Fraunhofer Institute for Surface Engineering and Thin Films IST
Slurry pot with abrasive in artificial seawater (3.5% NaCl solution).
Investigation into tribocorrosive stresses: Interactions of wear and corrosion
Tribocorrosive stresses occur in numerous applications – from the water and energy industries, through medical technology, and on to production engineering – for example in the extrusion of new materials. The tribological investigation of such stresses is a core area of expertise at the Fraunhofer IST. It enables the evaluation of application-specific loads and forms the basis for targeted adjustments to the surfaces and edge layers of components and tools. Through the expansion of this expertise to include the analysis of tribological interactions under additional corrosive stress, the Fraunhofer IST is able to exploit new fields of application. The in-depth investigation into tribocorrosion also opens up the potential for a unique selling point in industry-related stress analysis.
The tribocorrosive mass loss (T) is comprised of the specific wear values for the individual effective components used in the application
Combined wear and corrosion on components and tools
In tribocorrosive environments, corrosion and mechanical wear act simultaneously and have a mutually reinforcing effect, which significantly influences the degree of damage. Conventional test methods that only consider either wear or corrosion individually are therefore inadequate. As tribocorrosive processes often progress slowly, time-scalable test methods are necessary in order to realistically model the damage mechanisms and to systematically investigate new material concepts.
Pilot model test bench for the investigation of tribocorrosion
With the aid of an in-house-developed test bench, the test environment at the Fraunhofer IST enables the realistic and accelerated recording of wear under increased corrosion conditions. The high flexibility of the test bench allows customized test sequences - from fundamental investigations to determine mass loss all the way through to detailed analyses of individual tribocorrosive components.
Significant parameters, such as the angle of incoming media flow and the composition of the abrasive media and electrolytes, can be adapted to specific application conditions in order to simulate the stress as realistically as possible. In addition to the tribocorrosion parameters determined (derived from the mass losses and current-density potential curves), the scientific documentation for the customer also includes high-resolution images of the altered topography of the material samples. This enables a comprehensive evaluation of the influences of the respective application environment. Taking the material 1.4404 as an example, an increasing current density can be detected as an indicator of an increased corrosion rate as soon as additional mechanical stress occurs.
The superposition of chemical and mechanical influences creates a higher material loss than would be expected from the individual components (synergism).
Test conditions for application-specific investigations
In order to accurately map the application environment, defined framework conditions must be fulfilled to use the test bench:
Sample materials: The materials to be analyzed should have conductive properties. We have acquired particular expertise in the analysis of diffusion-treated steels (austenites) and nickel-based alloys (Alloy 718).
Sample holder: This can be customized by means of 3D printing, but requires clear geometric specifications. Previously investigated round specimens had diameters of 30 to 35 mm and a height of 4 to 10 mm.
Abrasive media: The range of utilized abrasive media is diverse and can be adapted to the respective application conditions. Defined types of sand and coarser abrasive media (e.g. barrel-finishing media) up to a size of approx. 1 cm³ are deployed.
Electrolytes: These are adapted to the specific application conditions. Experience has been acquired with 3.5% NaCl solutions. In addition, defined contents of sulphuric acid (H2SO4) or alkaline solutions (NaOH, KOH) are possible.
Extension of the range of investigations: In future, gaseous environments (CO2) and temperature control up to 70 °C are also envisaged.
Clamped specimen with counter electrode during testing without mechanical wear component.
Construction of a series test bench
The test bench developed in-house has already successfully confirmed in pilot tests that the elaborated test routines provide substantiated information on hybrid wear stress and on the evaluation of different material conditions. On this basis, the test bench is being further developed in terms of design and technology in order to make it usable as a series test bench with the established investigation routines. To increase efficiency, parallelization of the sample processing will be implemented. In addition, sensors are being integrated to facilitate the recording of significant test parameters. The acquired data will be digitally recorded and stored in a database in order to enable complete documentation and further investigations.