The influence of ultraviolet illumination on corrosion rate, surface film tightness, crystalline conversion, and defect density in Zn--Al--Mg coated steel under an extremely cold polluted atmosphere was investigated. The coating metal consisted of $\sim$20~$\mu$m thickness with composition of 4.4~wt.\% Mg, 6.8~wt.\% Al, 4.8~wt.\% Si, 0.5~wt.\% P, and balance Zn. The test atmosphere consisted of cycles of cooling to $-60$~$^\circ$C, heating to 20~$^\circ$C, humidity control to 95\% relative humidity, and drying for 12~h daily. Deposition rate of sodium bisulfite was 5.09~mg~cm$^{-2}$~d$^{-1}$ each 7~day, whereas ultraviolet illumination at wavelength of 340~nm and power of 5~W~m$^{-2}$ was applied continuously throughout testing duration. Measurement techniques employed included gravimetric measurement, electrochemical impedance spectroscopy, in-situ impedance measurement, x-ray diffraction, scanning electron microscopy, and Mott-Schottky analysis to correlate rate with surface-film continuity and defect-mediated ion permeation. The rate of corrosion after 7~days was negligible, but dark exposure caused greater mass loss compared with that exposed to ultraviolet light from days 14 to 35. The effect of ultraviolet illumination inhibition fell from 25.7\% at day 14 to 11.2\% at day 35, showing that ultraviolet irradiation provided only a temporary barrier improvement. The polarization resistance was achieved to be $2.36\times10^{4}$~$\Omega$~cm$^{2}$ in dark after 21~days and to be $4.80\times10^{4}$~$\Omega$~cm$^{2}$ under ultraviolet illumination after 14~days. X-ray diffraction confirmed no formation of a new crystalline compound by ultraviolet illumination but a delay in converting Zn$_4$SO$_4$(OH)$_6\cdot$4H$_2$O-dominated products to Zn$_2$(SO$_3$)(SO$_4$)$\cdot$3H$_2$O products. Observation under scanning electron microscope revealed a thickness of 3.77~$\mu$m loosely packed products after 7~days in dark while thickness under ultraviolet illumination condition was lower at 1.93~$\mu$m. Mott--Schottky analysis verified n-type products and revealed a reduction of donor density from $1.65\times10^{20}$~cm$^{-3}$ without ultraviolet illumination to $4.88\times10^{19}$~cm$^{-3}$ under ultraviolet illumination. In conclusion, the coating behavior resulted from two major mechanisms namely ultraviolet illumination enhanced hydroxysulfate product layer densification and conversion of sulfate products to sulfite products by sodium bisulfite deposition with resultant freeze-thaw cracking.

Zn--Al--Mg coated steel,atmospheric corrosion,ultraviolet illumination,sodium bisulfite,freeze-thaw cycling,electrochemical impedance spectroscopy,Mott--Schottky analysis