Selective Zinc Phosphation of Aluminum-Zinc Composites
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Solution Overview
Problem
Existing methods for anticorrosive treatment of composite metal constructions with aluminum, zinc, and iron surfaces face challenges in preventing the formation of phosphate crystal clusters on aluminum surfaces, specks on zinc surfaces, and cryolite precipitation, which can lead to corrosion and paint faults, while also dealing with the poisoning of the phosphation bath by titanium, zirconium, and hafnium.
Innovation Solution
A multistage method involving a zinc phosphation composition with controlled fluoride content, boron concentration, and pH levels to selectively form a crystalline zinc phosphate layer on zinc and iron surfaces without depositing significant amounts on aluminum, followed by an acidic passivation step to create a homogeneous passive layer on aluminum surfaces, while minimizing cryolite precipitation and bath poisoning.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional phosphation solutions are used to treat composite metal constructions, then phosphate layers form on all metal surfaces, but this causes unwanted phosphate crystal clusters on aluminum surfaces and specks on zinc surfaces
Solution Approach 1:
The patent modifies the chemical parameters of the phosphation solution by adding water-soluble inorganic boron compounds (0.01-5 g/l) and controlling fluoride content (5-200 mg/l), free acid (0.6-3.0 points), and temperature (20-65°C) to achieve selective phosphation that prevents crystal cluster and speck formation while maintaining phosphate layer deposition on steel and galvanized surfaces
Solution Approach 2:
Boron compounds act as an intermediary substance in the phosphation solution that modifies the chemical behavior of the bath, preventing unwanted phosphate deposition on aluminum surfaces and speck formation on zinc surfaces while allowing selective phosphation of steel and galvanized steel surfaces
2Productivity
If free fluoride content is increased to improve phosphation efficiency, then phosphate layer formation is enhanced, but cryolite precipitation on aluminum surfaces increases
Solution Approach 1:
The patent optimizes the free fluoride content to a specific range (5-200 mg/l) and combines it with boron compounds and controlled pH (0.6-3.0 points) to maintain adequate phosphation rate while preventing cryolite precipitation on aluminum surfaces through modified solution chemistry
3Ease of manufacture
If phosphation solution is used repeatedly, then process economics improve, but the bath becomes poisoned by titanium, zirconium, and hafnium
Solution Approach 1:
Boron compounds serve as protective intermediaries in the phosphation bath that reduce the poisoning effect of titanium, zirconium, and hafnium, allowing the bath to be reused longer while maintaining consistent phosphation performance and selectivity
4Area of stationary object
If aluminum surfaces are phosphated along with zinc and iron, then complete surface coverage is achieved, but the aluminum surface is no longer available for subsequent passivation treatment
Solution Approach 1:
The phosphation solution is designed to exhibit different chemical behavior at different metal surfaces through the addition of boron compounds and controlled fluoride content, achieving local selectivity where steel and galvanized surfaces are phosphated while aluminum surfaces remain unaffected and available for subsequent passivation
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
This approach effectively prevents the formation of unwanted phosphate layers and specks, reduces corrosion, and improves paint adhesion and durability, while maintaining process economics and controlling bath parameters for enhanced selectivity and reduced contamination.
Implementation Method 1
selective phosphation of the zinc and iron surfaces of the composite metal construction without deposition of significant amounts of zinc phosphate on the aluminum surfaces
Implementation Method 2
the aluminum surface is available, in a subsequent method step, for passivation with, for example, conventional acidic and optionally silane-containing passivation compositions that generate a homogeneous, thin passive layer that provides protection from corrosion
Implementation Method 3
cryolite precipitation on the surface of the composite metal constructions, especially on the aluminum parts thereof
Data Source
AI summary
The present disclosure relates to a method of chemical pretreatment and selective phosphation of a composite metal construction comprising at least a portion made of aluminum and at least a portion made of zinc and optionally a further portion made of iron, which includes(I) treating the composite metal construction with an aqueous zinc phosphation composition that results in the formation of a surface-covering crystalline zinc phosphate layer and then—with an intervening water rinse operation—(II) applying an aqueous acidic passivation composition,The present disclosure also relates to a corresponding zinc phosphation composition, to a concentrate for production thereof, to a corresponding composite metal construction and to a method of using thereof.