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Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


Solving this question and understanding the mechanism that are at play, means that you can turn a negative synergy into a positive one. So there is a growing need to study tribocorrosion.


How to investigate tribocorrosion? In principle, it is not very difficult to do tribocorrosion measurements. It is enough to bring a friction mechanism into an aqueous medium or electrolyte, to connect parts to electrochemical measurement equipment (such as a potentiostat) and measure simultaneously the force interaction (friction) and the electrochemical mechanisms at the surface of the materials. After solving some practical issues (making an electrochemical cell with a friction test inside of it), the combined effects of friction and corrosion can be easily measured. Most tribometer manufacturers offer ‘tribocorrosion’ cells (more or less complex reservoirs in some kind of polymer) and stand-alone or integrated potentiostats to enable tribocorrosion measurements. Examples of tribocorrosion cells and setups can be found all over the internet. They all have one thing in common: measure the electrochemical activity of a surface while friction and wear tests are done on it.


The difficulty lies in the interpretation of these measurements. Long books and articles have been written on the interactions between the mechanisms, and the conclusion is that each occurrence of tribocorrosion in practice has been studied in a particular way, very often by simulating the application as closely as possible. The only general approach to studying the synergetic effects of tribology and corrosion, was done in the field of abrasive tests, and culminated in standard ASTM G119. Unfortunately, the parameters of the standard are often quite different from field applications and therefore questionable as a prediction of field mechanisms. Furthermore, a proper lab scale simulation of field mechanisms will often not allow the application of the standard.


No.107 page 2


The interpretation of corrosion mechanisms under a mechanical load on the surface requires at first a solid understanding of electrochemical processes in static conditions, and the insight in how these mechanisms may be changed by mechanical influences on the surface.


Some tribocorrosion effects can be easily measured and monitored, for instance the destruction of a passive and protective surface layer by sliding friction can have a pronounced effect on the electrochemical surface potential E (Figure 2)


But the challenge lies in those conditions where the mechanism of tribocorrosion is not so clear or understood, due to the complexity of surface interactions taking place. The dynamic interaction between formation of surface layers induced by friction and surface changes due to corrosive processes, and how they influence each other, makes each process different and unique. Whether a synergy will be positive (reducing friction and wear, or corrosion damage or both) or negative (accelerating either corrosion, or wear, or both) isn’t easy to predict nor to measure.


Tribocorrosion in sliding contacts


In these tests, we link the sliding wear behaviour of bulk materials or coatings, to the simultaneously measured electrochemical properties during a friction experiment. By immersing the material in an electrolyte, the in situ electrochemical surface state of the material and its evolution during sliding tests is continuously monitored. That surface state can be determined by different electrochemical techniques, such as open circuit potential measurements, polarization measurements and impedance measurements. The information obtained by the in situ electrochemical techniques is then linked to the in situ mechanical measurements, like friction force recorded during sliding tests and/or to acoustic noise measurements revealing the formation of cracks.


Figure 2. Generalized measurements of Surface Potential E or corrosion current, when a tribological experiment takes place. Corrosion current increases, or potential E shifts to more active (=corrosive) conditions by the surface action of the tribology process.


LUBE MAGAZINE NO.136 DECEMBER 2016 33


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