Alloying Elements and Additives

The concentration of alloying elements and additives is generally different in the metal and in the natural oxide film [31]. The rate of formation and the surface properties of the oxide, therefore, depend on its own composition, and not on the composition of the metal underneath. 

As an example, the oxidation rate of a metal can be decreased by an alloying element that reacts preferentially with oxygen and forms an oxide layer across which diffusion is slowed down. 

Certain elements will strengthen the protective properties of the oxide film by forming mixed oxides, if their stmctures are compatible. This is the case of magnesium. For this reason, alloys of the 5000 series have excellent corrosion resistance. On the other hand, certain elements such as copper will weaken these protective properties. This explains the poor corrosion resistance of copper-containing alloys of the 2000 and 7(X)0 series. 

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According to the old French standard, NF A 02-400, each alloying element and additive was designated by a letter. 

The concentrations of alloying elements and additives are given in Table A.3.7. The standardised compositions of the most common alloys are given in Table A.3.8, and their physical properties in Table A.3.9. 

Influence of Alloying Elements and Additives Allotropic Modihcations Properties... 

The durabihty and versatility of steel are shown by its wide range of mechanical and physical properties. By the proper choice of carbon content and alloying elements, and by suitable heat treatment, steel can be made so soft and ductile that it can be cold-drawn into complex shapes such as automobile bodies. Conversely, steel can be made extremely hard for wear resistance, or tough enough to withstand enormous loads and shock without deforming or breaking. In addition, some steels are made to resist heat and corrosion by the atmosphere and by a wide variety of chemicals. 

A recent American development is the use of quenched and tempered carbon steels for subzero temperature duties. Such steels are fortified with minor additions of alloy elements and remain notch ductile down to -75°F (-59°C). 

The potentiostat has supplied an experimental tool for the study of anodic protection. The elucidation of passive behaviour made possible by poten-tiostatic anode polarisation curves allowed investigators to determine the conditions necessary for maintaining a metal in a stable passive condition by provision of a suitable environment, addition of cathodic alloying elements , and/or maintenance of the required potential by means of external anodic polarisation... 

The diagrams which have just been described are of only limited value because the presence of an alloying element in solution in the iron influences the thermodynamic behaviour of the solute. Thus it is well known that the solubility of gases in metals at constant pressure is changed by addition of alloying elements and since this is only another way of saying that the activity coefficient of the gas atoms in the solution has been changed, we... 

The case structure of nitiided steel depends on its type, concentration of alloying elements and particular conditions of nitiiding treatment. The diffusion zone is the original core microstructure with the addition of nitride precipitates and nitrogen solid solution. The surface compoxmd zone is the region where y (Fe4N) and e (Fe2-3N) intermetallics are formed. The corrosion resistance of steel varies with nitrided layer structure. The surface "white layer" can contain e nitride, y nitride or a two phase mixture e+y, below that is the diffusion zone. In add solutions the iron nitrides corrode more slowly than iron and when the "white zone" is formed on the steel surface the improvement of the corrosion resistance is a fact. In Fig. 1 is shown the typacal structure of nitrided plain carbon steel (Minkevich, 1965). 

Epit should be high, in other words we select alloying elements and environmental conditions in which E has a noble potential e.g. addition of copper and silicon to the aluminum matrix and sodium nitrate to the environment make pjt more noble (Figs. 3-8 and 3-9). 

With the focus on RSP for production of Mg alloy components, a greater interest has been shown in the role of all alloying elements and their effects on mechanical properties and corrosion resistance. The discovery that R additions to the RSP Mg alloy AZ91 dramatically increased the corrosion resistance (Chang et al. 1986), have stimulated a number of further investigations into the effects of R additions on the corrosion resistance of Mg alloys (Hehmann et al. 1988). 

Sn-5Sb solder has a near-peritectic composition and a relatively high-liquidus temperature, around 238°C, which is significantly higher than the melting point of eutectic Sn-Pb solder, 183°C. Unlike other alloying elements, the addition of Sb to Sn raises the melting point of Sn gradually as the solid solubility of Sb in Sn increases [49]. The peritectic reaction, which occurs near 246 ° C, produces Sn- Sb microstructures which are complex. A typical cast microstructure of Sn-5Sb alloy solidified rapidly is shown in Fig. 2. This microstructure exhibits coarse dendrites of Sn-rich solid solution with Sn Sb intermetallic compounds dispersed between the dendrites. 

AU the additions listed above lower the liquidus temperatures and, usually more pronounced, also the solidus temperatures of alloys which effectively means an increase of the width of the melting range. Too much of one or more of the alloying additions therefore increases the risk for formation of hot tears during solidification, due to segregation of alloying elements and development of... 

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