A look through the microscope: corrosion products as artefacts
10. October 2019
Fig. 5: Corrosion artefact.
During a wear investigation on a damaged ball bearing, fatigue spalling in the raceway of an outer ring was first documented (Fig. 1).
At higher magnification, numerous strikingly uniform spheres with a finely structured surface were visible within the spalled areas (Figs. 2 and 3).
Unusual structures under the microscope
At first glance, these structures resemble resolidified metal beads, such as those that can form during electrical discharges in ball bearing raceways, a phenomenon we discussed in an earlier article. Only further analysis clarified the situation.
Energy-dispersive X-ray spectroscopy (EDX) established that, unlike genuine resolidified metal beads, the spherical structures contain a significant proportion of oxygen (Fig. 4). This chemical composition clearly indicates that they are not metal that has melted and then resolidified.
Corrosion products rather than the cause of damage
The shape and composition of the structures instead suggest that they are corrosion products, in other words rust, which formed subsequently within the areas of fatigue spalling. These features are therefore not relevant to the cause of the bearing wear being investigated, but are merely artefacts.
An important point in materials analysis: critically examine the findings
This example clearly shows that investigations in materials engineering must always consider whether a finding is genuine.
Not every unusual structure is automatically part of the original damage mechanism. Some form only after the actual damage has occurred or while the component is in use.
W.S. Werkstoff Service – expertise in damage analysis
We offer two seminars for anyone wishing to deepen their knowledge of technical damage analysis:
- Technical damage analysis – mechanical damage: https://www.werkstoff-service.de/en/trainingcenter/kurse-und-seminare-buchen/semtsa-1-damage-analysis-mechanical-damage/.
- Technical damage analysis – corrosion: https://www.werkstoff-service.de/en/trainingcenter/kurse-und-seminare-buchen/semtsa-2-damage-analysis-corrosion/.
