Zn-Mg Coated Substrate Resisting Liquid Metal Embrittlement
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Solution Overview
Problem
Zinc-based coated steel sheets experience liquid metal embrittlement (LME) and reduced corrosion resistance when subjected to high-temperature processes like press hardening or welding, leading to cracks and decreased mechanical properties.
Innovation Solution
A coated metallic substrate with a specific composition and structure, comprising a first aluminum coating with a thickness between 2 and 4µm, a second coating with 0.5 to 5.9% magnesium and the balance zinc, and an optional intermediate layer, is developed to provide enhanced barrier and sacrificial cathodic protection while resisting LME.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high Mg content (10-15% by weight) is used in Zn-Mg coating to improve corrosion resistance, then sacrificial cathodic protection is enhanced, but surface defects occur and Mg oxide forms on the coating surface reducing corrosion resistance
Solution Approach 1:
The patent changes the Mg content parameter from the conventional high range (10-15% by weight) to an optimized range (0.5-5.9% by weight). This parameter optimization prevents Mg oxide formation and surface defects while maintaining adequate sacrificial cathodic protection, resolving the contradiction between corrosion resistance and surface quality.
Solution Approach 2:
The patent creates a composite coating system consisting of an Al-Si alloy film layer (0.1-1.0μm thickness) combined with a Zn-Mg alloy film layer (0.5-10μm thickness with 0.5-5.9% Mg). This composite structure provides both barrier protection from the Al-Si layer and sacrificial protection from the Zn-Mg layer, achieving comprehensive corrosion resistance without the drawbacks of high Mg content alone.
2Reliability
If Al-Si alloy film with thickness 0.1-1.0μm and Zn-Mg alloy film with 10-15% Mg and thickness 0.5-10μm are combined to improve corrosion resistance, then barrier effect is enhanced, but sufficient protection against corrosion is not achieved and LME issues persist
Solution Approach 1:
The patent optimizes the Mg content parameter to 0.5-5.9% by weight (reducing from 10-15%), which fundamentally changes the coating's interaction with molten metal during heating processes. This parameter change eliminates LME while maintaining adequate sacrificial protection, resolving the contradiction between corrosion protection and LME resistance.
Solution Approach 2:
The patent employs a composite coating system with Al-Si alloy film (0.1-1.0μm) and optimized Zn-Mg alloy film (0.5-10μm with 0.5-5.9% Mg). The synergistic combination provides robust barrier protection from the Al-Si layer and controlled sacrificial protection from the Zn-Mg layer, achieving comprehensive corrosion resistance without LME issues.
3Reliability
If zinc based coatings are used to provide cathodic protection, then sacrificial protection is achieved, but liquid metal embrittlement occurs during high temperature processes leading to cracks
Solution Approach 1:
The patent modifies the Mg content parameter in the Zn-Mg alloy coating to 0.5-5.9% by weight. This optimized parameter range maintains the coating's sacrificial cathodic protection capability while preventing LME during high-temperature processes, thereby preserving the mechanical properties and eliminating cracks.
Solution Approach 2:
The patent creates a composite coating system combining Al-Si alloy film and optimized Zn-Mg alloy film. The Al-Si layer provides barrier protection that prevents corrosive atmosphere contact, while the Zn-Mg layer with optimized Mg content provides sacrificial cathodic protection without causing LME, thus maintaining mechanical strength during high-temperature processing.
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 coated metallic substrate achieves high resistance to liquid metal embrittlement and improved corrosion protection, maintaining mechanical properties and preventing surface defects associated with high Mg content.
Implementation Method 1
The barrier effect is obtained by the application of a metallic coating on steel surface. Thus, the metallic coating prevents the contact between steel and corrosive atmosphere.
Implementation Method 2
sacrificial cathodic protection is based on the fact that zinc is a metal less noble that steel. Thus, if corrosion occurs, zinc is consumed preferentially to steel.
Implementation Method 3
These cracks appear with the following conditions: high temperature; contact with a liquid metal having a low melting point (such as zinc) in addition to stress; heterogeneous diffusion of molten metal with substrate grain bulk and boundary.
Data Source
Figure 1~2
AI summary
The present invention relates to a coated metallic substrate comprising at least a first coating consisting of aluminum, such first coating having a thickness below 5µm and being directly topped by a second coating comprising from 0.5 to 5.9% by weight of magnesium, the balance being zinc.