After Etching, My Specimen Has An Area That Looks Stained!
There is a certain sort of frustration that comes with putting a completed multi-step metallographic preparation beneath the microscope, taking a look, and seeing a discoloured or stained surface. While a few types of stains can be ignored, many stains mean something went wrong during the preparation.
What are some of the main causes of stains?
Many times, stains result from grinding/polishing/etching liquids that “bleed out” of their hiding places in the mount after preparations are complete, such as gaps between the mounting material and the specimen. These can be avoided by using proper mounting practices (more about this in a later post). Sometimes the voids are associated with the specimen itself (e.g. porosity, weld defects, specimen construction, etc.); these are more difficult, or impossible, to prevent. Staining can also occur if the specimen is not properly cleaned and dried after polishing and/or etching. When rinsing specimens, cold (not hot) water should be used, as should anhydrous ethyl alcohol. Drying should be accomplished with dry air (not containing oil or water).
What can I do during hot mounting to prevent staining?
Producing a tight bond between the mounting material and your specimen will go a long way toward preventing polishing and etching problems. To assure that the mounting material adheres to the specimen, be sure to clean the specimen thoroughly with cold water, then ethyl alcohol, and then dry it completely. During the cooling process in a mounting press, both the specimen and the mounting material shrink. Bakelite can shrink to such an extent that it pulls away from the specimen; this is especially true if the pressure is relieved before the mount reaches 70 °C (158 °F). Epoxy is stickier than Bakelite and much less prone to pull away from specimens. This makes it a better choice when even small gaps cannot be tolerated.