How SEM helps perform automated quality control on phosphate coatings

By Karl Kersten - March 16, 2018

We are surrounded by products that, for either decorative or functional purposes, are covered with coatings; from paintings and lacquers, to adhesive or protective coatings, optical, catalytic or insulating coatings. Of all these coatings, conversion phosphate coatings play an important role, especially in the automotive industry: they are used for corrosion resistance and lubricity. Since these coatings are used for critical parts, the coating process must undergo thorough quality checks. These checks consist of the analysis of the morphology of the coating as well as the percentage of coverage. In this blog, we describe and analyze how automated tools combined with SEMs can be helpful in quality checking phosphate coatings.

Conversion phosphate coatings

Coatings are not only used as a decorative feature, such as paint finishes or lacquers; most of the time have a functional purpose. Coatings can:

  • Serve as an adhesive,
  • Have optical, electrical or magnetic properties,
  • Be catalytic or light sensitive, such as those used to make photographic film.

One of the biggest categories is that of protective coatings, ranging from insulation to waterproof and wear resistant to anti-corrosion. Coatings can be applied through chemical vapor deposition, physical vapor deposition, spraying, or chemical and electrochemical techniques, such as electroplating.

Within this wide range of functionalities, materials and coating techniques, we focus our attention on conversion phosphate coatings. These are typically used in the automotive industry and serve as a protection layer on steel parts, preventing corrosion and providing lubricity. The main types of coatings are manganese, iron and zinc. The coating is applied by immersing the part in a bath containing a phosphoric acid, which causes the growth of a crystalline zinc, manganese or iron phosphate layer.

Because of the critical use of the coated parts, the coating process must undergo thorough quality checks to ensure the performance of the coating. The quality check consists of the analysis of the coating morphology and the percentage of coverage. One way to carry out this analysis is by using a scanning electron microscope (SEM).

How SEM helps perform quality control inspections on coating processes

SEM is an ideal choice for quality checking of conversion coatings and for the analysis of the crystal morphology. Moreover, imaging with the BSE detector is the most suitable technique to analyze the coverage of the coated sample because of the difference in atomic number between the phosphate coating and the steel.

Since steel is an alloy of iron, and therefore has a higher atomic number than the phosphate coating, it will appear brighter in the backscattered image. Fig.1 shows a BSE image overlaid with the colored EDX map, where the yellow areas consist of iron and the light blue refers to zinc. The brighter areas of the BSE images overlay with the yellow, demonstrating the effectiveness of using these images for the measurement of coating coverage.

Fig. 1: SEM image in backscatter mode of a steel sample covered with zinc phosphate coating overlaid with a colored EDX map, showing the coated (yellow-iron) areas and the coating (light blue-zinc)

Moreover, at the same time, BSE images also reveal the crystal structure of the coating, enabling the coating morphology to be analyzed. Fig. 2 shows an SEM image of the crystal structure of zinc phosphate coatings.

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Fig. 2: SEM images in backscatter mode of asample covered with zinc phosphate coating, showing the different morphology of the crystal structure.

Why automated quality control is key

In quality control processes, it is ideal to have a system that enables the user to get fast results and avoid long downtime on the production line. In most cases, the analysis of coated samples is done in a laboratory that is located far from the production line, causing unwanted delays in instances of negative feedback. Having a fast and reliable way to check the quality of phosphate on site is important. Desktop SEMs are the ideal choice in this case, having a small footprint and being easy to use.

However, this may be not sufficient. An automated tool for quality checking that does not require a dedicated and experienced user gives more advantages in terms of time saving and the reliability of the results. The Phenom Programming Interface (PPI) is the right platform to implement an automated tool for quality checks.

Fig. 3 shows the process flow of the automated tool designed for the quality checking of the coating coverage. It enables the user to scan a large area of the sample by collecting a set of BSEs images at low magnification and saving them in a selected folder.

These images are then automatically analyzed by applying a threshold on the grey level, effectively separating the coating (appears darker) and the uncovered steel (appears brighter). The average coverage percentage and the statistics of the measurement are then saved in a report that the user can print out later. It is also possible to load a set of images, select the correct threshold, do the analysis, and generate the report.


Fig. 3: Process flow of the automated tool for the quality check of the phosphate coatings coverage.

Automated quality control on coatings in practice

If you’d like to learn more about how SEM helps perform automated quality control on coating processes, our free application note on coating scripts is worth reading. It details how SEM makes for a fast, yet cost-effective instrument for quality control analyses of coatings — and how it helps increase the efficiency and reliability of production processes as a result.

Download the application note here:

   Learn more about automated quality control through SEM

About the author

Karl Kersten is head of the Thermo Scientific Phenom Desktop SEM Application Team at Thermo Fisher Scientific. He is passionate about the Phenom Desktop SEM product and likes converting customer requirements into product or feature specifications so customers can achieve their goals.

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