Correlation of Microstructural Events with Stress Corrosion Cracking Initiation Behaviour in Additively Manufactured Ni-Based Alloy 718: Microcapillary Electrochemical Technique Implementation

Abstract

During the past decade, metal additive manufacturing has gained tremendous attention for the production of substitutive components for oil, gas, and nuclear industries as a consequence of the possibility of fabricating complex full-density components with fewer manufacturing processes compared to conventional methods. Laser powder bed fusion (L-PBF) is one of the most precise additive manufacturing techniques based on layer-by-layer production of the final component with a high-power laser beam. Most investigations to dat focus on comparing the mechanical properties of additively manufactured components with conventional counterparts. Process parameter optimization has been extensively studied for obtaining almost full-density components with comparable and in some cases even better mechanical performance compared to conventionally manufactured components. However, the corrosion and more specifically stress corrosion cracking (SCC) behaviour of L-PBF and in general, additively manufactured components has received less attention. Since the major concern in critical industries is to reduce the risk of SCC occurrence, the mechanistic study of the effect of various micro and submicron structural alterations on SCC risk assessment is of paramount importance.It is well recognized in the corrosion community that the most critical stage of SCC is related to the incubation/initiation stage. From the information gathered by various authors in the past decades, having a prolonged incubation/initiation stage of SCC could significantly increase the lifetime of components, which results from the balance between the formation and destruction reactions occurring in the passive layer-environment interface. Thus, it is essentially always the case that by retarding the initiation of cracks, we significantly increase the life cycle. Until now, traditional electrochemical techniques are not able to detect the SCC initiation stage as a consequence of the large surface area in contact with the solution and the shortcomings of the used electrochemical techniques. Thus, during the past decades, extensive research has been done on the crack propagation stage by utilizing various techniques and statistical models for the prediction of crack growth rate. This study reports for the first time the correlation of electrochemical polarization behaviour with the susceptibility of the material to SCC initiation. Such an approach fulfils the knowledge gap covering the initiation mechanism by proposing the microcapillary method as the method of choice for mechanistic analysis of initiation susceptibility.In the current investigation, various electrochemical techniques combined with high-resolution microstructural analysis were implemented to elucidate the importance of submicron characteristics of L-PBF fabricated components on SCC initiation risk assessment. L-PBF fabricated Ni-Fe-Cr based alloy 718 with a slight variation of laser power in the range of achieving full density was used for elucidating the effect of slight alterations in micro and nanostructure in corrosion and SCC initiation behaviour. It was found that even with a slight variation of L-PBF process parameters such as laser power, various alterations occurred in micron and submicron scale namely, a slight decrease in hardness, an increase in subgrain width, porosity, dislocation density, the density of nanovoids adjacent to subgrain boundaries, and presence of carbide particles. Such alterations consequently lead to significant electrochemical behaviour and SCC initiation susceptibility. Furthermore, the underlying mechanisms responsible for the observed variations in SCC initiation susceptibility had been explained as a consequence of alterations in subgrain width, dislocation density, intrinsic nano cavities density adjacent to subgrain boundaries, and the presence of nanoscale carbide particles adjacent to subgrain boundaries. From the obtained results, the microcapillary technique proved to be a very useful and powerful tool for assessing SCC initiation susceptibility for additive manufactured components with submicron scale structures.