Deeparekha Narayanan

This study evaluated the influence of scan rate (4.23 mm/s [S10] and 6.35 mm/s [S15]) on the localized corrosion and tribocorrosion behavior of a laser engineered net shaping (LENS)-produced stainless steel 316L (SS316L) in a phosphate-buffered saline (PBS) solution. Electrochemical impedance spectroscopy (EIS) was performed by applying an AC signal from 10⁵ to 10⁻² Hz and cyclic potentiodynamic polarization (CPP) was performed by sweeping from −150 mV to +1.5 V (vs. open circuit potential) and back to characterize passivation and pitting susceptibility. Potentiostatic tribocorrosion tests were conducted using a reciprocating tribometer integrated with a potentiostat to probe material response in passive and cathodic regimes. S15 exhibited manufacturing-related defects that served as preferential pit initiation sites, with pits in both S10 and S15 showing evidence of cell-interior dissolution. Electrochemical results indicated that the charge transfer resistance was reduced by 66% for S15 and that the repassivation potential decreased by 35% compared to S10. Under tribocorrosion, material degradation was dominated by mechanical wear for both samples. However, sliding significantly accelerated electrochemical dissolution in S15, with the corrosion rate affected by wear (V_c-w) increasing by 46.8%. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) of wear scars revealed plastic deformation, abrasive grooves, and bio-tribofilm formation composed primarily of phosphates. Micro-pits associated with processing defects were observed exclusively in S15. Overall, lower scan rate processing (S10) produced a more defect-resistant microstructure with improved resistance to localized corrosion and tribocorrosion in PBS.

Commercial stainless steel 316L (316SS) is widely used in the industry due to its good mechanical properties, corrosion resistance, and abrasion resistance. Among non-conventional synthesis routes, ball milling is an effective solid synthesis technique that can also serve as a preparatory step for additive manufacturing (AM), where processing parameters strongly influence the resulting nanostructure. This study investigated the effect of milling conditions on the microstructure and corrosion behavior of 316SS powders. Milling was performed at 300 rpm for 20 and 40 h using ball-per-ratio (BPR) of [6:1] and [10:1]. The electrochemical analysis was performed using the techniques of open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization tested on an aggressive environment of 3.5 wt% NaCl, pH 8. The results showed no clear trend indicating an optimal combination of BPR and milling time; however, the BPR [10:1] 20 h sample showed the best electrochemical response, followed by the BPR [6:1] 40 h sample. Overall, all ball-milled samples showed reduced corrosion resistance compared to commercial and untreated 316SS, which was attributed to the formation of martensitic phases and increased porosity in the milled pellets.

The effect of temperature on the localized corrosion resistance and passive film characteristics of laser powder-bed fusion (LPBF) 316L (UNS S31603) was studied in a buffered 3.5 wt% NaCl solution at 25, 50, and 75 °C. DC techniques such as cyclic potentiodynamic polarization showed lower passive current densities, high breakdown potentials, and a higher resistance to initial breakdown compared with wrought 316L samples at all temperatures. However, LPBF 316L was more susceptible to metastable pitting at potentials before film breakdown and higher damage accumulation post film breakdown. AC techniques, such as Mott–Schottky analysis and electrochemical impedance spectroscopy, showed that the formed passive film was more robust on the LPBF 316L samples at all temperatures, accounting for the higher initial resistance to pitting. However, with increasing temperatures, the film formed had an increasing concentration of defect density. Passive compositions at the various test temperatures studied using X-ray photoelectron spectroscopy (XPS) showed that the LPBF samples showed higher amounts of Cr and Fe oxides and hydroxides compared with the wrought samples, which made the passive films on the LPBF samples more compact and protective. Investigation of the pits formed on the LPBF showed the preferential regions of attack were the melt-pool boundaries and cell interiors due to their being depleted of Cr and Mo when compared with the boundaries and matrix.

The effect of varying temperatures (15, 35, and 45 °C) on the corrosion behavior of a new single-phase distorted face-centered cubic (fcc) Al₂Cr₅Cu₅Fe₅₃Ni₃₅ multi-principal element alloy (MPEA) was studied in simulated soil environments (NS4 solution). X-ray diffraction and electron backscattered diffraction were used to confirm the presence of a single phase throughout the microstructure. Electrochemical tests such as electrochemical impedance spectroscopy and linear polarization resistance were performed to evaluate the interfacial behavior and corrosion rate at various test conditions. Additionally, cyclic potentiodynamic polarization (CPP) tests were also carried out at the selected temperatures to study the pitting behavior of the MPEA. Surface characterization techniques such as X-ray photoelectron spectroscopy and scanning electron microscopy were used to identify the nature of the passive film formed in such complex materials as well as study the pitting characteristics after CPP testing. A stable passive film was found to be present all tested temperatures with the absence of preferential sites for pitting due to homogenous element distribution.

In this work, the passivation and localized corrosion of selective laser melted (SLM) stainless steel 316 L when exposed to high pressures of CO₂ with the presence of H₂S and Cl⁻ at 25 °C and 125 °C were studied. Depletion of Cr/Mo was observed at the cell interiors and melt-pool boundaries (MPBs) compared to the cell boundaries. Volta potential differences obtained from scanning Kelvin probe force microscopy (SKPFM) showed that the MPBs were 8–20 mV lower than the matrix, while the cell interiors were 20–50 mV lower than the cell boundaries. Electrochemical impedance spectroscopy (EIS) and Mott–Schottky tests indicated a more defective passive film at 125 °C, and X-ray photoelectron spectroscopy (XPS) confirmed the formation of a less protective film with an increased S/O ratio at 125 °C than 25 °C. Initiation of localized corrosion was observed at the MPBs and pits formed after a week of immersion were wider by an order of magnitude at 125 °C than 25 °C, with evidence of cell-interior dissolution. While passivity was observed even at elevated temperatures, local chemical heterogeneities compromised the stability of the film and contributed to localized corrosion in SLM SS316L.

The influence of the microstructure on the resistance to pitting potential in austenitic stainless steel UNS S31603 is evaluated to explain the effect of the distribution of features such as carbides. Different microstructures were obtained by processing, via surface laser melting (SLM) and sensitized at 600°C, 700°C, and 800°C. The test solution used for electrochemical testing included a buffer pH 8 brine at room conditions. The characterization of the passive conditions is done by using potentiodynamic, potentiostatic, and Mott Schottky techniques. The results show that the UNS S31603 samples produced by SLM have higher passive layer stability. The correlation with the microstructural features attributes this to a lack of inclusions and carbides characteristic of the SLM process. The analysis of the experimental results using the point defect model description of the passive layer behavior indicates that the stability of the passive layer is a priori inversely proportional to both the metal cation and anion vacancy diffusivities. Experiments reveal the close dependence and explain the properties of the passive layer with respect to a point defect model.