Unveiling the impact of laser power variations on microstructure, corrosion, and stress-assisted surface crack initiation in laser powder bed fusion-processed Ni-Fe-Cr alloy 718

Corrosion and stress-corrosion related failures often compromise the integrity of critical metallic components during their service, raising significant concerns. It is crucial to comprehend the crack initiation mechanism and the impact of alloy microstructure on this crack initiation process. It is known that the introduction of unique microstructures through metal additive manufacturing brings new challenges. This study aims to investigate, for the first time, the effects of microstructural alterations resulting from fluctuations in laser power during laser powder bed fusion on the surface cracking initiation mechanism and electrochemical behaviour of Ni-Fe-Cr alloy 718, which is widely used in applications that require exceptional strength and corrosion resistance. To carry out this investigation, microcapillary electrochemical methods were combined with high-resolution techniques (TEM, SEM, AFM). The findings emphasize the existence of an optimal range of process parameters that effectively mitigate corrosion and crack initiation susceptibility. This work demonstrated that slight deviations in laser power from this optimal value result in diverse alterations at the micro and submicron scales. These alterations include increased subgrain width, porosity, dislocation density, density of nanovoids, and distribution of carbides. Importantly, these changes, particularly in dislocation and nanovoid densities caused by minor variations in process parameters, significantly affect the material's susceptibility to corrosion initiation and stress-assisted surface cracking

Microstructural Features, Defects, and Corrosion Behaviour of 316L Stainless Steel Clads Deposited on Wrought Material by Powder- and Laser-Based Direct Energy Deposition with Relevance to Repair Applications

This work analyses the microstructural defects and the corrosion behaviour of 316L stainless steel clads deposited by laser metal deposition on wrought conventional material, which is a highly relevant system for repair applications. The different defects and microstructural features found in these systems were identified and analysed from a perspective relevant to the corrosion performance of these materials. The role of these features and defects on the corrosion process was evaluated by exposure of the samples to corrosive media and further examination of the corrosion morphology. The heat-affected zone, located on the wrought base material in close vicinity of the deposited clad, was identified to be the primary contributor to the corrosion activity of the system due to the large depletion of alloying elements in this region, which significantly decreased its pitting resistance. Alongside the heat-affected zones, relatively small (<30 µm in diameter) partially un-melted powder particles scattered across the surface of the clad were systematically identified as corrosion initiation spots, possibly due to their relatively high surface energy and therefore high reactivity compared to larger powder particles. This work highlights the need for more investigations on as-built surfaces of additively manufactured parts to better explore/understand the performance of the materials closer to their final applications. It demonstrates that the surface defects resulting from the additive manufacturing process, rather than the presence of the refined sub-granular cellular structure (as highlighted in previous works), play the predominant role in the corrosion behaviour of the system

Effect of inclusions modified by rare earth elements (Ce, La) on localized marine corrosion in Q460NH weathering steel

In this work the initial stages of the pitting corrosion in Q460NH weathering steel in a marine environment was studied. To elucidate the effects of inclusions modified by rare earth (RE) elements on pitting corrosion, field emission-scanning electron microscopy-energy dispersive spectrometry (FE-SEM-EDS) analyses, scanning Kelvin probe force microscopy (SKPFM) tests, and a series of immersion tests were conducted. Two main types of inclusions were formed in the steel, and different pit morphologies were observed. The pitting corrosion was initiated by the dissolution of (RE)(2)O2S-(RE)xSy in both types of inclusions due to the lower potential of this phase compared to the matrix, which indicated that the inclusions in the Q460NH weathering steel had a lower pitting corrosion resistance than the matrix

Influence of Thermal Oxide Layers on the Hydrogen Transport through the Surface of SAE 1010 Steel

Most research on the hydrogen embrittlement of steel dealt with the interaction of hydrogen with the metal bulk microstructural features, whereas the first contact with hydrogen-containing environments occurs at the metal surface. Steel (when un-polarized) is always covered with an oxide layer, varying in composition and thickness. The impact of the oxide layer on the hydrogen transport is, however, not fully understood. This study focused on the effect of controlled pre-formed thermal oxide layers at the exit side on the hydrogen transport through the surface of SEA 1010 steel, considering two distinct thermally produced oxide types as test cases. Results demonstrated that thermal oxides can greatly limit hydrogen diffusion, with bilayers (hematite/magnetite) having a greater effect compared to magnetite layers. Increased oxide thickness resulted also in greater limiting diffusion. The main objective of this manuscript is to provide experimental evidence concerning the effect of oxide layers on the hydrogen transport through steel. Model thermal oxide layers were used to emphasize the importance of considering the surface characteristics when investigating hydrogen transport through metallic components

Revealing the stress corrosion cracking initiation mechanism of alloy 718 prepared by laser powder bed fusion assessed by microcapillary method

Stress corrosion cracking (SCC) behaviour of L-PBF processed alloy 718 in chloride-containing solution was revealed, utilizing the microcapillary method under constant tensile loading. Results were compared with conventionally produced counterparts. Passive layer resistance was compared quantitatively by implementing electrochemical polarization experiments, in as-received and under tensile loading states. Superior corrosion and SCC resistance of L-PBF specimens under tensile loading were identified. Submicron cracks were initiated adjacent to the subgrain boundaries and the underlying mechanisms were elucidated as the synergistic effect of various microstructural factors. Data availability: The raw data with the findings of this article cannot be shared since there is ongoing research on this topic

Comparative Method To Quantify Dielectric Constant at Nanoscale Using Atomic Force Microscopy

We propose a comparative method to measure the quasi-static dielectric constant of relatively thick dielectric films (approximately 500 nm or thicker) with comparatively low dielectric permittivity (1 < ε_r < 10) at nanoscale by using the force spectroscopy technique of atomic force microscopy (AFM). Based on the relevance of analytical expression of the force spectroscopy on the dielectric susceptibility, the dielectric constant could be estimated by comparing with a reference sample of comparable dielectric permittivity. The validity of the approach was verified by good agreement between the reported values in the literature and the experimental results obtained on different materials, such as muscovite mica, SiO₂ film, poly­(methyl methacrylate) (PMMA), and polystyrene (PS). The comparative scheme avoids the complex simulation involving irregular shape of AFM tips, providing a facile approach for quantitative analysis of dielectric properties of a number of materials at the nanometer scale

Scanning Kelvin probe force microscopy study of the effect of thermal oxide layers on the hydrogen release - Experiments and finite element method modelling

International audienceScanning Kelvin Probe Force Microscopy (SKPFM) was used to study the hydrogen diffusion through a surface designed for simultaneous mapping of multiple areas containing different thermal oxides, all covered with Pd. Potential maps were obtained simultaneously on an area of bare iron as the reference, an area covered with a bilayer oxide (inner magnetite and outer hematite) and an area covered with a magnetite layer (obtained by removing the outer hematite layer of a bilayer oxide). After hydrogen charging at the bottom side of the specimen, a contrast was obtained in the potential mapping on the covering Pd layer due to differences in hydrogen release through these distinct areas on the specimen surface. A finite element method (FEM) model of hydrogen diffusion across the different phases was developed to simulate the experiment. The modelling showed that both a lower diffusion coefficient and a lower solubility in the oxide can explain the contrast obtained in SKPFM. Cross diffusion in the ferritic bulk underneath the thermal oxide was found to have an influence on the spatial distribution of the hydrogen release

Effect of Thermal Treatment on Corrosion Behavior of AISI 316L Stainless Steel Manufactured by Laser Powder Bed Fusion

The effect of post-processing heat treatment on the corrosion behavior of AISI 316L stainless steel manufactured by laser powder bed fusion (L-PBF) is investigated in this work. Produced stainless steel was heat treated in a broad temperature range (from 200 &deg;C to 1100 &deg;C) in order to evaluate the electrochemical behavior and morphology of corrosion. The electrochemical behavior was investigated by potentiodynamic and galvanostatic polarization in a neutral and acidic (pH 1.8) 3.5% NaCl solution. The microstructure modification after heat treatment and the morphology of attack of corroded samples were evaluated by optical and scanning electron microscopy. The fine cellular/columnar microstructure typically observed for additive-manufactured stainless steel evolves into a fine equiaxed austenitic structure after thermal treatment at high temperatures (above 800 &deg;C). The post-processing thermal treatment does not negatively affect the electrochemical behavior of additive-manufactured stainless steel even after prolonged heat treatment at 1100 &deg;C for 8 h and 24 h. This indicates that the excellent barrier properties of the native oxide film are retained after heat treatment