Why wear and corrosion must be tested together
Many alloys survive corrosive environments only because a thin passive oxide film protects them. Sliding contact scrapes that film away. Fresh metal is exposed, corrodes rapidly until the film reforms, and is then scraped again. Corrosion products and a weakened surface can in turn accelerate mechanical wear. Because of this synergy, a material ranked by a dry wear test and a static corrosion test separately can still fail quickly in tribocorrosion.
The problem is common in biomedical implants sliding in body fluids, in marine and offshore hardware, pumps and valves, and in food, chemical and pharmaceutical processing.
How the module works
The corrosion module is a three-electrode electrochemical cell installed on the tribometer’s sample stage:
- Working electrode: the sample itself;
- Counter electrode: a platinum wire;
- Reference electrode: Ag/AgCl in saturated NaCl.
Wear and corrosion run simultaneously in a ball-on-plate sliding configuration, with the contact fully immersed in the test electrolyte. Linear reciprocating tests run up to 40 N; rotational tests are available as a custom configuration, also up to 40 N.
Reading the signals
Two signals recorded together describe the process:
- Open-circuit potential (OCP). A drop in OCP at the start of sliding indicates that the protective film has been damaged and active metal is exposed. Recovery after sliding stops shows repassivation.
- Coefficient of friction. Changes in COF mark the stages of wear development, such as breakthrough of a coating or the build-up of corrosion products in the track.
Correlating OCP with COF shows when protection fails, not just how much material was lost by the end.
Separating mechanical and chemical damage
Tests can also run under controlled potential. Anodic polarization accelerates corrosion and cathodic polarization suppresses it, so comparing wear under each with wear at open circuit separates the contributions of mechanical wear, corrosion and their interaction. ASTM G119 provides a framework for quantifying this wear–corrosion synergism. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) characterise the corrosion behaviour of the surface before, during or after sliding.
Measuring the wear
After the test, the wear track is measured by chromatic confocal 3D non-contact profilometry to obtain worn volume. Combined with the electrochemical record, this gives a quantitative account of total material loss under tribocorrosion.
