MCrAlY Coating Strip Process Using Chloride-Free Acid Solution
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Solution Overview
Problem
Current methods for removing MCrAlY coatings from turbine engine components using mineral acids, such as hydrochloric acid, pose environmental concerns and risk damaging the base alloy, while also being inefficient and not entirely effective in stripping the coating without chloride-based chemicals.
Innovation Solution
A process involving an acid solution comprising nitric acid, phosphoric acid, and ammonium bifluoride, maintained at 140-160 degrees Fahrenheit, is used to strip MCrAlY coatings from base alloys like nickel, cobalt, or iron-based substrates, ensuring the solution is chloride-free and effective in removing the coating without damaging the substrate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If hydrochloric acid is used to remove MCrAlY coating, then coating removal efficiency is improved, but environmental emissions and safety hazards worsen
Solution Approach 1:
The patent changes the chemical composition parameters of the stripping solution by replacing hydrochloric acid with a combination of nitric acid, phosphoric acid, and ammonium bifluoride. This substitution maintains effective coating removal while eliminating chloride-based emissions, directly resolving the contradiction between productivity and environmental harm.
2Productivity
If mineral acid is used to strip coating, then coating removal is achieved, but base alloy corrosion risk increases
Solution Approach 1:
The patent applies local quality by creating a chemically selective environment where the acid mixture preferentially attacks the MCrAlY coating while being substantially inhibited from attacking the base alloy. The specific combination of nitric acid, phosphoric acid, and ammonium bifluoride creates localized chemical conditions that differentiate between coating and substrate, enabling effective stripping without base metal corrosion.
3Loss of time
If chloride-based stripping composition is used, then coating removal speed is improved, but environmental compliance difficulty worsens
Solution Approach 1:
The patent modifies the chemical parameters of the stripping system by eliminating chloride ions from the solution composition. The replacement acid mixture maintains rapid coating removal kinetics while producing emissions that are compliant with environmental regulations, thereby resolving the contradiction between process speed and regulatory compliance.
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 method achieves complete removal of MCrAlY coatings within 2-4 hours with minimal attack on the underlying alloy, outperforming traditional chloride-based methods in efficiency and safety, while eliminating environmental hazards associated with chloride emissions.
Implementation Method 1
the acid leaches preferentially the aluminum in coatings containing relatively elevated levels of aluminum
Implementation Method 2
the mineral acids used preferentially attack the coating without significant chemical attack or corrosion of the base alloys
Implementation Method 3
the acid solution comprising nitric acid, phosphoric acid and ammonium bifluoride... removing the MCrAlY coating without damaging the part
Data Source
AI summary
The present disclosure relates generally to strip process for removing a coating from a substrate comprising the steps of: providing a base alloy and a MCrAlY coating deposited over the base alloy substrate; and removing the MCrAlY coating by bringing the MCrAlY coating in contact with an acid solution comprising nitric acid, phosphoric acid and ammonium bifluoride in an aqueous solution, and maintaining the acid solution contact for sufficient time and at sufficient temperature to permit the coating to be stripped from the base alloy substrate.