The effect of carbon dioxide ͑CO 2 ͒ on the NaCl-induced atmospheric corrosion of copper was studied using in situ Fourier transform infrared microspectroscopy, in situ scanning Kelvin probe, and scanning electron microscopy/energy-dispersive analysis by X-ray. The copper surface was contaminated with a single NaCl particle and then exposed to 80 ± 2% relative humidity clean humidified air with two concentrations of CO 2 ͑Ͻ5 and 350 ppm͒. After formation of an electrolyte droplet secondary spreading of electrolyte from the peripherical parts of the droplet was observed. The secondary spreading effect, which was much larger at Ͻ5 ppm CO 2 than at 350 ppm, was a consequence of the formation of a galvanic element between a local cathode outside the edge of the droplet and an anode in the droplet. This lead to alkaline conditions in the secondary spreading area and transport of Na + ions to the local cathode. The large secondary spreading at low CO 2 concentration was possible due to lowering of the surface tension of the electrolyte/metal oxide interface at the peripheral parts of the droplet. Carbonate formation lowered the pH when the CO 2 concentration was 350 ppm and resulted in a decrease of the pH and inhibition of the secondary spreading.
The effect of carbon dioxide
(CnormalO2)
on sodium chloride (NaCl) induced atmospheric corrosion of copper was studied in laboratory exposures using microgravimetry, ion chromatography, Fourier transform infrared spectroscopy, and scanning electron microscopy with X-ray microanalysis. With lower amount of NaCl particles on the copper surface
(<15μg∕cm2)
, the corrosion rate was higher with
<1ppm
CnormalO2
than with
350ppm
CnormalO2
, and for higher amount of NaCl
(15μg∕cm2)
, the corrosion was higher with
350ppm
CnormalO2
. With lower amount of NaCl and low
CnormalO2
concentration, a secondary spreading of electrolyte occurred from the droplets that formed at the particle clusters. This led to a larger effective cathodic area and a higher corrosion rate. However, at higher surface concentration of NaCl a spatial interaction effect between the local corrosion sites counteracted the increase in the corrosion rate due to overlap of the cathodic areas from the particles. Another factor, which influenced the corrosion process, was the effect of
CnormalO2
on the pH of the surface electrolyte. Higher pH (
<1ppm
CnormalO2
concentration) increased the formation of CuO, which improved the corrosion resistance of the corrosion product layer but hindered the formation of insoluble CuCl, whereby more soluble chloride ions were available for triggering localized corrosion and accelerating the initial atmospheric corrosion of copper. Hence, the overall influence of
CnormalO2
and NaCl depends on at least three identified mechanisms.
The corrosion behaviors of pure copper were studied in initial exposure to a simulated marine atmosphere. A two-electrode cell system with two identical pure copper plates had been employed in electrochemical tests of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The copper electrodes were deposited with 70 µg cm-2 NaCl particles and then exposed in a humidified pure air of 97% RH at 25 °C. The time-evolution of corrosion current density obtained from the cathodic polarization curves had a trend of firstly decreasing, then increasing and finally getting a relative stable stage. The EIS data showed that the transition of two time constants corrosion stage to three time constants corrosion stage. The initial atmospheric corrosion behaviors of pure copper could be divided into three stages, including rapid corrosion stage, diminished corrosion stage and balanced corrosion stage.
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