Unraveling the Formation Mechanism of a Hybrid Zr-Based Chemical Conversion Coating with Organic and Copper Compounds for Corrosion Inhibition
Authors: Liu Xiaoyang , Vonk Donald , Kisslinger Kim , Tong Xiao , Halada Gary P. , Petrash Stanislas , Foster Kate , Chen-Wiegart Yu-chen Karen Karen 
Published: December 2020

Corrosion protection is of great importance, since corrosion on metals and alloys happens in a wide range of applications and causes economic and environmental issues. Among various corrosion protection methods, chemical conversion coatings applied by a simple solution immersion process on different metals as a pretreatment has great advantages. The technique is easy to operate and economic. The resulting conversion coatings then act as an effective barrier to corrosion, as well as improving the adhesion between the metal/alloy substrate and the subsequent organic coating. As conventional chromate and phosphate conversion coating treatment has health and environmental issues, there is a strong motivation to develop novel environmentally friendly chemical conversion coatings with comparable functionality. Specifically, zirconium-based (Zr-based) conversion coatings created by immersing alloy substrates in hexafluorozirconic acid (H2ZrF6) solution is an outstanding candidate because of its lower operational cost and fast deposition, while offering good anti-corrosion and adhesion properties.1 Prior works of Zr-based conversion coatings have been focusing on further improving their anti-corrosion performance by controlling the coating deposition parameters including temperature, time, pH value and incorporating organic (silane, phosphonic acids) or inorganic (copper, cerium) compounds. In this study, a novel organic-inorganic hybrid Zr-based conversion coating with copper (Cu) and polyamidoamine (PAMAM).2 Incorporating organic component PAMAM has been shown to enhance the corrosion resistance of the conversion coating, as well as to provide binding media for the subsequent paint coating. Despite evidence showing superior performance, the film growth and reaction mechanisms of the hybrid conversion coatings, when incorporating these additives, is not well understood.
Here, we studied the effect of the additive PAMAM concentration in the coating solution on the morphology, structure and chemical composition of the coating by a multimodal X-ray and electron analysis. First, we discovered an inverse relationship between the concentration of PAMAM and the thickness of the coating. A higher PAMAM concentration was also found to reduce the Cu-containing cluster formation. Elemental distribution analysis by Scanning Transmission Electron Microscopy with Energy-dispersive X-ray Spectroscopy revealed different microstructure in the surface cluster formation; Cu-containing nanoparticles formed in coatings without PAMAM additives, whereas a more homogenous distribution of Cu species was found in samples with PAMAM. The atomic ratio of elements characterized by X-ray Photoelectron Spectroscopy (XPS) varied with coating conditions. Notably a higher ratio of organic elements indicated the incorporation of PAMAM into the conversion coating with possibly some further adsorption. High-resolution XPS revealed the existence of amide and amine in the coating and different oxidation states of Cu; the results indicate that the reduction reaction of the Cu ions was suppressed by PAMAM, because the complexation between Cu and PAMAM decreased the reduction potential of Cu. In addition, the corrosion of the hybrid coatings in alkaline environment was characterized by synchrotron X-ray Absorption Near Edge Structure (XANES) Spectroscopy showing a better corrosion resistance of hybrid coating with less dissolution of clusters. Overall, the study discussed the interaction between Cu ions and PAMAM and the subsequent effect on the coating morphology and composition as well as the anti-corrosion property. The work furthers the fundamental understanding on how organic and inorganic additives may synergistically act to enhance the properties of the conversion coating. We intend to shed light on the critical processing-structure-property relationship that need be considered and tailored for applying and designing novel conversion coatings for corrosion protection applications.
Figure caption:
Surface (top row) and cross-sectional morphology (bottom row) of the conversion coatings with 0 to 200 ppm PAMAM concentration in the coating solution. From the surface view, the cluster visually decreases with increase of PAMAM concentration. The flake-like morphology could be observed on sample with higher PAMAM concentration. From the cross-sectional view, the voids and thickness of the coating decreases with increasing PAMAM concentration.
Reference:
(1) Xiaoyang Liu, D. V., Hua Jiang, Kim Kisslinger, Xiao Tong, Mingyuan Ge, Evgeny Nazaretski, Bruce Ravel, Kate Foster, Stanislas Petrash, Yu-chen Karen Chen-Wiegart. Environmentally Friendly Zr-Based Conversion Nanocoatings for Corrosion Inhibition of Metal Surfaces Evaluated by Multimodal X-ray Analysis. ACS Appl. Nano Mater.
2019, DOI: 10.1021/acsanm.8b02309.
(2) Donald Robb Vonk, T. S. S., Alvaro Bobadilla Thin Corrosion Protective Coatings Incorporating Polyamidoamine Polymers. Feb. 8, 2018.

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