Industrial Etching - Etch Techniques

Etch Techniques

Metallographic etches exist for examining the microstructure of steel and related materials. A few very common etchants and use are listed here.

Nital - This is a common etch for revealing grain boundaries especially in carbon steels. It is made through the mixture of Nitric Acid (70% reagent grade) and an alcohol which may be ethanol, methanol, or mineral spirits. Methanol is seldom used due to its toxicity and the most common alcohol used is ethanol (Note: be cautious when mixing; always add the acid to alcohol while stirring and never store concentrations of Nital greater than 5% as at 10% it is comparable to rocket fuel in its volatility). This etchant is relatively simple to use, simply wash the polished specimen in the Nital for a few seconds then rinse with water and ethanol, dry and then the specimen is ready to be examined. For this etchant it is essential that the metal be polished with no imperfections to allow a proper etch.

Macroetchants- While there are many macroetchants around one technique very commonly used to reveal dendritic patterns and segregation impurities in steel is a hydrochloric acid solution. There are a few variations of this etch, but in the simplest form it is a mixture of approximately 38% HCl (reagent grade) with distilled and deionized water at a ratio of 1:1 (again, add the acid to the water). This mixture is heated to 70 to 80 degrees celsius and the specimen is immersed in the solution for anywhere between 10 minutes to over an hour depending on the desired result. After the desired time has passed the specimen is rinsed in cool water and desmutted with a vegetable brush and then rinsed in alcohol and dried. The rinse step of this etchant is extremely important as any remaining acid in the specimen will result in rapid oxidation and rusting, to avoid this an oil protectant or clear coat may be applied for storage.