Zn-Al-Mg Plated Steel Microstructure for Corrosion and Crack Resistance
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Solution Overview
Problem
Conventional plated steels face issues with crack resistance and corrosion resistance at worked parts due to the formation of coarse primary Zn phases and insufficient eutectic phases, especially when magnesium content is increased to enhance corrosion resistance, leading to oxidation and operational difficulties.
Innovation Solution
A method involving a plating process with a steel substrate in a bath containing aluminum, magnesium, and zinc, with a controlled cooling rate to form a plating layer comprising 1 to 3wt% aluminum, 1 to 2wt% magnesium, and the balance zinc, where the primary Zn phase is structured as a multi-layer form, ensuring a weight ratio of aluminum to magnesium is 1.2 or greater, and the layer thickness is between 15µm to 50µm.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the magnesium content is increased to enhance corrosion resistance, then the sacrificial anode efficiency is improved, but oxidation occurs due to reaction with oxygen in the air, resulting in deterioration of surface quality and operational difficulties
Solution Approach 1:
An aluminum-containing intermediate layer is introduced between the steel substrate and the magnesium-containing zinc plating layer. This intermediate layer acts as a barrier that prevents oxygen from reaching the magnesium, thereby preventing oxidation while allowing the magnesium to provide its corrosion-resistant function. The aluminum layer serves as a protective intermediary that resolves the contradiction between enhancing corrosion resistance and preventing oxidation.
Solution Approach 2:
The invention creates a composite plating structure consisting of multiple layers with different compositions and functions. The plating layer contains zinc as the base metal, aluminum to prevent oxidation and control crystal growth, and magnesium to enhance sacrificial anode efficiency. This composite material approach allows each element to contribute its specific properties while mitigating the drawbacks of individual components.
2Object-affected harmful factors
If the amount of magnesium is reduced to minimize oxidation problems, then the surface quality improves, but the primary Zn phase becomes coarse and the eutectic phase containing magnesium decreases, making it difficult to achieve high corrosion resistance
Solution Approach 1:
The invention applies local quality by creating distinct layers with different compositions and functions. The aluminum-containing intermediate layer locally provides oxidation protection and controls crystal growth, while the magnesium-containing outer layer locally provides enhanced sacrificial anode efficiency. This local differentiation allows each region of the plating to optimize its specific function without compromising the overall performance.
Solution Approach 2:
The plating structure is segmented into multiple functional layers: an aluminum-containing intermediate layer and a magnesium-containing outer layer. This segmentation separates the functions of oxidation prevention and corrosion resistance enhancement into distinct regions, allowing each layer to be optimized independently while working together to solve the technical contradiction.
3Reliability
If the eutectic phase containing magnesium is increased to improve corrosion resistance, then the sacrificial anode efficiency is enhanced, but the plating layer develops cracks during forming because the eutectic phase has high hardness and acts as initiation points for cracks
Solution Approach 1:
The invention changes the parameters of the plating process, specifically controlling the cooling rate to 10 to 30°C/sec and maintaining an aluminum-to-magnesium weight ratio of 1.2 or greater. These parameter changes control the solidification process to form a multi-layer primary Zn phase structure that reduces crack initiation while maintaining the beneficial eutectic phase for corrosion resistance.
Solution Approach 2:
The aluminum-containing intermediate layer is formed first as a preliminary action before applying the magnesium-containing plating layer. This preliminary layer establishes a controlled solidification structure that prevents crack formation during subsequent forming operations, while still allowing the outer magnesium layer to provide enhanced corrosion resistance.
4Object-affected harmful factors
If a deoxidizing chamber and inert gas are used to minimize oxygen contact during the wiping process, then oxidation is reduced, but additional processes are required and operation becomes more complex and difficult
Solution Approach 1:
The aluminum in the plating composition serves a dual function: it provides the desired plating properties and simultaneously acts as a self-contained deoxidizing agent that prevents oxidation of magnesium. This self-service approach eliminates the need for separate deoxidizing chambers and inert gas systems, simplifying the overall process while maintaining protection against oxidation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method results in a plated steel with excellent corrosion resistance and crack resistance at worked parts, demonstrated by a red rust occurrence time of at least 1100 hours in a salt spray test and no visible cracks after a 3T bending test, with a multi-layer primary Zn phase structure.
Implementation Method 1
during solidification of the plating layer, the primary Zn phase that forms is relatively coarse in shape
Implementation Method 2
cooling the plating layer at a cooling rate of 10 to 30°C/sec; wherein the weight ratio of aluminum to magnesium in the plating layer is 1.2 or greater
Implementation Method 3
an increase in the magnesium (Mg) content leads to the formation of oxides due to a reaction with oxygen in the air
Implementation Method 4
When hot-dip galvanized steel is exposed to a corrosive environment, zinc (Zn) acts as a sacrificial anode for any exposed steel areas, resulting in the loss of zinc from the plating layer
Data Source
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AI summary
The plated steel of the present invention comprises a steel substrate; and a plating layer formed on the steel substrate, the plating layer consisting of 1 to 3wt% aluminum (Al), 1 to 2wt% magnesium (Mg), and the balance being zinc (Zn) and unavoidable impurities, wherein the weight ratio of aluminum to magnesium in the plating layer is 1.2 or greater, wherein the plating layer comprises a primary Zn phase structure and an eutectic phase structure, and the primary Zn phase structure has a multi-layer structure in the thickness direction of the plating layer, wherein the area fraction of the primary Zn phase structure having a multi-layer structure in the plating layer is 30% or greater, and wherein the thickness of the plating layer is in a range of 15µm to 50µm.