Intercrystalline cracking

The behaviour of a wide range of a, a-0 and /3 brasses in various corrosive environments was studied by Voce and Bailey and the results summarised by Whitaker . Penetration by mercury and by molten solder was intercrystalline in all three types of brass. In moist ammoniacal atmospheres the penetration of unstressed brasses of all types was intercrystalline. Internal or applied stresses accelerated the intercrystalline penetration of a brasses and initiated some transcrystalline cracking, and also caused severe transcrystalline cracking of /3 alloys and transcrystalline cracking across the 0 regions in the two-phase brasses. Immersion in ammonia solution, however, caused intercrystalline cracking of stressed 0 brasses. 

While lead of purity in excess of 99.99% is commercially available, it is very rarely used owing to its susceptibility to grain growth and fatigue failure by intercrystalline cracking, and indifferent mechanical properties. Because of its generally superior corrosion resistance, pure lead to BS 334 1982 type A, shown in Table4.13, is occasionally used in chemical plant, but only if there is no suitable alternative. 

Changes of environment over small distances on metal surfaces can lead to specific types of corrosion some of which, once propagated, may be self-sustaining. The nature of the metal surface itself can lead to the production of different environments along its surface for example, an intercrystalline crack or inhomogeneity in the surface will lead to preferential cathodic or anodic reaction which, in turn, will cause changes in the chemistry of the local solute/solvent. These "micro environments and types of corrosion are closely related and some common occurrences are described below. 

In stress corrosion cracking, the material breaks as the result of mechanical stress under the influence of a corrosive medium. Stress corrosion cracking is characterized by the presence of deep intergranular or intercrystalline cracks that are generally not externally evident. It can be caused by inherent stress, which can be due to cold working or arise near a welding seam. 

Because of the limited accuracy of the AE source location, there was no direct proof that the cracking indicated by AE events occurs on grain boundaries. But combinations of all cited observations with our results lead us to the assumption that most AE events are caused by intercrystalline cracking with tensile opening of the cracks. This happens on grain boundaries parallel to the specimen axis. 

Cracking or fracturing that occurs between the grains or crystals in a polycrystalline aggregate. Also called intercrystalline cracking. Contrast with transgranular cracking. 

Embrittlement embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). Other steels in caustic nitrates and some chloride solutions. Brass in aqueous ammonia and sulfur dioxide. physical environments. bases of small corrosion pits, and cracks form with vicious circle of additional corrosion and further crack propagation until failure occurs. Stresses may be dynamic, static, or residual. stress relieve susceptible materials. Consider the new superaustenitic stainless steels. 

Only certain specific environments appear to produce stress corrosion of copper alloys, notably ammonia or ammonium compounds or related compounds such as amines. Mercury or solutions of mercury salts (which cause deposition of mercury) or other molten metals will also cause cracking, but the mechanism is undoubtedly differentCracks produced by mercury are always intercrystalline, but ammonia may produce cracks that are transcrystalline or intercrystalline, or a mixture of both, according to circumstances. As an illustration of this, Edmundsfound that mercury would not produce cracking in a stressed single crystal of brass, but ammonia did. 

High-strength a-0 brasses containing up to about 5Vo A1 (with small amounts of Fe, Mn, Sn, etc.) used for propellers, parts of pumps, nuts and bolts, etc. usually give good service but occasionally suffer intercrystalline failure, for instance in contact with sea-water. Examination of such failures usually reveals thin dezincified layers along the cracks, but it is difficult to decide whether the crack or the dezincification occurred first. 

Thompson and Tracy carried out tests in a moist ammoniacal atmosphere on stressed binary copper alloys containing zinc, phosphorus, arsenic, antimony, silicon, nickel or aluminium. All these elements gave alloys susceptible to stress corrosion. In the case of zinc the breaking time decreased steadily with increase of zinc content, but with most of the other elements there was a minimum in the curve of content of alloying elements against breaking time. In tests carried out at almost 70MN/m these minima occurred with about 0-2% P, 0-2% As, 1% Si, 5% Ni and 1% Al. In most cases cracks were intercrystalline. 

M.A. Timonova, Corrosion Cracking of Mg Alloys and Methods of Protection in Intercrystalline Corrosion and Corrosion Metals under Stress, I.A. Levin (ed.), Translated from Russian, Consultant Bureau in New York, 1962, pp. 263-282. 

Nevertheless, even for defective zeolite membranes (i.e., presence of cracks and pinholes), higher n-butane/isobutane separation factors [95] could be potentially achieved by intercrystalline adsorption and capdlary condensation. 

Branched cracks, transgranular except for caustic embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). 

Strong acid sites of the zeolite with and without silica binder were measured by the chemisorption of pyridine at 400 C. The acid sites were also measured in terms of the activity of the zeolite catalysts in acid catalyzed model reaction, disproportionation of toluene at 500 C. Acid sites on the external surface of zeolite crystals or intercrystalline acid sites of the zeolite catalysts were measured in terms of the iso-octane (which cannot enter in ZSM-5 zeolite channels even at 400°C [18, 19]) cracking activity at 400 C [11]. The results showing the influence of silica binder on both the intracrystalline and intercrystalline acidity of the zeolite catalyst are presented in Tables 1 and 2. 

Both the intracrystalline and intercrystalline (or external) acid sites of the zeolite are decreased by the silica binder. The changes in the intracrystalline acidity of the zeolite are reflected in its propane aromatization activity the activity is reduced significantly by the silica binder. The aromatics selectivity and the dehydrogenation / cracking and aromatization / cracking activity ratios and aromatization/(methane + ethane) mass ratio are also affected appreciably by the silica binder. The shape selectivity of the zeolite is increased markedly by the silica binder. Also because of the binder, the deactivation rate constant for initial fast deactivation is decreased but for the later slow deactivation is increased. 

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