Metals reactions, 596 alloys

Reviews of batch calorimeters for a variety of applications are published in the volume on Solution Calorimetry [8] cryogenic conditions by Zollweg [22], high temperature molten metals and alloys by Colinet andPasturel [19], enthalpies of reaction of inorganic substances by Cordfunke and Ouweltjes [16], electrolyte... 

Reaction with Inorganic Compounds. Most metals and alloys react with aqueous hydrochloric acid via... 

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. 

Aqueous Corrosion. Several studies have demonstrated that ion implantation may be used to modify either the local or generalized aqueous corrosion behavior of metals and alloys (119,121). In these early studies metallic systems have been doped with suitable elements in order to systematically modify the nature and rate of the anodic and/or cathodic half-ceU reactions which control the rate of corrosion. 

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. 

At high temperatures, aluminum reduces many oxygen-containing compounds, particularly metal oxides. These reactions, of the type shown in equation 6, are used in the manufacture of certain metals and alloys, as well as in the thermite welding process. ... 

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 ... 

Reactions of boron ttihalides that are of commercial importance are those of BCl, and to a lesser extent BBr, with gases in chemical vapor deposition (CVD). CVD of boron by reduction, of boron nitride using NH, and of boron carbide using CH on transition metals and alloys are all technically important processes (34—38). The CVD process is normally supported by heating or by plasma formed by an arc or discharge (39,40). 

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. 

The study of corrosion is essentially the study of the nature of the metal reaction products (corrosion products) and of their influence on the reaction rate. It is evident that the behaviour of metals and alloys in most practical environments is highly dependent on the solubility, structure, thickness, adhesion, etc. of the solid metal compounds that form during a corrosion reaction. These may be formed naturally by reaction with their environment (during processing of the metal and/or during subsequent exposure) or as a result of some deliberate pretreatment process that is used to produce thicker films or to modify the nature of existing films. The importance of these solid reaction products is due to the fact that they frequently form a kinetic barrier that isolates the metal from its environment and thus controls the rate of the reaction the protection afforded to the metal will, of course, depend on the physical and chemical properties outlined above. 

This represents a special case of high-level turbulence at a surface by the formation of steam and the possibility of the concentration of ions as water evaporates into the steam bubbles . For those metals and alloys in a particular environment that allow diffusion-controlled corrosion processes, rates will be very high except in the case where dissolved gases such as oxygen are the main cathodic reactant. Under these circumstances gases will be expelled into the steam and are not available for reaction. However, under conditions of sub-cooled forced circulation, when cool solution is continually approaching the hot metal surface, the dissolved oxygen... 

The terms hot corrosion or dry corrosion are normally taken to apply to the reactions taking place between metals and gases at temperatures above 100 C i.e. temperatures at which the presence of liquid water is unusual. The obvious cases of wet corrosion at temperatures above 100 C, i.e. in pressurised boilers or autoclaves, are not considered here. In practice, of course, common metals and alloys used at temperatures above normal do not suffer appreciable attack in the atmosphere until the temperature is considerably above 100 C. Thus iron and low-alloy steels form only the thinnest of interference oxide films at about 200 C, copper shows the first evidence of tarnishing at about 180 C, and while aluminium forms a thin oxide film at room temperature, the rate of growth is extremely slow even near the melting point. 

Consideration will also be given to attack arising from contact with solids such as refractories, and with molten materials such as salts, glasses, and lower-melting-point metals and alloys. On a fundamental basis, the distinction between some of these latter reactions and normal-temperature aqueous corrosion is not always clear, since galvanic effects may be of significance in both cases, but for practical purposes a distinction can be made on the basis of the temperature involved. 

The thermodynamic phase stability diagrams appear to be preferred by corrosion scientists and technologists for the evaluation of gas-metal systems where the chemical composition of the gaseous phase consisting of a single gas or mixture of gases has a critical influence on the formation of surface reaction products which, in turn, may either stifle or accelerate the rate of corrosion. Also, they are used to analyse or predict the reason for the sequence of formation of the phases in a multi-layered surface reaction product on a metal or alloy. 

Corrosion may be described as the undesirable reaction of a metal or alloy with its environment and it follows that control of the rate of process may be eflFected by modifying either of the reactants. In corrosion inhibition , additions of certain chemicals are made to the environment, although it should be noted that an aqueous environment can, in some cases, be made less aggressive by other methods, e.g. removal of dissolved oxygen or adjustment of pH. 

Since corrosion involves a reaction of a metal with its environment, control may be effected through either or both of the two reactants. Thus control could be based entirely on the selection of a particular metal or alloy in preference to all others or the rejection of metals in favour of a non-metallic material, e.g. by a glass-reinforced polymer (g.r.p.). At the other extreme control may be effected by using a less corrosion-resistant material and... 

Oxidation of a metal or alloys to its (lower energy state) oxides or cations. In effect, the wastage or other damage to a metal caused by one or more of several types of chemical or electrochemical reactions. Takes many forms such as galvanic, crevice, pitting, underdeposit, and biologically induced corrosion. 

Xin and co-workers modified the alkaline EG synthesis method by heating the metal hydroxides or oxides colloidal particles in EG or EG/water mixture in the presence of carbon supports, for preparing various metal and alloy nanoclusters supported on carbon [20-24]. It was found that the ratio of water to EG in the reaction media was a key factor influencing the average size and size distribution of metal nanoparticles supported on the carbon supports. As shown in Table 2, in the preparation of multiwalled carbon nanotube-supported Pt catalysts... 

Velocity Most metals and alloys are protected from corrosion, not by nobility [a metal s inherent resistance to enter into an electrochemical reaction with that environment, e.g., the (intrinsic) inertness of gold to (almost) everything but aqua regia], but by the formation of a protective film on the surface. In the examples of film-forming protective cases, the film has similar, but more limiting, specific assignment of that exemplaiy-type resistance to the exposed environment (not nearly so broad-based as noted in the case of gold). Velocity-accelerated corrosion is the accelerated or increased rate of deterioration or attack on a metal surface because of relative movement between a corrosive fluid and the metal surface, i.e., the instability (velocity sensitivity) of that protective film. 

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