Ultra-Fast Zinc Phosphate Coatings Obtained By Pulsed Electrochemical Deposition
Authors: Lissandrello Federico , Magagnin Luca 
Published: November 2022

Phosphating is one of the most common conversion coatings for steels and other ferrous alloys. Owing it to the process simplicity and unique morphology of the coatings, this treatment has cemented itself in a wide variety of applications, ranging from painting to corrosion resistance and cold working operations. In recent years, the need for more sustainable technologies has however driven the research for new alternatives to phosphating. One of the main drawbacks of this process is in fact the formation of a sludge byproduct composed mainly of phosphates of heavy metals, which must be continuously filtered off from the deposition tanks and disposed properly. These operations take a big toll on the environmental burden of the process and are the reason why an alternative will eventually become a necessity.
One possible solution to this problem is to study conversion coatings with a different chemical nature. Formulations based on vanadium [1] and zirconium [2] have already been proposed as an alternative to phosphating, showing good results in terms of corrosion protection and coating adhesion; however it’s still unclear whether their application at the industrial scale would have a lower environmental impact than phosphating.
A completely opposite solution is instead to keep the same, well-known chemistry of phosphating baths and change the method of application. In electrolytic zinc phosphating the coating process is carried out in an electrochemical cell, where the conditions required for phosphate precipitation are promoted by the application of a cathodic potential. In this scenario, substrate dissolution is prevented thanks to the cathodic polarization applied and the formation of the sludge byproduct is completely avoided. [3]
By applying a cathodic polarization, hydrogen evolution is promoted at the substrate-electrolyte interface, leading to a local increase in pH, which triggers zinc phosphate precipitation. Therefore, the deposition rate in electrolytic phosphating is related to the speed at which the ideal pH conditions are created. In principle, it’s therefore possible to increase the current density to enhance the deposition rate, similarly to common electroplating systems. In practice, a current density too high, would lead to accumulation of hydrogen bubbles on the substrate surface and growth of dendritic metallic zinc, which would drastically reduce the quality of the final coating. [4]
To overcome these limitations, in this work we demonstrate the use of pulsed current deposition as a mean to achieve higher coating weights in electrolytic zinc phosphating on mild steel substrates. The high current density applied during the impulse promotes the formation of the proper pH conditions for phosphate precipitation to occur, while the off-time allows for hydrogen bubbles to leave the surface, improving the overall coating quality. To highlight the effects of the impulse parameters on the final coatings, surface characterization is carried out with multiple techniques, such as SEM imaging, EDS mapping, XRD and GDOES. Moreover, corrosion resistance of the zinc phosphate coatings is evaluated with both destructive and non-destructive techniques in a NaCl 3.5% environment.

Bibliography

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[2] A. Ghanbari and M. M. Attar, “Surface free energy characterization and adhesion performance of mild steel treated based on zirconium conversion coating: A comparative study,” Surf. Coatings Technol., vol. 246, pp. 26–33, May 2014.
[3] S. Jegannathan, T. S. N. Sankara Narayanan, K. Ravichandran, and S. Rajeswari, “Performance of zinc phosphate coatings obtained by cathodic electrochemical treatment in accelerated corrosion tests,” Electrochim. Acta, vol. 51, no. 2, pp. 247–256, Oct. 2005.
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