Mechanisms of hydrogen-induced cracking in ultrahigh-strength steels

There has been a significant increase in the application of high-strength steels in the engineering and offshore industry. Nanostructured bainitic steels (BS-200 and BS-350) and austenitic twinning-induced plasticity (TWIP) steel have emerged as two promising materials for engineering and offshore applications, due to their extraordinary mechanical properties. Generally, steel corrosion is a major problem, especially in the offshore industry. Hence, several corrosion protection techniques such as protective coating, alloying and cathodic protection have widely been employed. The cathodic protection system is preferred by corrosion experts in the offshore industry as it provides a more effective method of protecting steels from general corrosion. However, high-strength metallic materials, in general, are prone to a localized form of corrosion known as hydrogen-induced cracking (HIC). The diffusion of hydrogen can occur during the cathodic protection potentials and cause catastrophic failure of the material under tensile loading. HIC undermines the integrity of structural components and leads to huge financial losses, as well as potential environmental pollution. The degree of HIC susceptibility is influenced by the microstructure of the material. Generally, steels containing a predominantly ferrite phase are more susceptible to HIC than those containing an austenite phase. Ultrahigh-strength steels generally contain refined microstructures with different phases, grain sizes and other features such as dislocations, twinning and precipitates. These features can trap or enhance the mobility of hydrogen and thus affect the steel's susceptibility to HIC. Therefore, it is critical to study the HIC susceptibility of ultrahigh-strength nanostructured bainitic steels and TWIP steels to determine their potential applications for steel pipeline and structural components in the offshore oil industry. This thesis aims to determine the HIC susceptibility of these ultrahigh-strength steels by experimentally calculating the hydrogen diffusivity in these materials, and by mechanical property testing in conducive environments. Nanostructured bainitic steels comprised of refined microstructures including ferrite with a large component of dislocation density and austenite phases. The microstructural features contribute to the steel's superior tensile strength (˃ 800 MPa) and ductility (≥ 30 pct). To understand the HIC susceptibility of the nanostructured bainitic steels, electrochemical hydrogen permeation tests and micro-hardness tests were carried out on the steel, and the results were compared to those from a nominal mild steel. Electrochemical hydrogen permeation results showed that the nanostructured bainitic steel containing 79 pct of ferrite phase (BS-200) exhibited lower effective hydrogen diffusivity compared to steel containing 47 pct ferrite phase (BS-350) and mild steel by about two orders of magnitude. The effective hydrogen diffusivity for each steel was found to increase as the cathodic charging current density was increased. However, the increase was not significant in BS-200 compared to the other steels. This was attributed to the trapping effect of the refined microstructural constituents: bainitic ferrite laths, retained austenite films and higher dislocation density in the bainitic ferrite of the BS- 200 steel. In order to understand how hydrogen diffusion and hydrogen concentration in the steel affect the mechanical properties, micro-hardness tests were performed on charged samples. The results showed softening in nanostructured bainitic steel and hardening in mild steel. The BS-200 and BS-350 nanostructured bainitic steels softened by ~ 5% and 12%, respectively. The softening was higher in BS-350 nanostructured bainitic steel. This was attributed to the interaction of hydrogen with dislocations, which enhanced dislocation mobility and hence softening. For the mild steel, the hardness was attributed to supersaturation of dissolved hydrogen that gave rise to the formation of voids and cracks. This generated additional stress and new dislocation that enhanced hardening. The effect of hydrogen diffusion on hardness was, however, found to be limited to the subsurface region of BS-200 compared to BS-350 nanostructured bainitic steel and mild steel. In TWIP steel, the microstructure is comprised of a single austenite phase stabilized by a high amount of manganese. TWIP steel has an excellent combination of strength and elongation which is achieved through work hardening, deformation twinning and small grains size ~ 5 μm. The hydrogen permeation rate in TWIP steel was about three orders of magnitude lower than the permeation rate in mild steel. Interestingly, the effective hydrogen diffusivity in TWIP steel was about two times higher compared to mild steel. The higher hydrogen diffusivity in TWIP steel compared to mild steel can be attributed to diffusion been primarily through the low energy grain boundaries of TWIP steel, as austenite grains have very low effective hydrogen diffusivity. Tensile tests were carried out in pre-charged and in-situ hydrogen charged conditions to evaluate the HIC susceptibility of the steel. The resistance to HIC based on the susceptibility indices (IHIC) for elongation (Ɛf) and reduction in area (RA) showed that TWIP steel was about 60% more resistant in its pre-charged condition than mild steel. The fractured surface analysis of the pre-charged sample revealed ductile failure in TWIP steel characterized by a uniform distribution of fine dimples and micro-voids. In mild steel, the surface showed brittle fractures. However, in the in-situ condition, TWIP steel exhibited brittle failure with a mix of dimples and large cracks attributable to more cathodically evolved hydrogen available for diffusion along the grain boundaries for crack propagation. The findings from this study have been disseminated through the following publications

Electrochemical corrosion behaviour of nanostructured bainitic steel

Nanostructured bainitic steels are gaining high interest due to their excellent mechanical properties. However, high carbon content in nanostructured bainitic steels can influence their general and localized corrosion susceptibility. In this study, the corrosion behaviour of nanostructured bainitic steel was compared with the well-known martensitic steel in chloride-containing solution using electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation. EIS results showed that the polarisation resistance (R(p)) for nanostructured bainitic steel (3400 Ω cm²) was higher than that of martensitic steel (2000 Ω cm²). Potentiodynamic polarisation results showed an 85% lower corrosion current density (i(corr)) for nanostructured bainitic steel as compared to martensitic steel. Interestingly, galvanostatic polarisation of the steels showed different corrosion morphology, i.e., martensitic steel revealed intergranular corrosion (IGC) and the nanostructured bainitic steel exhibited lamellar structure suggesting selective dissolution.\ud \ud In order to understand the corrosion mechanism of the nanostructured bainitic steel, two different isothermal temperatures were used to produce nanostructured baintic steel with different percentages of retained austenite (RA) and bainitic ferrite (BF). Nanostructured bainite formed at 200 °C (RA: 21%) exhibited marginally higher corrosion resistance compared with that at 350 °C (RA: 53%). Post-corrosion analysis of the galvanostatically polarised samples revealed localised corrosion for both the steels, but the degree of attack was higher in the 350 °C steel than in the 200 °C steel. The localised corrosion attack was due to the selective dissolution of the RA phase. The higher volume fraction and larger size of RA in the 350 °C steel as compared to that of the 200 °C steel contributed to the pronounced corrosion attack in the 350 °C steel.\ud \ud To enhance the corrosion resistance of the nanostructured bainitic steel, a conducting polymer, polyaniline (PANI), was coated on the steel using galvanostatic method. Samples coated for 10 mins with lower current density (5 mA cm⁻²) exhibited higher R(P) (3.2 × 10⁴ Ω cm²) as compared to the samples coated with 20 mA cm⁻² (9.82 × 10³ Ω cm²). Although the R(P) of the coating increased with increase in the coating thickness (i.e., by increasing the coating time), under long-term exposure the R(P) of the coated samples dropped drastically. This can be attributed to the large pores in the coatings. To reduce the porosity in the coating, the coating process was performed under stirred-condition. The stirred-condition coating (20 mA cm⁻² for 20 mins) exhibited only a few fine defects and the R(P) was almost two orders of magnitude higher than that of the unstirred-condition coated sample. Long-term EIS results for the stirred-condition coated sample showed an initial increase in R(P) (4.3 × 10⁶ Ω cm²) after 78 h exposure, and then gradually decreased to 7.0 × 10⁵ Ω cm² after 168 h exposure. However, the R(P) was significantly higher than that of the bare metal. Potentiodyanamic polarisation results confirmed the higher corrosion resistance of the stirred-condition coated sample as compared to the unstirred-condition coated sample.\ud \ud The study suggests that nanostructured bainitic steel exhibited higher corrosion resistance than martensitic steel in chloride-containing solution, but the RA in the nanostructured bainitic steel dissolved selectively. PANI coating using galvanostatic method under stirred-condition, however, improved the general and localized corrosion resistance of nanostructured bainitic steel significantly

Effect of cathodic hydrogen-charging current density on the hydrogen diffusivity in nanostructured bainitic steels

The effect of cathodic hydrogen-charging current on the effective hydrogen diffusivity in nanostructured bainitic steels produced at transformation temperatures 200 degrees C (BS200) and 350 degrees C (BS350) was investigated and compared to that of mild steel. The effective hydrogen diffusivity at 10 mA cm(-2) was the lowest for BS200, followed by BS350 and mild steel, due to the finer microstructure and higher dislocation density in the bainitic ferrite of BS200. Increase in the hydrogen-charging current density, i.e. 20 and 30 mA cm(-2), increased the effective hydrogen diffusivity of mild steel by 37 and 135%, and BS350 by 49 and 150%, respectively. For BS200, the increase was not significant (2%) at 20 mA cm(-2), but increased by 34% at 30 mA cm(-2)

Effect of polyaniline coated galvanized steel electrodes on electrokinetic sedimentation of dredged mud slurries

An experimental study on electrokinetic improvement of dredged marine sediments to accelerate their sedimentation for land reclamation purposes is presented. Electrokinetic stabilization is currently used to improve soils, however, its use on soils with marine sediments with low permeability is still questionable due to the deterioration of anodes caused by electrolysis reaction. A number of traditional methods are employed in literature to reduce the corrosion degradation of metals, conducting polymers such as polyaniline is of engineering interest due to its properties such as ease of preparation and its high environmental stability. For this purpose, the anodes used herein is coated with polyaniline to investigate its effect on this method. Two series of experiments were performed using a polyaniline coated galvanized steel anode, and two series of experiments with non-coated galvanized steel anodes. Polyaniline coating increased the power consumption during the electrokinetic stabilization compared to the case where the same electric potential is applied using the uncoated electrodes. However, when 5 V electric potential is applied to the soil through the polyaniline coated anode, its settlement and electroosmotic permeability are equivalent to what was observed with 30 V electric potential applied through the non-coated anode, with 3 times less energy consumption.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels

Hydrogen depth profiles and microhardness of the electrochemically hydrogen-charged nanostructured bainitic steels (produced at two different transformation temperatures, i.e. 200 degrees C (NBS200) and 350 degrees C (NBS350)) were obtained using elastic recoil detection analysis (ERDA) technique and Vickers microhardness testing, respectively, and compared to that of mild steel. The ERDA results showed that the subsurface hydrogen concentration was higher in NBS200, followed by NBS350 and mild steel. However, the microhardness data of the hydrogen-charged steels revealed material softening in NBS200 and NBS350, whereas the mild steel exhibited material hardening effect. The microhardness along the cross-sectional depth of the steels showed that the softening effect in NBS200 was closer to the hydrogen-charged surface compared to that of NBS350. The plausible mechanisms for the softening effect in the NBS200 and NBS350, and hardening effect in mild steel have been discussed in this paper. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved

Hydrogen permeation in nanostructured bainitic steel

Hydrogen permeation of nanostructured bainitic steel, produced at two different transformation temperatures, i.e., 473.15 K (200 A degrees C) BS-200 and 623.15 K (350 A degrees C) BS-350, was determined using Devanathan-Stachurski hydrogen permeation cell and compared with that of mild steel. Nanostructured bainitic steel showed lower effective diffusivity of hydrogen as compared to the mild steel. The BS-200 steel, which exhibited higher volume fraction of bainitic ferrite phase, showed lower effective diffusivity than BS-350 steel. The finer microstructural constituents (bainitic ferrite laths and retained austenite films) and higher dislocation density in the bainitic ferrite phase of BS-200 steel can be attributed to its lower effective diffusivity as compared to BS-350 steel and mild steel