Hydrogen induced cracking (HIC) testing of low alloy steel in sour environment: Impact of time of exposure on the extent of damage

Hydrogen induced cracking (HIC) of line pipe steel was investigated through immersion testing and hydrogen permeation measurements. At constant pH and hydrogen sulphide partial pressure (pH₂S), the extent of HIC was found to depend on exposure time until a stable level was reached. The time to reach this stable value is affected by pH and pH₂S. Results of permeation experiments confirmed that HIC is linked with the increase of hydrogen concentration in the steel. It is also shown that low severity requires longer exposures to reach equilibrium. This must be taken into account for HIC testing in mildly sour environment.     

Effect of microstructure on the hydrogen trapping efficiency and hydrogen induced cracking of linepipe steel

The hydrogen trapping efficiency in different microstructures is compared, and the critical hydrogen flux for hydrogen induced cracking (HIC) is determined for API X65 grade linepipe steel. By controlling the start cooling temperature (SCT) and the finish cooling temperature (FCT) in thermomechanically controlled process (TMCP), three different kinds of microstructure such as ferrite/degenerated pearlite (F/DP), ferrite/acicular ferrite (F/AF), and ferrite/bainite (F/B) are obtained. A modified ISO17081(2004) standard method is used to evaluate the hydrogen trapping by measuring the permeability (J_ssL) and the apparent diffusivity (D_app). Microstructures affecting both hydrogen trapping and hydrogen diffusion are found to be DP, AF, BF and martensite/austenite (M/A) constituents. The hydrogen trapping efficiency is increased in the order of DP, BF and AF, with AF being the most efficient. HIC is initiated at the local M/A concentrated region when the steel has such microstructures as F/AF or F/B. Although the trapping efficiency of bainite is lower than that of AF, bainite is more sensitive microstructure to HIC than to AF.     

Hydrogen embrittlement susceptibility and permeability of two ultra-high strength steels

Slow displacement rate tensile tests were carried out in a saturated H₂S solution to investigate the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of two ultra-high strength steels (PH 13-8 Mo stainless steel and T-200 maraging steel). Hydrogen permeation properties were determined by an electrochemical permeation method. The results of permeation tests indicated that over-aged specimens showed a lower diffusivity/hydrogen flux and higher solubility than those solution-annealed. The great increase in reverted austenite (irreversible hydrogen traps) together with numerous precipitates at the expense of dislocations (reversible) in the over-aged specimen led to such a change in permeability. Ordinary tensile tests indicated that four tested specimens had roughly the same yield strength level. Hence, the hydrogen embrittlement susceptibility of the material could be related to their permeation properties. The uniform distribution of strong hydrogen traps in over-aged specimens instead of weak traps in the solution-annealed impeded the hydrogen transport toward the strained region, thus, the resistance to sulfide stress corrosion cracking was improved in over-aged specimens.     

Notch tensile properties of laser-surface-annealed 17-4 PH stainless steel in hydrogen-related environments

Slow displacement rate tensile tests were performed to determine the notched tensile strength (NTS) of 17-4 PH stainless steel with various microstructures in hydrogen-related environments. Solution-annealed (SA), peak-aged (H900), over-aged (H1025), and laser-annealed (LA) specimens were included in the study. Based on the results of NTS in air, the NTS loss in both gaseous hydrogen and H₂S-saturated solution was used to access the detrimental effects of hydrogen in 17-4 PH steel subjected to different treatments. Electrochemical permeation tests were also employed to determine the hydrogen permeation characteristics of the 17-4 PH steel plate with various microstructures. The result indicates that all the specimens have low NTS loss in gaseous hydrogen but significantly suffer from sulfide stress corrosion cracking (SCC), especially for the soft SA specimen. It was deduced that high hydrogen diffusivity and less trapped hydrogen atoms in the SA matrix provided rapid transport of massive hydrogen atoms into highly stressed region, and deteriorated the NTS tested in the saturated sulfide solution. On the other hand, H1025 specimen consists of the blocky austenite together with Cu-rich precipitates uniformly distributed in the grain interior; dense and coarse precipitates are also observed along prior austenite grain boundaries. Hydrogen atoms tend to be trapped along grain boundaries, and lead to the formation of intergranular fracture for H1025 specimen tested in the H₂S solution. Fine and homogeneously distributed precipitates in the H900 matrix result in uniformly trapping of hydrogen atoms, so it behaves superior properties than other specimens. The decohesion of precipitate/matrix interfaces induces quasi-cleavage fracture of the H900 specimen tested in H₂S solution. Finally, the application of laser-annealing treatment on the H900 specimen cannot improve its resistance to sulfide SCC, because the laser-annealed zone is susceptible to hydrogen embrittlement in the H₂S solution.     

Corrosion and hydrogen absorption of commercially pure zirconium in acid fluoride solutions

The corrosion and hydrogen absorption of commercially pure zirconium have been investigated in acidulated phosphate fluoride (APF) solutions. Upon immersion in 2.0% APF solution of pH 5.0 at 25 °C, a granular corrosion product (Na₃ZrF₇) deposits over the entire side surface of the specimen, thereby inhibiting further corrosion. In 0.2% APF solution, marked corrosion is observed from the early stage of immersion; no deposition of the corrosion product is observed by scanning electron microscopy. A substantial amount of hydrogen absorption is confirmed in both APF solutions by hydrogen thermal desorption analysis. The amount of absorbed hydrogen of the specimen immersed in the 2.0% APF solution is smaller than that in the 0.2% APF solution in the early stage of immersion. The hydrogen absorption behavior is not always consistent with the corrosion behavior. Hydrogen thermal desorption occurs in the temperature range of 300–700 °C for the specimen without the corrosion product. Under the same immersion conditions, the amount of absorbed hydrogen in commercially pure zirconium is smaller than that in commercially pure titanium as reported previously. The present results suggest that commercially pure zirconium, compared with commercially pure titanium, is highly resistant to hydrogen absorption, although corrosion occurs in fluoride solutions.     

The effect of grain size and aging on hydrogen embrittlement of a maraging steel

Notched tensile tests were conducted under a slow displacement rate to evaluate the influences of grain size and aging on hydrogen embrittlement (HE) of T-200 maraging steel. In addition, an electrochemical permeation method was employed to measure the effective diffusivity (D_eff) and apparent solubility (C_app) for hydrogen of various heat-treated specimens. The results indicated that the aged (482 °C/4 h) specimens comprised of numerous precipitates led to a raised C_app and a decreased hydrogen diffusivity as compared to those of the solution-treated ones. The solution-treated specimens were resistant to gaseous HE, whereas aged specimens were susceptible to it, implying the strength level was the controlling factor to affect the HE susceptibility of the specimens. Nevertheless, all specimens suffered from sulfide stress corrosion cracking (SSCC) severely but to different degrees. The aged specimens were more likely to form intergranular (IG) fractures in H₂S but quasi-cleavage (QC) in H₂. For the solution-treated specimens, a fine-grained structure was susceptible to HE in H₂S and revealed mainly QC that differed from the IG fracture of the coarse-grained one. The fracture mode of the specimens could also be related to the transport path and / or the supply of hydrogen to the plastic zone of notched specimens in hydrogen-containing environments.     

Hydrogen-induced cracking of pure titanium in sulphuric acid and hydrochloric acid solutions using constant load method

The hydrogen-induced cracking (HIC) of the commercial pure titanium (Ti) has been investigated as functions of applied stress and test temperature in sulphuric acid and hydrochloric acid solutions by using a constant load method. From the results obtained, HIC was hydrogen embrittlement (HE) related to the fracture of hydride. We have also found that the steady state elongation rate obtained from corrosion elongation curve becomes a relevant parameter for predicting time to failure and a criterion to assess whether HIC takes place or not. A parameter, t_sst_f⁻¹ was also found to become an indicator to assess whether HE takes place or not. Furthermore, it was deduced that HIC was qualitatively explained in terms of hydride formation and a localized deformation, which was basically based on a hydride formation-rupture event at crack tips.     

The effect of hydrogen sulphide on ammonium bisulphite when used as an oxygen scavenger in aqueous solutions

The effectiveness of ammonium bisulphite (ABS) as oxygen scavenger and the behaviour of H₂S in brine/ABS solutions are examined. Deionised water, 1 wt.% NaCl and 3.5 wt.% NaCl with different ABS concentrations are bubbled with H₂S gas, while the dissolved oxygen, sulphide, E_H and pH are measured. With the exception of natural seawater, ABS concentration much greater than 100 ppmw (parts per million by weight) is needed to completely scavenge dissolved oxygen in all the solutions considered. The reaction between ABS and H₂S leads to increase in sulphide. The implications of the results for environment assisted cracking of oil and gas production tubings are discussed.     

Hydrogen entry into steel during atmospheric corrosion process

Hydrogen entry and permeation into iron were measured by an electrochemical method during atmospheric corrosion reaction. The hydrogen permeation was enhanced on passive films because the hydrogen adsorption increased by the hydrogen evolution mechanism which is different from that on a bear iron surface. The permeation rate during a wet and dry corrosion cycle showed a maximum in the drying process depending upon the surface pH and the corrosion potential. The pollutant such as Na₂SO₃ which decreases the pH and the corrosion potential causes an increase in the permeation rate. The mechanism of the change in the permeation rate during the wet and dry cycles is explained by the polarization diagram of the electrode covered by thin water layer.     

Measurement of steady-state hydrogen electrode reactions on Alloys 600 and 690 tubes

The steady-state polarization measurements for HER and HOR were carried out in order to obtain the effects of hydrogen pressure, solution pH, and temperature on the current densities of Alloys 600 and 690, respectively. Optimization was performed to obtain the electrokinetic parameters of HER and HOR on Alloys 600 and 690 such as forward and reverse transfer coefficients and equilibrium corrosion densities. From the optimization process, the activation energies, E_ac, of both hydrogen reactions on the surfaces of the Alloys 600 and 690 tubes were obtained as 30.5 kJ/mole on the surface of Alloy 600 tube and 35.6 kJ/mole on Alloy 690 tube. Furthermore, the equilibrium exchange current densities of hydrogen electrode reaction, i₀(H₂), on the surface of the Alloys 600 and 690, respectively, were proposed as functions of hydrogen pressure, solution pH, and temperatures.     

Embrittlement of T-200 maraging steel in a hydrogen sulfide solution

Slow displacement rate tensile tests were carried out to investigate the effect of hydrogen embrittlement (HE) on notched tensile strength (NTS) and fracture characteristics of aged T-200 maraging steel. Hydrogen diffusivity, permeation flux and apparent hydrogen solubility were determined by an electrochemical permeation method, and correlated with the HE susceptibility and microstructures of the specimens. The results indicated that all aged specimens were susceptible to HE in the saturated H₂S solution, to different degrees. The susceptibility in the decreasing order of severity was observed to be under-aged, peak-aged, and over-aged conditions. The main trend was that the specimen with the highest diffusivity and permeation flux of hydrogen had the greatest NTS loss. Reverted austenite, if present in the microstructure, acted as irreversible traps for hydrogen and hence, improved the HE resistance. At similar strength and hydrogen solubility level, the more reverted austenite the less susceptibility to HE of specimens was resulted. The detailed microstructures of distinct specimens and their performances in hydrogen-containing environments are discussed.     

Dissolution of Ti wires in sulphuric acid and hydrochloric acid solutions

Ti wire electrodes were immersed in acidic solutions containing H₂SO₄ and HCl of various concentrations at 353 K to evaluate corrosion rate by measurement of electric resistance change (resistometry). Addition of hydrochloric acid to sulphuric acid solution promoted depassivation of Ti. After depassivation, the immersion potential dropped to the hydrogen evolution potential and a hydride layer was formed on the surface. The hydride layer dissolved continuously in the acidic solution. SEM observation showed that Ti wires dissolved almost uniformly in the early stage and that the dissolution then trace became irregular due to nonuniform growth of the hydride layer. Dissolution rate of a Ti wire was estimated almost accurately by resistometry.     

Evaluation of hydrogen absorption behaviour during acid etching for surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys

The hydrogen absorption behaviour during acid etching for the surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys has been investigated on the basis of the surface morphology, electrochemical behaviour and hydrogen thermal desorption analysis. To simulate the conventional acid etching for the improvement of the biocompatibility of Ti alloys, the specimens are immersed in 1 M HCl, 1 M H₂SO₄ or 0.5 M HCl + 0.5 M H₂SO₄ aqueous solution at 60 °C. Upon immersion, commercial pure Ti absorbs substantial amounts of hydrogen irrespective of the type of solution. In H₂SO₄ or HCl + H₂SO₄ solutions, the hydrogen absorption occurs for a short time (10 min). For Ti-6Al-4V alloy, no hydrogen absorption is observed in HCl solution, whereas hydrogen absorption occurs in other solutions. For Ni-Ti superelastic alloy, the amount of absorbed hydrogen is large, resulting in the pronounced degradation of the mechanical properties of the alloy even for an immersion time of 10 min, irrespective of the type of solution. The hydrogen absorption behaviour is not necessarily consistent with the morphologies of the surface subjected to corrosion and the shift of the corrosion potential. The hydrogen thermal desorption behaviour of commercial pure Ti and Ni-Ti superelastic alloy are sensitively changed by acid etching conditions. The present results suggest that the evaluation of hydrogen absorption is needed for each condition of acid etching, and that the conventional acid etching often leads to hydrogen embrittlement.     

Vergleich von Bauteil- und Laboratoriumsversuchen im System Baustahl-H2S-haltige,feuchte Gase bzw. wäßrige Prüflösungen

Full scale tests and laboratory tests of low alloy steels in wet and liquid H₂S containing environments The results of five hydrogen induced cracking(HIC)-Round Robin tests of steels exhibiting different levels of HIC susceptibility involving nine laboratories and a smaller number of hydrogen induced stress corrosion cracking (HSCC)-Round Robin tests are presented. These results are compared with the results of full scale tests using the same steels and the same liquid environment as in the Round Robin tests. This full scale tests were followed by full scale tests with humidified gaseous environments saturated with H₂S and CO₂.     

Roles of carbon dioxide and steam on the hydrogen embrittlement of 3Cr tube steel in synthetic natural gas environment

The hydrogen embrittlement behaviour of 3Cr has been investigated under mixed H₂ with CO₂ at different strain rates, hydrogen partial pressures, and in the presence/absence of steam. The slow strain rate test results show that the HE susceptibility of 3Cr increased with increasing hydrogen partial pressure, and the plasticity of 3Cr obviously decreased in the presence of steam. The effect of strain rate was negligible in H₂/CO₂ environment but showed a significant difference in H₂/CO₂/steam environment. The fracture was a ductile fracture mode in N₂ environment and a brittle fracture mode in H₂/CO₂/steam environment. The reason for the severe plasticity loss of 3Cr in H₂/CO₂/steam environment was probably that the steam has a preferential adsorption onto the 3Cr surface compared with H₂ and CO₂. Consequences in CO₂ combined with H₂O to form H₂CO₃, which accelerated the anodic dissolution of 3Cr, and the physical adsorption of H₂ on steel was enhanced.     

The negative difference effect of magnesium and of the AZ91 alloy in chloride and stannate-containing solutions

The negative difference effect of pure magnesium and of the alloy AZ91 was investigated by volumetric tests in NaCl with and without addition of Na₂SnO₃. Hydrogen comes from two sources: H₂ which accompanies localized magnesium dissolution inside the pits and H₂ from H₂O reduction at the passive surface outside the pits. By separating the two parts it could be shown that the rate of hydrogen evolution inside the pits is quantitatively consistent with a two-step EC-mechanism of magnesium dissolution with hydrogen evolution coupled to the second dissolution step. Addition of Na₂SnO₃ does not influence the second step.     

The effect of H2S concentration on the corrosion behavior of carbon steel at 90 °C

The electrochemical behavior of SAE-1020 carbon steel in 0.25 M Na₂SO₄ solution containing different concentrations of H₂S at 90 °C was investigated using the methods of weight loss, electrochemical measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that the corrosion rate of carbon steel increased significantly with the increase of H₂S concentration. H₂S accelerated the corrosion rate of SAE-1020 carbon steel by a promoted hydrogen evolution reaction. Severe corrosion cavities were observed on the carbon steel surface in the solutions containing H₂S due to cementites stripped off from the grain boundary. The loose corrosion products formed on the steel surfaces were composed of mackinawite.     

Acoustic emission during the electrochemical corrosion of 304 stainless steel in H2SO4 solutions

Acoustic emission (AE) behaviour during the electrochemical corrosion of 304 stainless steel (304SS) in H₂SO₄ solutions was studied. AE signals which related to transpassive dissolution are detected in solutions with low pH, and are very slightly influenced by current density and pre-strain. During hydrogen bubble evolution, a weak correlation exists between the AE signal amplitude and the hydrogen bubble diameter. The concept of potential – pH – AE diagram is proposed and such a diagram is drawn based on AE activity and b-values. The main mechanisms of AE sources which are transpassive dissolution and hydrogen bubble evolution, are also discussed.     

Film properties and stability influence on impedance distribution during the dissolution process of low-carbon steel exposed to modified alkaline sour environment

In this work, we characterized the anodic dissolution and the hydrogen transport within carbon steel (SAE 1018) samples immersed in alkaline sour solutions (CN⁻, polysulfide-base inhibitor and H₂S(aq)). The evolution of interfacial and transport processes could be quantified by Electrochemical Impedance Spectroscopy (EIS) and hydrogen permeation measurements. EIS experimental data were analyzed and fitted by using Transmission Line Model (TLM); this latter helped to propose the mechanisms through the porous layer of the corrosion products formed. The area influencing the dissolution and the mass transfer process was quantified by the pores number, pores thickness and the interfacial passive electrical elements describing the mechanisms in different regions within the pores of the corrosion product layer. The TLM was used to analyze the active-mass transport processes occurred at different spatial positions of the porous layer, such as the mass transfer at the wall and the active-mass transfer at the base of the cylindrical pore of the non-stoichiometric Fe_xS_y.     

Acoustic emission during pitting corrosion of 304 stainless steel

Acoustic emission (AE) during pitting corrosion of 304 stainless steel (304 SS) in H₂SO₄ solutions with different pH values and Cl⁻ concentrations was studied. Two types of AE signals are detected in all solutions. Each type of signals is characterized by AE parameters (rise time, counts number, duration and amplitude) and waveform carefully. It is believed that the hydrogen bubbles evolution inside the pits is the AE source.