Tarnish films

The tarnishing of copper and silver in dry air containing traces of hydrogen sulphide (Table 2.6) is another example of film growth by lattice diffusion at ambient temperatures. In these cases defects in the sulphide lattice enable the films to grow to visible thicknesses with the consequent formation of tarnish films which are aesthetically objectionable and may have a significant effect on the behaviour of the metals in particular applications, e.g. electrical contacts. 

Fig. 14. Tarnishing of copper in iodine vapor. Ordinate thickness of tarnishing film, measured interferometrically abscissa time in minutes. Curve 1 with influence of supersonic vibrations (coupled to a supersonic drive, 300 kHertz) curve 2 without supersonic vibrations.

Stress-corrosion cracking of copper-zinc alloys can occur in environments other than ammoniacal solutions (Ref 114, 147, 151, 152). Included are nitrogen-bearing compounds such as amines and aniline, as well as sulfates, nitrates, nitrites, acetates, formates, and tartrates. These environments can produce tarnish films of Cu20 similar to the films formed in ammoniacal solutions. Both the rate of formation and... 

Corrosion tunneling Plasticity Tarnish Film-induced mpture cleavage... 

A trace amount of hydrogen sulfide in contaminated atmospheres causes the observed tarnish of silver and may also cause tarnish of copper. The tarnish films are composed of Ag2S and a mixture of CU2S + CuS + CU2O, respectively. 

With extremely thin films, tunneling from metal to metal may be the dominant mechanism even at high temperatures, and will certainly be so for low temperatures, where thermally activated processes are frozen out. The theory of this mechanism was considered many years ago by Holm and others in connection with the low resistance of electrical contacts where a thin oxide or other tarnish film will usually be present. 

The slip dissolution model assumes that plastic deformation at the crack tip is responsible for the activation. But other mechanisms can have the same effect. Tensile stress at the crack tip could, for example, break a brittle tarnish film or passive oxide film, thereby exposing the base metal to the electrolyte. Selective dissolution of alloy components at the crack tip could locally weaken the metal matrix and thus permit... 

Tierney, V., The Nature and Rate of Creepage of Copper Sulfide Tarnish Films over Gold, Journal of the Electrochemical Society, Vol. 128, 1981, p. 1321. 

Humidity plays a major part in the corrosion of magnesium and its alloys (Loose, 1946). Water vapor in air increases corrosion (Froats et al., 1987). The rate of attack is negligible at low humidity, but increases considerably above ca 90% relative humidity (RH) (Whitby, 1933 Froats et al., 1987). At RH up to 90% minor corrosion results from the formation of a nearly invisible film of amorphous Mg(OH)2. As the humidity increases beyond this level, heavier tarnish films develop, and the principal corrosion product is crystalline Mg(OH)2 (Froats et al., 1987). 

Stainless steels, on the other hand, develop a relatively thick tarnish film with a thin, powdery surface film. Both films have a nominal composition of where M represents iron, nickel, or chromium. The corrosion rate of stainless steel is not greatly affected by temperature in the range of 260 to 400°C, and it does not exhibit the marked effect of temperature on corrosion rate that is characteristic of Zircaloys. 

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