation measurements on iron

Abstract Electrochemical permeation experiments have been carried out with iron samples, the detection side being coated or not with palladium. It is clearly seen that when the quantity of hydrogen which diffuses through the iron membrane is important, the imposition of a potential higher than the hydrogen equilibrium potential at the detection side is not a sufficient condition to ensure the complete oxidation of hydrogen atoms. A part of hydrogen atoms coming to the detection side combine and give molecular hydrogen; these atoms are not measured in the anodic oxidation current recorded at the detection side. Then this anodic current is not a reliable measurement of the hydrogen flux. On a palladium coated detection side, the oxidation of hydrogen atoms seems to be complete even in drastic charging conditions. The use of a palladium coating on the detection side of ferrous samples is necessary for a reliable exploitation of the results.

The electrochemical permeation of hydrogen in steels without palladium coating. Part I: Interpretation difficulties

There is no general agreement on the experimental conditions used in the hydrogen electrochemical permeation method. Particulary, the necessity of a palladium coating on steel is not yet recognized. To answer some questions concerning the boundary conditions, experiments were carried out on pure iron and low alloyed steels without palladium coating. It is shown clearly that in this case surface phenomena are not controlled. Especially, the passivation layer on the exit side induces variation of the hydrogen concentration on this side with time. Consequently no quantitative evaluation can be made for diffusion parameters, as long as the mathematical model issued from the usual stationary boundary conditions is used.

A thin palladium coating on iron for hydrogen permeation studies

For the sake of the hydrogen electrochemical permeation method, the possibility to obtain a thin and reliable palladium coating on iron has been studied. Experiments were carried out at room temperature with a PdCl2 solution. Electrochemical kinetic measurements were done to find out the potential range in which the only possible reaction is the palladium reduction. A well adherent palladium coating, covering homogeneously the whole iron surface and thinner than 0.1 μm, has been obtained.

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. I. Characterisation of hydrogen evolution on vertical unstressed electrodes

For investigating stressed carbon-steel electrode (AISI 4140) cracking induced by hydrogen embrittlement with the electrochemical noise (EN) technique, the contribution of the evolving hydrogen bubbles to the EN has to be determined. Under cathodic current control, various reasons may explain the fluctuations of the electrode potential. In the first paper of this series the electrolyte resistance (ER) fluctuations, which yield ohmic-drop fluctuations, were analysed for unstressed vertical tensile specimens. A simplified model was proposed to tentatively derive the characteristic parameters of the gas evolution, such as the bubble mean size and evolution mean rate, from the power spectral density (PSD) of the ER fluctuations. However, results in only qualitative agreement with optical observations were obtained, indicating that the complicated screening and dragging effects of rising bubbles have to be taken into account in the modelling for vertical electrodes. The electrode potential fluctuations of unstressed tensile specimens were investigated in the second paper of this series and the influence of a stress was examined in the third paper.

The electrochemical permeation of hydrogen in steels without palladium coating. Part II: Study of the influence of microstructure on hydrogen diffusion

Abstract The presence of graded microstructures such as those encountered in heat affected zones (HAZ) may modify the material susceptibility to hydrogen induced cracking. In an attempt to clarify the role played by the different microstructures present in HAZ, large simulated HAZ were produced using a Gleeble machine and studied by the hydrogen electrochemical permeation technique. The method was applied without palladium coatings but the surface electrochemical conditions were strictly controlled. The permeation and outgassing rates are clearly related to the volume fraction of martensite. In ferrite-pearlite structures, the influence of small A1N- and NbC-type precipitates on permeation rate was also shown.

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. III. Influence of hydrogen absorption for stressed electrodes

In this third part of the study of the electrochemical noise generated by hydrogen evolution on cathodically-polarised AISI 4140 steel specimens in 0.5 M sulphuric acid, a tensile stress was applied to the specimens. The two components of the measured potential fluctuations DeltaV, namely ohmic-drop fluctuations DeltaR(e)I and faradaic potential fluctuations DeltaE, were analysed. After bubble size homogenisation, the effect of an applied stress was an increase in the level of the power spectral densities (PSD) Psi(V) and Psi(E) of the Delta V and DeltaE fluctuations, indicating a change in metal-hydrogen-interactions related to damages in the metal induced by the combined actions of stress and hydrogen embrittlement. In the meanwhile, the PSD Psi(ReI) of the ohmic-drop fluctuations did not exhibit any change, revealing that the departure rate and size of hydrogen bubbles were not modified by the internal damages in the specimen. The time evolution Of Psi(E) up to fracture could be explained by the enhancement of hydrogen penetration into the metal induced by the increase in the density of microdefects or crack advances inside the metal.

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. II. Identification of potential fluctuation sources for unstressed electrodes

In this second part of the study of electrochemical noise (EN) generated by hydrogen evolution on a vertical cylindrical AISI 4140 steel electrode under galvanostatic control in 0.5 M sulphuric acid, the potential fluctuations induced by the growth and detachment of hydrogen bubbles at the electrode surface were analysed. They could be related to fluctuations of various quantities: electrode active surface due to bubble screening effects, concentration of dissolved hydrogen in the electrolyte close to the electrode surface, and metal-hydrogen interactions (MHI) on or beneath the electrode surface. The existence of MHI and their influence on faradaic potential fluctuations could be revealed by comparing the noise features on steel and platinum. The influence on EN of the charging cathodic current density, the presence of dissolved oxygen in the solution, and the electrode rotation speed was investigated in the absence of stress applied to the electrode. In the third paper of this series, the effect of hydrogen embrittlement on potential fluctuations of stressed electrodes will be examined.

The electrochemical permeation of hydrogen in palladium: Boundary conditions during a galvanostatic charging under low charging current densities

The application of the hydrogen electrochemical permeation in palladium can help to better understand the phenomena involved in the permeation process. With this metal surface phenomena can be controlled and stationary conditions as well as a good reproducibility can be obtained. The validity of the boundary conditions during a galvanostatic charging was verified and the hydrogen efficiency was evaluated. The concentration gradient and the amount of hydrogen which is found in the sample during the stationary conditions were quantitatively determined.

Behaviour of hydrogen in FeNiC alloys

Abstract A particular cathodic charging technique was used to evaluate the behaviour of hydrogen on Fe25.66Ni0.31C alloy. This technique is based on the electrolysis of water injected in a molten salts bath. The electrolytic charging conditions were chosen as −2.05V/Ag and 300°C. The quantities of hydrogen extracted from specimens of different diameters after electrolysis over different durations were used to calculate both the substantial surface concentration ( C 0 = 3 cm 3 H 2 /cm 3 metal) and the hydrogen concentration profiles in austenite. After quenching at −65°C, a linear relationship between the hydrogen concentration and the amount of retained austenite was determined. A critical concentration C k = 0.06 cm 3 H 2 /cm 3 metal, initiated microcracks in martensite. The behaviour of hydrogen on austenite is discussed in terms of grain boundary and dislocation trapping.

Hydrogen absorption by cathodically protected underground steel piping in thiosulfate containing media

This paper deals with the influence of the cathodic protection conditions on the hydrogen absorption by underground steel pipes for natural gas transport in media containing thiosulfate. Hydrogen electrochemical permeation tests were performed in soil-like media with different concentrations of thiosulphate. The effect of applied potential as well as that of the medium composition on the hydrogen absorption were analyzed. The deposit formed during cathodic polarization on the protected steel surface was examined by scanning electron microscopy and X-ray diffraction. The results confirmed the catalytic effect of the thiosulphate on the hydrogen absorption. However it was shown that a high negative electrode potential as well as a calcium-magnesium deposit formed during the cathodic polarization can inhibit this catalytic action. The mechanism of the inhibition seemed to be unique and was attributed to a distance effect between the electronegative catalytic species and the metallic surface due to either an electrical field (created by a high negative potential) or a physical barrier (created by a calcium-magnesium deposit).