Effects of Sodium Chloride Particles, Ozone, UV, and Relative Humidity on Atmospheric Corrosion of Silver

Abstract

The corrosion of Ag contaminated with NaCl particles in gaseous environments containing humidity and ozone was investigated. In particular, the effects of relative humidity and UV light illumination were quantitatively analyzed using a coulometric reduction technique. The atmospheric corrosion of Ag was greatly accelerated in the presence of ozone and UV light. Unlike bare Ag ͑i.e., with no NaCl particles on the surface͒, Ag with NaCl exhibited fast corrosion even in the dark, with no UV in the presence of ozone. Samples exposed to different outdoor environments and samples exposed in a salt spray chamber were studied for comparison. Ag corroded at extremely low rates in a salt spray chamber partly because of the combined absence of light and oxidizing agents such as ozone. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3310812͔ All rights reserved. Despite their widespread use, the results of accelerated atmospheric corrosion tests such as ASTM B117 often do not correlate well with field exposures.1 As shown below, silver is an example of this phenomenon as silver oxidizes readily during outdoor field exposure but is barely attacked in a salt spray chamber. Clearly, some controlling factors of the chemical and electrochemical reactions in the field exposures are not accurately reproduced in the salt spray chamber environment.Atmospheric corrosion of silver is affected by factors such as relative humidity ͑RH͒, 2-4 airborne pollutants, 5,6 and temperature. Traditional atmospheric corrosion studies of Ag have focused mainly on sulfidation 3,7-9 as silver reacts strongly with sulfurcontaining species. The absence of such species in the ASTM B117 test is one reason why silver is relatively inert in the salt spray environment. However, there has been little focus on the effects of chloride and photoassisted corrosion.In a previous paper, 10 the effects of ozone, UV radiation, and RH on the atmospheric corrosion of bare silver were studied. Atomic oxygen generated by the photolysis of ozone by UV radiation reacted quickly with bare silver to form silver oxide. However, 254 nm UV radiation or ozone alone did not cause any corrosion of Ag within the exposure period. The corrosion rate of bare silver was relatively independent of RH, and it was suggested that the initial chemisorption of atomic oxygen or OH radical under wet conditions is relatively unaffected by RH. The effects of chloride on these reactions are not known. Furthermore, the atmospheric corrosion rate of metals in the field is typically dependent on RH as a result of the interactions of water with salts on the surface. 11Sea-salt aerosols can play a critical role in the atmospheric corrosion process. Cole et al. described marine aerosol formation, chemistry, reaction with atmospheric gases, transport, deposition onto surfaces, and reaction with surface oxides.12 The small size of sea-salt aerosols allows them to be carried away by the wind to places far away from the sea.13 These aerosols are created by physical processes such as the bursting of air...

Analysis of Ag Corrosion Products

“…Previous research indicated that Ag 2 O, AgCl, and Ag 2 S are reduced at around −0.15, −0.28, and −1.15 V MSE, respectively, before hydrogen evolution reaction (HER) occurs at −1.75 V MSE. 14,15,18 Figure 1 shows plateaus near −0.15, −0.28, and −1.15 V MSE for the products mentioned above as well as at −0.08 V MSE and −1.55 to −1.6 V MSE. These latter two plateaus have not been reported in the literature and the species responsible for them are unknown.…”

“…It has been proven in lab exposure tests that UV has a significant accelerating effect on Ag corrosion by the generation of atomic oxygen and OH radicals from photolysis of ozone. 14 However, the results suggest that the accelerating effect of UV on Ag corrosion in the field maybe more significant.…”

“…Some interesting and maybe unique aspects of Ag outdoor corrosion have been reported. However, the details of how Ag corrodes in the field are not yet clear.Previous research shows that AgCl and Ag 2 S are the two most common corrosion products in the field, with AgCl usually dominating [14][15][16] except near a volcano with high emission of H 2 S where Ag 2 S is dominant.17 This might result from a higher deposition rate of chloride outdoors and is different than indoor exposure of Ag where Ag 2 S is more dominant than AgCl.12 Surprisingly, no AgCl was observed on the surface of Ag after exposure in an ASTM B117 salt spray chamber.14 One reason for studying the corrosion behavior of Ag is to clarify this discrepancy in behavior between field exposure and a supposedly accelerated laboratory test. Enhancing the understanding of Ag might help to clarify why the behavior of other metals and alloys can be different in field exposures and lab tests.…”

“…Previous research shows that AgCl and Ag 2 S are the two most common corrosion products in the field, with AgCl usually dominating [14][15][16] except near a volcano with high emission of H 2 S where Ag 2 S is dominant.…”

“…Previous research shows that AgCl and Ag 2 S are the two most common corrosion products in the field, with AgCl usually dominating [14][15][16] except near a volcano with high emission of H 2 S where Ag 2 S is dominant. 17 This might result from a higher deposition rate of chloride outdoors and is different than indoor exposure of Ag where Ag 2 S is more dominant than AgCl.…”