Electrochemical Permeation Technique Research Articles

Quantification of Hydrogen Flux from Atmospheric Corrosion of Steel Using the Scanning Kelvin Probe Technique

The atmospheric corrosion of high-strength steels can lead to hydrogen absorption directly linked to hydrogen embrittlement or delayed fracture phenomena. A scanning Kelvin probe (SKP) and electrochemical permeation technique (EPT) were applied to correlate the potential of an oxidized surface with the flux of hydrogen across a thin steel membrane. The side of the membrane opposite the corroding or electrochemically charged area was analyzed. The potential drop in the oxide was calibrated in terms of surface hydrogen activity, and SKP can be applied in situ for the mapping of hydrogen distribution in the corroding metal. A very low flux of hydrogen can be characterized and quantified by SKP, which is typically observed under atmospheric corrosion conditions. Therefore, hydrogen localization that drives steel durability under atmospheric corrosion conditions can be evaluated.

Effects of Ni-flash coating on hydrogen evolution, ad/absorption, and permeation behaviors of advanced high-strength steel during electro-Zn plating

The effects of a thin Ni-flash coating, tens of nanometers thick, on hydrogen evolution, ad/absorption, and permeation of advanced high-strength steel were examined for a deeper understanding of the hydrogen infusion behavior in the steel substrate during electro-Zn plating. The electrochemical permeation technique and impedance spectroscopy were used under cathodic polarization in a step-up manner. In addition to the electrochemical analyses, the hydrogen microprinting technique was employed to identify the distribution of Ag particles (locating hydrogen atoms) in the electro-Zn plated steels with and without a thin intermediate Ni-layer. The results revealed that despite the higher hydrogen evolution rate on Ni-flash coating layer than on bare steel, the intermediate Ni-layer decreased the hydrogen infusion considerably in the steel substrate during electro-Zn plating, due primarily to the lower hydrogen ad/absorption rate on the Ni-flash coating layer, and the predominant hydrogen trapping at the multi-interfacial areas of the Zn-layer/Ni-layer/steel substrate. These results could provide insights into the precise role of a thin (tens of nanometers) Ni-flash coating on the resistance to hydrogen embrittlement of ultra-high-strength steel alloys during electro-Zn plating.

Quantification of subsurface hydrogen in corroding mild steel using Scanning Kelvin Probe calibrated by electrochemical permeation technique

Scanning Kelvin probe (SKP) can be applied for mapping of subsurface hydrogen in steels. The good spatial resolution is combined with poor quantification. Controversy, the electrochemical permeation technique (EPT) is extremely sensitive to hydrogen flux but has low spatial resolution. Thus, a local hydrogen quantification method using SKP measurements calibrated by EPT was developed. The fixed amount of hydrogen flux in mild steel membrane was obtained by cathodic polarization and was detected using the two methods. A semi-logarithmic relationship between SKP potential drop and the hydrogen sub-surface concentration underneath of the corroding surface was established. SKP quantification was applied for mapping the subsurface hydrogen in steel corroding under various atmospheric corrosion conditions.

Critical verification of the effective diffusion concept

Knowing the hydrogen distribution c(x,t) and local hydrogen concentration gradients grad(c) in ferritic steel components is crucial with respect to hydrogen embrittlement. Basically, hydrogen is absorbed from corrosive or gaseous environments via the surface and diffuses through interstitial lattice sites into bulk. Although, the lattice diffusion coefficient DL∼0.01mm2/s is in the order of magnitude of those for well-annealed pure iron, trapping sites in the microstructure retard the long-range chemical diffusion jL=−Dchem(c)grad(c), causing local hydrogen accumulation in near surface regions in limited time. Considering pure ferritic crystals without trapping sites in the microstructure, the limited characteristic diffusion depth xc∼Defft is proportional to the square root of the effective diffusion coefficient Deff and of time t. Effective diffusion coefficients are measured independently for hydrogen using the electrochemical permeation technique. For pure crystals, the effective diffusion coefficient is constant at given temperature and allows accurate calculations of the diffusion depths. However, with trapping sites in the microstructure the effective diffusion coefficient is not a material property anymore and becomes dependent on the hydrogen charging conditions. In the present work, the theory of hydrogen bulk diffusion is used to verify the concept of effective diffusion. For that purpose, the generalized bulk diffusion equation was solved numerically by using the finite difference method (FDM). The implementation was checked using analytical solutions and a comprehensive convergence study was done to avoid mesh and time dependency of the results. It is shown that effective diffusion coefficients can vary by magnitudes depending on the sub-surface lattice concentration. This limits the application of the effective diffusion concept and also the calculation of the characteristic diffusion depth.

Influence of microstructure on hydrogen trapping and diffusion in a pre-deformed TRIP steel

Austenitic steels are known to exhibit a low hydrogen diffusion coefficient and hence a good resistance to hydrogen embrittlement. Therefore, it is an experimental challenge to investigate their hydrogen diffusion properties. In this study, the electrochemical permeation technique is used to determine the hydrogen diffusion coefficients in different pre-deformed states (φ = 0, 0.32, 0.39, 0.49) of the high-alloy austenitic TRIP steel X3CrMnNiMoN17-8-4 in a temperature range of 323 K–353 K. In combination with microstructural analysis, a correlation between phase transformation from γ-austenite to α′-martensite and dislocation density is shown. As a result of the lattice transformation from fcc to bcc, the diffusion rate of hydrogen is significantly increased (Dapp,φ = 0 = 3.6 × 10−12 cm2 s−1, Dapp, φ = 0.32 = 1.6 × 10−11 cm2 s−1at 323 K). With higher degrees of deformation, the dislocation density also increased in the martensite islands, resulting in a degressive growth of the diffusion coefficient (Dapp, φ = 0.39 = 5.3 × 10−11 cm2 s−1, Dapp, φ = 0.49 = 1.1 × 10−10 cm2 s−1at 323 K). Moreover, detailed calculations are performed to describe the way of hydrogen trapping and to give a possible mechanism of diffusion.

Hydrogen permeation through steel during cathodic polarization of lubricating oils in a modified Devanathan–Stachurski cell

In lubricated tribo-contacts, hydrogen ingress in steel is possible due to chemical reactions of lubricant components like base oils or additives, and/or contamination upon service particularly water, and/or corrosion processes, and/or electrostatic fields or current flow. Absorbed by the metal, atomic hydrogen may cause serious deleterious effects on the physical–chemical and mechanical properties, reducing the material’s ability to withstand the design loads. The present research work is focused on analyzing the influence of electric field on lubricating oils in contact with steel surface. In order to evaluate the possibility of atomic hydrogen generation and permeation into the steel under cathodic polarization of lubricating oils the electrochemical permeation technique developed by Devanathan and Stachurski is used. The input cell of a Devanathan–Stachurski set up is appropriately modified by realizing a very close distance between the working electrode (steel membrane) and a Pt counter electrode with the oil between. This significantly increases the sensibility of the set up and allows the application of larger voltage and higher temperature to enable hydrogen generation from lubricating oils. The complex effects of cathodic polarization, temperature, additives and presence of water in model lubricating oils on atomic hydrogen permeation into steel is discussed.

Characterization of Hydrogen Diffusion in Offshore Steel S420G2+M Multi-layer Submerged Arc Welded Joint

As onshore installation capacity is limited, the increase in the number of offshore wind turbines (OWT) is a major goal. In that connection, the OWTs continuously increase in size and weight and demand adequate foundations concepts like monopiles or tripods. These components are typically manufactured from welded mild steel plates with thickness up to 200 mm. The predominant welding technique is submerged arc welding (SAW). In accordance with the standards, the occurrence of hydrogen-assisted cracking is anticipated by either a minimum waiting time (MWT, before non-destructive testing of the welded joint is allowed) at ambient or a hydrogen removal heat treatment (HRHT) at elevated temperatures. The effectiveness of both can be estimated by calculation of the diffusion time, i.e., diffusion coefficients. In this study, these coefficients are obtained for the first time for a thick-walled S420G2+M offshore steel grade and its multi-layer SAW joint. The electrochemical permeation technique at ambient temperature is used for the determination of diffusion coefficients for both the base material and the weld metal. The coefficients are within a range of 10−5 to 10−4 mm2/s (whereas the weld metal had the lowest) and are used for an analytical and numerical calculation of the hydrogen diffusion and the related MWT. The results showed that long MWT can occur, which would be necessary to significantly decrease the hydrogen concentration. Weld metal diffusion coefficients at elevated temperatures were calculated from hydrogen desorption experiments by carrier gas hot extraction. They are within a range of 10−3 mm2/s and used for the characterization of a HRHT dwell-time. The analytical calculation shows the same tendency of long necessary times also at elevated temperatures. That means the necessary time is strongly influenced by the considered plate thickness and the estimation of any MWT/HRHT via diffusion coefficients should be critically discussed.

Role of Hydrogen in the Separation of Interfaces in S13Cr Supermartensitic Stainless Steel

This paper outlines experimental procedures and numerical analyses to investigate hydrogen embrittlement (HE) of S13Cr supermartensitic stainless steel (SMSS) subjected to various cathodic potentials. The hydrogen diffusion behavior was investigated using two electrochemical permeation techniques, namely, the double potentiostatic method (DPM) and the step method (SM), along with thermal desorption spectroscopy (TDS) tests. The apparent hydrogen diffusion coefficients varied from 1.4 × 10−13 to 4.7 × 10−12 m2/s, and TDS revealed the existence of deep traps, such as interfaces around precipitates and between retained austenite and ferritic matrix. The effects of HE in terms of a reduction in ductility were analyzed through tensile tests and fractographic analysis. A maximum reduction in elongation of approximately 14% was measured, and a majority of brittle fracture along the entire net section was observed in test samples pre-charged under -1.5 V/SCE and -1.7 V/SCE. A computational simulation was performed using the finite element method to predict the loss of ductility using the obtained experimental data. The computational model used a fracture-controlled method under static structural conditions, which links the reduction in elongation of the tensile specimens to the decrease in critical fracture energies, which, in turn, were calculated through a new mean field approach that used thermodynamic excess variables. The observations presented here suggest that S13Cr has good resistance to HE and that the computational model is reliable because only slight deviations in the magnitude (5%) were observed between the experimental and numerical results.

Study on Hydrogen Diffusion and Sulfide Stress Cracking Behaviors of Simulated Heat-Affected Zone of A516-65 Grade Pressure Vessel Carbon Steel

Hydrogen diffusion and sulfide stress cracking of simulated heat-affected zone (HAZ) of A516- 65 grade steel were examined using an electrochemical permeation technique, glycerin volumetric method, and constant loading method. HAZ samples were fabricated using a metal thermal cycle simulator with a welding heat input of 20, 35, and 50 kJ/cm. The fractions of bainite and martensite-austenite (M-A) constituent in coarse-grained HAZ (CGHAZ) and intercritical HAZ (ICHAZ) obtained by a simulated thermal cycle with a low heat input (20 kJ/cm) were higher than those with a higher heat input. These fractions contributed to the increase in the reversible hydrogen trap density (N<sub>[H]rev</sub>) and reversibly trapped hydrogen concentrations (C<sub>rev</sub>). Although CGHAZ had higher N<sub>[H]rev</sub> and C<sub>rev</sub> meaning that it is more likely to be vulnerable to brittle failure by hydrogen, actual fracture by sulfide stress cracking (SSC) occurred in ICHAZ composed of a mixture of soft ferrite/pearlite, and hard bainite and M-A. The hydrogen diffusion/trapping parameters, which were obtained from the electrochemical permeation or glycerin method, cannot be directly indicative of the resistance to SSC of the steel in a H<sub>2</sub>S environment. The susceptibility to SSC was more influenced by the level of M-A-localization and localized corrosion attack, acting as a stress intensifier under a tensile load.

The Role of Hydrogen on the Separation of Interfaces of  Supermartensitic Stainless Steel S13Cr

This paper outlines experimental procedures and numerical analyses to research Hydrogen embrittlement (HE) of the supermatensitic stainless steel (SMSS) S13Cr submitted to different cathodic potentials. Hydrogen diffusion behavior was investigated by means of two electrochemical permeation techniques, namely the double potentiostatic method (DPM) and the step method (SM), and it was also performed thermal desorption spectroscopy (TDS) tests. Apparent Hydrogen diffusion coefficients vary from 1.4 x 10-13 m²/s to 4.7 x 10-12 m²/s and TDS reveals the existence of deep traps, such as interfaces of precipitates and between retained austenite and ferritic matrix. HE effects in terms of reduction of ductility were analyzed through tensile tests and fractographic analysis, where a maximum reduction of elongation of around 14% was measured and a majority of brittle fracture along entire net section was observed in test samples pre charged under -1.5 V/SCE and -1.7 V/SCE. A calculation procedure based on computational simulation via finite element method has been performed, in order to predict mentioned loss of ductility using the experimental data mentioned before. The computational model used a fracture-controlled method under static structural condition, which links reduction of elongation of tensile specimens to the decreasing of critical fracture energies, which, in turn, were achieved through a new mean field approach regarding thermodynamic excess variables. Herein presented observations make it possible to suggest, that S13Cr has good resistance to HE and that the mentioned calculation procedure is reliable, since slight deviations in the magnitude of 5% were observed.

Hydrogen diffusion in low alloy steels under cyclic loading

Abstract The aim of this work is to analyze hydrogen transport in a low alloy steel by applying the electrochemical permeation technique to matrices subject to cyclic loading conditions, up to the yield strength and beyond this limit. The results indicate that, with an increase in the applied maximum stress, a decrease in the apparent diffusivity takes place, along with a marked and instantaneous reduction of diffusing hydrogen in the lattice and an increase in hydrogen solubility. An effect on the permeation current was observed, together with a variation of hydrogen diffusion kinetics ascribable to the activation on new trapping sites, with an appreciable effect under cyclic loading already at 55% of the yield limit, which becomes more relevant in the plastic domain.

Comparative study of devices for hydrogen permeation measurements

New devices for measurement of hydrogen permeability in steels using electric resistance sensors have been assessed. Diffusion coefficients of two types of advanced high strength steels obtained with the traditional electrochemical permeation technique using the Devanathan–Stachurski permeation setup and the novel techniques are compared. The EPT technique is found to be more appropriate for laboratory assessment of hydrogen permeability due to generally better control over the experiment and data evaluation. The novel setups can be a good choice for comparative hydrogen measurements in real environments, including atmospheric ones, because of simpler sample preparation and small size of hydrogen probes.

Techniques for investigation of hydrogen embrittlement of advanced high strength steels

AbstractProduction volumes of advanced high strength steels (AHSS) are growing rapidly due to material and energy savings they provide in a number of application areas. In order to use their potential fully, it is necessary to minimize any danger of unexpected failures caused by hydrogen embrittlement. It is possible only if deeper understanding of underlying mechanisms is obtained through further research. Besides description of main grades of AHSS and mechanisms of HE, this paper reviews available tools for determination of hydrogen content and susceptibility to HE focusing on atmospheric conditions. Techniques such as slow strain rate testing, constant load testing, electrochemical permeation technique, scanning Kelvin probe and scanning Kelvin probe force microscopy have already been used to study the effect of hydrogen entered under atmospheric exposure conditions. Nanoindentation, hydrogen microprint technique, thermal desorption spectroscopy, Ag decoration or secondary ion mass spectrometry can be also conducted after atmospheric exposure.

Effect of Hydrogen Trapping and Poisons on Diffusion Behavior of Hydrogen in Low Carbon Steel

Effect of hydrogen trapping and poisons on diffusion behavior of hydrogen in commercial cold-rolled low carbon steel was investigated by means of electrochemical hydrogen permeation techniques. The experimental results reveal that diffusion rate and diffusion flux of hydrogen in the materials gradually increase with increasing the number of hydrogen charging and outgassing, and lag time significantly shortens with them, therefore, hydrogen trapping impede diffusion behavior of hydrogen in the materials. Different poisons in the hydrogen charging solution have also resulted in a certain influence on the assessment of hydrogen diffusion behavior.

Effect of bainite in microstructure on hydrogen diffusion and trapping behavior of ferritic steel used for sour service application

Abstract

Effect of the cathodic current density on the sub-surface concentration of hydrogen in X80 pipeline steels under cathodic protection

The effect of the cathodic current density (i) on the permeation of hydrogen through API X80 pipeline steels under cathodic protection was investigated using an electrochemical permeation technique. A proper equation was presented for determining the sub-surface concentration of hydrogen (c0). With the aim to enhance the predictive capabilities of the proposed equation, actual environmental parameters such as temperature were considered. The experimental results reported herein clarified the relationship between c0 and i. It was clearly shown that the ratio of c0 to the specimen thickness (L) was proportional to √i when i was lower than its critical value (iH). On the other hand, for i values above iH, c0 was equal to the saturation hydrogen content because of reaction equilibrium. Furthermore, iH was found to be dependent on temperature (i.e., iH increased with the test temperature).

Hydrogen permeation behavior in relation to microstructural evolution of low carbon bainitic steel weldments

The microstructures of high strength bainitic steel weldments were characterized, and their hydrogen permeability was investigated by the electrochemical permeation technique. The effective diffusion coefficient in the welded joint increases with heat input, while the hydrogen concentration at the sub-surface decreases with increasing heat input, mainly due to coarsening grain and inclusion sizes. Compared with as-welded specimens, the tempered membranes have lower effective diffusion coefficient and higher density of hydrogen traps due to newly formed carbides acting as hydrogen trapping sites that are derived from the decomposition of martensite-austenite constituents. These were further confirmed by the partial permeation technique.

Numerical study on hydrogen permeation of ferritic steel evaluated under constant load

Hydrogen permeation of two kinds of ferritic steel with different level of 2nd phase particles are compared utilising electrochemical permeation technique evaluated under various loads. This study indicates that tensile stress in plastic domain induces expansion of the interface between the 2nd phase particles and steel matrix, and formation of blister crack. Such microstructural-discontinuity provides additional hydrogen traps, resulting in slower diffusion and lower steady-state permeation flux. These phenomena appear predominantly in steel containing coarse-sized inclusion and precipitate having sharp extremities. The numerical analysis also supports that the steel containing coarse-sized 2nd phase particles with sharp extremities exhibits higher parameters of hydrogen capture rate per irreversible traps and blister crack formation rate than the steel containing fine-sized ones with spherical shape.

Effect of cathodic hydrogen-charging current density on mechanical properties of prestrained high strength steels

The present work investigated the effect of cathodic hydrogen-charging current density on mechanical properties of prestrained high strength steels. This was done by tensile tests on both hydrogen-charged and -uncharged prestrained specimens at a cross-head displacement speed of 0.03mm/min. The influence of prestrain on hydrogen behavior was also studied using an electrochemical permeation technique. The results show that the relationship between ultimate tensile strength of the hydrogen-charged specimens (UTS-H) and prestrain depends on current density. The UTS-H decreases with increasing current density independent on prestrain. With an increase in prestrain the diffusion coefficient of hydrogen gradually decreases, which is attributed to increasing dislocations density acted as hydrogen traps. SEM fractograph reveals that hydrogen charging causes a change from ductile to brittle failure.

Simulation of hydrogen diffusion in welded joint of X80 pipeline steel

Hydrogen diffusion coefficients of different regions in the welded joint of X80 pipeline steel were measured using the electro-chemical permeation technique. Using ABAQUS software, hydrogen diffusion in X80 pipeline steel welded joint was studied in consideration of the inhomogeneity of the welding zone, and temperature-dependent thermo-physical and mechanical properties of the metals. A three dimensional finite element model was developed and a coupled thermo-mechanical-diffusion analysis was performed. Hydrogen concentration distribution across the welded joint was obtained. It is found that the postweld residual hydrogen exhibits a non-uniform distribution across the welded joint. A maximum equivalent stress occurs in the immediate vicinity of the weld metal. The heat affected zone has the highest hydrogen concentration level, followed by the weld zone and the base metal. Simulation results are well consistent with theoretical analysis.