Copper Corrosion Mechanisms of Polysulfides
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*The Lubrizol Corporation, 29400 Lakeland Boulevard, Wickliffe, OH 44092-2298 [email protected]
"**Molecular
Simulations, Inc., 9685 Scranton Road, San Diego, CA 92121-9990
ABSTRACT Corrosive behavior of commercial organopolysulfides of the type R-(S),,-R is known to increase dramatically towards copper-based alloys when n > 4, whereas di- and trisulfides are essentially inert. Three possible reasons for this behavior are examined using local and nonlocal densityfunctional theory as well as post-Hartree-Fock theory at the MP2 level. The hypotheses are: the shorter chains are protected from the surface by steric hindrance of the terminal organic groups, longer polysulfides can chelate copper atoms and remove them from the surface, and S-S bonds become weaker and more reactive as the polysulfide chains become longer. We find the predominant reason for the increase in corrosive behavior to be a dramatic decrease in S-S bond dissociation energy when n > 4 due to stabilization of the unpaired electron via delocalization in RSS* thiyl radicals. The thiyl radicals are thus the species which is capable of attacking and corroding the copper surface. Chelation is eliminated as a possible mechanism, and steric protection and oil solubility afforded by the organic terminal groups are found to play a minor role. INTRODUCTION Organopolysulfide lubricant additives which effectively passivate and protect ferrous metals often corrode copper-containing metal alloys such as bronze and brass. This is a serious limitation as ferrous and non-ferrous metals are commonly used to fashion different parts of the same mechanical system. The ultimate goal is to minimize the corrosive behavior of the additives while preserving their effective wear protection performance.
In this study we focus on the mechanisms
of copper corrosion. Polysulfides are linear molecules with dihedral angles of approximately 900. Disulfides can exist in either d or I conformations, and polysulfides can form either right or left-handed helices. Chains with at least five atoms can be described as either cis or trans depending on whether the two terminal atoms are on the same or opposite side of the plane formed by the three central atoms. Fibrous sulfur S. exists in the all-trans conformation, forming a helical structure [I]. In this investigation we use polysulfides in the all-trans conformation, analogous to S-, and with twofold symmetry. In commercial organopolysulfides of the type R-Sn-R, where n=2-6, the longer sulfide chains are known empirically to be much more corrosive with respect to copper than the shorter chains where n < 3. R is typically a hydrocarbon used to impart oil solubility to the polysulfide. Currently little is known about the corrosion mechanisms involved which can explain this difference in reactivity. We propose three hypotheses in an effort to determine why the corrosive behavior of polysulfides dramatically increases when n > 4. The first hypothesis considered is that the steric bulk of the hydrocarbon side chains can inhibit corrosion
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