Phosphoric Acid Coating with Magnesium Passivation
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Solution Overview
Problem
The use of chromates and molybdates in phosphoric acid-based coating compositions for ferrous surfaces is problematic due to environmental concerns and toxicity, as they react violently with aluminum, causing interference in the formation of protective coatings, and there is a need for alternatives that provide similar corrosion protection without these toxic substances.
Innovation Solution
A bonding solution comprising phosphoric acid, magnesium ions, and a leachable corrosion inhibitor, with a pH range of 2 to 4.5, is used to create a stable coating composition that is inert to aluminum, eliminating the need for chromates and molybdates, and is combined with particulate metallic material like aluminum powder to form a protective coating.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If chromates or molybdates are added to the acidic bonding solution to passivate it toward aluminum, then the reactivity between the acidic solution and aluminum is controlled, but toxic substances are introduced into the coating composition
Solution Approach 1:
Magnesium ions serve as an intermediary substance that passivates the acidic bonding solution toward aluminum, replacing the toxic chromates and molybdates. The magnesium ions form a protective layer that controls reactivity without introducing carcinogenic hexavalent chromium or other toxic substances, thus resolving the contradiction between effective reactivity control and environmental safety
Solution Approach 2:
The invention changes the chemical composition parameters of the bonding solution by substituting toxic passivating agents (chromates, molybdates) with non-toxic magnesium ions. This parameter change maintains the essential function of passivation while eliminating the harmful toxic effects, achieving both reactivity control and environmental compatibility
2Object-affected harmful factors
If the acidic bonding solution is used without passivation agents, then no toxic substances are introduced, but the solution reacts violently with aluminum causing burning, explosion, or conversion to salts
Solution Approach 1:
Magnesium ions act as a mediating substance between the acidic bonding solution and aluminum particulate material. This intermediary prevents direct violent reaction while maintaining the essential acidic bonding function, eliminating burning and explosion risks without requiring toxic chromates or molybdates for passivation
Solution Approach 2:
The bonding solution is pre-treated with magnesium ions before contact with aluminum to establish a stable, passivated state. This preliminary action prevents violent reactions from occurring in the first place, ensuring safe and controlled coating formation without toxic additives
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 provides stable and effective corrosion protection for ferrous metal alloy surfaces, especially at high temperatures, without the use of toxic metals, maintaining or exceeding the performance standards of existing Allen-type coatings, and is suitable for applications like turbine compressor components.
Implementation Method 1
the addition of chromates or molybdates to the acidic bonding solution effectively passivated the solution toward aluminum and inhibited the oxidation of metallic aluminum
Implementation Method 2
effectively passivated the solution toward aluminum and inhibited the oxidation of metallic aluminum
Implementation Method 3
the acidic bonding solution can react with the metallic material. Such reactions can be very violent, causing the flake and/or powder to burn or even explode, or less violent, simply resulting in the conversion of the metallic material into various salts
Implementation Method 4
provide high quality coatings which protect ferrous metal alloy surfaces from oxidation and corrosion, particularly at high temperatures
Implementation Method 5
the coated surface may be cured at a second temperature, generally above 1000° F. (538° C.) to form the final protective coating
Data Source
AI summary
An aqueous phosphoric bonding solution consists essentially of phosphoric acid, a source of magnesium ions, and a leachable corrosion inhibitor. The bonding solution is stable with respect to inorganic metal particles, such as aluminum, which are admixed to the bonding solution for the preparation of a coating slurry. Metal parts coated with the coating compositions have very satisfactory properties such as heat and corrosion resistance.