Sacrificial Dissolution of Zinc Electroplated and Cold Galvanized Coated Steel in Saline and Soil Environments: A Comparison.

Ameeq Farooq,Kotiba Hamad,Ahsan Saleem,Kashif Mairaj Deen,Rafiq Ahmad,Umer Masood Chaudry

doi:10.3390/ma14040744

Ameeq Farooq, Kotiba Hamad + Show 4 more

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https://doi.org/10.3390/ma14040744

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Abstract

To protect steel structures, zinc coatings are mostly used as a sacrificial barrier. This research aims to estimate the dissolution tendency of the electroplated and zinc-rich cold galvanized (ZRCG) coatings of a controlled thickness (35 ± 1 μm) applied via brush and dip coating methods on the mild steel. To assess the corrosion behavior of these coated samples in 3.5% NaCl and 10% NaCl containing soil solutions, open circuit potential (OCP), cyclic polarization (CP), and electrochemical impedance spectroscopy (EIS) tests were performed. The more negative OCP and appreciably large corrosion rate of the electroplated and ZRCG coated samples in 3.5% NaCl solution highlighted the preferential dissolution of Zn coatings. However, in saline soil solution, the relatively positive OCP (>−850 mV vs. Cu/CuSO4) and lower corrosion rate of the electroplated and ZRCG coatings compared to the uncoated steel sample indicated their incapacity to protect the steel substrate. The CP scans of the zinc electroplated samples showed a positive hysteresis loop after 24 h of exposure in 3.5% NaCl and saline soil solutions attributing to the localized dissolution of the coating. Similarly, the appreciable decrease in the charge transfer resistance of the electroplated samples after 24 h of exposure corresponded to their accelerated dissolution. Compared to the localized dissolution of the electroplated and brush-coated samples, the dip-coated ZRCG samples exhibited uniform dissolution during the extended exposure (500 h) salt spray test.

Highlights

Coatings are widely used to protect metals from corrosion, but their performance depends on how much they physically restrict the approach of water and oxygen to the coating/substrate interface [1]
In contrast to electroplated Zn coating, the controlled dissolution tendency of the zinc-rich cold galvanized (ZRCG) samples and their larger stability in both 3.5% NaCl and soil environment was related to their composite structure
Compared to ZRCG coated samples, the very high dissolution rate of the Zn-electroplated steel sample (4.72 mm/year) in 3.5% NaCl solution corresponded to its limited life and short term protection of the steel substrate

Summary

Introduction

Coatings are widely used to protect metals from corrosion, but their performance depends on how much they physically restrict the approach of water and oxygen to the coating/substrate interface [1]. Coating experts say that even an excellent coating has internal defects and that any localized mechanical damage of the coating may deteriorate the integrity of the structure [2]. For the protection of a steel structure, the application of coatings has a dual character, i.e., sacrificial nature and barrier characteristics are considered important. Zinc-based galvanic coatings can fulfill the requirement by providing good sacrificial protection in the initial stage of service life. When the zinc-based coatings are exposed to aqueous solutions for an extended period, these show good barrier characteristics due to the formation of insoluble species on the surface that are alkaline, e.g., Zn(OH)2 [3]. Under an acidic and saline environment, these coating may deteriorate rapidly and their protection capability is limited

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