The surfaces to be etched should be smoothed by filing, machining, or grinding on metallographic papers. With different alloys and tempers, the etching period will vary from a few seconds to several minutes, and should be continued until the desired contrast is obtained. As a protection from the fumes liberated during the etching process, this work should be done under a hood. After etching, the specimens should be thoroughly rinsed and then dried with a blast of warm air. Coating the surface with a thin clear lacquer will preserve the appearance.
For Ferrous Metals
Etching solutions suitable for carbon and low alloy steels, together with directions for their use, are suggested as follows.
Hydrochloric Acid. Hydrochloric (muriatic) acid and water, equal parts, by volume. The solution should be kept at or near the boiling temperature during the etching process. The specimens are to be immersed in the solution for a sufficient period of time to reveal all lack of soundness that might exist at their cross-sectional surfaces.
Ammonium Persulfate. One part of ammonium persulfate to nine parts of water, by weight. The solution should be used at room temperature, and should be applied by vigorously rubbing the surface to be etched with a piece of cotton saturated with the solution. The etching process should be continued until there is a clear definition of the structure in the weld.
Iodine and Potassium Iodide. One part of powdered iodine (solid form), two parts of powdered potassium iodide, and ten parts of water, all by weight. The solution should be used at room temperature, and brushed on the surface to be etched until there is a clear definition or outline of the weld.
Nitric Acid. One part of nitric acid and three parts of water, by volume.
CAUTION: Always pour the acid into the water. Nitric acid causes bad stains and severe burns.
The solution may be used at room temperature and applied to the surface to be etched with a glass stirring rod. The specimens may also be placed in a boiling solution of the acid, but the work should be done in a well-ventilated room. The etching process should be continued for a sufficient period of time to reveal all lack of soundness that might exist at the cross-sectional surfaces of the weld.
For Nonferrous Metals
The following etching reagents and directions for their use are suggested for revealing the macrostructure.
Aluminum and Aluminum-Base Alloys
Hydrochloric acid (concentrated) 15 ml Hydrofluoric acid (48%) 10 ml Water 85 ml
This solution is to be used at room temperature, and etching is accomplished by either swabbing or immersing the specimen.
For Copper and Copper-Base Alloys:
Cold Concentrated Nitric Acid. Etching is accomplished by either flooding or immersing the specimen for several seconds under a hood. After rinsing with a flood of water, the process is repeated with a 50-50 solution of concentrated nitric acid and water.
In the case of the silicon bronze alloys, it may be necessary to swab the surface to remove a white (SiO 2 ) deposit.
For Nickel and Nickel-Base Alloys
Material Formula Nickel Nitric Acid or LepitoÆs Etch Low Carbon Nickel Nitric Acid or LepitoÆs Etch Nickel-Copper (400) Nitric Acid or LepitoÆs Etch Nickel-Chromium-Iron Aqua Regia or LepitoÆs Etch (600 and 800)
MAKEUP OF FORMULAS FOR AQUA REGIA AND
Aqua LepitoÆs Regia Etch [(1), (3)] [(2), (3)]
Nitric Acid, Concentrated - HNO3 1 part 3 ml Hydrochloric Acid, Concentrated -- 2 parts 10 ml HCL Ammonium Sulfate -- . . . 1.5 g (NH4 )2 (SO4 ) Ferric Chloride - FeCl3. . . 2.5 g Water . . . 7.5 ml
(1) Warm the parts for faster action. (2) Mix solution as follows: (a) Dissolve (NH4 )2 (SO4 )in H2 O. (b) Dissolve powdered FeCl3 in warm HCl. (c) Mix (a) and (b) above and add HNO3 . (3) Etching is accomplished by either swabbing or immersing the specimen.
For Titanium KrollÆs Etch KellerÆs Etch Hydrofluoric acid (48%) 1 to 3 ml 1/2 ml Nitric acid (concentrated) 2 to 6 ml 2 1/2 ml Hydrochloric Acid . . . 1 1/2 ml (concentrated) Water To make 100 ml To make 100 ml
Hydrofluoric acid 3 ml Nitric acid (concentrated) 22 ml Water 22 ml
Apply by swab and rinse in cold water. These are general purpose etchants which are applied at room temperature by swabbing or immersion of the specimen.