It’s common, especially in the aerospace industry, for parts that are fracture critical to be Liquid Penetrant Inspected (LPI) prior to installation. Also known as Dye Penetrant Inspection (DPI) or simply Penetrant Testing (PT), this method is used to detect micro-cracks or other defects that could serve as an initiation site for failure. In order to properly execute a penetrant test, the surface of a metal part must be thoroughly clean of any debris, smeared metal, or any amorphous layer that may be hiding a hairline crack, thus yielding a false reading.
Electropolishing is the ideal process for this essential preclean step because it can be easily controlled to remove a precise amount of material in order to achieve the goal of eliminating displaced metal and exposing the true surface.
Other types of chemical etching baths that do not involve electrical current are harder to control. As the bath heats up, the chemicals can become more aggressive, and the material removal rate may vary and be harder to keep under control. Parts with close tolerances can be easily turned to scrap with an etching procedure.
Electropolishing uses an electrolytic solution in combination with rectified current. As soon as the current is turned off, there is no more electropolishing action/material removal taking place, even while the parts are still within the electrolytic bath. With these controls, electropolished parts can be expected to have a tolerance of +/- 0.0002 inch stock removal from part to part or lot to lot. For example, if the aim is to remove 0.0006 inch total off of a given diameter or thickness, the range of total stock removal expected over the course of the production run should never vary outside of 0.0004 to 0.0008 inch.
Electropolishing is also superior to chemical etching in that some material alloys do not respond favorably to chemical etching. It has been reported that A286 material, for example, was completely destroyed by a chemical etch bath that had worked fine on other materials. Volume 2 of the “Liquid Penetrant Testing Non-Destructive Testing Handbook” states also that “abrasive cleaning methods should be avoided” as these methods smear the metal and close discontinuities that the penetrant test would have detected.