Surface Tension of Binary Metal Surface Active Solute Systems under Conditions Relevant to Welding Metallurgy R SAHOO, T. DEBROY, and M.J. McNALLAN Since the fluid flow, heat transfer, and the resulting weld properties are significantly affected by interfacial tension driven flow, the variation of interfacial tension in dilute binary solutions is studied as a function of both composition and temperature. Entropy and enthalpy of adsorption of surface active components such as oxygen, sulfur, selenium, and tellurium in Fe-O, Fe-S, Fe-Se, Cu-O, Cu-S, Cu-Se, Cu-Te, Ag-O, and Sn-Te systems were calculated from the analysis of the published data on interfacial tension of these systems. For these calculations, a formalism based on the combination of Gibbs and Langmuir adsorption isotherms was used. Interfacial tensions in Cr-O, Co-S, and Ni-S systems, where the data are scarce, were predicted by using certain approximations. The computed values were found to be in reasonable agreement with the data available in the literature. Temperature coefficients of interfacial tensions were calculated for several binary systems. It was demonstrated that in dilute solutions, the temperature coefficient of interfacial tension is strongly influenced by the heat of adsorption which, in turn, is influenced by the difference in electronegativity between the solute and solvent ions. I. INTRODUCTION RECENT studies have demonstrated that, in many cases, the fluid flow, heat transfer, and the resulting weld shape, size, and properties are significantly influenced by the Marangoni convection. I~'2~Presence of a surface active ele- ment significantly alters the surface tension of the solvent. Furthermore, these elements often change the temperature dependence of surface tension (dy/dT) from a negative value for pure solvents to a positive value for binary sys- tems. This change in the sign of dy/dT has a significant implication in several practical systems. Of particular prac- tical significance is the reversal in the direction of circu- lation in the weld pool which occurs in steels in the presence of small amounts of surface active impurities. The flow reversal results in deeper weld penetration at particular welding speeds and power levels during welding 131 and it is believed that steels containing small amounts of surface active elements may be more easily fabricated than clean steels which contain very low levels of impurities. While this effect has been identified, the details of the interactions between temperature, impurity levels, and sur- face tension are not well understood and optimum impurity levels for different welding processes cannot be specified. Most of the impurity elements which affect surface tension driven flow have deleterious effects on the mechanical prop- erties of steel. If such elements were to be present in steels for improved weldability, it would be important for the steel to contain the lowest level of the surface active element necessary to produce improved weld penetration. The surface tension driven flow in weld pools is influ- enced by the temperature dependence of the surface tension. The fluid flow within the weld pool is strongly affected by the steep temperature gradients in the pool and by the tern- P. SAHOO, Graduate Student, and T. DEBROY, Associate Professor of Metallurgy, are with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. M.J. McNALLAN is Associate Professor, Department of Civil Engineering, Mechanics and Metallurgy, The University of Illinois at Chicago, P. O. Box 4348, Chicago, IL 60680. Manuscript submitted June 3, 1987. perature dependence of the surface tension of the metal. In pure metals, the surface tension decreases with increasing temperature. In a dilute alloy containing a single surface active component, the surface tension is affected by the segregation of the surface active component to the surface and may increase or decrease with temperature depending on the thermodynamics of the segregation process. In this paper, we have reviewed the thermodynamics of adsorp- tion of surface active elements on metal surfaces with the intention of defining the relationships between surface tension, temperature, and composition under conditions relevant to welding metallurgy. II. BACKGROUND The temperature coefficient of surface tension of pure metals and their alloys can be estimated from a knowledge of factors such as melting point, molar volume, atomic number, and heat of vaporization. However, this approach cannot be easily extended to systems with a surface active component because of the radical changes in the nature of the surface layer when these surface active elements are present. One of the first major publications dealing with the effect of surface active agents on the surface tension of solutions was given by Szyszkowski I41in 1908. He derived the following empirical relation: ~ - b In + 1 [1] where y~ is the surface tension of the pure solvent, y is the surface tension of the solution, C2 is the molality of the solution, and b and C are constants. Bernard and Lupis tSj developed a statistical model on the basis of formation of a compound A,_ ~ B at the surface when the solute element, B, provides maximum surface coverage. The model takes into account the net attractive forces between the adsorbed solute atoms. Application of their formalism to the Fe-O and Ag-O systems produced a semi-empirical correlation between sur- face activity and solute concentration. Although the cot- METALLURGICAL TRANSACTIONS B VOLUME 19B, JUNE 1988--483