Stripping Knife Edge Seal Coatings

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One may already know that plasma sprayed coatings are quite common in knife edge seals used in the aero-engine industry. These knife edge seals are a critical component in aircraft engines and are generally coated with an abrasive coating. They are used to form a seal for the gas flows and form a mating surface with an abradable coated surface. That is one of the beauties of the thermal spray coating process. You can thermal spray one part with an abrasive material and coat its mating part with an abradable material. Is that neat or what? Sorry, got carried away praising the process.

The abrasive coating used on knife edge seals is generally plasma sprayed aluminum oxide – titanium oxide over a bond coat of nickel-aluminum. The coating thickness of the bond coat is usually two to four thousandths of an inch and the top coat is usually anywhere from four to twelve thousandths of an inch thick. Coating these knife edge seal components can be very interesting indeed, with almost no one that does it manually. Automation is a virtual necessity in this case. While the nickel-aluminum-alumina-titania is the most common, there are also knife edge seals that are coated with chrome-carbide-nickel-chrome; in this case the coating thickness generally runs around five thousandths of an inch thick.

There are a variety of reasons why one might have a need to strip these coatings. Without getting into too much of a discussion as to the reasons for the stripping, the quick ones that come to mind are A. standard overhaul and repair procedure and B. One of the million reasons for quality discrepancy. And in this article, we will consider the method of stripping such coatings.

Grind stripping is a very very expensive route and generally not considered at all due to the configuration of these parts. Grit blast stripping can destroy the knife edges and then you have a component that becomes unusable. The best method to strip plasma sprayed coatings on knife edge seals is therefore ( do I hear a drum-roll ……) chemical stripping.

In the case of plasma sprayed alumina-titania over nickel-aluminum types, one might want to consider dipping in a hot diluted acid bath. What this does is that the acid gets through the porosity of the top coat and removes it away from the bond coat. Once this is done, the component needs to be washed with water and then dipped in a hot diluted alkaline bath to attack the bond coat material. After the coating is removed, rinse the part with water and then a gentle abrasive dust blast followed by a glass bead blast operation could be considered.

In the case of plasma sprayed chrome carbide nickel chrome coated seals, about the only method that might work is electrolytic stripping using electricity in an electrolytic bath that is generally aqueous alkaline material. The process takes a long time to complete but then such parts are very expensive to manufacture anyway.

As you may have noticed, what is presented above is basic chemistry. One will have to develop the proper dilution ratios, proper temperature settings and time in the solution parameters before claiming to be an expert. Additionally, one of the most important aspects to consider is the ability of the base material to withstand the chemical environment. If the chemicals used to strip the coating attacks the base material too, then it becomes meaningless. Testing and experimenting on sample hardware is highly recommended. An additional point to consider is to evaluate the sample material ( which incidentally must be the SAME as the actual hardware ) by testing metallurgically for evidence of any intergranular attack after the stripping process. This kind of attack greatly reduces the metallurgical integrity of the component and no evidence of such attack is permissible.

Of course, once the stripping is successful, the part can go back to the normal re-coating procedure which will involve everything including starting with incoming inspection. There are a variety of other parameters to be considered also. For example, how often can the solution be used before it gets depleted: secondly how often can one simply add acid or caustic before the bath is basically spent. In the case of electrolytic stripping, obviously current and voltage parameters need to be developed also.

Regardless of the stripping process used, it is of imperative importance to get your customer approval before you go around dipping your customer hardware into chemicals. And process approval must include all the details of the stripping process including all key parameters. And parts that have been stripped and re-coated must be identified and marked as such for traceability reasons. Hardware safety is of paramount importance in the aerospace industry and no short cuts are allowed.

Of course, the easy solution ( no pun intended ) is simply to send out the hardware to a firm that specializes in coating removal rather than doing it in-house. This is something to evaluate also from a business stand-point depending upon the volume of stripping that is involved. Because there are other hidden costs such as hazardous waste disposal costs and environmental compliance costs that need to be considered. That is where technology meets regulations.. isn’t that wonderful !

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