Oxidation of metals tarnishing

In accordance with the expressed purpose of this monograph, we shall now discuss various models to explain the chemical and physical processes of tarnishing, and we shall not be concerned with presenting the details for particular systems. Such information can be found in a large number of special monographs on this subject [1, 2, 3]. 

The particular rate law which will be obeyed by the reaction 

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General Corrosion, or Uniform Attack. This type of corrosion includes the commonly recognized rusting of iron or tarnishing of silver. Fogging of nickel and high-temperature oxidation of metals are also examples of this type. 

A metal resists corrosion by forming a passive film on the surface. This film is naturally formed when the metal is exposed to the air for a period of time. It can also be formed more quickly by chemical treatment. For example, nitric acid, if applied to austenitic stainless steel, will form this protective film. Such a film is actually a form of corrosion, but once formed it prevents further degradation of the metal, provided that the film remains intact. It does not provide an overall resistance to corrosion because it may be subject to chemical attack. The immunity of the film to attack is a fimction of the film composition, temperature, and the aggressiveness of the chemical. Examples of such films are the patina formed on copper, the rusting of iron, the tarnishing of silver, the fogging of nickel, and the high-temperature oxidation of metals. 

The discussion of Section 17.2 treated the corrosion of metallic materials in terms of electrochemical reactions that take place in aqueous solutions. In addition, oxidation of metal alloys is also possible in gaseous atmospheres, normally air, in which an oxide layer or scale forms on the surface of the metal. This phenomenon is frequently termed scaling, tarnishing, or dry corrosion. In this section, we discuss possible mechanisms for this type of corrosion, the types of oxide layers that can form, and the kinetics of oxide formation. 

The metal has a bright silvery metallic luster. Neodymium is one of the more reactive rare-earth metals and quickly tarnishes in air, forming an oxide that spalls off and exposes metal to oxidation. The metal, therefore, should be kept under light mineral oil or sealed in a plastic material. Neodymium exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at 863oC. 

On a fresh surface the metal has a steely lustre but rapidly tarnishes in air as a result of surface formation of oxide and carbonate species. For protection against oxidation the metal is usually stored in a light mineral oil. When made finely divided, eg, on being cut, it can be strongly pyrophoric, and, for this reason is used, as the ferro-alloy mischmetal, in lighter flints and ordnance. Cerium reacts steadily with water, readily dissolves in mineral acids, and is also attacked by alkafl it reacts with most nonmetals on heating. 

Nonstoichiometric oxide phases are of great importance in semiconductor devices, in heterogeneous catalysis and in understanding photoelectric, thermoelectric, magnetic and diffusional properties of solids. They have been used in thermistors, photoelectric cells, rectifiers, transistors, phosphors, luminescent materials and computer components (ferrites, etc.). They are cmcially implicated in reactions at electrode surfaces, the performance of batteries, the tarnishing and corrosion of metals, and many other reactions of significance in catalysis. ... 

Metals tarnish when their surface atoms react with gaseous substances in the air. Oxygen is a highly reactive element, as we saw in the previous chapter, and it combines with iron to form the ruddy oxide compound we recognize as rust. Copper reacts with oxygen and carbon dioxide to form a greenish patina of copper carbonate. Silver resists the advances of oxygen but will slowly combine with sulphur compounds in the air to form black silver sulphide. 

Rubidium is a silver-white, very soft metal tarnishes instantly on exposure to air, soon ignites spontaneously with flame to form oxide best preserved in an atmosphere of hydrogen rather than in naphtha reacts vigorously with H2O forming rubidium hydroxide solution and hydrogen gas, Discovered by Bunsen and Kirchhoff m 1860 by means of the spectroscope. 

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