Corrosion Resistance of Zinc Alloy Coatings Containing 1-3wt% Mg and Al at Small Radius Bends
Authors: Brooks Matthew Alexander , Penney David , Sullivan James , Malla Amar , Mills Christopher , Challinor Clive 
Published: December 2024

Environmental targets to increase the whole-life efficiency of many products have led to an increased focus on improvements in metallic coating performance, with view to increase durability and minimise consumption of raw materials. Zinc Magnesium Aluminium (ZMA) alloy galvanised coatings have been the focus of numerous research studies over the past two decades. Research has predominantly been based around the improved corrosion resistance of these coatings for automotive and construction products exposed to external environments. A variety of compositions exist, each forming complex microstructures containing primary zinc within eutectics rich in magnesium and aluminium. These compositions generally include, but are not limited to, alloy additions that range between 1 and 2 wt% to zinc. The alloying additions enhance the corrosion resistance and pressing performance. It has been previously identified that limited systematic research has been undertaken to date to understand what influences adverse mechanical behaviours of these microstructures. Studies have shown that eutectic phases are susceptible to cracking on forming. More recent research has attributed the genesis of the cracks, specifically to the twinning locations within the eutectic phase, MgZn2.

This study investigates four Zinc Alloy coatings containing: 1Mg1Al; 3Mg3Al; 1Mg3Al and 3Mg1Al wt% additions. These compositions formed different morphologies, with varying volumes of primary and eutectic phases. All four specimen coatings are approximately 30 microns thick and have been created using identical heat treatment processing parameters and cycle. The galvanic alloy coatings have been mounted upon 0.7mm gauge Interstitial Free (IF) steel substrate. This substrate has been selected as typically used for car body panels in the automotive industry, and cold formed light gauge structural sections in the construction industry. IF steels offer good tensile strength, as well as good formability capability. A Hot Dip Processing Simulator was used to apply the IF steels with the four separate specimen coatings, to isolate variables, such as: substrate grade, substrate thickness, coating thickness, heat treatment cycles and post coating cooling.

Uniaxial tensile tests are governed by the structural substrate mechanical properties. Typically, with elongation limits of approximately 20%. Traditional soft-tempered galvanic coatings can achieve as much as 65% elongation, which is desirable when forming pre-galvanised sheet material. However, bending material presents a stress distribution of both tension and compression. Elongations as much as 100% (theoretical 0T bend) can be induced on the coated material surface, without total structural failure of its substrate. BS EN 13523-7 provides a Standard for testing and specifying a minimum bend radius for coated sheet material. As bend radius and material thickness govern bending tensile strain, bends are categorised relative to material thickness.

A systematic study was undertaken that incrementally induced relatively high levels of strain to the material surface, by bending the pre-coated steel sheet material. The cracked microstructures of the four zinc alloy coating specimens were quantified before accelerated corrosion tests were undertaken. All specimen samples were immersed in 1wt% NaCl electrolyte solution and test data recorded over a 24-hour period, at room temperature. A GAMRY Potentiostat recorded Open Circuit (OCP) and Linear Polarisation Resistance (LPR) data of both strained and unstrained samples and results were compared.

The results showed that for a comparable strain, each of the cracked coating specimens were cathodically polarised. The depths of coating cracks were observed using a scanning electron microscope, during a fracture analysis exercise of cross-sectional images. The cracks were observed to be the full depth of the coating full thickness, exposing the surface of the steel substrate. The characteristics of the coating fractures correspond with the electrochemical test results. The corrosion resistance of these zinc alloy coating specimens was compromised when the coatings cracked on the tension surface at small radius bends, when compared to flat unstrained samples.