Nitric acid is widely used for etching copper and brass. It is also used for etching steel and stainless steel. Metals used for jewelry, like silver, can also be etched with nitric acid to get results better than other etchants. Nitric acid for etching commonly comes as concentrated 67-70% strength that is extremely harmful and its handling requires safety gear. One common recipe for a nitric acid solution for etching is 3 parts water to 1 part concentrated nitric acid. Solutions having varying (5, 8, or 10 water to 1 nitric acid) strengths of nitric acid can also be used depending upon the type of metal and level of etching. In all cases, it's extremely important to add nitric acid slowly to the water rather than the other way around. This is because mixing nitric and water releases intense heat that can sometimes lead to boiling of solution. This requires protective clothing (goggles, rubber gloves, apron, etc.) and reasonable care during mixing.
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Stainless steels are highly resistant to corrosion, however, they can undergo surface damage under long-term harsh environments without routine cleaning and maintenance. Moreover, the cleanliness of stainless steel is essential for applications like foods, chemicals, and pharmaceuticals. Routine cleaning of stainless steel can be simply done using soap or a mild detergent in warm water. Any soft cloth or fine nylon sponge can be used followed by a rinse and drying with a soft cloth. Heavier staining like heat discoloration, corrosion, and rust stains require cleaning with an acid solution. Nitric acid is the only mineral acid that is safe for cleaning stainless steel. A commonly used solution for this purpose constitutes 1 part of concentrated nitric acid added to 9 parts water. This is a weak solution of nitric acid known as 10% solution and may be obtained from most chemists. Rubber gloves and safety goggles should be used when working with nitric acid and its solutions. Glass containers are most suitable for mixing and storing nitric acid solutions. Stainless steel cleaning with nitric acid often requires thorough washing with plenty of water and the spent acid should be drained with thorough flushing.
Clean laboratory glassware forms the basis of successful chemical analysis. Contaminated or dirty glassware can lead to inaccurate results and loss of valuable time. Cleaning glassware is tricky, just because something looks clean it is not necessarily clean. A simple way to confirm the cleanliness of glassware is by performing a surface wetting test. In this test, distilled water is used to wet the surface of the glass. Water will uniformly wet the clean surface but will form beads on the walls of dirty glassware. This test by itself should not, however, be used as the sole criterion for clean glass. The best time to clean glassware is just after use. One usual practice is to start with the gentlest method which is to scrape off any residue and use brushes with normal soaps or detergents. If basic cleaning doesn’t get the job done, a next step could be to soak in a gentle solvent with heating or mild agitation. Occasionally, more aggressive cleaning methods may be necessary when laboratory glassware is contaminated with chemicals or stains. For this purpose, along with organic solvents and bases, acids and their diluted solutions are also used. Nitric acid is one of the commonly used acids that is used in varying concentrations for cleaning glassware. For example, new glassware is soaked in a 1-2% solution of nitric acid for several hours before conducting sensitive chemical tests. Likewise, acid baths, a suitable container that holds up to several liters of acid, often use 10% nitric acid solution (1 part of concentrated nitric acid added to 9 parts water) to clean glassware. In some cases, 1 part of concentrated nitric acid is mixed with 3 parts of concentrated hydrochloric acid to form aqua regia. It is an aqueous-based acidic oxidizing solution for cleaning glassware contaminated with strong organic compounds and metal particles. Aqua regia and similar solutions are hazardous and should be used while wearing personal protective gear.
Silver has long been valued for its white metallic luster, its ability to be readily worked, and its corrosion resistance. Silver refining by using acid is a simple process requiring almost no investment in time, equipment, or supplies. It can be done by almost anybody. The most common refining method uses 67-70% nitric acid to dissolve silver, which is precipitated as silver chloride by the addition of un-iodized table salt. Nitric acid, along with dissolved silver and other base metals, is separated from gold or other metals. The sludge of contaminants settled at the bottom and is separated by pouring the contents into another container. Silver salt is turned back into silver by adding silver precipitant crystals and nitric acid is poured off. Purified silver is washed with water and dried afterward. This process, however, requires the use of concentrated nitric acid that produces corrosive fumes. A 4-5% nitric acid solution formed by adding 3 to 4 parts of distilled water and 1 part of concentrated nitric acid can also be used for the purification of silver. When done properly, this procedure enables silver recovery of up to 99.95%. The refining and purifying procedures, however, must be done out-of-doors with protective clothing (goggles, rubber gloves, apron, etc.) and reasonable care must be taken while draining spent acid.
Inductively coupled plasma mass spectrometry (ICP-MS) is an elemental analysis technology. It can detect a majority of the elements in the periodic table in trace (milligram to nanogram) quantities. ICP-MS uses an inductively coupled plasma to ionize the sample forming detectable atomic and small polyatomic ions. It is used in a variety of industries such as environmental monitoring, geochemical analysis, metallurgy, pharmaceutical analysis, and clinical research. Samples for the ICP-MS study are relatively simple to prepare. Biological samples are usually diluted or thermally digested before analysis. Depending on the sample type, usually, 5 mL of reagent is added to a test tube along with 10–500 microliters of sample. This mixture is then vortexed until mixed well and then loaded for analysis. Common diluents used in the ICP-MS study are dilute acid (e.g. nitric acid, hydrochloric acid) or alkali (e.g. ammonium hydroxide) solutions. Among acids, nitric acid (1% to 5%) is preferred for metal dissolution and stabilization of ICP-MS analysis. This is because all nitrate salts are soluble in water. Using sulfuric acid or hydrochloric acid will produce sulfate or chloride salts. Some metal sulfates and chlorides are not very soluble in water limiting their application in sample preparation. ICP-MS analysis requires high purity nitric acid for sensitive determination of elements in trace concentration. The presence of impurities in nitric acid will strongly influence the trace analysis. Therefore, the selection of a suitable grade is essential for reliable results.
Gold is a valuable metal with sources that are not only limited but also rarely pure. Either as freshly mined ore or in jewelry in refined form, gold often contains contaminants, unwanted minerals, and other metals. Gold rings, chains, and necklaces are often blends of gold with a percentage of silver and copper. While various procedures can purify gold, today nitric acid combined with hydrochloric acid is a convenient method for gold recovery. This combination produces aqua regia, a corrosive reagent that is used in gold refining processes. It is made by mixing nitric acid and hydrochloric acid in 1:3 ratios. Gold refining with aqua regia involves steps such as dissolving, filtration, and retrieval of gold from the substances bonded to it. Aqua regia refinement can also be used to recover 99.95% pure gold from smelted gold. Smelt gold, even though purer than the original ore, contains impurities such as silver, copper, and platinum. One has to be very careful when handling the various substances involved in the stages of the gold refining and retrieval process. Parting, a commercial-scale method, is also used for separating silver and gold. It is performed to dissolve metallic silver from gold alloys of less than 30% gold by boiling with 30-40% nitric acid solution. Nitric acid is not able to (fully) extract silver and other impurities from an alloy with a content of gold greater than 30%. It only reacts with silver and impurities resulting in pure gold (very close to 99.5% pure) upon completion of the process.
Gold testing is essential to avoid buying gold that is hallmarked, but not authentic. A few scientific methods can precisely determine karat gold purity. These include destructive (fire assay) and non-destructive (X-ray fluorescence) methods. But costly equipment and special procedures make it difficult to use them in routine gold testing. Acid test, a simpler and cheaper method, can be conveniently used for determining gold purity in jewelry. Gold testing by using nitric acid is relatively nondestructive to jewelry and offers quick results. Nitric acid is freely available and testing usually takes a few minutes. First, a gold-colored item is rubbed on a black stone leaving visible marks. The mark is tested by applying nitric acid, which dissolves the mark of any item that is not gold. If the mark remains, it is tested by applying aqua regia, a mixture of concentrated nitric acid and hydrochloric acid in 1:3 ratios. Aqua regia is used to test higher karat purity through the process of comparison and elimination. If the mark is removed, then this test dissolves the gold, proving the item to be genuine gold. The purer the gold (e.g. 24k), the stronger the acid required to dissolve it. Gold testing can be routinely done by using an acid testing kit. Such gold testing incorporates the use of acids, so paying close attention to personal safety and safety gear (rubber gloves, safety goggles, apron) is a must. Nitric acid gold testing, though reasonably accurate at authenticating gold, poses some limitations. First, it only tests the surface layers, and, secondly, the test is limited in accuracy to rough karat counts such as 10k, 14k, 18k, etc. The acid is pre-made to test for a certain karat and one has to watch out for false positives.https://www.laballey.com/collections/nitric-acid
Stainless steel is a tough metal that has corrosion resistance through natural passivation. Passivation is a widely-used metal treatment method to prevent corrosion. Passivation of stainless steel is done for removing free iron and other contaminants resulting from the handling, fabrication, pickling, or welding operations. Stainless steel can lose natural passivation through mechanical, heat, or chemical damage. Passivation may need to be performed regularly to prevent rust. Nitric acid passivation is a proven and established process for stainless steel. A two-step passivation procedure is usually recommended for appropriate corrosion resistance. The first step is freeing stainless steel from contamination and foreign matter. Nitric acid is the only mineral acid that is safe for stainless steel cleaning. A commonly used solution for this purpose constitutes 1 part of concentrated nitric acid added to 9 parts water. This is a weak solution of nitric acid known as 10% solution and may be obtained from most chemists. Second step is the immersion of stainless steel in the nitric acid bath. It dissolves free iron from the surface while leaving the chromium intact. The acid chemically removes the free iron, leaving behind a uniform surface with a higher proportion of chromium than the underlying material. Mostly, the passivation approach should be based on the chrome content and machinability characteristics of the stainless steel grade, along with the prescribed acceptance criteria.
Buy Nitric Acid NowGrowing use of specialty alloys with high resistance to corrosion has challenged many laboratories to refine the way they etch alloy samples for microstructural evaluation. The big problem – the alloys resist the etchants just as they do the corrosive conditions encountered in service.
Microstructure of a stainless 330 sample in annealed condition @ 100x, using a tint etch consisting of a solution of 40 ml hydrochloric acid (MCL) + distilled water (H2O) + one gram potassium meta bisulfite (K2S2O5) + 4 grams ammonium biflouride (NH4F–HF) at room temperature.Such high-performance alloys have been used increasingly for critical applications in bio-medical, aerospace and defense industries, among others. Due to the usually demanding material requirements, laboratory evaluation of the finished microstructure is often desired, if not required.
The difficulty in etching these materials has focused attention also on employing the most effective procedures for etching a wide variety of iron-, nickel- and cobalt-based alloy systems. Ensuing discussion, therefore, will offer guidelines for processing stainless steels, high temperature alloys, tool and alloy steels, and magnetic and expansion alloys.
Light microscopy to evaluate the microstructure of metals is employed extensively by quality control and failure analysis laboratories. Selection of the proper etchant depends largely upon alloy composition, heat treatment and processing. The etchants used in metallographic examination are solutions of acid and chemicals and are applied selectively to attack a highly polished surface, thus permitting microstructural examination.
There are three basic methods of etching alloy samples – immersion, swabbing and electrolytic. In the first method, the sample is immersed in the etching solution until the desired structure is developed. Samples may be immersed in stain etchants to highlight specific microstructural features.
In the second, the sample is swabbed with cotton that has been immersed in the etchant. In electrolytic etching, a D.C. source or a rectifier serves as the power supply, and the specimen is the anode in the electrolytic cell. Power requirements can be adjusted as needed, depending on sample size, anode-to-cathode spacing, electrolyte, etc.
The following rules should be observed to obtain a true and representative microstructure:
Table 1 lists 28 etchants commonly used in the laboratory, along with their compositions and guidelines for use. Table 2 lists various specialty alloys, along with the suggested etchants for each and application notes. These tables can be used to quickly select the appropriate etching procedure for the alloys most often used today. Additional etchants are reported in ASTM specification E-407-99 “Standard Practice for Microetching Metals and Alloys” and ASM Metals Handbook (Volume 9, Metallography and Microstructures).
The etchants for each alloy are presented in order of preference and successful experience. In general, the most benign etchant is shown first, followed by those that grow progressively stronger. If a weaker etchant is tried first and it does not yield satisfactory results, the investigator only has to buff the surface slightly to obtain a good polish for examination with a stronger etchant.
If the first etchant attempted is too strong, far more preparation is required to restore the surface to a workable condition. If one is not familiar with the etchants recommended, it is usually a good idea to start with the weakest solution.
Before etching, the examiner should first consider what s/he is looking for. If the intent is to evaluate non-metallic inclusions, sulfide distribution or morphology, for example, the sample is best examined in the as-polished condition since etching can remove various inclusions and attack structures such as ferrite stringers in an austenitic matrix.
On the other hand, if the search is designed to examine grain size, precipitation or cold- work deformation, the sample must be etched.
The condition of an alloy and its heat treatment play an important part in the selection and application of etchants. After confirming the alloy grade and analysis, consider whether it has been annealed, aged, cold worked, tempered and/or is in the as-hardened condition.
Special techniques are required to effectively prepare highly corrosion resistant alloys for microstructural examination. Prominent in this group are alloys such as Micro-Melt® BioDur® CCM Plus®, MP35N* (UNS R) and Custom Age 625 Plus® (UNS N).
*MP35N is a registered trademark of SPS Technologies Inc.
These nickel-base and cobalt-base alloys, with their superior corrosion resistance, are etched using Waterless Kalling’s, Glyceregia, Acetic Glyceregia and Ralph’s. If the etchants typically used for highly resistant stainless and high temperature alloys do not work, HCL + H2O2can be used. These alloys usually should be etched slowly.
All require a fresh polish to avoid “flashing,” which would require a complete re-polish. Best results are obtained by lightly re-rubbing one or two samples at a time on the final polishing wheel (normally a 0.05 micron alumina slurry).
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The challenge with these alloys is to offset their natural tendency to self-passivate rapidly in the presence of oxygen. To effectively process these grades, etching must be done immediately after final polishing. Rinsing, drying and etching should be performed without delay.
When etching these highly resistant alloys, the etchant should be prepared in advance of the polishing process. This advance planning will minimize the length of time between polishing and etching, thus permitting a more effective microstructural evaluation. Samples should be polished and etched individually rather than processed in batches.
Five light microscopy methods of illumination can be used in microstructural examination: bright-field illumination, dark-field illumination, oblique illumination, differential interference-contrast (DIC) and polarized light (Fig. 1). The well-equipped laboratory should have all five because their capabilities are complementary. Their role in microstructural evaluation cannot be overestimated.
Visual results for the same optical field at 100x magnification are shown via different lighting techniques.
(a) Stain etchant used to produce color (b) Bright field – typical way of examining microstructure, shows grain structure and “twins,” but some areas not clearly definedDIC, using bright-field microscopes that change the way light is deflected, gives a three-dimensional image. It is the ultimate in light optical microscopy, showing grain boundaries very clearly in highly corrosion resistant alloys. It is the best performing means of showing relief in structure, and is capable of identifying the effects of cold work, mechanical damage, etc. Compared with the other methods, DIC provides increased image contrast.
Dark-field, oblique and differential interference-contrast illumination methods can aid in delineating grain boundaries and other microstructural features that are only weakly visible in bright field illumination. A polarizing filter can be used in conjunction with a stain etch to enhance color contrast.
In dark-field technology, the grain seen is dark and the grain boundaries, light. With the bright-field microscope, the grain is light and the boundaries, dark. Oblique illumination brings light in on an angle, producing shadows on microstructural features.
When etching austenitic stainless steels, DIC illumination will better show the grain structure and help find cold-work deformation, when such exists. Waterless Kalling’s reagent can be used to show the general structure of many austenitic stainless alloys. Other agents such as Glyceregia or Acetic Glyceregia may be required to retain ferrite, carbide precipitation.
If the only interest is grain structure in an austenitic stainless, start etching with a more aggressive etchant. However, start etching with Glyceregia for a shorter time if interested in features other than grain boundaries, such as carbides, ferrite stringers, second phases and duplex structure.
Ralph’s reagent normally provides a good etch for general structures in ferritic stainless steels. Waterless Kalling’s and Glyceregia also can yield good results. Even after a good polish, scratches may still be visible. They may or may not be a problem.
Ralph’s is usually best for the precipitation hardenable stainless steels; however, Vilella’s reagent will work fine if a light etch is preferred. Etching time will vary because the alloy in the aged condition will react quicker to the etchant. The higher the aging temperature, the quicker the response. An alloy aged at °F (590°C) will etch darker and quicker than one aged at 900°F (480°C).
The annealed structure in PH stainless steels requires either an aggressive etchant for a short time or a less aggressive etchant for a longer time. Here, Ralph’s reagent could be used for a few seconds, or Vilella’s, which is less aggressive, for a longer time.
Vilella’s reagent is preferred for martensitic stainless steels. Etching time and response will vary depending on whether the alloy has been annealed, hardened or tempered. Annealed samples usually require the longest etching time because everything is in solution, with not much to be seen. It is best to stop etching while the specimen is still on the light side. Etching has gone too long when the sample starts going black. A little experience will help the examiner to stop etching at the right time. Hardened and tempered samples usually require less etching time than annealed stock.
Etching procedures for high temperature alloys can vary greatly depending on condition of the material and what evaluation is required. High temperature alloys, which exhibit different aging conditions with a range of aging responses, phases, precipitates, etc., are typically more difficult to etch than austenitic stainless steels.
If uncertain about which etchant to use, start with Glyceregia and increase in severity until the desired result is obtained. This procedure requires only the final polishing step between etchants, instead of a complete re-polish. Glyceregia is mixed at time of use, and becomes more aggressive with the passage of time.
Waterless Kalling’s, another choice, works well with alloys such as Waspaloy(UNS N), 718 (UNS N) and A-286 (UNS K). It can be stored, and is thus more convenient to use.
Stain etchants and electrolytic etchants can be used to show specific aspects of structure in high temperature alloys. Stain etchants, used with immersion techniques, have been effective in highlighting structural features such as second phases.
If the etchants typically used for high temperature alloys do not work, HCL + H2O2can be used just as they can for stainless steels. Precipitation can be evaluated using bright field microscopy, but DIC is better for showing grain structure.
Nital (2 to 5 percent) is useful for showing carbide structure in tool and alloy steels, but re-etching the treated sample afterward with Vilella’s darkens the matrix and provides a clearer view of the microstructure. Many times, samples can be etched using Nital, examined and then etched with Vilella’s without re-polishing. Alloy condition will determine etching time.
Most magnetic and expansion alloys can be treated as suggested for one of the stainless steel families, or the tool and alloy steels. When examining the Ni-Fe alloys, for example, use the procedures for austenitic stainless steels. Many of the alloys used for their D.C. magnetic properties (such as stainless Type 430F) can be etched using procedures for ferritic stainless steels. Others, such as Fe and Si Core Irons, can be etched like low alloy steels using Nital.
Some etchants, like Waterless Kalling’s, can be made in bulk and stored for use as needed. This is convenient for the examiner. Other reagents, like Glyceregia and Acetic Glyceregia, must be prepared each time they are required. Both Glyceregia solutions undergo a continuous chemical reaction as they age, becoming aggressive in less than one hour. Special care must be taken then because the aggressive solution can affect etching time and procedure. It is a good idea, therefore, for the examiner to use the solution as soon as possible after mixing it.
As shown in Tables 1 and 2, many different etchants can be used successfully for specific alloys and special circumstances when examining basic microstructure. The choice of which reagent to use, however, does not have to be difficult.
Table 1 List of Etchants
Note: Please see ASTM for proper handling of all chemicals.
Table 2 List of Alloys
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NOTE: Etchants listed in order of preference. All Etchants should be used on a freshly polished surface.