Corrosion Resistant Phosphate Conversion Coatings Analysis

By Matt Jobbins, 4 minutes to read
Industry: Technology:

Rusting of Automobile Frames & Undercarriage

For over a decade the automotive industry has been facing a billion-dollar problem of undercarriage corrosion or frame rot. Frame rot occurs when the engine cradle, chassis, and/or frame of an automobile begins to rust over time. Frame rot occurs naturally over time and can be accelerated depending on the climate or humidity of the region. The automobile industry takes preventive measures during the manufacturing process by coating these parts with a phosphate conversion coating.

truck rot example corrosion damage
Fig. 1 – Corrosion of automobile undercarriage commonly known as frame rot in the automobile industry.

Corrosion Resistant Phosphate Conversion Coatings Analysis

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.

Scanning Electron Microscopy Analysis of Phosphate Conversion Coatings

The Phenom desktop SEM employs unique attributes that assist in the analysis of phosphate conversion coatings including:

  • Back-scatter electron detection coupled with automated image acquisition allows for extremely 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 image of Zn3(PO4)2 conversion coating
Fig. 2 – BSE image of Zn3(PO4)2 conversion coating.
SE map analysis of a Zn3(PO4)2 conversion coating
Fig. 3 – SE image of Zn3(PO4)2 conversion coating.

Automated Image Acquisition and Analysis with the Phenom Scanning Electron Microscope (SEM)

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
Fig. 4 – Quantitative image processing to measure zinc phosphate coating coverage.
False color overlay highlighting exposed iron.
Fig. 5 – False color overlay highlighting exposed iron.
Ten images acquired at random to quantitatively analyze the Zn3(PO4)2 conversion coating coverage
Fig. 6 – Ten images acquired at random to quantitatively analyze the Zn3(PO4)2 conversion 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.

Acquiring these results over an equivalent 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 accommodate up to 36 samples allowing for extremely fast batch acquisitions.

Determination of Elemental Composition of Phosphate Conversion Coatings 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.
Fig. 7 – SED map analysis of a Zn3(PO4)2 conversion coating.
EDS map analysis of a Zn3(PO4)2 conversion coating.
Fig. 8 – EDS map analysis with color coated element identification of Zn3(PO4)2 conversion coating.


On-Demand Demo – Coating Coverage Analysis from Nanoscience Instruments on Vimeo.

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