Tint Etching

The following article describes the theory and technique of tint etching. It was written by George Vander Voort and originally appeared in the March, 1985 edition of Metal Progress "the monthly magazine of ASM International from 1930 to 1986." It is reproduced here with the kind permission of Margaret Hunt, Editor of Advanced Materials and Processes.
There are 11 accompanying micrographs and a table listing 21 tint etchants.

Tint etchants have been developed to color etch many metals and alloys -- cast irons, steels, stainless steels, nickel base alloys, copper base alloys, molybdenum, tungsten, lead, tin, and zinc. Limited success has been obtained with tint etching of aluminum and titanium alloys.

A selected list of etchants is given in Table I; additional information can be obtained in Ref 1 and 2.

The most widely applicable tint etchant is that developed by Klemm which colors ferrite in steels, reveals overheating or burning in steels, and develops the grain structure of copper and many copper alloys, as well as those of lead, tin, and zinc.

Some of the Basics Of Tint Etching

Satisfactory tint, or stain, etchants are balanced chemically to produce a stable film on the specimen surface. This is contrary to ordinary chemical etchants where the corrosion products produced during etching are redissolved into the etchant. Tint etchants have been classified as anodic, cathodic, or complex systems depending on the nature of the film precipitation.¹

Etching is a controlled corrosion process based on electrolytic action between surface areas of different potential. For pure metals and single phase alloys, a potential difference exists between grains with different orientations, between grain boundaries and grain interiors, between impurity phases and the matrix, or at concentration gradients in single phase alloys. For multiphase alloys, a potential also exists between the various phases present. These potential differences alter the rate of attack, thus revealing the microstructure when chemical etchants are used.

For a two phase alloy, the potential of one phase is greater than that of the other. During etching, the more electropositive (anodic) phase is attacked while the more electronegative (cathodic) phase is not attacked appreciably. The magnitude of the potential difference between two phases is greater than the potential differences existing in single phase alloys. Hence, alloys with two or more phases etch more rapidly than do single phase metals and alloys.

With most chemical etchants, the same phase will usually be anodic or cathodic. Indeed, it is rather difficult with standard etchants to reverse the attack; that is, make the anodic phase cathodic. This was demonstrated by Kehl and Metlay in studies of the etching behavior of alpha-beta brass (Cu-40% Zn) using nine different commonly used etchants.³ In eight of these etchants, beta was anodic to alpha. In only one (equal parts of NH₄OH, 3% H₂O₂, and H₂O) was the alpha phase anodic to the beta phase.

Only with the potentiostatic method can phases be etched selectively in the same electrolyte by changing the applied voltage.

Tint etchants generally color either the anodic or the cathodic phases. Some success has been obtained in developing tint etchants for steels that are selective to the phases that are normally cathodic. Most tint etchants, however, color the anodic phases. Tint etchants are usually acidic solutions using either water or alcohol as the solvent. They have been developed to deposit a thin film, generally 40 to 500 nm thick, of an oxide, sulfide, complex molybdate, elemental selenium, or chromate on the specimen surface.

Colors are developed by interference in the same manner as with heat tinting or vacuum deposition. Tint etchants work by immersion, never by swabbing, as this would prevent film formation. Externally applied potentials are not used.

Film thickness controls the colors produced. As the film thickness increases, interference creates colors (viewed with white light) in the usual sequence: yellow, red, violet, blue, and green. With anodic systems, the film forms only over the anodic phase but the thickness of this film can vary with crystallographic orientation of the phase.

For cathodic systems, the film thickness over the cathodic phase is generally constant so that only one color is produced. This color, however, will vary as this film grows during etching. Hence, to obtain the same color each time, the etch time must be held constant. This is usually accomplished by timing the etch and by watching the macroscopic color of the sample during staining

Beraha has developed tint etchants that deposit a thin sulfide film on a wide range of metals: cast irons, steels, stainless steels, nickel base alloys, copper, and copper alloys.⁴ The sulfide films are produced in two ways. For reagents containing either potassium or sodium metabisulfite, the iron, nickel, or cobalt cation in the sulfide film comes from the sample and the sulfide anion comes from the reagent after decomposition.

The second type of film is produced by a metal-thiosulfate complex in the reagent which consists of an aqueous solution of sodium thiosulfate, citric acid (organic acid), and either lead acetate or cadmium chloride (metal salt). In such etchants, the specimen acts as a catalyst and the film formed is either lead sulfide or cadmium sulfide. These reagents color the anodic constituents only -- that is, the film is not formed over the cathodic features.

Beraha also developed tint etchants that utilize reduction of the molybdate ion.⁵ Sodium molybdate is employed. Molybdenum in the molybdate ion, MoO₄^-2, has a valence of +6. In the presence of suitable reducing compounds, it can be partially reduced to +4. A dilute (1%) aqueous solution of sodium molybdate is made acidic (pH 2.5 to 4.0) by addition of a small amount of nitric acid which produces molybdic acid, H₂MoO₄. Addition of a strong reducing agent, such as SnCl₂, would color the solution blue, while a weaker reducing agent, such as FeSO₄, would color the solution brown.

When the 1% aqueous sodium molybdate solution, made acidic with nitric acid, is used to tint etch steels, molybdate is reduced at the cathodic cementite phase producing a yellow-orange to brown color depending on the etching time. If a small amount of ammonium bifluoride is added, the carbides are colored red-violet and ferrite is colored yellow.

The lngredients In Tint Etchants

Common ingredients in tint etchants include: sodium metabisulfite (Na₂S₂O₅), potassium metabisulfite (K₂S₂O₅), and sodium thiosulfate (Na₂S₂O₃· 5H₂O). These are used with water as the solvent and generally color anodic phases. To tint more acid resistant metals, hydrochloric acid is added. Tint etchants containing these compounds produce sulfide films. During use, the odor from sulfur dioxide and hydrogen sulfide can be detected. Although this is only a minor nuisance, etching should be conducted under a hood.

Tint etchants based on either selenic acid (H₂SeO₄) or sodium molybdate (Na₂MoO₄· 2H₂O) generally color cathodic constituents, such as cementite in cast irons and steels. Selenic acid is a rather dangerous acid to handle and its use should be restricted to those thoroughly versed in techniques for handling dangerous materials. Fortunately, the reagents based on sodium molybdate are relatively safe to use and are quite effective. Some reagents contain a small addition of ammonium bifluoride (NH₄FHF). This should be handled very carefully.

lnstructions for Mixing of Reagents

With most chemical etchants, precise adherence to the stated formula is not required. However, with tint etchants, the etch formula must usually be followed closely. For some, the order of mixing the various etch components is also critical. It is best to follow the developer's recommendations closely.

Many tint etchants can be made up in 500 to 1000 mL quantities as stock solutions. In some cases, one ingredient is left out until the quantity needed for etching is poured into a beaker. Then, the activating agent is added. Klemm's I reagent can be used in this manner. However, after mixing, this reagent can be saved for many days simply by covering the beaker tightly with aluminum foil to prevent evaporation (if this occurs, crystals will form which are very difficult to dissolve). When ammonium bifluoride is to be added to a tint etch, use a polyethylene beaker.

Specimen Preparation For Tint Etching

Very carefully prepared samples are required to get the most from tint etchants. Control of scratches is the most challenging problem, particularly for alloys such as brass. It is not uncommon to observe a rather dense scratch pattern after tint etching such alloys, even when they appeared to be scratchfree before polishing. This is a characteristic problem with methods that utilize interference effects to produce an image.

The photomicrographs shown in this article were prepared using automatic polishing techniques. All samples were mounted; the type of mount is not critical unless edges are to be examined. Methyl methacrylate (Du Pont Co.'s Lucite) is generally avoided because of its poor chemical resistance to some of the solvents used.

Samples were ground and polished using a Struers Abrapol machine in the following sequence: 120, 240, 320, 400, and 600 grit SiC with water as the lubricant/coolant; 6 micron diamond on canvas, and 1 micron diamond on a medium nap synthetic cloth using Struers' diamond extender lubricant. The diamond was applied via aerosol cans with occasional recharging during polishing. Applied pressures and times were adjusted to suit the sample being prepared.

After the 1 micron diamond polish, a light omnidirectional scratch pattern is present which must be removed by final polishing. To achieve the desired final polish quality, two types of automatic final polishing systems could be used. For some of the alloys shown, attack polishing was employed.

Vibratory polishing with a device such as the Syntron unit made by FMC Corp.'s Material Handling Equipment Div. can be employed. Another suitable device, and the one I used, is Leco Corp.'s Fini-Pol automatic polishing attachment. Polishing with this device is more rapid than with vibratory polishing and results are excellent. Because the bowl is plastic, undesired reactions do not occur during attack polishing.

All of the samples shown here were final polished using colloidal silica, also known as Syton, developed by Monsanto. Syton is available through Supplies Ltd.,⁶ and is called Final polishing solution. Colloidal silica is also offered by Buehler Ltd. as Mastermet⁷ and by Struers as OP-S.⁸

A medium nap synthetic cloth was used to line the Fini-Pol bowl and colloidal silica and distilled water, approximately equal parts, were added to just cover the cloth. Specimens were placed in the holding fixtures and inserted inside the bowl. Fixtures rotate against rollers while the wheel is slowly rotated. A rather low speed was used.

For all of the copper alloys shown a few millilitres of 1% aqueous ferric nitrate was added to the polishing solution. This should just lightly color it a pale yellow. Too much attack polishing solution produces excessive etching. With a little experience, superb results are obtained.

By itself, colloidal silica produces excellent final polishing results for most metals. It is particularly useful for ferrous alloys. Two precautions must be followed, however.

First, never let the cloth dry out after use. If this happens, silica particles precipitate and will scratch any samples subse- quently polished. After polishing, rinse out the cloth.

Second, before pouring the polishing solution from the container, wipe the top to remove any particles that might have precipitated from prior use. Surfaces polished with colloidal silica appear to be more lustrous than those polished with other abrasives. The polishing solution can be used to introduce a minor amount of relief in two phase structures depending on the applied pressure and polishing time. Minor etching has been observed with some two phase alloys which, in some cases, obviates the need for chemical etching.

Etch Technique: What Is lnvolved

The desired tint etch is mixed according to the formula (see Table I or Ref 1 and 2), or the stock solution is poured into a beaker and activated in the specified manner. I generally use a small glass or polyethylene beaker and about 100 to 200 mL of solution. The properly prepared sample must be cleaned carefully before etching because any residue on the surface will interfere with film formation. Because many of the tint etchants require 60 to 90 s immersion, the sample is placed on the bottom of the beaker, face up. Then, gently swirl the solution, being careful not to splash it on your hands.

After about 20 to 40 s, depending on the sample and the solution, the surface begins to color. At this point, the beaker is held motionless until the surface is colored red to violet. The sample is removed, washed under warm water, sprayed with ethanol, and dried.

Do not touch the sample surface. For tint etchants that work fairly fast, the sample is held in the solution with tongs and gently agitated until the surface is darkened. For these etchants, the macroscopic surface color is generally a gray-black color.

Sample Examination: How It's Carried Out

Samples are now ready for viewing with an upright (or inverted) microscope and for photographing. A tip on flattening the sample: placing tissue paper over the etched sample when it is on a piece of clay on a microscope slide may leave portions of tissue paper on the surface and scratch the film. To avoid the problem, take a 1 or 1.25 in. (25 or 32 mm) in diameter aluminum ring form and flatten it slightly in a vise. This can be placed on top of the mounted sample, resting only on the mounting materials, to flatten the sample in the hand press. If an inverted microscope is used, take care in placing the sample to avoid scratching the film.

Samples are examined first with bright field illumination, using no filters other than neutral density types to control brightness. Use of a complementary colored filter may enhance contrast between phases in certain cases. In many instances, coloration may be intensified by using crossed, or nearly crossed, polarized light.

Black and white or color photographs of any desired type or format may be obtained. If black and white film is used, remember that orthochromatic types are not sensitive to red colors. To capture the true color contrasts on black and white film, use panchromatic films. In certain instances, orthochromatic film may prove quite useful even when reds are present as they will appear quite dark on the print.

For color photography numerous films may be used to produce either transparencies or prints.

Several Applications of Tint Etching