Optical microscopy revealed that both AlSi10Mg and Alloy C achieved similar bulk densities of approximately 99.8–99.9% relative density, yet notable differences emerged at the sample borders. AlSi10Mg exhibited fewer pores compared to Alloy C, a disparity attributed to the absence of contour printing in Alloy C. Introducing contours during printing significantly reduced porosity. Measurements were performed using cut-section analysis in ImageJ with a 1 µm detection limit, a destructive approach unlike CT scanning or Archimedes methods.
Electron microscopy provided further insight into microstructural distinctions. AlSi10Mg displayed a eutectic Al–Si rich solidification structure, with subtle variations in interdendritic spacing between melt pool boundaries in the bulk and borders, indicating differences in cooling rates. EDS mapping confirmed silicon segregation at the Al–Si eutectics. Alloy C, in its as-printed state, contained numerous nanometric Mn-rich precipitates, with rod- or needle-shaped precipitates (<100 nm) in melt pool centers and larger spherical precipitates (200–400 nm) at boundaries. Border regions contained precipitates up to 1–2 μm, enriched in Mn, with some Cr enrichment in bulk boundaries. Heat treatment produced fine Al–(Mn,Cr) precipitates concentrated at grain boundaries, contributing to increased hardness. Scanning Kelvin probe force microscopy (SKPFM) characterized electrochemical heterogeneities, revealing that Si particles in AlSi10Mg and Al–(Mn,Cr) precipitates in Alloy C were cathodic relative to the Al matrix. The Volta potential difference (ΔV) varied with particle size and location. For AlSi10Mg, Si particles in melt pool boundaries had a median ΔV of 0.23 V, higher than bulk regions at 0.18 V. In Alloy C AP, MPB precipitates had the lowest median ΔV, while in Alloy C HT, MPB particles showed higher ΔV than bulk. Particle geometry analysis indicated that AlSi10Mg possessed the largest cumulative cathode area and micro-galvanic couple interface length, despite having fewer discrete particles due to its interconnected Si network. Alloy C HT contained more numerous but smaller particles than Alloy C AP. Short-term immersion tests in 3.5% NaCl demonstrated that corrosion initiated near cathodic particles via micro-galvanic coupling. AlSi10Mg showed widespread attack along the eutectic network, while Alloy C exhibited localized corrosion around precipitates. Alloy C HT’s finer particles and lower ΔV reduced pitting likelihood compared to Alloy C AP. Porosity and coarser precipitates at borders acted as corrosion initiation sites, with contour printing mitigating this in AlSi10Mg. After 24 hours in NaCl, AlSi10Mg’s corrosion progressed to extensive surface coverage by corrosion products. Alloy C maintained localized damage, though intensity increased. Alloy C AP suffered greater cumulative attack area than Alloy C HT, underscoring the influence of particle density on long-term corrosion severity. Potentiodynamic polarization in NaCl and natural seawater revealed that Alloy C, both AP and HT, had significantly lower corrosion and passive current densities than AlSi10Mg. In seawater, AlSi10Mg lacked a passive region and remained active, whereas Alloy C maintained stable passivity and higher pitting potentials. Seawater’s diverse anion composition accelerated passive film breakdown compared to NaCl. Heat treatment slightly reduced Alloy C’s pitting potential in NaCl, likely due to grain-boundary precipitates. Electrochemical impedance spectroscopy confirmed these trends. In NaCl, Alloy C’s low-frequency impedance reached 272 kΩ (AP) and 446 kΩ (HT), far exceeding AlSi10Mg’s 3.7 kΩ. Charge transfer resistance differences mirrored corrosion current densities. In seawater, impedance differences narrowed but Alloy C retained superiority. Lower capacitance values for Alloy C indicated denser passive films. Phase angle analysis suggested Alloy C’s passive films possessed higher barrier properties and resistance. Overall, Alloy C’s coarser microstructure and Mn–Cr–Zr precipitate distribution favored the formation of protective passive layers, enhancing corrosion resistance in both NaCl and seawater. AlSi10Mg’s extensive cathodic Si phase area fraction led to poor passivity and high susceptibility to corrosion initiation, with damage spreading rapidly across the eutectic network. In contrast, Alloy C’s corrosion remained localized, and heat treatment improved resistance by refining precipitate dispersion.