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The corrosion behavior of Ag in outdoor environments and in a salt spray chamber was investigated. The coulometric reduction technique was applied to identify various silver corrosion products including Ag 2 SO 4 and AgO. These corrosion products were observed on samples exposed in certain field environments. A plateau in the coulometric reduction curve associated with metastable hydrogen evolution was observed under conditions in which a considerable amount of Ag 2 S was formed and reduced. Soluble silver chloride complexes were the dominant corrosion product, instead of insoluble AgCl, when the chloride deposition rate was high enough. Under those conditions, the corrosion rate measured by coulometric reduction is lower than the actual corrosion rate. Ag particles formed on the surface during exposure to different environments. During exposure in a marine environment, photodecomposition of AgCl corrosion product resulted in the formation of Ag nanoparticles embedded in AgCl. Isolated Ag particles of a few micrometers in size formed during exposure in a salt spray chamber due to the high exchange current density for the Ag/Ag + reaction. Ag is used in electronics applications, utensils, and decorative arts. Its corrosion behavior in indoor environments 1-3 and in lab chambers containing sulfur 4-11 has been studied. Ag 2 S is the main corrosion product and the kinetics of its formation rate usually are linear. The sulfidation rate of Ag increases as the concentration of H 2 S and carbonyl sulfide increase. Both of them generate HS − on the Ag surface in the presence of water and then HS − reacts with Ag to result in sulfidation.12 Therefore, humidity is critical for Ag sulfidation. The sulfidation rate stays low as long as the relative humidity (RH) is less than 35% but it increases significantly as the RH increases up to 95%. 6Recently it was reported that the corrosion rate of other metals such as carbon steel or aluminum alloys correlates with the concentration of atmospheric chlorides as assessed by the amount of AgCl formed on Ag.13 Ag has been used as an atmospheric corrosion monitor to measure environmental corrosivity, 13 which has attracted attention to the corrosion behavior of Ag in the field. 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...

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A Mg-rich primer (MgRP) consisting of metallic magnesium pigment was developed for the protection of AA2024-T351. 1-10 Both a MgRP and a MgO-rich primer (MgORP) with topcoats were tested in laboratory primer cycle test and long-term field exposure conditions to evaluate coating barrier properties, scribe protection, and sacrificial anode-based cathodic protection in the case of MgRP. Results show that detectable amounts of Mg pigment in MgRP is preserved through 4.25 y of field exposure. Furthermore, cathodic protection afforded by these Mg pigments remains a viable form of protection in MgRP for as long as 2.5 y in coastal marine field exposure at Kennedy Space Center. Beyond this, scribe coverage of Mg 2+-based products was observed to increase during the course of a 4.25 y coastal marine exposure. Mg 2+ repartitioning from the coating into the scribe was also observed for MgORP after 2.5 y of exposure. This observation forms the basis for the explanation of how a MgORP functions as a protective primer. The performance of a MgORP was also studied to assess chemical protection effects of stored and released Mg 2+ (both in solution and corrosion product) and to investigate how MgORP functions for corrosion protection. Several observations indicated favorable effects of Mg 2+ on the corrosion protection of AA2024-T351 as Mg 2+ repartitioning was also observed. Scribe damage was found to be similar for both MgRP and MgORP after 2.5 y of exposure.