Effect of Zinc, Magnesium, and Manganese Phosphate Coatings on the Corrosion Behaviour of Steel

TL;DR

This study compares how well zinc, magnesium, and manganese phosphate coatings protect steel from corrosion in river water and seawater.

Contribution

The study systematically compares the corrosion resistance of three phosphate coatings using identical application methods and real-world environments.

Findings

Zinc phosphate coatings provided the best corrosion protection, significantly reducing corrosion rates in both river water and seawater.

Zinc-phosphated samples showed no major morphological changes and achieved corrosion rates of 0.258 µm/year and 3.060 µm/year in river water and seawater, respectively.

Magnesium phosphate coatings showed the least corrosion resistance among the three tested coatings.

Abstract

This study provides a systematic comparison of three types of phosphate coatings, applied by identical immersion phosphating processes and tested under two different environmental conditions representative of real industrial scenarios. The focus of this study is the investigation of the corrosion behaviour of zinc, magnesium, and manganese phosphate coatings on reinforcing steel in two different corrosion environments: river water and seawater. The phosphate coatings were obtained via the immersion phosphating technique. Various techniques, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), potentiodynamic polarization curve (PDP) testing, and electrochemical impedance spectroscopy (EIS), were used to evaluate the morphology and corrosion resistance of the coatings. The overall corrosion protection performance of the coatings followed the order of Zn phosphate > Mn phosphate > Mg phosphate. The results indicate that the samples with the Zn-phosphated coating showing the highest improvement. This coating showed no major morphological changes and achieved significantly reduced corrosion rates—0.258 µm/year in river water and 3.060 µm/year in seawater—compared to the typical corrosion rate of uncoated steel, which is about 45 µm/year. These findings emphasize the effectiveness of Zn phosphate coatings in mitigating corrosion in both river water and marine conditions.

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