Efficient Electrochemical Stripping of WC-Co Wear Coatings from Inconel® 718 Substrates
Authors: Skinn Brian , Snyder Stephen , Hall Timothy D , Inman Maria , Taylor E. J. 
Published: February 2020

A significant cost driver in maintenance operations for aerospace parts is the stripping of functional coatings either due to damage or to allow for periodic inspection/maintenance of the parts. Typical coating processes include electroplating of metals such as chrome, nickel, zinc-nickel, cadmium and silver, and thermal processes such as plasma/flame spray or high-velocity oxygen fuel (HVOF) for deposition of tungsten, cobalt, chrome and carbide-based alloys/composites. The coatings must be stripped in as efficient and rapid a fashion as possible, while leaving the underlying part undamaged and ready for re-coating and return to service. The ideal coating stripping process will be fast, inexpensive, and innocuous to the base material; electrochemical stripping processes are well established for various applications [[1],[2],[3],[4]]. An additional consideration is that functional coatings often contain valuable elements, and recovery of these materials as part of the stripping operation is thus desirable. For metals that remain soluble in the stripping solution, electrowinning is a viable recovery method; filtration, centrifugation, and other approaches are available for retrieval of insoluble materials.
This talk will present data generated from stripping of WC-Co HVOF wear coatings from Inconel® 718 (IN718) substrates using a citrate-percarbonate electrolytic system. The subject study was motivated by difficulty expressed by a client in achieving rapid, complete stripping of WC-Co coatings with this system. Electrolytic stripping tests on pristine and thermally aged (60 h at 500 °C in air) commercial WC-Co coatings were performed on individual IN718 coupons and on sets of four coupons (see Figure 1) to examine the effect of apparatus configuration on stripping efficacy. The experiments performed indicate proper design of part racking/fixturing to be crucial for efficient stripping, and highlight an appreciable effect of thermal aging on stripping performance. Significant, rapid decomposition of the hydrogen peroxide constituent of the stripping solution was observed, though this decomposition minimally affected stripping performance. The potential for in situ electrowinning to enable recovery of the cobalt and solubilized tungsten constituents of the dissolved coatings will be briefly discussed, and preliminary data will be presented suggesting that the use of pulsed electrolytic waveforms may afford appreciable process enhancement.

Figure 1 Caption

(Top) WC-Co coated IN718 part. (Bottom) Electrode configuration in the four-part electrolytic stripping tests. Each part was staged at a different distance from the SS316 counterelectrode. The parts were wired in parallel with split leads to allow individual measurement of the current passed to each part.

References

[1]. F. Passal. “Electrolytic method of stripping nickel, chromium, copper, zinc, cadmium, silver, tin, and lead electrodeposits from ferrous basis metals, and compositions for use therein.” United States Patent No. 2,561,222 (1948).
[2]. Capt. Matthew Whitaker. “Speed is good: New cleaning process nets huge gains in efficiency.” Air Force Print News Today (9 Jan 2015). Available online: http://www.tinker.af.mil/news/story_print.asp?id=123435922. Accessed 16 Dec 2015.
[3]. D.C. Fairbourn, M. Sorenson. “Apparatus, methods, and compositions for removing coatings from a metal component.” United States Patent No. 8,262,870 (2012).
[4]. D.R. Gabe. “Metal strippers: their science and technology.” Trans Inst Metal Finishing
85(2): 72 (2007).

Figure 1