Silver alloys corrosion

Silver reduces the oxygen evolution potential at the anode, which reduces the rate of corrosion and decreases lead contamination of the cathode. Lead—antimony—silver alloy anodes are used for the production of thin copper foil for use in electronics. Lead—silver (2 wt %), lead—silver (1 wt %)—tin (1 wt %), and lead—antimony (6 wt %)—silver (1—2 wt %) alloys ate used as anodes in cathodic protection of steel pipes and stmctures in fresh, brackish, or seawater. The lead dioxide layer is not only conductive, but also resists decomposition in chloride environments. Silver-free alloys rapidly become passivated and scale badly in seawater. Silver is also added to the positive grids of lead—acid batteries in small amounts (0.005—0.05 wt %) to reduce the rate of corrosion. 

Lead—silver alloys are used extensively as soft solders these contain 1—6 wt % silver. Lead—silver solders have a narrower free2ing range and higher melting point (304°C) than conventional solders. Solders containing 2.5 wt % silver or less are used either as binary alloys or combined with 0.5—2 wt % tin. Lead—silver solders have excellent corrosion resistance. The composition of lead—silver solders is Hsted in ASTM B32-93 (solder alloys) (7). 

The most common form of corrosion is uniform corrosion, in which the entire metal surface degrades at a near uniform rate (1 3). Often the surface is covered by the corrosion products. The msting of iron (qv) in a humid atmosphere or the tarnishing of copper (qv) or silver alloys in sulfur-containing environments are examples (see also SiLVERAND SILVER ALLOYS). High temperature, or dry, oxidation, is also usually uniform in character. Uniform corrosion, the most visible form of corrosion, is the least insidious because the weight lost by metal dissolution can be monitored and predicted. 

Barnard, K. N., Christie, G. L. and Gage, D. G. Service Experience with Lead Silver Alloy Anodes in Cathodic Protection of Ships , Corrosion, 15, 11, 581-586 (1959) Peplow, D. B. and Shreir, L. L. Lead/Platinum Electrodes for Marine Applications , Corr. Tech. Apr. (1984)... 

The outstanding characteristics of the noble metals are their exceptional resistance to corrosive attack by a wide range of liquid and gaseous substances, and their stability at high temperatures under conditions where base metals would be rapidly oxidised. This resistance to chemical and oxidative attack arises principally from the Inherently high thermodynamic stability of the noble metals, but in aqueous media under oxidising or anodic conditions a very thin film of adsorbed oxygen or oxide may be formed which can contribute to their corrosion resistance. An exception to this rule, however, is the passivation of silver and silver alloys in hydrochloric or hydrobromic acids by the formation of relatively thick halide films. 

PMS liquids are corrosion-inert substances. Under normal conditions and heated to 100-150 °C they do not cause corrosion and for a long period of time do not change in airflow when in contact with aluminum and magnesium alloys, bronzes, carbon and doped steels, as well as titanium alloys. PMS liquids do not change their properties under 100 °C in air for 200 hours in contact with the above-listed alloys as well as with beryllium, bismuth, cadmium, Invar alloy, brass, copper, mel-chior, solder, lead, silver. The stability of the properties of PMS liquids in these conditions is usually accompanied by the absence of metal and alloy corrosion, although the colour of the metal surface may slightly change. 

Nitric acid reacts energetically at ordinary temperatures, with formation of the nitrate and of the oxides of nitrogen. The behaviour of the metal with hydrochloric acid is similar to that of lead. The action of concentrated sulphuric acid and of nitric acid finds application in the separation of alloys of gold and silver. The corrosive effect of acids is accelerated by the presence of an oxidizer.5... 

Table 2.5. Effect of tin and silver on corrosion resistance of lead-calcium-tin alloys. (Corrosion weight loss in mg cm . )...

What is the effect of silver on the anodic corrosion of lead It is revealed hy the rate of oxidation of the metal expressed in current density units (Fig. 2.45). Let us consider the case when the polarization is carried out at a constant potential (e.g., 1500 mV). When Ag ions are introduced into the solution, the oxygen over-voltage decreases. When silver is alloyed in the metal, a weaker effect on the rate of oxygen evolution is observed, but the corrosion rate of the lead—silver alloy is considerably reduced. Hence, the introduction of silver by both methods accelerates the oxygen evolution reaction, but it affects differently the anodic corrosion of the metal. 

G.S. Duffo, J.R. Galvele, Surface-mobdity stress corrosion cracking mechanism in silver alloys, in R.P. Gangloff, M.B. Ives (Eds.), Environment-induced Cracking of Metals, NACE, Houston, TX, 1990, pp. 261—264. 

Cast lead-silver alloys are extremely corrosion-resistant, but in general have a life of only 2-4 years. The short life is in general related to the presence of casting defects within the body of the anode. The internal corrosion of these defects, as seen in Figure 2, can lead to the production of holes and uneven corrosion conditions in certain areas of the anode leading to bending, warping and short circuits. 

A lead-silver alloy containing 0.5 - 1% Ag has been used as the insoluble anode in the electrolytic production of pure zinc for many years (1). During the casting of the Pb-Ag alloy anodes, a more slowly cooled region is formed in the upper center of the anode plate. Industrial practice shows that the slowly cooled area is much more susceptible to corrosion leading to the perforation of the anodes. Although this is a serious problem, there is a lack of information about the mechanism of the localized accelerated corrosion. In this work, we attempt to compare the electrochemical behavior of the two parts of the anode and to elucidate the corrosion mechanism using several dc and ac electrochemical methods. 

Silver and silver alloys are used for electrical contacts, connecting leads in semiconductor devices, solders and brazes, corrosion-resistant structural parts, batteries, oxidation catalysts, optical and heat reflecting mirrors, table ware, jewellery, dentistry, and coins. Silver halides are base components in photographic emulsions. 

Yau, T. L., Andrews, J. A., Henson, H. R., and Holmes, D. R., Practice for Conducting Corrosion Coupon Tests on Zirconium and Its Alloys, Corrosion Testing and Evaluation Silver Anniversary Volume, ASTM STP 1000, ASTM International, West Conshohocken, PA, 1990, pp. 303-311. 

The membrane conducted protons and therefore sulphuric acid was a natural choice for the anolyte. The anode was a lead-silver alloy, the silver reducing the oxygen overpotential and increasing the corrosion r itaiioe,... 

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