Scanning Electron Microscopy of Conversion Coating

Conversion coatings often use manganese, iron, and zinc. Zinc phosphate coatings, for example, are widely used in the automotive industry on components that require high corrosion resistance. They can be applied through immersion or spray methods. Manganese phosphate coatings, on the other hand, are primarily applied via immersion and help prevent corrosion and enhance lubricity. Depending on the material of the component, iron coatings, applied by either immersion or spray coating, may also be required as a base for subsequent coatings.

Evaluating the quality of these coatings requires detailed morphological analysis to see if they completely cover the base material. Uncoated areas as small as even several micrometers can compromise the overall quality of the coating — they can be the starting points for corrosion and reduce lubricity. SEMs make it possible to see these miniscule features and unambiguously assess quality. 

SEMs use backscatter electron (BSE) imaging to capture high-magnification, high-contrast images that show exactly where a coating has or has not been applied. For example, image shows a BSE image of a steel surface with a zinc phosphate coating. The darker areas show the coating, and the brighter areas show where the steel remains uncoated. This clear difference makes it easy to see that, in this case, the coating was poorly applied. Proper coating would cover very close to 100% of the product surface.

Because many production facilities lack in-house SEM analysis capabilities, they often outsource this critical task. The production team extracts a sample from the coating process and sends it to an external testing facility. Then they have to wait a day or more to know if the coating was properly applied.

If the analysis reveals that the coating was improperly applied, all parts produced in the interim are at risk. They would require additional scrutiny, possible re-coating, or even disposal — a significant waste of resources and a major disruption to production schedules.

To address these risks, we developed a workflow for the Thermo Scientific Phenom XL-G2 Desktop SEM that automatically assesses the quality of zinc phosphate coatings. It simplifies the entire analysis procedure. 

After you insert a sample, the Phenom Desktop SEM takes over, automatically aligning the sample and identifying optimal testing locations. Once you confirm the alignment and testing locations, the system automatically acquires and analyzes the images to determine the coverage of the coating. Finally, it generates a report that states if the coating was applied correctly or not. 

This entire process, from acquisition to analysis and results, takes less than 15 minutes — markedly faster than the day or more required when outsourcing. The Phenom Desktop SEM’s automated workflow delivers a clear advantage: definitive data in less time.

Whether you’re applying conversion coatings or producing any other product that relies on microscopic quality, robust quality control can help you mitigate the risks of wasted time and materials. The Phenom XL-G2 Desktop SEM can help you significantly improve your quality checks, minimize time waiting for results, and quickly respond to even the slightest imperfection.