NASF Technical Education Director, Scientific Control Laboratories, Chicago, Ill.
The stripping of electrodeposited and flame-sprayed coatings is a necessary part of surface finishing. Stripping may be required to allow for the re-plating of a part rejected due to defective finishing or as part of rework processes such as those commonly conducted by airline/aerospace facilities. This paper discusses stripping methods and operational considerations in their use.
Keywords: stripping metallic coatings, electrochemical stripping, chemical stripping, mechanical stripping, aerospace finishing
Stripping metallic coatings in aerospace applications The stripping of electrodeposited and flame-sprayed coatings (along with organic coatings such as paints and lacquers) is a necessary part of surface finishing. Stripping may be required to allow for the re-plating of a part rejected due to defective finishing or as part of rework processes such as those commonly conducted by airline/aerospace facilities. Airline/aerospace facilities conduct stripping of a number of metallic/semi-metallic coatings. Some of those more commonly encountered are:
Stripping these coatings facilitates the inspection of the parts, allows for refinishing with a newer more effective coating, or allows the application of a replacement coating.
The basic chemical reaction conducted when stripping a metal is that of oxidation, i.e., the metal is converted from the zero valent state to a higher valent (ionic) state. Essentially, this is the reverse of the deposition process. To allow such a chemical reaction to proceed, we have basically two choices:
React the metallic coating(s) with a solution that is powerful enough to oxidize the metal (dissolve it).
React the metallic coating(s) with a combination of electrical power and a chemical solution that will keep the metallic coating in either the dissolved state, or will allow the dissolved metal to crystallize out and be removed from the process as a salt or sludge.
Over the 150+ years that the electroplating industry has been active, a number of stripping solutions have been developed. Some of these are popularly employed "generic" formulations that can be obtained from most any text book, and from articles published in the surface finishing journals. Of course, there are also commercial stripping products that may be safer, more effective and more economical, and may allow for the generation of recyclable forms of the stripped metals.
Chemical immersion stripping is preferred over other methods because the process can strip complicated shapes (stripping occurs wherever the solution touches the surface). Chemical immersion stripping also does not require rectifiers, bus bars, racking/rack design and the paying of an electricity bill. However, chemical strippers tend to slow down in stripping rate as the solution is used and builds up in metal content. Immersion strippers typically cost more to use than electrolytic.
In the airline/aerospace industry, equipment manufacturers have approved lists of generic and proprietary stripping solutions, and only these can be employed under the specified conditions after referring to the latest revision of the manufacturer's specification(s). Care should be taken not to exceed maximum immersion times prescribed by the specifications, as etching, pitting or roughening of the basis metal may result from exceeding such immersion times.
Electrochemical stripping is employed when immersion solutions are ineffective, unavailable or too difficult to use (too slow, too uneconomical or too aggressive on the basis metal). The part is connected to the positive terminal of the rectifier (anode bus bar on the tank). The counter electrodes are typically steel or stainless steel. As in electroplating, careful control over the voltage (and therefore the current density) on the part is critical to success. Too high a voltage during electro-stripping can cause localized pitting of the basis metal (usually at the high current density areas).
Electrochemical stripping typically removes metal faster at the high current density surfaces and may leave traces of the coating to be stripped in recesses and other low current density areas. Electrolytic strippers are less expensive and provide more steady rates of stripping than immersion types. Electrolytic strippers are more prone to pitting problems than immersion types. Airline / aerospace specifications typically prescribe the current density range or maximum voltage to be applied during electrochemical stripping.
Stripping by mechanical means
It is important that we point out that mechanical means can be and often are also utilized to remove organic and inorganic coatings, especially when the part shape makes the application of abrasive methods practical. Mechanical finishing may also be employed in addition to chemical stripping to produce a more uniform surface finish on the stripped part and/or to remove smut and other residues that may be left behind by chemical methods.
Blasting, machining, grinding and vibratory finishing are the most commonly employed mechanical methods of stripping or treating parts after chemical or electrochemical stripping. Machining and grinding are commonly employed methods of stripping or finishing parts with simple geometries, such as shafts and cylinders, while blasting is employed on more complicated part shapes.
In the airline / aerospace industry, equipment manufacturers provide specifications that describe the approved methods of mechanical stripping/finishing. These must be carefully followed since machining and grinding in an uncontrolled manner can cause surface defects before and after re-plating. As an example, Boeing has reported that grinding the stripped surface of a part by using too much pressure and not enough lubrication can cause "chicken cracks" to appear in the hard chromium deposit (Fig. 1).