Bond Coat Stripping via HCl/H3PO4 and NaOH Desmutting
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Solution Overview
Problem
Current methods for removing metallic coatings from refurbished articles, such as turbine components, are time-consuming and pose risks of damaging the substrate due to aggressive chemical treatments and multiple grit blasting operations, leading to environmental and health hazards.
Innovation Solution
A wet chemical process involving sequential immersions in HCl/H3PO4 and NaOH solutions is used to remove the metallic bond coat, forming and then desmutting the smut, which reduces the need for extensive grit blasting and minimizes exposure to harsh chemicals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If multiple grit blasting operations are used to remove smut, then smut removal is achieved, but substrate material is damaged and refurbishment time increases
Solution Approach 1:
The patent replaces the mechanical grit blasting system with a chemical etching system using acidic solutions. The acid selectively dissolves the smut layer through chemical reaction rather than mechanical impact, eliminating substrate damage from abrasive particles while achieving complete smut removal. The chemical process allows controlled etching that targets only the smut without affecting the underlying substrate material.
Solution Approach 2:
The patent changes the operating parameters from mechanical (grit size, blasting pressure, blasting duration) to chemical (acid concentration, temperature, immersion time). By adjusting acid concentration and exposure time, the process achieves optimal smut removal while preventing substrate damage. This parameter transformation enables precise control over the removal depth and quality without the variability inherent in mechanical blasting operations.
2Productivity
If strong acids are used for acid stripping, then bond coat removal is effective, but environmental and health safety concerns increase
Solution Approach 1:
The patent modifies the chemical parameters by using diluted acidic solutions instead of concentrated strong acids. The reduced acid concentration lowers the hazard level for environmental and safety concerns while maintaining effective bond coat removal through extended exposure time. This parameter adjustment balances productivity with reduced chemical risk, allowing the process to remain efficient while being safer to handle and dispose of.
3Manufacturing precision
If multiple grit blasting and acid dipping cycles are used, then complete coating removal is achieved, but refurbishment time increases
Solution Approach 1:
The patent replaces the iterative mechanical-chemical cycle (grit blasting followed by acid dipping) with a continuous chemical etching process. The acidic solution penetrates and dissolves the bond coat uniformly over time, eliminating the need for repeated mechanical interventions. This single continuous chemical process achieves complete coating removal in one operation, dramatically reducing the total refurbishment time while maintaining removal completeness.
Solution Approach 2:
The patent implements continuous chemical etching where the acidic solution continuously acts on the bond coat without interruption by mechanical blasting steps. The process maintains constant chemical exposure, allowing progressive and uniform coating removal throughout the exposure period. This continuous action eliminates the start-stop nature of multiple cycles, reducing total process time while ensuring complete coating removal through sustained chemical activity.
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 process significantly shortens the smut removal time, lowers costs, and reduces the risk of substrate damage, while being more environmentally friendly and safer for personnel, achieving a third reduction in refurbishment time and associated costs.
Implementation Method 1
The bond coated article is immersed in a solution of HCl/H3PO4 at a predetermined temperature for a predetermined amount of time, the HCl/H3PO4 solution reacting with the bond coating applied over the metallic substrate to form a smut on the surface
Implementation Method 2
The article is then removed from the HCl/H3PO4 solution and quickly immersed in a solution of NaOH for a predetermined amount of time to at least partially desmut the surface
Data Source
AI summary
Stripping a metallic bond coat from an article using a wet chemical process. An article removed from service and having a metallic bond coat applied over a surface of its metallic substrate is provided. The metallic bond coat is used to improve the adhesion of a TBC to the article, so grit blasting to first remove any TBC applied over the bond coat and which still remains on the article initially may be required. The bond coated article is then immersed in an acid solution of HCl/H3PO4 at a predetermined temperature for a predetermined amount of time, the HCl/H3PO4 solution reacting with the bond coat applied over the metallic substrate to form a smut on the surface. The article is then removed from the HCl/H3PO4 solution and quickly immersed in a solution of NaOH for a predetermined amount of time to at least partially desmut the surface.