<p>This study investigates the effect of wire diameter (<i>Φ</i> = 1.2, 1.6, and 2.0&#xa0;mm) on the microstructure and properties of Al-based coatings deposited via plasma-enhanced high-velocity arc spraying onto steel substrates. The microstructure, mechanical properties, and electrochemical behavior of the resulting coatings were systematically characterized, with a focus on investigating the influence of wire diameter on the microstructural evolution, mechanical performance, and corrosion resistance of the coatings. The results demonstrate that an increase in wire diameter leads to the formation of larger molten droplets during the melting and atomization process, which in turn affects the droplet spreading behavior and the overall coating structure. While the phase composition of the coatings remains unaffected by the wire diameter, both surface roughness and porosity initially decrease and subsequently increase, achieving the lowest values at a wire diameter of <i>Φ</i>1.6&#xa0;mm. The microhardness and bonding strength exhibit a slight fluctuation, showing an initial increase followed by a decrease; however, the overall variation is minimal, with average values of approximately 35 HV 0.1 and 37&#xa0;MPa, respectively. Electrochemical analysis reveals that the corrosion resistance of the coatings is strongly influenced by their surface morphology and internal density. The corrosion resistance ranking is as follows: <i>Φ</i>1.6&#xa0;mm &gt; <i>Φ</i>1.2&#xa0;mm &gt; <i>Φ</i>2.0&#xa0;mm. Among the tested diameters, the coating fabricated using <i>Φ</i>1.6&#xa0;mm wire demonstrates the best overall performance.</p>