Stainless Steel In Air Research Articles

Chemistry of Crystals

THE thirty-fourth Bedson Lecture was delivered at Armstrong College, Newcastle-upon-Tyne, on May 8, by Dr. C. H. Deseh, his subject being “The Chemical Properties of Crystals”. Dr. Desch said that a solid metal is a crystalline mass of which the chemical, as well as physical properties are anisotropic; acid attack, for example, not being uniform, but producing geometrically shaped pits indicating the symmetry of the constituent crystals. This constitutes a part of the evidence for a secondary or mosaic structure in crystals. The boundaries between the individual crystals have different chemical properties from the mass. Their form corresponds with that of foam cells, being determined by surface tension, and in certain alloys they can be separated by the action of specific reagents. Certain brasses containing aluminium, for example, fall to pieces, the grains separating like sand, when placed for a few seconds in a solution of mercurous nitrate. In the stainless steels, such intercrystalline disruption is explained by the segregation of carbides, but in other instances no boundary constituent has been detected. The stability of iron, aluminium and stainless steel in air is explained by the presence of a surface film of oxygen or oxide. The greatly increased chemical activity of a metallic surface under high local pressures, as in abrasion, was also discussed. The effect known as ‘corrosion fatigue’ is caused by the combined action of alternating stresses and chemical attack, in which the properties of the film and of minute cracks both play a part. Dr. Desch concluded with an account of the transformations within solid solutions, including the work of Prof. W. L. Bragg on the changes from disorder to order in a lattice.