![]() This is thought to occur by the loss of a proton from coordinated water, followed by coordination of the OH - to a second cation: One of the most obvious characteristics of Cr(III) is that it is acidic i.e it has a tendency to hydrolyse and form polynuclear complexes containing OH - bridges in a process known as OLATION. pH 2-6 HCrO 4 - and Cr 2O 7 2- orange-red.World production of chromite ores approached 12 million tonnes in 1995. ![]() The sodium chromate produced in the isolation of chromium is itself the basis for the manufacture of all industrially important chromium chemicals. Alternatively, the Cr 2O 3 can be dissolved in sulfuric acid to give the electrolyte used to produce the ubiquitous chromium-plating which is at once both protective and decorative. The main use of the chromium metal so produced is in the production of nonferrous alloys, the use of pure chromium being limited because of its low ductility at ordinary temperatures. The P-B ratio of Zirconium alloys can vary between 1.48-1.56, meaning that the oxide is more voluminous than the consumed metal.\] The P-B ratio is important when modelling the oxidation of nuclear fuel cladding tubes, which are typically made of Zirconium alloys, as it defines how much of the cladding that is consumed and weakened due to oxidation. Many of the exceptions can be attributed to the mechanism of the oxide growth: the underlying assumption in the P-B ratio is that oxygen needs to diffuse through the oxide layer to the metal surface in reality, it is often the metal ion that diffuses to the air-oxide interface. However, the exceptions to the above P-B ratio rules are numerous. 1 R PB 2: the oxide coating chips off and provides no protective effect (example iron).On the basis of measurements, the following connection can be shown: Scheme of the oxide structure and the Pilling-Bedworth ratio. Conversely, the metals with the ratio higher than 1 tend to be protective because they form an effective barrier that prevents the gas from further oxidizing the metal. The oxide layer would be unprotective if the ratio is less than unity because the film that forms on the metal surface is porous and/or cracked. They ascribed the protectiveness of the oxide to the volume the oxide takes in comparison to the volume of the metal used to produce this oxide in a corrosion process in dry air. Bedworth suggested in 1923 that metals can be classed into two categories: those that form protective oxides, and those that cannot. The P–B ratio is defined as: R P B = V o x i d e n ⋅ V m e t a l = M o x i d e ⋅ ρ m e t a l n ⋅ M m e t a l ⋅ ρ o x i d e – number of atoms of metal per molecule of the oxide On the basis of the P–B ratio, it can be judged if the metal is likely to passivate in dry air by creation of a protective oxide layer. The Pilling–Bedworth ratio ( P–B ratio), in corrosion of metals, is the ratio of the volume of the elementary cell of a metal oxide to the volume of the elementary cell of the corresponding metal (from which the oxide is created).
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