Metals, or alloys

XJsorption of gases on to transition metal surfaces is important, and transition metals or alloys are often used as heterogeneous catalysts. 

This is essentially a corrosion reaction involving anodic metal dissolution where the conjugate reaction is the hydrogen (qv) evolution process. Hence, the rate depends on temperature, concentration of acid, inhibiting agents, nature of the surface oxide film, etc. Unless the metal chloride is insoluble in aqueous solution eg, Ag or Hg ", the reaction products are removed from the metal or alloy surface by dissolution. The extent of removal is controUed by the local hydrodynamic conditions. 

Disadvantages associated with some organic solvents include toxicity flammabiHty and explosion ha2ards sensitivity to moisture uptake, possibly leading to subsequent undesirable reactions with solutes low electrical conductivity relatively high cost and limited solubiHty of many solutes. In addition, the electrolyte system can degrade under the influence of an electric field, yielding undesirable materials such as polymers, chars, and products that interfere with deposition of the metal or alloy. 

Skiving is a variant in which the base metal surface oxides are mechanically removed foUowed immediately by pressure rolling of a precious metal or alloy strip. This is commonly used for inlays for electrical contacts and for jewelry fabrication. The common inlay materials include gold, silver, copper, brass, and solder. No heat is needed, and the coating is appHed only to designated areas so there is Htde waste (3,50). 

Multilayered composite sheets and plates can be bonded in a single explosion, and cladding of both sides of a backing metal can be achieved simultaneously. When two sides are clad, the two prime or clad metals need not be of the same thickness nor of the same metal or alloy. 

Qualitative Analysis. Nitric acid may be detected by the classical brown-ring test, the copper-turnings test, the reduction of nitrate to ammonia by active metal or alloy, or the nitrogen precipitation test. Nitrous acid or nitrites interfere with most of these tests, but such interference may be eliminated by acidifying with sulfuric acid, adding ammonium sulfate crystals, and evaporating to alow volume. 

Reactive Evaporation. In reactive evaporation (RE), metal or alloy vapors are produced in the presence of a partial pressure of reactive gas to form a compound either in the gas phase or on the substrate as a result of a reaction between the metal vapor and the gas atoms ... 

Where appropriate, the direct precipitation of hexavalent chromium with barium, and recovery of the Cr(VI) value can be employed (166). Another recycling (qv) option is ion exchange (qv), a technique that works for chromates and Finally, recovery of the chromium as the metal or alloy is... 

Corrosion inhibitors are substances which slow down or prevent corrosion when added to an environment in which a metal usually corrodes. Corrosion inhibitors are usually added to a system in small amounts either continuously or intermittently. The effectiveness of corrosion inhibitors is partiy dependent on the metals or alloys to be protected as well as the severity of the environment. For example, the main factors which must be considered before apphcation of a corrosion inhibitor to an aqueous system are the compatibility of the inhibitor and the metal(s), the salt concentration, the pH, the dissolved oxygen concentration, and the concentration of interfering species such as chlorides or metal cations. In addition, many inhibitors, most notably chromates, are toxic and environmental regulations limit use. Attention is now being given to the development of more environmentally compatible inhibitors (37). 

Gold and gold alloys serve the needs of dentistry better than any other metals or alloy systems. Gold alloys have a broad range of working characteristics and physical properties, coupled with excellent resistance to tarnish and corrosion ki the mouth. 

Composites. Another type of electro deposit in commercial use is the composite form, in which insoluble materials are codeposited along with the electro-deposited metal or alloy to produce particular desirable properties. Polytetrafluoroethylene (PTFE) particles are codeposited with nickel to improve lubricity (see Lubrication and lubricants). SiHcon carbide and other hard particles including diamond are co-deposited with nickel to improve wear properties or to make cutting and grinding tools (see Carbides Tool materials). 

Intergranular Corrosion Selec tive corrosion in the grain boundaries of a metal or alloy without appreciable attack on the grains or crystals themselves is called intergranular corrosion. When severe, this attack causes a loss of strength and ductility out of proportion to the amount of metal actually destroyed by corrosion. 

Anodic Protection This electrochemical method relies on an external potential control system (potentiostat) to maintain the metal or alloy in a noncorroding (passive) condition. Practical applications include acid coolers in sulfuric acid plants and storage tanks for sulfuric acid. 

Electroplating—the process of electrodeposition onto a metallic substrate of a thin adherent layer of a metal or alloy having desirable chemical, physical and/or mechanical properties. 

Passivity—a condition of a metal or alloy in which the material is normally thermodynamically unstable in a given electrolytic solution but remains visibly unchanged for a prolonged period. The electrode potential of a passive metal is always appreciably more noble than its potential in the active state. 

These elements that insert in a pipe or specially fabricated cylindrical holder can usually be fabricated from any workable and weldable metal or alloy. In addition, most plastic that can be fabricated by molding, cutting, heat welding, or even bolting can be used. This wide array of fabrication materials allows the units to fit an extremely wide range of corrosive applications. 

If a metal or alloy combination is to be selected, choose combination of metals as close together in the galvanic series as possible. 

Choose the metal or alloy so that the anode area is larger than the cathode area. 

Conditions necessary for the onset of corrosion are quite often provided by heterogeneities. These heterogeneities may very well exist within the metal or alloy or may be imposed by external factors. These heterogeneities can give rise to variations in potential on a metal surface immersed in an electrolytic fluid. The galvanic cell thus formed gives rise to flow of current that accompanies corrosion [188]. 

One of the most effective methods of preventing corrosion is the selection of the proper metal or alloy for a particular corrosive service. Once the conditions of service and environment have been determined that the equipment must withstand, there are several materials available commercially that can be selected to perform an effective service in a compatible environment. Some of the major problems arise from popular misconceptions for example, the use of stainless steel. Stainless steel is not stainless and is not the most corrosion-resistant material. Compatibility of material with service environment is therefore essential. For example, in a hydrogen sulfide environment, high-strength alloys (i.e., yield strength above 90,000 psi or Rc 20 to 22) should be avoided. In material selection some factors that are important to consider are material s physical and chemical properties, economics and availability. 

While a metal or alloy may be selected largely on the basis of its mechanical or physical properties, the fact remains that there are very few applications where the effect of the interaction of a metal with its environment can be completely ignored, although the importance of this interaction will be of varying significance according to circumstances for example, the slow uniform wastage of steel of massive cross section (such as railway lines or sleepers) is of far less importance than the rapid perforation of a buried steel pipe or the sudden failure of a vital stressed steel component in sodium hydroxide solution. 

The interaction of a metal or alloy (or a non-metallic material) with its environment is clearly of vital importance in the performance of materials of construction, and the fact that the present work is largely confined to a detailed consideration of such interactions could create the impression that this was the sole factor of importance in materials selection. This, of course, is not the case although it is probably true to say that this factor is the one that is the most neglected by the design engineer. 

In the case of non-metallic materials, the term corrosion invariably refers to their-deterioration from chemical causes, but a similar concept is not necessarily applicable to metals. Many authorities consider that the term metallic corrosion embraces all interactions of a metal or alloy (solid or liquid) with its environment, irrespective of whether this is deliberate and beneficial or adventitious and deleterious. Thus this definition of corrosion, which for convenience will be referred to as the transformation definition. 

On the other hand, corrosion has been defined as the undesirable deterioration of a metal or alloy, i.e. an interaction of the metal with its environment that adversely affects those properties of the metal that are to be preserved. This definition —which will be referred to as the deterioration definition —is also applicable to non-metallic materials such as glass, concrete, etc. and embodies the concept that corrosion is always deleterious. However, the restriction of the definition to undesirable chemical reactions of a metal results in anomalies which will become apparent from a consideration of the following examples. 

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