Flux Composition for Hot-Dip Galvanizing Reactive Steel
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Solution Overview
Problem
Batch hot-dip galvanizing processes face issues with reactive steels, resulting in fragile coatings, excessive zinc consumption, and surface defects due to uneven Si and P distribution, which are not adequately addressed by existing methods, particularly with the use of toxic ZnCl2 in flux solutions that violate environmental regulations.
Innovation Solution
A flux solution with reduced ZnCl2 concentration, containing ZnCl2, NH4Cl, and BiCl3, is used to minimize reactivity in steel pieces before immersion in a Zn-Al alloy bath, maintaining ZnCl2 below environmental limits, with controlled pH and temperature, ensuring adherence and easy removal of salts, allowing for uniform and defect-free coatings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If ZnCl2 is used in flux solution for hot-dip galvanizing, then coating quality and reactivity control are improved, but environmental compliance deteriorates due to toxic ZnCl2 restrictions
Solution Approach 1:
The patent changes the chemical composition parameters of the flux solution by replacing toxic ZnCl2 with non-toxic alternatives (NH4Cl, Zn(NO3)2, ZnSO4) while adjusting concentrations and adding organic additives to maintain coating quality. This allows achieving the same technical effect without the harmful substance.
Solution Approach 2:
The patent extracts and removes the toxic ZnCl2 component from the flux solution formulation, eliminating the harmful factor while retaining the essential fluxing functionality through alternative salt combinations that provide similar chemical effects without environmental toxicity.
2Strength
If Al is added to molten Zn bath to reduce reactivity, then interface strength is improved, but coating defects increase due to not-coated areas
Solution Approach 1:
The patent optimizes the Al concentration parameter in the molten Zn bath, using very low amounts (0.01-0.1 wt%) rather than higher concentrations. This controlled parameter change provides sufficient reactivity control while minimizing the formation of not-coated areas and other defects.
Solution Approach 2:
The patent introduces a specifically formulated flux solution as an intermediary between the steel substrate and the Al-modified Zn bath. This flux layer mediates the interaction, allowing the benefits of Al addition (reactivity control) while preventing the harmful effects (coating defects) through controlled chemical reactions during the galvanizing process.
3Reliability
If flux solution contains high ZnCl2 concentration, then wetting and coating adhesion are improved, but environmental harm increases due to toxic compound presence
Solution Approach 1:
The patent uses a flux solution with a different chemical basis (NH4Cl, Zn(NO3)2, ZnSO4) that can be applied, performs its function during the galvanizing process, and then is easily rinsed away. This disposable approach allows achieving good adhesion during processing while eliminating persistent toxic residues that would remain with ZnCl2.
Solution Approach 2:
The patent extracts the toxic ZnCl2 from the flux solution formulation and replaces it with non-toxic alternative salts. This removal eliminates the harmful factor while the alternative salts maintain the necessary fluxing properties for adequate wetting and coating adhesion.
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 solution enables uniform, defect-free coatings on reactive steels with reduced zinc consumption and improved corrosion resistance, compliant with environmental regulations, by optimizing the flux composition and processing conditions in hot-dip galvanizing.
Implementation Method 1
the selection of the appropriate chemical solution composition together with special operating conditions, guarantee the satisfactory wetting of the steel part during the immersion into the molten Zn-afloy
Implementation Method 2
The salts from the flux solution remain adherent to the steel pieces and have a fusion temperature well below the temperature of the molten galvanizing bath and are therefore easily eliminated upon immersion forming ashes and dross.
Data Source
AI summary
The present invention aims at contributing to the solution of potential problems arising from the presence of ZnCl2 in the galvanizing shop, by means of a procedure for the preparation of surface of pre-fabricated steel pieces to be hot-dip galvanized in a Zn-Al-alloy, according to which the concentration of ZnC12 into the flux solution is maintained below the limit prescribed by the law and preferably down to zero. According to this procedure, the steel pieces, after pickling are immersed into an aqueous solution containing ZnCl2, NH4Cl and Bi2O3. Using the flux solution according to the present invention it is possible to coat in a batch galvanizing bath containing a Zn-Al-alloy, carbon steels and steels with high Si content, and/or Mn, and/or P (i e. steels commercially known as Sandelin or Iper-sandelin steels), usually considered difficult steels for galvanizing. obtaining coatings with uniform thickness, with no surface defects and with a smooth surface without non-homogeneous areas.