ASPECTS

B.

OF CORROSIVE

Dimitrov

WEAR

IN

LUBRICATING

MEDIA

UDC

620.193:539.538:621.892.096.1

Corrosive wear is the loss of m a t e r i a l from rubbing surfaces due to the simultaneous or successive action of (a) aggressive factors in the environment and (b) m e c h a n i c a l loads. In corrosive wear, the c h e m i c a l factors act continuously on the rubbing surfaces, whereas the m e c h a n i c a l loads act periodically [4]. Wear is produced by r e m o v a l of corrosion products that are formed on the rubbing surfaces during a rest period ( c h e m i c a l corrosion) and during operation of the friction pair ( m e c h a n o c h e m i c a l corrosion) as a result of t r i b o m e c h a n i c a l processes [1]. In the work reported here, we have examined variom aspects of corrosive wear and its influence on friction and wear as a whole. Particular attention is allotted to the tribological effect of antiwear and extreme-pressure additives used to decrease adhesive wear and thereby decrease t o t a l wear [7, 9]. In the development of additive compositions, it is e x t r e m e l y important to know the eorrosivity of lubricating otis [6, 10], since not all c h e m i c a l corrosion inhibitors are able to prevent m e c h a n o c h e m i c a l corrosion [5]. Otherwise, it m a y prove that the corrosive wear obtained by the use of these oils will exceed the t o t a l of all other types of wear [2, 8]. The tribometer used in this work is shown s c h e m a t i c a l l y in Fig. 1. The rubbing pair consists of a rotating copper cylinder 1 and a fixed steel disk 3 with line contact between the two parts. The m a t e r i a l used in preparing the cylindrical speckmens was e l e c t r o l y t i c copper containing 1% indium. The specific activity of the copper s p e c i mens was at least 150 ~ C i / m g with respect to I n - l 1 4 . The disk was made of R. K.-100 alloy steel. Standard f r i c tional surfaces for the rubbing pairs were obtained by a method proposed in [3]. The robbing pair was immersed in white oil containing the test additive, in a glass bath. T h e base oil was treated in the laboratory to remove a l l a c t i v e substances present in the oil. A s p e c i a l l y designed glass fitting 10 was usedto bubble air or inert gas through the oil and to take periodic oil samples for measurement of activity. Strain gages in the unit were used to provide continuous measurements of the frictional force. The total wear of the c y l i n d r i c a l specimen was determined r a d i o m e t r i c a l l y . The load on the rubbing pair F N = 15 daN, corresponding to a Hertz load of 1250 daN/era z. The sliding speed of the cylindrical specimen was 2.2 m / s e e , bulk oil temperature 150~ When no load is applied, the tribometer becomes an oxidation-corrosion unit in which the c h e m i c a l corrosion of the copper specimen can be d e t e r m i n e d . The products of c h e m i c a l corrosion remain in the form of a layer on the surface of the rubbing pair and can be removed only under the influence of dynamic loads, thus influencing the m e c h a n o c h e m i c a l corrosion. In the first stage of these studies, the oil was bubbled with an inert gas (nitrogen). Here it was established that certain conventional additives are corrosive m a i n l y at the first moment of interaction with m e t a l ; the most active is dibenzyl disulfide, the least corrosive trieresyl phosphate. In the second stage of this investigation, c h e m i c a l corrosion rates of copper were determined in various otis with air bubbled through the oil (Fig. 2). Also determined were the corrosivities of various additives when oxygen was bubbled through, as welt as the influence of oxidation products and t h e r m a l decomposition of the base oil and additive on the c h e m i c a l corrosion rate. Each test was run for 4 h at 150~ The value chosen for the m a x i m u m allowable corrosion rate of copper with air bubbling through the additive oil was A =10 "z m g / ( c m z 9h) (in accordance with GOST SRR 5272-50). The least corrosive of the test additives were tricresyl phosphate and zinc dialkytdithiophosphate. It was established that the formation of a layer of reaction products on the m e t a l surface does protect the m e t a l and thereby decreases the c h e m i c a l corrosion rate. Even though it was impossible to determine the importance of corrosive wear in the dew.iJpment of the other Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 2, pp. 50-53, February, 1975.

9 1975 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy o f this article is available from the publisher for $15.00.

139

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Fig. 1. Diagram of tribometer: 1) cylindrical specimen; 2) shaft; 3) disk; 4) holder; 5) strain gages; 6) potentiometer; 7) recording potentiometer; 8) glass bath; 9) stirrer; 10) glass sampling apparatus; 11) air blower; 12) nitrogen cylinder; 13) drying column; 14) flowmeter; 15) trap; 16) aspirator; 17) cylindrical furnace; 18) rheostat; 19) metal cover; 20) thermocouple, 9 = 1 mnm; 21) dc electric motor; 22) tachometer; 23) glass test tube; 24) scintillation pickup; 25) radiometer; 26) recorder. Fig. 2. Effects of additives on rate of chemical corrosion of copper (with oxygen bubbling): 1) white oil without additives; 2) white oil + 1% chlorinated wax; 3) white oil + 19o tricresyl phosphate; 4) white oil + 1% dibenzyl disulfide; 5) white oil + 1% zinc alkyldithiophos phate. types of wear under the action of loads, it was possible to study the influence of various factors governing the corrosion process, i.e., the nature of the additive, the presence of oxygen, and the test time. On the basis of numerous experiments, copper/steel was selected as the friction pair, these materials providing the greatest sensitivity in the determinations. The other parameters were selected so that the rubbing pair would operate under conditions of mixed boundary and hydrodynamic lubrication. Results are shown in Figs. 3 and 4 from the simultaneous determination of friction coefficient and wear for the copper/steel pair in various oils. The main results from these studies are as follows: 1. Under the conditions adopted for operation of the friction pair, the factors in question have but little influence on the coefficient of friction. 2. In certain cases (see Fig. 3), a comparatively low rate of initial wear williacrease gradually due to the growth of adhesive and corrosive wear processes, leading to a roughening of the surface and a shift of the operating conditions into the boundary friction region. This takes place both with noncorrosive oils that do not destroy the protective layer of metal oxides on the frictional surface (curve 1) and with corrosive oils that form stable layers of reaction products of additives and metal oxides on the surfaces (curves 2 and 3), i.e., the first group of lubricating oils. When nitrogen is bubbled after mechanical abrasion of the reaction product layers, we observe an increase in the intensity of adhesive wear, the intensity of the corrosion process decreasing when oxygen is absent. Subsequently, since the rubbing surface is only weakly protected, the introduction of oxygen first reinforces the corrosive wear and tends to roughen the surface, which in turn accelerates the wear process. Gradually, owing to chemical and mechanical polishing of the contact surface, as well as the formation of a protective layer of reaction products, the wear process becomes less intense and the rate becomes constant. 140

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Fig. 4

Fig. 3. Influence of additives (first group) on processes of friction and wear: 1) white oil without additive; 2) white oil + 1% chlorinated wax; 3) white oil + 1% tricresyl phosphate. Fig. 4. Influence of additives (second group) on processes of friction and wear: 4) white oil + 1 5 dibenzyl disulfide; 5) white oil + 1% zinc dialkyldithiophosphate. 3. In other cases, the wear rate increases rapidly from the very start, then decreases gradually, this decrease being slower in nitrogen than in oxygen (see'Fig. 4). This occurs when the materials used as additives are sulfur compounds (curves 4 and 5), the second group of lubricating oils. Apparently these additives first interact with the oxide layer and adsorbed oxygen on the metal surface, this interaction failing to provide suitable corrosion protection. This brings about the development of a severe wear process, generating comparatively high temperatures in the contact microzones. These high temperatures favor the development of tribochemieal reactions leading to the formation of stable protective layers, and this produces a decrease in the wear rate. CONCLUSIONS The corrosion protection of metals through the action of oils and additives is based on limiting the development of corrosion on the metal surface through the formation of dense layers of reaction products that are c h e m i c a l ly and mechanically stable. The presence of oxygen favors the formation of protective layers, even though sometimes the decrease in adhesive wear will be accompanied by severe increases in corrosive wear. Tribomechanical loads accelerate the corrosion proeess through erosion of the protective layer and activation of the metal. However, this type of loading may bring about certairi;tribochemical reactions that will in turn establish protective layers on the metal surface. LITERATURE lo

2. 3, 4.

CITED

V. Dimitrov, Rev. Roum. Sci. Techn.-Mec. Appl., 1_~8,No. 3, 571-585 (1973). I. B. Goldman, Wear, 14__.,No. 6, 481-444 (1969). I. Iliue, Rev. Roum. Sci. T e e h n . - Mec. Appl., 15_._,No. 1, 235-244 (1969). /3. Pavelescu, Conceptii Noi, Calcul si Aplicatii in Frecarea si Uzarea Solidelor Deformabile, [in Roumanian], Ed. Academiei R.S.Ra (1971), pp. 96-109.

141

5.

6. 7. 8. 9. 10.

142

D.L. Rakhmankulov, Korroziya Zashchita Neftegaz Prom., No. 3, 13-14 (1973). P.I. Sanin, A. M. Kuliev, V. V. Sher, and K. K. Papol~, World Petrol. Congr., 5th (1959), Proc. Sect. VI, Paper 20. T. Sakurai and K. Sato, ASLE Trans., --9, No. 1, 77-87 (1966). D. Tabor, Proc. Internat. Syrup. Lubr. Wear (1965), pp. 753-759. K. Wanger and H. Jost, Schrnierungstechnik, 2, No. 6, 169-173 (1971). Yu. S. Zaslavskii, G. I. Shot, R. N. Shneerova, and F. B. Lebedeva, in: Friction and Wear in Machinery [in Russian], Vol. 15, Izd. Akad. Nauk SSSR~ Moscow (1962), pp. 486-494.