Standardless XRF Measurements of Zirconium and Titanium Based Conversion Coatings

Conversion coatings are coating layers that are created over the surface of a metal substrate either by an electrochemical or chemical process to give a decorative color and to enhance the chemical and physical properties like paint adhesion and corrosion resistance.

Chromate, anodization, or phosphate conversion coatings are common examples. For the pretreatment of car bodies and automotive components made of steel and galvanized steel, zinc phosphate and related compound conversion coatings have long been state-of-the-art.

New coatings based on titanium (Ti) and zirconium (Zr) have been more frequently utilized in the past decade, with the aim of replacing phosphate-based coatings as a pre-treatment layer.1,2 The Zr based and Zr/Ti-based coatings have a number of advantages over zinc and zinc-manganese-nickel-phosphates, these include:1,2

  • Lower operating cost (less waste, lower temperature, less chemical consumption)
  • Lower environmental impact and wastewater treatment (no regulated heavy metals, phosphate-free)
  • Better corrosion resistance (better salt spray resistance)
  • Improved maintenance (less scale, less sludge, low foaming)
  • Thinner coatings

Producers of coating chemicals around the globe have utilized the Thermo Scientific™ Niton™ XL3 handheld XRF analyzer in addition to those at numerous factories in the automotive and general industry.

The Niton XL3 analyzer has supplied quality control of Zr- and Ti-based coatings on substrates like cold-rolled steel (CRS), aluminum alloys, electro-galvanized steel (EG), and hot-dip galvanized steel (HDG).

The new Thermo Scientific™ Niton™ XL5 Plus handheld XRF analyzer will be more powerful for these applications and, thanks to a fundamental parameter algorithm, the workflow for application development and analysis will also be much more simple and flexible.

Zr- and Ti-based conversion coatings improve paint adhesion and enhance corrosion protection of automotive bodies made of aluminum

Zr- and Ti-based conversion coatings improve paint adhesion and enhance the corrosion protection of automotive bodies made of aluminum. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers 

Niton XL5 Plus Analyzer

The Niton XL5 Plus analyzer is the most lightweight and smallest of all the high-performance X-ray fluorescence (XRF) metal analyzers currently on the market.

The large area detector with graphene detector, compact measurement geometry, and powerful 5W X-ray tube supply best-in-class sensitivity for the most demanding applications like the measurement of thin coating layers.

The fundamental parameter-based, standardless Coatings Mode of the Niton XL5 Plus analyzer accurately establishes coating thickness of up to four layers of pure element or alloy coatings over numerous substrates consisting of alloys, pure elements, plastics or wood.3

The Niton XL5 Plus analyzer can measure the coat weight of Ti- and Zr-based conversion coatings easily over alloys like aluminum or steel or coated materials such as galvanized steel as a result.

Users are able to operate the instrument ‘out of the box’ with no requirement for advanced training or multiple calibration samples. For users who wish to fine tune their analyzer with their own reference materials, an easy-to-use type standardization feature is available.

The Niton XL5 Plus Handheld XRF Analyzer

The Niton XL5 Plus Handheld XRF Analyzer. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers 

Test Technique and Results

The Niton XL5 Plus analyzer was employed to analyze a number of previously characterized samples of Zr- and Ti-based conversion coatings over CRS, HDG and aluminum alloys with no previous adjustment.

Only the nature of the substrate (like steel or aluminum grades and coating elements like Zinc (Zn) for galvanized steel, Zr or Ti) in addition to the measurement time and unit of measurement were entered in the measurement profiles (analytical methods).

The correlation plots between laboratory-obtained reference values and measurements performed with a Niton XL5 Plus analyzer for Zr and Ti on different substrates are shown in Figures 1a-d. Ti and Zr are typically present in the coatings at levels equivalent to a few atomic layers.

Correlation curves measured versus reference values obtained without any calibration adjustment for a) Zr over cold rolled steel, b) Zr over hot dip galvanized steel, c) Zr over aluminum (coating # 1), d) Ti over electro-galvanized steel, Ti over aluminum.

Figure 1. Correlation curves measured versus reference values obtained without any calibration adjustment for a) Zr over cold rolled steel, b) Zr over hot dip galvanized steel, c) Zr over aluminum (coating # 1), d) Ti over electro galvanized steel, Ti over aluminum. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers 

Figures 2a-d show the correlation coefficient R2, the slope and the intercept of the linear regression. Where a perfect correlation would have an R2 of 1, the R2 value is a measure of how closely the data sets correlate with each other.

If there is a good correlation, then the slope is an indicator of the accuracy of the analyzer response; the slope is ideally equal or near to 1. High values of intercepts of the calibration curve can be a sign of the presence of the coating element in the substrate.

All of the values measured with the Niton XL5 Plus Coatings Mode ‘out of the box’ had an R2 higher than 0.98. This demonstrates a strong correlation with lab reference values for Ti and Zr over different substrates.

The slopes were also close to 1 for Zr on HDG (Figure 2b) and for Zr and Ti over aluminum (Figures 2c and 2d), with values of approximately 0.93. A slope of 0.804 was gathered for Zr over CRS (Figure 2a).

This departure from the ideal response of 1 can be because elements and compounds other than Zr are present in the coatings and have an effect on the density and mass absorption coefficient of the matrix and so on the Zr signal.

In this instance, a simple post-calibration adjustment can be made for a given formulation to enhance the accuracy by utilizing the type standardization feature. With a value of 9.03 mg/m2, it is worth noting that in the case of Zr over aluminum, the intercept of the linear regression significantly differs from 0, which is due to the presence of Zr in the substrate.

Zr and also Ti are commonly present in aluminum alloys at trace levels, which are not necessarily specified by the alloy grade standard. So, it is enough to measure one coated sample and a non-coated sample for a given batch and formulation of Zr-based conversion coatings and then calculate the difference of the results.

An example where measured coat weight for a Zr-coated aluminum was 23.9 mg/m2 versus an expected value of 15 mg/m2 is shown in Figure 3a. As shown in Figure 3b, the non-coated substrate of the same batch gave a value of 9.0 mg/m2 Zr, which should be taken away from the result of the coated sample.

a) Analytical results for an aluminum alloy grade AA5082 sample coated with 15 mg/m2 Zr, b) Non-coated sample of the same batch of alloy AA5082.

Figure 3. a) Analytical results for an aluminum alloy grade AA5082 sample coated with 15 mg/m2 Zr, b) Non-coated sample of the same batch of alloy AA5082. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers 

The resulting Zr (Zr net) value of 14.9 mg/m2 is extremely near to the expected value of 15 mg/m2. A subtraction such as this can be performed only when the layer is thin, as is the case for Ti- and Zr-based conversion coatings.

Conclusion

The Niton XL5 Plus analyzer is ideally suited for modern conversion coating applications. Good agreements and correlations were gathered between measured and expected values of Ti and Zr across multiple substrates like galvanized steel, steel, or aluminum alloys.

Compared to empirical calibrations which are available on the Niton XL3 analyzer, the fundamental parameter-based Coatings Mode of the Niton XL5 Plus analyzer is more flexible, easier to use and does not need many reference samples.

The accuracy of the standardless calibration can be assessed by utilizing few samples per coating type and, if required, the analytical response of the analyzer can be fine-tuned easily by the user for optimum accuracy.

In the automotive and metal finishing industries, the Niton XL5 Plus analyzer has been shown to be perfect for controlling the coat weight of Ti- and Zr-based coatings.

Quick return on investment because of:

  • Non-destructive analysis. The analyzer moves to the sample so measurements can be performed on finished products directly without the requirement to cut a sample and transport it to the lab.
  • Enhanced productivity. The Niton XL5 Plus analyzer shows the measured coat weight within seconds, in real time, enabling the immediate control of the coating process and quick decision-making in quality control of finished or semi-finished products.
  • The analytical performance matching lab instrumentation with low operational costs and smaller initial investment.
  • Versatility. The Niton XL5 Plus analyzer can be utilized to establish the alloy grade versus specification of non-coated material like aluminum alloys.
  • Ease of use. Technique development and operation of the Niton XL5 Plus analyzer do not need skilled lab personnel.

References and Further Reading

  1. Gardobond® X 4707 Product Info, www.chemetall.com, Chemetall GmbH, Frankfurt am Main, Germany 2012.
  2. I. Milosev, G. S. Frankel, Review—Conversion Coatings Based on Zirconium and/or Titanium, Journal of The Electrochemical Society, 165 (3) C127-C144 (2018).
  3. M. Bauer, Application Note: Measuring Metal Coating Thickness at Line using the Thermo Scientific Niton XL5 Plus, Thermo Fisher Scientific, Tewksbury, MA, USA.

Acknowledgments

Produced from materials originally authored by M. Bauer from Thermo Fisher Scientific.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.

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