Atmospheric corrosion protection of LPBF manufactured AlSi10Mg by combining SiOx-CVD and PDMS grafting

Information on processing PM-IRRAS data and the experimental setup: PM-IRRAS was employed to characterize sample surfaces after each modification step. A Bruker Vertex 70 (Bruker Optics, Germany) was used. The measurement angle between the detector and the surface normal of the samples was fixed at 80°. For infrared beam polarization, an aluminum wire grid was used and modulated at 50 kHz with a ZnSe Photo-Elastic-Modulator (PMA50, Bruker Germany). To account for the wavelength-dependent optical throughput and detector response of the PMA50 setup, spectra were recorded in two separate acquisitions with the optical focus optimized around 3000 cm⁻¹ and 1500 cm⁻¹, respectively, in order to maximize the signal-to-noise ratio in both the C–H stretching and fingerprint regions. Reflected light from the substrates was focused with a ZnSe lens onto a cryogenic mercury cadmium telluride (MCT) detector. All modified samples were measured with 512 scans, a resolution of 4 cm-1 and a mirror speed of 10 kHz. An aperture of 1.0 mm was used for the Reference, SiOx-CVD, and SiOx-CVD+PDMS samples, whereas an aperture of 1.5 mm was used for the PDMS sample. Data treatment was done with the OPUS 7.8 software (Bruker Optics, Germany), where a polynomic background correction was applied to all spectra in a similar way to remove the background of the Bessel function. Information on processing XPS data and the experimental setup: To study the chemical composition of the cleaned and modified surfaces, X-ray photoelectron spectroscopy (XPS) has been performed. For this, an Omicron ESCA+UHV System (Scienta Omicron NanoTechnology GmbH, Germany) has been utilized. A monochromatic Al Kα (1486.3 eV) X-ray source was used. The effective measured sample surface area is approximately 3.1 mm2. Every recorded measurement was done with a base pressure of below 4 x 10-10 mbar. The emitted photoelectrons have been detected with a detection system oriented at an angle of 30° relative to the surface plane. The angle between the X-ray source and the detector system is 102°. For each (modified) specimen, two survey spectra, in the range of 0 eV – 1350 eV, have been recorded with a step size of 0.5 eV and a pass energy of 100 eV. These survey spectra scans have been utilized for elemental data calculation and are implemented into the supporting information (SI, Figure S1). The atomic composition of the (modified) specimen has been evaluated, after a Shirley (O 1s, C 1s, Si 2p and Al 2p) and linear (F 1s, N 1s) background subtraction, by using relative sensitivity factors (RSFs) integrated into CASA XPS software (CASA Software Ltd). The used effective RSFs are 2.93, 1.0, 0.69, 0.45, 4.43 and 1.8, respectively. Additional core-level spectra for Si 2p and Al 2p were recorded using a pass energy of 20 eV with a step size of 0.1 eV and a dwell time of 0.5 s. The FWHM of the Ag 3d5/2 peak (368.1 eV) of a freshly sputtered Ag sample measured under these conditions equals 0.77 eV. Each component was fitted with a GL(30) function, which is a convolution of a 70 % Gaussian- and a 30 % Lorentzian product function. The Si (0) and Al (0) component in the Si 2p and Al 2p spectra have been fitted with a Gaussian/Lorentzian product formula modified by an asymmetric form, which is defined as A(a,b,n)Gl(30), with a and b defining the asymmetric form of the fit and n defines the width of the Gaussian to convolute the basic shape of the profile. A Shirley background is used in all cases. Information on FE-SEM/EDX experimental setup: Field-emission scanning electron microscopy has been conducted to get topographical data of the surface investigated. Images of different magnifications are obtained employing a NEON 40 FE-SEM microscope (Carl Zeiss SMT AG) equipped with an InLens detector and an SE2. To identify elemental species, energy-dispersive X-ray spectroscopic (EDX) measurements have been done in the same device as for FE-SEM. The applied acceleration voltage was 4 kV and the aperture size was 60 µm. The used detector is called “Ultra Dry” (Thermo Fisher Scientific Inc.). Information on processing electrochemical data and the experimental setup: The used electrochemical (EC) set-up is established in a droplet-cell, which is attached to an OCA 15 Plus system (DataPhysics Instruments GmbH, Germany). The droplet-cell employed a three-electrode configuration, in which the sample served as the working electrode, exposed to an area of approximately 0.071 ± 0.01 cm². The electrode area was determined using the integrated camera system of the OCA 15 Plus setup and, where possible, was additionally verified based on dried electrolyte residues or visible surface degradation using a digital microscope (VHX-7000, Keyence Deutschland GmbH). A gold counter electrode and an Ag/AgCl (3 M KCl) reference electrode were used. As an electrolyte, 0.1 M NaCl in ultra-pure water was used. The open circuit potential (OCP) was recorded for 300 seconds before electrochemical measurements, serving as a compromise between electrolyte evaporation and the equilibration of the electrochemical system. Electrochemical impedance spectroscopy (EIS) has been applied with an AC voltage of 20 mV in a frequency range of 100 kHz to 0.1 Hz. Linear Sweep voltammetry (LSV) has been done by applying a cathodic voltage of -0.15 V vs. OCP until a final voltage of +0.5 V vs. OCP is reached with a scan rate of 2 mV∙s-1. Anodic LSV measurements were performed by ramping the potential from OCP to +0.7 V vs. OCP with a scan rate of 2 mV∙s-1.Chronoamperometry has been applied to test the relaxation of the electrochemical system of the individual surface states. For this, 0.1 V vs. OCP has been applied for a maximum of 800 seconds. All electrochemical experiments have been conducted with a Gamry 1000 Interface potentiostat (Gamry Instruments, Inc, USA) and the data evaluation has been done with the Software “Gamry Echem Analyst” (Version 7.8.2). Information on atmospheric corrosion studies: Atmospheric corrosion processes have been simulated in a closed exicator with a constant relative humidity (r. h.) of 75 ± 2 %. The system was connected to a bubble counter filled with a saturated NaCl solution to allow gas exchange with the ambient environment. The composition of the laboratory atmosphere was not explicitly controlled or considered. The humidity was controlled by placing petri-dishes with saturated NaCl solution inside the exicator and observed by using a temperature and humidity sensor (SHT4x SmartGadget) from Sensirion AG. Before the samples were placed inside the exicator, they were spray-coated with a thin film of NaCl (≈ 127 ± 16.7 μg∙cm-2). The NaCl loading on the samples was determined gravimetrically. The spray-coating has been done with a ND-SP spray coater (Nadetech Innovations). The spray solution was a saturated methanolic NaCl solution. During the spray coating process, the samples are heated to about 75 ± 5 °C to ensure that the methanol is evaporating as fast as possible to get a homogeneous spray result. Both modified and unmodified samples were characterized by digital microscopy before and after NaCl loading, as well as after exposure to corrosive atmospheric conditions. Information on water contact angle measurements: Static water contact angle measurements were carried out with an OCA 15 plus (Dataphysics) at RT (≈ 20 °C). For each measurement, 5 μl water droplets were dispensed onto the respective surface by support of a motor-driven syringe system. For each surface, at least three samples with one to four measurements were conducted and evaluated. For each droplet several data points were acquired to account for artificial baseline positioning. Parameters other than the contact angle (CA(M)) (e.g., “IFT”, “Err”, and “Vol”) were automatically generated by the software and are not physically meaningful for the present measurements.