Conversion coatings are coating layers created over the surface of a metal substrate either by a chemical or electro-chemical process to provide a decorative color and to improve physical and chemical properties such as corrosion resistance and paint adhesion.
Common examples are anodization, chromate or phosphate conversion coatings. Zinc phosphate and related compound conversion coatings have long been the state-of-the-art in pre-treatment of car bodies and automotive components made of steel and galvanized steel.
Over the past decade, new coatings based on zirconium (Zr) and titanium (Ti) have been increasingly used with the goal of replacing phosphate-based coatings as a pre-treatment layer.1,2 The Zr based and Zr/Ti based coatings have numerous advantages over zinc- and zinc-manganese-nickel-phosphates such as:1,2
- increased corrosion resistance (better salt spray resistance)
- thinner coatings
- reduced environmental impact and wastewater treatment (phosphate-free, no regulated heavy metals)
- Improved maintenance (less sludge, less scale,low foaming)
- reduced operating cost (lower temperature, less waste and chemical consumption)
When it comes to quality of metal coatings, however, it is important that coating materials be applied in certain thickness specifications in order to express the desired characteristics. ASTM International has outlined standard practices for coatings on various metals. XRF coating thickness measurement method is covered by international standards ASTM B568-98 and ISO3497:2000.
Metals that are either over- or under-plated won’t perform as expected. Imagine that a wrong or insufficient amount of metal coating was used on automotive parts to reduce corrosion or stress. The results could be catastrophic. Over-plating also wastes expensive coating material. A thin layer multiplied by coating area and millions of parts adds up to a lot of profit being lost.
Thus, it is extremely important that when analyzing metals to help ensure the coatings meet specifications, it is crucial to analyze the substrates. One of the technologies used in the automotive industry to do an elemental analysis of the metals is portable X-ray fluorescence (XRF), which is non-destructive, so it is useful when one cannot see the multiple layers to ensure that all the plating is within specification.
XRF measures the fluorescent (or secondary) X-ray emitted from a sample when it is excited by a primary X-ray source. The strength of the signal can be used to determine the thickness of the coating; a second reading can determine the thickness of the substrate.
Some of the latest XRF analyzers use advanced algorithms based on physical constants (fundamental parameters) and can measure without need of additional calibration up to 4 layers atop a substrate, in which the layers and the substrate may be defined to consist of a single element or a multi-element alloy. The analyzer can be used for the measurement of a specified set of layered coatings’ thicknesses (or coating weights) on a specified substrate, for an identified metal or alloy in each layer.
Portable XRF technology can be used anywhere in the supply chain, from incoming inspection to confirmatory testing of completed subassembly parts, to final product quality control.
Zr- and Ti-based conversion coatings improve paint adhesion and enhance corrosion protection of automotive bodies made of aluminum – but they must be applied in specific amounts to customer requirements. Accurate analysis of the coatings on the production floor can help ensure that the base metal is protected, along with company profits.
Additional Resources and References:
- Application Note: Standardless Measurements of Titanium and Zirconium Based Conversion Coatings
- Application Note: Measuring Metal Coating Thickness at Line using the Thermo Scientific Niton XL5 Plus
- Gardobond® X 4707 Product Info, www.chemetall.com, Chemetall GmbH, Frankfurt am Main, Germany 2012.
- I. Milosev, G. S. Frankel, Review—Conversion Coatings Based on Zirconium and/or Titanium, Journal of The Electrochemical Society, 165 (3) C127-C144 (2018).
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