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New insights into corrosion initiation and propagation in a hot-dip Al-Zn-Mg-Si alloy coating via multiscale analytical microscopy.

1. Introduction

Corrosion of coated steel in coastal environments is a significant challenge in materials engineering, affecting structural integrity and longevity. Pre-painted hot-dip Zn-55Al-2Mg-1.5Si coated steel has been widely used due to its superior corrosion resistance; however, the underlying mechanisms governing its degradation remain inadequately understood. This study utilizes multiscale analytical microscopy to provide novel mechanistic insights into the corrosion initiation and propagation processes in this alloy system. By integrating electrochemical analysis with microstructural observations, this research elucidates how both chemical composition and phase morphology influence corrosion behavior.

2. The Role of Phase Morphology in Corrosion Initiation

Traditional corrosion studies primarily focus on the electrochemical properties of alloy phases, but this research highlights the critical role of phase morphology in corrosion initiation. Sub-micron Zn particles embedded within the Zn-Al binary eutectic structure are identified as the initial sites of corrosion due to their high dissolution propensity. The morphology and distribution of these Zn particles determine the localized electrochemical environment, influencing the onset of material degradation. Understanding this interplay between microstructure and electrochemical activity provides valuable insights into designing corrosion-resistant coatings.

3. Corrosion Propagation Mechanisms in Zn-55Al-2Mg-1.5Si Coatings

Following the initial dissolution of Zn particles, corrosion propagates through multiple pathways, including the dissolution of larger Zn-rich regions and the selective leaching of Mg from MgZn₂ and Mg₂Si intermetallic phases. This stage represents a transition from localized to more widespread material degradation. The study’s findings emphasize that Mg dealloying accelerates corrosion by disrupting the phase equilibrium, thereby creating localized anodic sites that promote further dissolution. These insights are crucial for predicting long-term performance and improving alloy formulations for enhanced durability.

4. Degradation of Al-Rich Dendrites and Corrosion Product Formation

In the later stages of corrosion, primary Al-rich dendrites undergo dissolution, leading to structural instability and volumetric expansion in the corrosion products. The transformation of Al-rich phases into corrosion byproducts affects the mechanical properties of the coating, contributing to crack formation and delamination. This phase-specific degradation mechanism highlights the limitations of Al-rich coatings in prolonged coastal exposure and suggests avenues for modifying the microstructure to enhance corrosion resistance.

5. Multiscale Analytical Microscopy for Corrosion Analysis

The study employs an advanced multiscale analytical microscopy approach, integrating scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS) to characterize corrosion progression at multiple length scales. These techniques provide high-resolution imaging and elemental mapping, allowing researchers to correlate phase-specific corrosion behaviors with microstructural features. This methodology establishes a comprehensive framework for studying complex corrosion systems and developing more resilient coatings.

6. Implications for Alloy Design and Future Research Directions

The findings from this research have significant implications for the design of next-generation corrosion-resistant coatings. By understanding the role of phase morphology in corrosion initiation and propagation, material scientists can develop alloy compositions with optimized microstructures to minimize early-stage degradation. Future research should focus on tailoring phase distributions, introducing nano-scale reinforcements, and exploring advanced surface treatments to enhance durability in aggressive environments. Additionally, in-situ electrochemical microscopy could provide real-time insights into dynamic corrosion processes, further advancing the field.

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