Corrosion resistance of Zn coatings produced by air arc deposition

Abstract

Pulsed air arc deposition (PAAD) is a process of depositing coatings using a chain of high current, short duration, pulsed electrical arcs to melt and evaporate material from a source anode and transport it to the workpiece-cathode which is held in close proximity. The depositions were conducted with a series of pulses with current magnitudes of 150 or 500 A, a pulse duration of 150 us, and a pulse repetition rate of 100 Hz. Zinc anodes of 2 and 5 mm diameters were employed. Coatings of Zn were applied to 18×18×5mm low carbon (0.2%) steel substrates. The influence of discharge energy and anode diameter on the coating process and the corrosion resistance of the Zn coating in a 3% aqueous NaCl solution were investigated. The coating thickness increased with increasing discharge energy and decreasing anode diameter. Coatings of 7 and 15 μm reduced the corrosion mass loss rate by factors of 4.2 and 8.5, respectively. First signs of corrosion were visible on uncoated samples after 1 d immersion in 3% aqueous NaCl solution. Application of a coat of Zn-containing spray paint extended the period to 4 d, while application of a 15 μm PAAD coating plus spray paint extended the period to 8 d. Salt fog testing showed that the PAAD coating had a higher corrosion resistance than a Zn spray paint coating. X-Ray diffraction studies showed the presence of metallic Zn, ZnO and the Fe 3Zn 10 intermetallic phase in the formed coatings.