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This wonderful phrase often marks the beginning of a search for a needle in a haystack. Despite digital measurement systems and control units, process controls, and countless hours of brainstorming, unfortunate chains of events can still lead to components developing defects during manufacturing or in use. And just like that, we find ourselves deep down the rabbit hole of failure analysis.
The term “damage” can mean all sorts of things here: an uneven surface on a mirror-finish decorative item, a shaft that breaks after just a few load cycles, or a pipe with holes in it. For every type of defect, there is an even greater variety of possible causes, and no two cases are alike. So, as you can imagine, there are virtually no limits to the complexity and the expertise required to untangle these issues.
Unfortunately, two interests conflict in damage analysis: the need for a quick resolution to prevent economic losses and even personal injury, and the need for a thorough and careful examination of the causes, because often enough, you only get this one chance.
But how does one begin a damage analysis? The best approach is to walk through the process using a real-world example.
Shown here is a connecting rod bearing that was in service for 10 years. The fundamental task was to determine the cause of corrosion and improve the component’s service life through better material selection.
First, all documentation is completed, meaning all photographs of the relevant areas are taken. From this, it is determined which locations can yield promising results using which testing methods.
It is evident on the component that, on the one hand, corrosive attack has occurred, and on the other hand, cracks caused by stress have developed.
Upon closer examination of the area surrounding the cracks, it becomes clear that the crack path is directly related to corrosion. Visibly, the grain boundaries of the metal matrix are being attacked and weakened, leading to microcracks and, later, macrocracks. The primary cause is therefore corrosion, with the cracks being a secondary effect.
Two testing methods were subsequently deemed appropriate:
Metallographic sections and microscopic examination of the corroded areas as well as the base structure to identify possible causes of the weakening of the grain boundaries
Analysis of the corrosion products under the SEM using EDX to identify foreign elements.
Microscopic examination confirmed that pitting corrosion is one of the causative forms of corrosion.
Grayish deposits are visible on the inner surfaces, occasionally with reddish components. Visually, it can be assumed that these are oxide and hydroxide reaction products.
The uniform distribution and clear demarcation of the corrosion pattern are already clearly visible on the unetched section. The corrosion surrounds the microstructure in a needle-like manner, suggesting a correlation with the microstructure and the choice of material itself.
This correlation is ultimately confirmed by the etched section. On the one hand, the needle-like structure of a martensitic microstructure is evident; on the other hand, there are signs of corrosion attack that primarily target the grain boundaries and subsequently the interfaces of the martensite needles. From a microstructural perspective, the atoms at these interfaces are more weakly bound to the matrix and are therefore more easily detached.
In addition to the expected elements, sulfur and chlorine are also detected by SEM. Chlorine is particularly relevant here, as it penetrates the protective passivation layer of chromium, thereby enabling progressive corrosion.
EDX mapping reveals that the granular, corroded areas contain an increased amount of oxides and, overall, few iron spots. From this, it can be concluded that the areas are covered with chromium oxide, i.e., the passivation layer, or that other oxidic reaction products are present.
The increased occurrence of the element carbon can have two causes and must therefore be interpreted with caution. This is because the mounting medium of the section also contains carbon and may be trapped in the interstices of the sample.
However, there are also interactions between CO₂-saturated aqueous solutions and iron that specifically attack the grain boundaries of the material. Determining whether this mechanism was at work here would require significant additional effort. However, this is not considered a viable course of action. Once all results that reliably support a hypothesis regarding the cause of the damage have been compiled, the investigation can be considered complete. The final step now is to develop a corrective measure.
To this end, the investigation results and all other findings must be compiled. According to the investigation, it is primarily important to curb corrosion, particularly pitting corrosion. Certain high-alloy materials can offer better or complete protection; for example, the addition of molybdenum can significantly improve resistance to pitting corrosion. This is important because while the rate of corrosion can be calculated and consequently controlled, the formation of cracks as a side effect can lead to a sudden and unpredictable failure with potentially far-reaching consequences. Whether reduced susceptibility is sufficient, or whether one should instead invest in more expensive materials with higher resistance, must ultimately be determined through a cost-benefit analysis.
