Oxide adherence

Another way to protect a metal uses an impervious metal oxide layer. This process is known as passivation, hi some cases, passivation is a natural process. Aluminum oxidizes readily in air, but the result of oxidation is a thin protective layer of AI2 O3 through which O2 cannot readily penetrate. Aluminum oxide adheres to the surface of unoxidized aluminum, protecting the metal from further reaction with O2. Passivation is not effective for iron, because iron oxide is porous and does not adhere well to the metal. Rust continually flakes off the surface of the metal, exposing fresh iron to the atmosphere. Alloying iron with nickel or chromium, whose oxides adhere well to metal surfaces, can be used to prevent corrosion. For example, stainless steel contains as much as 17% chromium and 10% nickel, whose oxides adhere to the metal surface and prevent corrosion. 

A small amount of palladous oxide adheres to the casserole and cannot be removed by the ordinary means. The oxide is not dissolved readily by aqua regia but is easily removed by boiling with 48 per cent hydrobromic add. 

This network can represent a wide variety of important classes of reactions. For example, oxidation reactions occurring in excess oxidant adhere to this reaction network, where B represents the partial oxidation product and C denotes the complete oxidation product CO2 ... 

A step forward in magnetic recording was to replace the wire with a thin steel ribbon. A further advance was the development of paper tape with a layer of iron oxide adhering to one side, which was introduced in Germany in 1930. A few years later, plastic tape replaced the paper. 

The residue is rinsed into a centrifuge tube with a little water. The suspension is warmed and centrifuged. The clear supernatant liquid is evaporated on a platinum lid and then ignited at a dull red heat, to convert any magnesium carbonate into the oxide. The product is moistened with a little water and carefully warmed. The alkali carbonates go into solution, but magnesium oxide adheres to the platinum (usually as a small stain). A drop of the perfectly clear solution is removed with a capillary and placed on litmus paper. If the paper turns blue, the presence of an alkali metal in the silicate is indicated. 

The most spectacular effect on the high temperature oxidation behaviour of metallic materials is observed when reactive elements (RE) are added in small quantities. They decrease the oxidation rate and drastically increase the oxide adherence to the doped metallic substrates. The protective barrier keeps its effectiveness even when submitted to thermal shocks it then guarantees an excellent lifetime of the materials under service conditions. The beneficial effect of reactive elements on the high temperature resistance of alloys has been known for more than 50 years [102,103]. The reasons why... 

Pivin J C, Delaunay D, Roques-Carmes C, Huntz AM and Lacombe P (1980), Oxidation Mechanism of Fe-Ni-20-25Cr-5 A1 AUoys-Influence of Small Amounts of Y on Oxidation Kinetics and Oxide Adherence, Corr Sci, 20, 351-373. 

Sigler D R (1989), Aluminum Oxide Adherence on Fe-Cr-Al Alloys Modified with Group IBB, IVB, VB and VIB Elements, Oxld Met, 32, 337-355. 

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