Electrochemical lubrication for metalworking using bisulphite solutions

Electrochemical lubrication for metalworking using bisulphite solutions

Surface and Coatings Technology, 35 (1988) 217 - 219 217 Short Communication Electrochemical lubrication for metalworking using bisulphite soluti...

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Surface and Coatings Technology, 35 (1988) 217

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219

217

Short Communication

Electrochemical lubrication for metalworking using bisulphite solutions T. L. GUEST Department of Combined Engineering, Coventry Polytechnic, Priory Street, Coventry CV1 5FB (U.K.) J. P. G. FARR Department of Metallurgy & Materials, University of Birmingham, P.O. Box 363, Birmingham B15 2TT (U.K.) G. W. ROWE* Department of Mechanical Engineering, University of Birmingham, P.O. Box 363, Birmingham B15 2TT (U.K.) (Received January 1, 1987)

In the majority of lubrication situations it is intended that the rubbing surfaces will be completely separated by a film of lubricant. As loading normal to the surface increases, this separation cannot always be maintained and increasing contact of the highest points on each surface takes place, leading to damage and ultimate seizure. In order to extend the range of permissible loadings many lubricants contain organic or inorganic additives which react with the rubbing surfaces to form protective films. The majority of these reactions result from the elevated temperatures developed at the rubbing contacts by the expenditure of frictional energy. Previous work [1J has been concerned with elucidation of the reaction mechanisms and the stability of the films under high load in machine applications. Studies of film reactions using the hot wire technique [2] have shown that the initial film formation typically takes from 30 to 60 s and the development of full protection has been followed for periods of several hours. This is in contrast to the situation in metal working or cutting, where in addition to the very high contact loads, which are sufficient to cause yielding of the bulk material, the contact times between tool and workpiece may be extremely short, possibly of the order of 30 50 j~is. Although most lubricants for these applications contain additives, usually chlorine or sulphur compounds, it is difficult to see how under these circumstances such relatively slow reactions are able to offer effective protection to surfaces. -

*I)eceased 0257-8972/88f$3.50

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218

Earlier work in this laboratory [3] has demonstrated the possibility of using electrochemical rather than thermal activation to initiate lubrication reactions prior to contact and at the same time to form films which contain lamellar compounds of established lubricating properties, including molybdenum disuiphide. The effectiveness of these films has been demonstrated in their application to wire drawing and metal sheet forming [4]. The films were deposited anodically from an acidified solution containing 0.1 M ammonium molybdate and 0.1 M thioacetamide or 0.1 M thiourea. To form MoS2 it is necessary to have the molybdenum ion in the IV valency state. Under other conditions of potential a molybdenum oxide was formed. The presence of these compounds was confirmed by electron diffraction. One difficulty in the wider application of this process is the cost and limited stability of thioacetamide or thiourea. More recently, organic sulphur compounds of this type have come under suspicion as potential carcinogenic materials and their use has been limited in some countries. The original process has now been modified by the discovery that useful lubricating films can be electrodeposited from baths containing sodium metabisuiphite and ammonium molybdate. Laboratory scale experiments have been performed with acidified solutions containing 0.1 M ammonium molybdate and either 0.1 M sodium metabisulphite or 0.1 M sodium suiphite. Measurements of current were made while the d.c. potential of molybdenum, mild steel and stainless steel testpieces, relative to the standard calomel electrode (SCE), was varied with time. With molybdenum and suiphite there was a wide passive region with small cathodic currents in the potential range as had been found in earlier work for forming disulphide. On the other hand, with bisulphite the metal was anodically active and on returning to potentials more cathodic than the SCE currents were higher and black films of apparent lubricity were formed. In experiments at fixed potential where current was measured over a period of time it was found that with bisulphite there was no decay and even a slight increase over the initial current value. Mild steel and stainless steel both remained passive and there was a rapid decay from the initial current. Wire-drawing experiments were performed on molybdenum wire at room temperature using a draw bench fitted with a load cell for measuring the drawing load. The wire was cleaned with ammonia to remove existing oxide films and was then treated in a bath 1.5 m long which had a stainless steel counterelectrode running the whole length of the tank. Direct currents were passed from a stabilized power source with the wire as the anode. Current was passed for 1 mm at a potential just less than the onset of gassing, typically 2 V with a current of about 0.2 A. There appeared to be no disadvantage attached to increasing the potential into the region of gassing; the film formed more quickly. There was a noticeable decrease in the rate of film formation when suiphide was used in place of bisuiphite. Table 1 shows the results obtained when molybdenum wire of initial diameter 0.76 mm was drawn with 15% reduction in area after various pretreatments.

219 TABLE 1 Results for molybdenum wire of internal diameter 0.76 mm drawn with 15% reduction in area after pretreatment Condition

Drawing load (N)

Coefficient of friction

As received Oxide removed Base grease Base grease + MoS

263 285 247

0.156 0.182 0.139

234 201 (seized almost immediately) 205

0.124 0.090 0.094

2 powder Dry MoS2 powder Electrolytic bisuiphite ÷ molybdate

In comparison with the process originally proposed this new formulation gives lower friction in wire drawing. The original solution containing thioacetamide required a load which was equivalent to a friction coefficient of 0.16 even though a smaller reduction was then being made on a softer wire. Drawing loads with this electrolytic film are comparable with those for MoS2 alone and significantly less than for a base grease carrying the same sulphide powder. There are also significant improvements over the original process. The bath now contains materials more acceptable with respect to environmental and health considerations and the pH of this bath is close to neutrality with little risk to operators or plant. There is also a significant reduction in their cost. 1 A. Dorinson and K. C. Ludema, Mechanics and Chemistry in Lubrication, Elsevier, 1985, Chapters 10 and 11. 2 T. Sakurai, K. Sato and Y. Yamamoto, Bull. Jpn. Pet. Inst., 7 (1965) 17. 3 D. R. Atkins, J. P. G. Farr and G. W. Rowe, ASLE Trans., 14 (1973) 248. 4 G. W. Rowe, Institute of Mechanical Engineers Paper C141/80, 1980.