Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
The ex-situ surface characterization (like Raman laser spectroscopy, X-ray diffraction, scanning and scanning or transmission electron microscopy) and wear measurements are then combined with the in-situ information to try to make a global picture of the degradation or protective processes taking place during the experiment. For this, we use an adapted Basalt-N2 precision tribometer. A view on the test instrument in Figure 2 shows the tester and the electrochemical cell used. The friction, tangential force, applied load and corrosion potential of the system are continuously monitored. This apparatus allows for a fast and accurate measurement of the frictional behaviour of material under various environments, whereas it is flexible and versatile in loading range (depending on the selecting cantilevers, load can range from 0.2 mN up to 100 N) and allows for different contact geometries (point, line, area contacts).
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during the tribocorrosion test. The mechanical load causes an immediate drop of corrosion potential towards more negative values, this indicates that the corrosion process is accelerated. Throughout the test, the corrosion potential remains stable and so does the friction coefficient, indicating a metastable process of surface oxide repassivation and passive layer destruction. After completion of the test (removal of the friction contact), the corrosion potential rises quickly to its initial value, indicating a total recovery of the surface passive layer. Accelerated corrosion takes thus place only during the simultaneous mechanical loading of the friction mechanism.
But other systems may not react like that and permanent damage to the surface could be the result, as in the case of the Zn-layers shown in Figure 5. Here the corrosion potential after the sliding test is lower than before, which indicates a permanent damage to the surface protective layers.
Figure 3. (left) N2 Basalt tester and (right) tribocorrosion cell.
As an example, we study the tribocorrosion of hard chromium coatings (transportation industry). For decades, hard chrome coatings are widely used in industrial components, due to their excellent oxidation/corrosion resistance and wear resistance. Hard Chromium spontaneously builds an oxide layer on the surface that passivates and protects against corrosion. At the same time, the oxide layer can protect the surface from adhesive wear damages. But when the mechanical load is too high, the passive film can crack or rupture, and an acceleration of the corrosion process takes place. The test instrument must therefore be capable of applying well controlled loads to the surface. A typical example is given in Figure 3 and shows the effect of mechanical loading on the corrosion potential of a hard chrome coating and the evolution of the frictional behaviour
Figure 5. Changes of corrosion potential by a friction experiment on Zn and ZnFe coatings (source : Comparative behaviour in terms of wear and corrosion resistance of galvanized and zinc-iron coated steels, Souza, Maria Eliziane Pires; Ariza, Edith; Ballester, Margarita; Rocha, Luís Augusto; Freire, Célia M. A., Matéria, (Rio J.) vol;12 no.4, 2007).
Tribocorrosion under abrasive conditions The interaction between an aqueous medium and abrasive particles, as in a slurry, has been widely investigated and standardized in the 1960-1970’s culminating in the Miller Number ASTM G75 standard.
Figure 4. (a) effect of mechanical loading on open circuit potential of hard chome coatings and (b) evolution of coefficient of friction of tribosystem during tribocorrosion testing.
34 LUBE MAGAZINE NO.136 DECEMBER 2016
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