Iron-Tin Alloy Coating Process for Packaging Steel
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Solution Overview
Problem
The increasing cost of tin and environmental concerns related to hexavalent chromium in conventional tinplate and Tin Free Steel (TFS) coatings necessitate the development of alternative metal coatings for packaging steels that maintain corrosion resistance and adhesion to organic coatings while reducing tin usage and eliminating hexavalent chromium.
Innovation Solution
A process involving diffusion annealing of an electrodeposited tin layer at temperatures above 513°C to form a robust, stable iron-tin alloy layer with at least 80 weight percent FeSn on recrystallization annealed steel substrates, which is then rapidly cooled in a non-oxidizing atmosphere, creating a substrate with enhanced adhesion and corrosion resistance without using hexavalent chromium.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the tin coating weight of conventional tinplate is reduced below approximately 1 g/m2, then the cost and resource consumption are reduced, but the corrosion resistance and welding range deteriorate
Solution Approach 1:
The patent changes the physical state and microstructure by creating a diffusion alloy layer through thermal diffusion processing. The alloy layer forms a metallic bond structure between iron substrate and tin coating, transforming the coating from a simple tin layer to a complex iron-tin intermetallic structure that enhances corrosion resistance
Solution Approach 2:
The patent creates a composite coating structure consisting of an iron-tin alloy layer with specific intermetallic phases (FeSn, FeSn2) combined with the iron substrate. This composite material approach provides superior corrosion resistance compared to pure tin coating at equivalent or lower tin weights
2Reliability
If thin nickel coatings are applied prior to tinplating to retain corrosion resistance at low tin weights, then corrosion resistance is maintained, but adhesion to organic coatings and retention of coating integrity at temperatures exceeding the melting point of tin becomes insufficient
Solution Approach 1:
The patent changes the surface chemistry parameters by forming an iron-tin alloy layer with controlled Sn content (1-98 wt%) and specific intermetallic phase distribution. This creates a surface composition that provides both corrosion resistance and organic coating adhesion without requiring nickel intermediate layers
Solution Approach 2:
The patent replaces the expensive nickel intermediate coating with a cost-effective iron-tin alloy layer that achieves the same functional benefits. The alloy layer serves as both the corrosion barrier and the adhesion promoter, eliminating the need for additional nickel material and processing steps
3Strength
If conventional TFS is used to achieve adhesion to organic coatings, then adhesion is excellent, but hexavalent chromium must be used which is hazardous to the environment and worker safety
Solution Approach 1:
The patent extracts and eliminates the hazardous hexavalent chromium component from the coating system while retaining the essential adhesion function through an iron-tin alloy layer. The alloy layer provides adhesion to organic coatings without requiring chromate conversion coatings
Solution Approach 2:
The patent converts the potentially harmful interaction between tin and organic coatings at high temperatures into a benefit by forming a stable iron-tin alloy layer that resists oxidation and maintains coating integrity up to and beyond the melting point of tin, eliminating the need for hexavalent chromium protection
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 resulting iron-tin alloy layer provides excellent corrosion resistance and adhesion to organic coatings, maintaining performance at elevated temperatures, and allows for heat-resistant welding, while significantly reducing tin usage and eliminating the need for hexavalent chromium, making it a sustainable alternative to conventional tinplate and TFS.
Implementation Method 1
diffusion annealing of an electrodeposited tin layer at temperatures above 513°C to form a robust, stable iron-tin alloy layer
Implementation Method 2
Diffusion annealing the blackplate substrate provided with said tin layer in a reducing gas atmosphere to an annealing temperature (Ta) of at least 513°C for an annealing time (ta) sufficient to convert said tin layer into an iron-tin alloy layer or layers
Implementation Method 3
rapidly cooled in a non-oxidizing atmosphere
Implementation Method 4
cooling in a non-oxidising cooling medium, while keeping the coated substrate in an reducing or inert gas atmosphere prior to cooling, so as to obtain a robust, stable surface oxide
Data Source
AI summary
A production process for producing an iron-tin alloy layer on a packaging steel substrate and to a substrate provided with said layer wherein one or both sides of a SR- or DR-blackplate substrate is coated with an iron-tin alloy layer which contains at least 80 weight percent (wt. %) of FeSn (50 at. % tin and 50 at. % iron).