Hydrogen Stress Cracking Research Articles

The Role of Nickel in Low Alloy Steels Exposed to H2S Containing Environments. Part II: Effect of the Electrochemical Potential and Stress Level on Trench Formation

In the first part of this paper, trenches were reported for Ni-containing steels tested using the slow strain rate test (SSRT) method in H2S-containing environments at the open-circuit potential (OCP). Trenches are deep elongated pits, and their appearance is more similar to small blunt cracks. These features can develop a sharp sulfide stress crack at their bottom under certain conditions, which are still not fully understood. In this second and final part, the effect of the electrochemical potential and the stress level in trench nucleation and growth was investigated for the same set of Ni-containing steels with up to 5 wt% Ni. The anodic nature of the trench formation mechanism was verified, and under an anodic polarization a critical stress value for trench formation, σtrench, was estimated from SSRTs and finite element modeling. Applying a cathodic potential suppressed trench formation, but not cracking, because cracks nucleated and propagated by hydrogen stress cracking. The resulting environmental and stress-level dependencies for Ni steels confirmed that trenches could be considered a form of environmental-assisted cracking. It is concluded that the main role of trenches is to provide favorable spots for hydrogen stress crack nucleation at OCP, but their presence is neither necessary nor sufficient for cracking occurrence.

Hydrogen stress cracking of Alloy 716 under cathodic protection—root cause analysis, and an evaluation of quality control methods

A subsea fastener made of Alloy 716 (UNS N07716) experienced a brittle fracture in service. The investigations described herein suggested that the failure was caused by hydrogen stress cracking (HSC), thus, adding to the number of reported HSC incidents on precipitation hardened nickel-based alloys over the last two decades. Incremental step loading testing on notched tensile specimens showed a net section threshold stress of <44% of the fastener materials actual yield strength. X-ray diffraction revealed that the grain boundary (GB) precipitates were predominantly the σ–related precipitate F-phase, which were suggested to have a detrimental effect on HSC resistance—even in low amounts. The low amounts of GB precipitates were indicated by a low degree of sensitization in the double-loop electrochemical potentiokinetic reactivation test. Two additional heats from the same alloy were investigated to elucidate the effect of different GB precipitates on HSC resistance and to evaluate quality control methods that are discussed in the research community.

Corrosion inhibition of steel by selected homologues of the class 3-alkyl-5-amino-1H-1,2,4-triazoles in acidic media

The use of hydrochloric acid in the treatment of the bottomhole formation zone leads to the significant corrosion of metals, as well as hydrogen and chloride stress cracking of pump compressor pipes. In order to solve this problem, corrosion inhibitors are added to a hydrochloric acid solution. This article presents the results of a study of the anticorrosive activity of a number of derivatives of the class of 3-alkyl-5-amino-1H-1,2,4-triazole under the conditions of hydrochloric acid corrosion of low-carbon steel. During the study, selected 3-alkyl-5-amino-1H-1,2,4-triazoles were synthesized. Their structure was confirmed and proved using NMR spectroscopy and HPLC/MS spectrometry. Regularities of the anticorrosive ction of the investigated compounds have been established using polarization electrochemical studies and gravimetric direct corrosion tests. Corrosion rates, inhibition coefficients and degrees of protection have been calculated for all inhibitors. The probable mechanism of the inhibitory action of the studied compounds has been substantiated using quantum chemical calculations based on the density functional theory using the Gaussian program. It was shown that the structure of the alkyl substituents has the greatest effect on the inhibitory activity of the studied compounds. The mechanism was proposed for the adsorption of the inhibitor, which explained the increase in protective properties with an increase in the length of the alkyl substituent. The high hydrophobicity of the aliphatic fragment, not involved in the chemisorption process, additionally prevents the acid solution from contacting the metal surface, while the heterocyclic moiety ensures the sorption of the inhibitor on the metal surface. As a result, it was shown that derivatives of the homologous series of 3-alkyl-5-aminotriazole are suitable as inhibitors of acid corrosion of ST-3 steel. The minimum length of a hydrocarbon radical at which significant inhibitory activity was observed is 7 carbon atoms. Protection degrees of 65–85% were achieved when 3-heptyl-5-amino-1H-1,2,4-triazole additives at a concentration of at least 2 g/L were added to the hydrochloric acid solution.

Hydrogen Stress Cracking Resistance and Hydrogen Transport Properties of ASTM A508 Grade 4N

Low alloy steels combine relatively low cost with exceptional mechanical properties, making them commonplace in oil and gas equipment. However, their strength and hardness are restricted for sour environments to prevent different forms of hydrogen embrittlement. Materials used in sour services are regulated by the ISO 15156-2 standard, which imposes a maximum hardness of 250 HV (22 HRC) and allows up to 1.0 wt% Ni additions due to hydrogen embrittlement concerns. Low alloy steels that exceed the ISO 15156-2 limit have to be qualified for service, lowering their commercial appeal. As a result, high-performing, usually high-nickel, low alloy steels used successfully in other industries are rarely considered for sour service. In this work, the hydrogen stress cracking resistance of the high-nickel (3.41 wt%), quenched and tempered, nuclear-grade ASTM A508 Gr.4N low alloy steel was investigated using slow strain rate testing as a function of applied cathodic potential. Results showed that the yield strength and ultimate tensile strength were unaffected by hydrogen, even at a high negative potential of −2.00 VAg/AgCl. Hydrogen embrittlement effects were observed once the material started necking, manifested by a loss in ductility with increasing applied cathodic potentials. Indeed, A508 Gr.4N was less affected by hydrogen at high cathodic potentials than a low-strength (yield strength = 340 MPa) ferritic-pearlitic low alloy steel of similar nickel content. Additionally, hydrogen diffusivity was measured using the hydrogen permeation test. The calculated hydrogen diffusion coefficient of the ASTM A508 Gr.4N was two orders of magnitude smaller when compared to that of ferritic-pearlitic steels. Hydrogen embrittlement and diffusion results were linked to the microstructure features. The microstructure consisted of a bainitic/martensitic matrix with the presence of Cr23C6 carbides as well as Mo- and V-rich precipitates, which might have played a role in retarding hydrogen diffusion, kept responsible for the improved HE resistance.

Hydrogen Stress Cracking Behaviour in Dissimilar Welded Joints of Duplex Stainless Steel and Carbon Steel

As the need for duplex stainless steel (DSS) increases, it is necessary to evaluate hydrogen stress cracking (HSC) in dissimilar welded joints (WJs) of DSS and carbon steel. This study aims to investigate the effect of the weld microstructure on the HSC behaviour of dissimilar gas-tungsten arc welds of DSS and carbon steel. In situ slow-strain rate testing (SSRT) with hydrogen charging was conducted for transverse WJs, which fractured in the softened heat-affected zone of the carbon steel under hydrogen-free conditions. However, HSC occurred at the martensite band and the interface of the austenite and martensite bands in the type-II boundary. The band acted as an HSC initiation site because of the presence of a large amount of trapped hydrogen and a high strain concentration during the SSRT with hydrogen charging. Even though some weld microstructures such as the austenite and martensite bands in type-II boundaries were harmless under normal hydrogen-free conditions, they had a negative effect in a hydrogen atmosphere, resulting in the premature rupture of the weld. Eventually, a premature fracture occurred during the in situ SSRT in the type-II boundary because of the hydrogen-enhanced strain-induced void (HESIV) and hydrogen-enhanced localised plasticity (HELP) mechanisms.

Microstructural aspects of hydrogen stress cracking in seawater for low carbon steel welds produced by flux-cored arc welding

This study investigated the role of weld microstructures on hydrogen stress cracking (HSC) in low carbon steels. HSC behaviours were compared for base metal (BM) and transverse-weld joints (WJs) using in-situ slow strain rate testing (SSRT) with hydrogen charging. The HSC resistivity of transverse WJs was inferior to that of BM. The transverse WJs were fractured in the BM during SSRT in hydrogen-free condition. The in-situ SSRT changed the fracture location to the inter-critical heat affected zone (ICHAZ) for transverse WJs and granular bainite in ICHAZ acted as the initiation site of HSC in transverse welds.

Influence of hydrogen on softened heat-affected zones during in-situ slow strain rate testing in advanced high-strength steel welds

Hydrogen stress cracking (HSC) of weldments should be evaluated as it occurs under environmental stress. This study investigated HSC reactions with regard to microstructures and hardness distribution in the transverse cross sections of welds deposited on an advanced high-strength steel through in-situ slow-strain rate testing (SSRT). SSRT in a hydrogen-free environment led to the fracture of the base metal, although a narrow and softened heat-affected zone (HAZ) was observed. However, during in-situ hydrogen charging SSRT, stress triaxiality increased in the softened inter-critical HAZ due to hydrogen trapped, and premature rupture was initiated from hydrogen-promoted voids with enhanced plasticity.

Evaluation of Stress Corrosion Cracking, Sulfide Stress Cracking, Galvanic-Induced Hydrogen Stress Cracking, and Hydrogen Embrittlement Resistance of Aged UNS N06625 Forged Bars

UNS N06625 is a nickel-based superalloy used for oil and gas applications and commonly produced according to NACE MR0175 in the annealed/solution annealed condition. The annealing/solution annealing treatment makes the material corrosion resistant in the most challenging environments, in the presence of sulfides and chlorides at high pressure and temperature. However, thanks to its chemical composition, UNS N06625 can also be considered as an age-hardenable material whose mechanical strength can be improved by promoting the metastable second phase γ′′ precipitation into the γ matrix. However, the corrosion behavior of the aged alloy has never been investigated in NACE environments. This paper aims to understand the suitability of the age-hardened condition of UNS N06625 for oil and gas applications through the evaluation of the material corrosion performance in NACE level VII environments by using NACE TM0177 tests. Three heats of UNS N06625 have been produced and forged in different bar diameters: 152 mm (6 in), 203.2 mm (8 in), and 254 mm (10 in). Afterward, the bars have been annealed and age-hardened according to optimized time-temperature parameters and finally tested to assess their mechanical properties and resistance to stress corrosion cracking, sulfide stress cracking, galvanic-induced hydrogen stress cracking, and hydrogen embrittlement.

Detailed mechanisms of hydrogen charging and hydrogen stress cracking of steel in liquid ammonia storage

When the unprecedented environmental cracking of steel in liquid ammonia was collectively studied, its undisputable “anodic character” was taken as the signature of astress corrosion crackingmechanism, which is effectively the case in aqueous corrosion. Conversely, when the metallurgical precautions proved to be the same as in sour service, this strongly suggested ahydrogen stress crackingmechanism. In aqueous corrosion, however, this can only occur by cathodic hydrogen charging at low potential, and for 50 years, this basic contradiction could never be overcome. Actually, it occurs that the liquid ammonia solvent (NH3) is 50% richer in hydrogen than the water solvent (OH2), so that hydrogen gas can also be produced by a partial oxidisation into ½ N2 + H2. This therefore induces a theoretical possibility of an “anodic” hydrogen charging, or more exactly a protonic cathodic reaction only running at high potential on passive iron in oxygen contaminated ammonia. And once the detrimental potential is achieved through appropriate combinations of oxygen and water traces, the charging process becomes an autonomous oxidation-reduction at the steel surface NH3 → ½ N2 + H2 + (H+ + e−)steel. In Part II (Jean-Louis Crolet,Matériaux & Techniques107, 402, 2019), this new assumption will be successfully confronted to all the factual data from both field and laboratory experience.

Detailed mechanisms of hydrogen charging and hydrogen stress cracking of steel in liquid ammonia storage

The available polarisation curves of the literature are analysed in view of the theoretical analysis of the corrosive medium in Part I (J.-L. Crolet, Matériaux & Techniques107, 401, 2019) and the new views on passivation and hydrogen charging. The accurate electrochemical conditions of the respective cathodic and “anodic” hydrogen charging are thus defined, or more exactly the paradox of a protonic cathodic reaction only running at high potential. Similarly in the available exposure test results, the locus of the ternary redox equilibrium between ammonia and its two decisive contaminants, oxygen and water, separates contaminated ammonia into two domains, on the one hand, a safe domain where the undersaturated oxygen cannot act as an oxidiser, hence no anodic HSC at the free corrosion potential, and on the other hand, a dangerous domain where oxygen is supersaturated, hence oxidising conditions, steel passivation, high potentials, anodic charging and anodic HSC. Likewise, all the known features of this environmental cracking are also explained, with no exception nor contradiction, including the differences between the liquid and vapour phases. An experimental method is also proposed to directly check the occurrence of anodic charging, and proposals are also made for at the same time improving safety and reducing operational or capital expenditures.

Effect of Nickel on the Hydrogen Stress Cracking Resistance of Ferritic/Pearlitic Low Alloy Steels

Nickel additions to low alloy steels improve mechanical and technological properties. However, Part 2 of ISO Standard 15156 limits the nickel content to a maximum of 1 wt% in oil and gas environments containing H2S because of controversial concerns regarding sulfide stress cracking. The objective of this work was to investigate the effect of nickel in solid solution in the ferrite phase on hydrogen stress cracking resistance. Ferritic/pearlitic research-grade low alloy steels with nominal nickel contents of 0, 1, 2, and 3 wt% were tested by the slow strain rate test method with cathodic hydrogen charging to −1.05 VAg/AgCl and −2 VAg/AgCl. No difference in fracture mode or morphology was found between the alloys. However, the plastic elongation ratios and reduction in area ratios decreased with increasing nickel content when tested at −2 VAg/AgCl. The direct and indirect effects of nickel, such as the influence of an increasing fraction of pearlite with increasing nickel content, are discussed.

Hydrogen stress cracking and crack initiation in precipitation hardened Ni-alloys

The two different precipitation hardened nickel alloys, Alloy 718 and Alloy 725, were tested in its as received condition. Their susceptibility to hydrogen stress cracking was examined by the use of stepwise increasing load testing. The mechanical properties of the two alloys and the effect of hydrogen were studied and compared. Both alloys were proven to be susceptible to hydrogen stress cracking, however it was not possible to distinguish the susceptibility of the two alloys. Investigation of the fracture surfaces revealed a change from pure micro void coalescence failure in the samples tested in air, to a more complex fracture surface for the samples tested under cathodic polarization. Here the area closest to the edge had brittle intergranular and transgranular feature, while the centre was dominated by ductile dimples. This was attributed to the hydrogen concentration gradient through the sample and how this affects which mechanism for hydrogen degradation is dominating. In addition, crack initiation in the presence of hydrogen was investigated through tensile testing in a micro-load cell. This test showed that the cracks initiated near the grain boundaries, and propagated in a transgranular manner. This was related to an observed non-uniform stress distribution in the grains.

Critical Hardness of Hydrogen Stress Cracking for Circumferential Weld Joint of Buried Pipeline

Critical Hardness of Hydrogen Stress Cracking for Circumferential Weld Joint of Buried Pipeline

DISLOCATION INDUCED MECHANISMS OF HYDROGENE EMBRITTLEMENT OF METALS AND ALLOYES

The paper discusses some models of hydrogen-stress cracking of metals and alloys. These models are based on hydrogen-dislocation interaction. It is shown that the critical role of dislocation emissions in AIDE mechanism is, in its turn, similar to HELP except for a higher localization of deformations compared with microvoids coalescence that is related with HELP, because that stresses needed for the dislocation propagation are high enough to boost general dislocation activity in deformation zones in front of cracks. This results in the formation of small voids on intersecting deformation bands. It has been observed that a crack is essentially growing due to the emission of dislocations. However the emission of dislocation towards the tip of a crack and the formation of voids in front of a crack contribute a lot to the process. Furthermore, the formation of voids in front of a crack makes for a short radius of the crack tip and low angles of the crack tip opening displacement The paper considers crack growing in inert media in plastic materials. Crack plastic growth takes place mainly due to dislocations that originate from the sources in the deformation zone in front of the crack tip and are propagating backwards along the crack tip plane with a small or zero emission of the dislocations that start from the crack tip. Small number of the dislocations that originate in the sources lying closest to the crack tip will intersect the tip of the crack precisely thus promoting the crack development while the majority of the dislocation will have either blunting effect or contribute to the deformation in front of the crack. Thus to cause a crack growth due to microvoid coalescence and deep cavities with shallow depressions therein on fracture surfaces there must be a large deformation in front of the crack. It is demonstrated that the cracking mechanism resulting from the AIDE mechanism will be either intergranular or transcrystalline depending on the location where the propagation of dislocations and formation of voids run mostly easily. In case of transcrystalline cracking alternative sliding motion along the planes on either side of the crack will tend to minimize the reverse stress caused by previously emitted dislocations. Then the macroscopic transcrystalline cracking plane will divide the angle between the slide planes and the crack front will be located on the intersection line of the crack planes and the slide planes. However, if there is a difference in the number of slides that occur on either crack side because of big differences in shear stresses on different slide planes, there will be deviations from the planes and directions with low refraction index. If the plane index is not low, there still can be deviations in the failure planes depending on the location of nucleus voids in front of the crack. A detailed description of the relationship between hydrogen effect on the behavior of dislocations and voids, sliding motion localization and hydrogen embrittlement is still lacking, moreover, it presents a serious problem that can be solved by describing the kinetics of hydrogen embrittlement process. Thanks to their sophisticated nature HELP and AIDE mechanisms can be embrittlement contributors both in cracking and in the formation of cavities due to ductile fracture.

PIPELINE CORROSION CONTROL IN OIL AND GAS INDUSTRY: A CASE STUDY OF NNPC/PPMC SYSTEM 2A PIPELINE

Corrosion in pipelines is one of the major challenges faced by oil and gas industries all over the world. This has made corrosion control or management a major factor to consider before setting up any industry that will transport products via pipelines. In this study the types of corrosion found on system 2A pipeline were; pitting, microbial, sulfide-stress cracking, hydrogen-stress cracking and hydrogen-induced cracking and these were caused by poor maintenance of the pipeline system, severe mutilation of the pipeline coatings, substrates due to vandalization and coating failures. The data from cathodic protection control method from Nigeria National Petroleum Corporation (NNPC)/ Pipeline and Product Marketing Company (PPMC) for system 2A line was analyzed and it was deduced that about 10.3km of the pipeline was well protected and possibly fit for use and about 62.7km is experiencing under protection which means corrosion is predicted to take place in that segment in a short time and finally about 16km of the pipeline is experiencing corrosion. From the results obtained, it can be deduced that the use of cathodic protection technique as a method of controlling corrosion in oil and gas pipelines is effective and efficient when compared to other methods and thus constant monitoring is needed to achieve optimum efficiency. http://dx.doi.org/10.4314/njt.v35i2.11

On the Contradiction of Applying Rolled Threads to Bolting Exposed to Hydrogen-Bearing Environments

Summary Numerous industries continue to experience bolting failures as a result of hydrogen stress cracking (HSC) when exposed to hydrogen-bearing environments such as seawater with cathodic protection (CP) or as a result of insufficient baking after plating operations. This paper describes the mistaken, but long-held, belief that because rolled threads are beneficial for fatigue resistance, they are at best not injurious to the performance of bolting from other causes of failure such as environmental cracking.

Hydrogen stress cracking in power generator M42 galvanized martensitic carbon steel screws

Screws are used to fasten parts of an assembly together, and can be used to join most classes of materials. The use of sufficient initial tension is very advantageous in reducing the fatigue effects when cyclic loads are present. In general, there is high applied stresses to preserve the joining, and in many practical applications the electrical contact of dissimilar metallic materials is unavoidable. On specific environmental condition, a potential difference produces electron flow between dissimilar materials, corroding the less corrosion-resistant metal. Thus, a better understanding of this joining process and environmental interactions are needed to predict failures in design, avoiding disasters and financial losses. An unexpected fracture in power generator M42 screws was investigated by failure analysis including visual, fractographic, microstructural, chemical and mechanical characterization. It was found that the fracture has no fatigue or plastic deformation signs, and the screws are free of microstructural or mechanical failures arising from raw material or manufacturing process. Intergranular fracture and grain boundary gaps were evidenced, due to this fact, localized fracture area and high-stress level in mounting condition, it was determined that failure cause was Hydrogen Stress Cracking (HSC). Hydrogen source was a galvanic couple between the screw material and cadmium coating. As recommendations, in acid medium presence, martensitic carbon steel screws should never be cathode in a galvanic couple, they must be replaced to obtain minimum levels of uniform corrosion and should not be painted.

FE simulation of hydrogen diffusion in duplex stainless steel

ABAQUS FE simulations of hydrogen diffusion in duplex stainless steel have been performed. Three models with different ferrite–austenite configurations have been applied and the hydrogen diffusion and the hydrogen coefficient have been evaluated as a function of austenite phase size and shape and the calculated diffusion coefficients compared to literature. Hydrogen concentration due to stress and plastic strain close to an embedded flaw has also been evaluated. An important observation is that the simulations show that when the austenite phases are saturated with hydrogen there is no large difference in the overall diffusion rate between the small and large phased models, i.e. no influence of tortuosity is observed. The work clearly demonstrates that both microstructure and flaws will influence the hydrogen diffusion and the hydrogen concentration and hence, must be taken into account when evaluating the susceptibility of hydrogen stress cracking in duplex stainless steels.

Latent Cracking of Tantalum-Titanium Welds Due to Hydrogen Embrittlement

Establishing electrical interconnects in implantable electronic medical devices frequently requires joining of dissimilar materials. A weld between a tantalum wire and titanium sheet metal on a contact module is presented as an example for dissimilar joining. Latent, brittle cracking was observed in the proximity of the weld upon pull testing. The weld cracking occurs by the mechanism known as hydrogen stress cracking (HSC) and is due to titanium hydride formation. Diffusion facilitated hydrogen transport into the weld area. Diffusing hydrogen accumulates preferably in regions of high stress, causing latent titanium hydride formation and embrittlement of the weld. A broad array of analytical tools such as scanning electron microscopy (SEM), transmission electron microscopy, electron backscattered diffraction, dynamic secondary ion mass spectroscopy, and nanoindentation were utilized to identify the root cause for HSC.

Effect of special features of distribution of nonmetallic inclusions in the metal of electric-welded oil conducting tubes on resistance to hydrogen-stress cracking

Results of an analysis of fracture surfaces obtained by the method of eccentric detachment of specimens of the metal of welded tubes from steel 09GSF in the initial state and after testing for hydrogen-stress cracking in a hydrogen sulfide medium are presented. Qualitative dependences of the susceptibility of the steel to hydrogen-stress cracking and of the kind of fracture in the initial stage and after testing for hydrogen-stress cracking on the purity of the metal with respect to nonmetallic inclusions in the zone of axial segregation are established. The dependences are used for developing a method for diagnosing rolled stock.