Surface active elements

Tribochemical reaction The formation of an iron sulfide tribofilm may be displaced by other surface active elements such as oxygen, where the oxide of the iron heat of formation AHf = -2.82 eV is thermodynamically more stable than the sulfide AHf = -1.04 eV. Using the data from Table 5.11, compare the heat of formation of molybdenum oxide and molybdenum sulfide. 

Wang, Y Shi, Q. Tsai, H.L. (2001). Modeling of the effects of surface-active elements on flow patterns and weld penetration. Metall. Mater. Trans. B, Vol. 32B, 145-161. 

It should also be noted that it is the soluble O and not the combined O (e.g. oxides) which affects surface tension. Certain elements such as Ca, Al, Mg react strongly with O and reduce the soluble O (denoted to very low levels e.g. (a few ppm) and form stable metallic oxides (Figure) [3]. The total O is not a measure of the soluble O (Q%) in such cases. Thus very low concentrations of these reactive elements (e.g. Ca) can have a marked effect on the process because of their effect on the surface active elements present. Thus for the Fe-Al-0 system in Figure 2a, 10 % of Al will lower the soluble O to < 5 ppm but 10 % Ca (10 ppm) (Fe-Ca-0 system) will reduce... 

Most substances have negative (dy/dT) temperature coefficients. However, when concentrations of surface active elements such as S or O in liquid metals exceed a certain critical concentration these systems exhibit positive (dy/dT) values. This is shown in Figure lb In some cases there is a limit to O solubility (e.g. Si) (i.e. saturation) and thus (dy/dT) remains negative (Figure 4a) and y has a fixed value for the 0-saturated condition. 

Surface active elements (e.g. S in Fe) reduce the surface tension, consequently, Marangoni flow occurs from regions of high to low surfactant concentrations. Local changes in surface tension may occur through evaporation or dissolution of a specific species or compound. 

The presence of surface active elements at the interface, such as sulphur, tends to block (or poison ) interfacial reactions. The classical case is the kinetics of nitrogenisation of iron where it has been shown that the rate is inversely proportional to the S content of the iron. 

Figure 7.2 shows the dependence of the surface energy of the Cu-Sb system at 950 °C as a function of the concentration of antimony in the alloys [2]. The rapid decrease in y with small Sb additions is the result of Sb being a surface-active element. The horizontal section of the plot... 

D. H. Buckley We are currently examining molecular structures which contain surface active elements including sulfur, chlorine and phosphorus. Some of these compounds contain all three elements. 

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