P.E.V. de Miranda

Hydrogen solubility in amorphous and crystalline materials

Abstract Electrochemical hydrogen permeation tests were performed in an electrolyte of 0.1 N NaOH at 313 K with the objective of studying the hydrogen solubility in metallic amorphous and crystalline materials. Samples were prepared from the amorphous metallic alloys Ni 81 P 19 and Fe 40 Ni 38 Mo 4 B 18 and crystalline pure nickel and palladium, as well as low carbon steel. It was shown that the hydrogen solubility in Ni 81 P 19 amorphous alloy is much bigger than in the Fe 40 Ni 38 Mo 4 B 18 glassy alloy and in the crystalline pure nickel, palladium and low carbon steel. Also that a terminal hydrogen solid solubility of 1232.0±334.1 mol H m - 3 is attained in the Fe 40 Ni 38 Mo 4 B 18 amorphous alloy by forming a hydride phase, while, under the same conditions, the Ni 81 P 19 glassy alloy dissolves 11645.0±341 mol H m - 3 as a partial hydrogen solid solubility without transforming into hydride. The cathodic hydrogen generation potential used in the hydrogen permeation tests was shown to influence the hydrogen permeation kinetics.)

Effect of thermal treatments on adhesive properties of a NiCr thermal sprayed coating

Abstract Thermal sprayed coatings are most often used to resist wear or as thermal barriers. In some situations they may have to resist to the combined effects of corrosion and wear at high temperature. Ni-base thermal coatings are being used successfully in this case. It has been demonstrated that adhesion may be modified after a thermal treatment. For example, a substantial increase in adhesive properties was obtained after annealing of a chromium carbide thermal sprayed coating. Using the interfacial indentation method we have studied the influence of an annealing treatment upon adhesion of NiCr coatings for different thicknesses. It was confirmed that annealing improves adhesion to a great extent. Thermal cycling between room temperature to 900°C (×5 times) and thermal shocks consisting in heating the samples to 900°C, then quenching them in oil at room temperature (×5 times), were also studied after the annealing treatment. It was found that a beneficial effect is obtained after such treatments since the critical load necessary to initiate a crack at the interface was increased by thermal shock and even more by thermal cycling. In addition, crack propagation at the interface was slowed down after the heat treatments. This behaviour is discussed and related to the role played by the residual stresses in the coating.

New methodology for the determination of hydrogen permeation parameters in layered materials

A new methodology for interpreting hydrogen permeation test data has been proposed with the objective of determining hydrogen permeability, solubility and diffusivity in materials containing surface-coating layers. The mathematical development of equations has been undertaken for steady-state hydrogen permeation conditions allowing the determination of all parameters of interest for the composite material and the coating layer itself. For a single-layered material, a set of three statistically significant permeation tests is enough to determine all variables for the substrate, the composite material and the coating layer. This methodology has been applied to published results on the hydrogen permeation in nitrogen ion-implanted extra-low carbon steel, showing that the hydrogen diffusion coefficient in such a coating layer is several orders of magnitude lower than that in the substrate. The hydrogen solubility in the layer is, by contrast, increased. The magnitudes of these effects depend on the nitrogen concentration in the surface layer.

Hydrogen diffusivity and solubility in crystalline and amorphous alloys

The hydrogen permeation behaviour of iron- and nickel-based amorphous alloys was characterized using electrochemical methodology and compared with the properties of crystalline metals and alloys. The materials studied were amorphous Fe40Ni38Mo4B18, Fe74Ni4Mo3B17, Fe78B13Si9 and Ni81P19, as well as a crystalline low-carbon steel, pure nickel and pure palladium. The double potentiostatic electrochemical hydrogen permeation tests were performed at 40°C using a 0.1 N NaOH solution as electrolyte. It was found that the hydrogen diffusivity in the iron-based amorphous alloys is a few orders of magnitude lower than in carbon steel and iron, while a much smaller difference exists between Ni81P19 and pure nickel. Furthermore, the amorphous alloys showed a strikingly greater capacity to dissolve hydrogen in solid solution compared with their crystalline counterparts. In some cases, their apparent hydrogen solubility was even greater than that for the liquid metal of the main element present in their chemical composition.The hydrogen permeation behaviour of iron- and nickel-based amorphous alloys was characterized using electrochemical methodology and compared with the properties of crystalline metals and alloys. The materials studied were amorphous Fe40Ni38Mo4B18, Fe74Ni4Mo3B17, Fe78B13Si9 and Ni81P19, as well as a crystalline low-carbon steel, pure nickel and pure palladium. The double potentiostatic electrochemical hydrogen permeation tests were performed at 40°C using a 0.1 N NaOH solution as electrolyte. It was found that the hydrogen diffusivity in the iron-based amorphous alloys is a few orders of magnitude lower than in carbon steel and iron, while a much smaller difference exists between Ni81P19 and pure nickel. Furthermore, the amorphous alloys showed a strikingly greater capacity to dissolve hydrogen in solid solution compared with their crystalline counterparts. In some cases, their apparent hydrogen solubility was even greater than that for the liquid metal of the main element present in their chemical composition.

Design of ion-implanted hydrogen contamination barrier layers for steel

Abstract The results of electrochemical hydrogen permeation tests are presented for a high strength low alloy steel of the type API 5L X-65 in the non-implanted and nitrogen-ion-implanted conditions. The material was implanted under a variety of conditions to produce three series of samples, each with a characteristic implanted layer depth. The results for the implanted materials, when compared with the maximum hydrogen permeability value determined for the non-implanted material, indicate the existence of a critical implanted layer thickness. When the depth of an implanted layer is less than this critical value, the hydrogen permeability of the composite material (implanted layer and substrate) is observed to increase (relative to the non-implanted material). For layer thicknesses equal to or greater than the critical value, the permeability is unnaffected or reduced respectively. It is concluded that nitrogen-ion-implanted layers on steel are well suited for impeding hydrogen contamination and that the techniques employed in this study may be exploited to design surface-engineered hydrogen contamination barrier layers for steel.

Influence of cementite morphology on the hydrogen permeation parameters of low-carbon steel

In this investigation the effect of cementite morphology and distribution on the hydrogen permeation parameters of a low-carbon steel [chemical composition (in wt%): C 0.07, Mn 0.28, Si ~< 0.01, S 0.017, P 0.009 and A10.079] was studied utilizing an improved electrochemical technique. A two-compartment electrochemical permeation cell (based on the traditional apparatus described by Bees and Zfichner [1]) with the necessary dedicated hardware and software was developed to permit automated, high-precision isothermal permeation tests with real-time computer monitoring and data analysis via a programmable multichannel control unit. The results presented here were obtained using the above experimental system, selecting the double-potentiostatic option as the mode of operation, and a 2 s data-sampling rate. The specimens used for this investigation were of the order of 0.6 mm thick, having been prepared from 1 mm-thick sheet material by the usual metallographic polishing technique. Both compartments of the electrochemical cell were filled with deaerated 0.1 M NaOH, continuous nitrogen bubbling being maintained throughout the test. Hydrogen was generated cathodically at the sample surface in one of the compartments, utilizing a constant potential of 1 .35V versus saturated calomel electrode (SCE) and detected by anodic polarization at the corrosion potential in the adjacent compartment after permeating through the thin-sheet metallic specimen which separated the two. The progress of the hydrogen permeation process was followed by monitoring the evolution, over time, of the anodic current flow, measured between the specimen and the counterelectrode of the detection compartment. Thermostatic control of both cells was maintained throughout the test, the temperature (300 K) being monitored and controlled with an accuracy of +0.1 °C via a system of silicon-transistor sensors. Relatively small variations in temperature may significantly effect the precision and scatter of the data obtained. An analysis of the resultant data in terms of the unidirectional solid-state diffusion of hydrogen in the thin metallic sheet, as described by Ficks laws, permits the determination of the three interrelated parameters which quantify the permeation behaviour characteristics of the material: the permeability [P(t)], the apparent diffusivity (Dapp) and the apparent solubility (Sapp) [1, 2]. The term solubility is used here to signify the total hydrogen content,

Design of ion-implanted coatings to impede hydrogen contamination of steel

Abstract The present work utilizes a recently proposed methodology for performing tests and analysing the data from electrochemical hydrogen permeation experiments so as to characterize the effect of surface coating layers on hydrogen ingress into the sample. This methodology has been successfully applied to nitrogen ion-implanted API-5L-X65 type pipeline steel. Hydrogen permeation experiments were conducted using a double electrolytic cell, thermostatically controlled at 308 K, containing a 0.1 N NaOH solution in both compartments. The hydrogen permeability behaviour of the low alloy high strength steel is significantly altered when it is coated with a nitrogen ion implanted layer. The implanted layer was shown to possess an apparent hydrogen diffusivity much smaller than that of the substrate steel, while presenting an apparent hydrogen solubility considerably greater than that of the uncoated substrate.

Texturing Nd–Fe–B magnets under high magnetic field

An original approach is explored in the preparation of anisotropic hard magnetic alloys. This constitutes a proof of principle toward the preparation of anisotropic bonded magnets. Nd–Fe–B ribbons (50% Nd2Fe14B+50% Nd–Cu alloy), constituted of Nd2Fe14B grains embedded in a Nd–Cu eutectic matrix, were annealed under an applied magnetic field up to 16 T at various temperatures above the Nd–Cu melting temperature. The grain orientation mechanism is described in terms of a competition between the aligning magnetic field torque acting on the solid magnetic grains and the friction counter torque at the interface between the grains and the liquid matrix. The large temperature effect on the orientation behavior is attributed to the associated increase in the liquid phase volume fraction.

Influence of hydrogen contamination on the tensile behavior of a plasma ion nitrided steel

Abstract The effect of hydrogen on the tensile mechanical properties was examined for samples taken from a nitrided layer (white layer plus diffusion zone) as well as from the substrate steel. An API X-65 type steel was plasma ion nitrided at 773 K during 16 h generating a white layer 5 μm thick and a diffusion zone measuring about 550 μm. Electrochemical tests were performed to obtain hydrogen permeation curves at 323 K, using a 0.1 N NaOH solution as electrolyte. This enabled hydrogen permeability, apparent solubility and diffusivity of the substrate, the nitrided layer and the diffusion zone to be determined. The nitriding treatment led to a strong decrease of hydrogen permeability compared with that of the as received steel. Three different phenomenological equations describing plasticity (Hollomon’s, Ludwig’s and Swift’s) were used to study the effect of hydrogen on the deformation stages. Ludwig’s equation was found to be most sensitive to the hydrogen effects. The main effects of hydrogen in the as received steel were to induce an ageing type yield drop and to reduce ductility by about 7%. For the nitrided material a ductility loss of about 70% was observed. In both materials, all deformation stages were affected by hydrogen with stages 2 and 3 more strongly diminished in extent than stage 1. This demonstrated that the entire plastic domain was affected by hydrogen.

Role of hydrogen on adhesion of NiCr thermal sprayed coatings

Hydrogen embrittlement was known for many years. Different theories explain how hydrogen plays the role of an accelerator for fracture when it was introduced into the material either during its elaboration or during its service as a mechanical part. In this latter case, the presence of a barrier to introduction of hydrogen into the material may delay or even impede the embrittlement process. The present study was devoted to the mechanical aspects of hydrogen contamination of this type of a NiCr thermal sprayed coating and in particular to the influence of hydrogen on the coating adhesion on a low carbon steel substrate. It was found that, besides the embrittlement of the coating, adhesion was also affected since the critical load necessary to initiate a crack at the interface was reduced in the presence of hydrogen. Using an apparent interface toughness concept it was also possible to point out the effect of the residual stresses in relation to the coating thickness as well as surface effects on the apparent interface toughness value.