Phosphate Conversion Coatings

Phosphate conversion coatings are crystalline coatings applied to steel in order to prevent corrosion and promote the adhesion of paints. Frequently used in the automotive industry, these coatings are important as the first line of defense in the prevention of rust on critical structural components of automobiles including chassis and engine cradles. Scanning Electron Microscopes (SEM) and Energy Dispersive X-ray spectroscopy (EDS) are invaluable tools for providing high resolution images of the crystalline structure of these materials, as well as determining the coverage and quality of these coatings.

The Phenom Desktop SEM employs many unique attributes that assist in the analysis of these coatings:

  • Back-scatter electron detection coupled with automated image acquisition allows for extremeily high-throughput data acquisition and quantitative post-processing based on the BSE signal.
  • The high-brightness CeB6 source produces unparalleled image quality while maintaining an extremely long lifetime.
  • The integrated EDS detector with mapping allows for the quick determination of coating quality with zero ambiguity.

Two types of detectors are routinely used to generate SEM images: back-scatter electron (BSE) detectors and secondary electron (SE) detectors. The key benefit of BSE detectors for this application is that voids in the coating are clearly visible, since the image contrast is sensitive to the elemental difference between the phosphate conversion coating and the underlying steel.

BSE and SE images of a Zn3(PO4)2 conversion coating


Automated Image Acquisition and Analysis

The Phenom SEM can be programmed to randomly acquire images and automatically quantify coating coverage based on the gray level difference seen in the BSE image. Programmable acquisition minimizes user bias and allows for significantly more area to be measured in much less time.

Quantitative image processing to measure zinc phosphate coating coverage


A custom script was written to acquire ten images at random and then perform a threshold analysis to quantitatively determine the Zn3(PO4)2 conversion coating coverage in this example. Coating coverage was measured over 1.4 mm2 with a 350 nm pixel resolution. The random nature of the acquisition and the consistent application of the brightness threshold create data with no user bias.

Void Count Average Void Size (px) % Coating Coverage
Image 1 975 11.942 92.106
Image 2 1011 11.788 91.92
Image 3 1032 12.689 91.122
Image 4 1035 11.988 91.588
Image 5 1037 12.122 91.477
Image 6 1002 12.303 91.642
Image 7 1006 13.296 90.932
Image 8 1056 11.554 91.728
Image 9 1030 12.515 91.261
Image 10 1040 11.929 91.589
Average Coverage 91.54%
Image Pixel Size 350 nm
Area Measured 1.4 mm2
Acquisition Time 60 s

Acquiring these results over an equilivant area with a conventional SEM would require approximately 30 minutes of effort. With the Phenom XL the whole process can be automated in under 2 minutes. Further, the large sample holder of the XL can accomidate up to 36 samples allowing for extremely fast batch acquisitions.

Determination of Elemental Composition by EDS

In addition to quantitative coverage mapping by BSE signal, it is also possible to measure coverage and spatially resolved elemental composition using energy dispersive X-ray spectroscopy (EDS). EDS is also extremely valuable for determining the chemical composition of contaminants on the surface of a material.

EDS map analysis of a Zn3(PO4)2 conversion coating


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