Lesson 5-2  METAL IDENTIFICATION

5-5. Metal-Identification Tests. Unknowingly, you identify metal everyday; for instance, a penny is made of copper, dimes and quarters are made basically of silver, and nickels are made of nickel. You can identify them by looking at them. Some metals are identified by their use, such as aluminum for aircraft skin, and copper for electrical wire. However, this general classification is not good enough for the purpose of corrosion control, because if you misidentify a metal type, the work you do may damage the metal or create a condition that will increase the possibility of corrosion. By misidentifying metal, you may waste your efforts, damage material, or cause corrosion. The first and most important step in fighting corrosion is identifying the material that is being attacked or will require protection from attack. There are other tests besides general classification that will help you determine what type of metal you are working with, such as visual examinations and mechanical, chemical, and complex tests.

     a. Visual Examination. This examination reveals numbers or color codes that will identify a metal. The Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI) have established a means of identifying steels with numbers and colors.

(1) Number system. The number system, which is a steel identification numerical index (Table 5-4), works in the following manner. A four- or five-digit number is used to indicate the steel type. For example, 2330 is a number code identifying a certain steel. The first digit, 2, indicates the main alloy element; in this case, nickel. The second digit, 3, indicates the percentage of the main alloy element other than carbon; thus, there is 3 percent nickel. The last two digits in the number indicate the amount of carbon expressed in hundredths of percent; the example indicates that the steel has 0.30 percent carbon. In the case of a five-digit number, the second and third digits are used to express the percent of the main alloy element when this figure requires more than one digit; for example, 1.50 percent and 18.00 percent. In the case of plain carbon steel, 1095, the same system of interpreting numbers is used except that the digit 1 represents carbon steel, and since there is no main alloy element other than carbon, the second digit is always 0.

Table 5-4. Steel identification numerical index

(2) Color-code system. This system is linked to the numerical system of identifying the various alloys and is used to mark them. At present, there are two color-code systems in use for identifying metals and alloys. The old SAE-AISI system is related primarily to steel metals; however, the new system changes the old system and adds aluminum and copper alloys.

The new system uses an identification-marking and code (Table 5-5) and has its own color-code breakdown. For each number used (1, 2, 3, 4, 5, 6, 7, 8, 9, and 0), there is a designated color (column 1). The numbers are listed in column 2, and the letters (F, H, O, T, W, A, B, C, D, and S) are listed in column 3. For example, blue is designated as the number 1 and the letter F.

Table 5-5. Identification marking and color code breakdown

A combination of these colors, which represents a combination of numbers and letters, is used to mark the material to denote its general composition or condition, where applicable. In the example shown in Figure 5-12, the aluminum tubing's identification markings for the base-metal color codes are green, black, green, and orange; and for the temper- and strain-handening color codes are yellow and red. By comparing these colors with the adjacent numbers and letters in columns 2 and 3 of Table 5-5, above, you can identify this metal as 20-25-T6.

Figure 5-12. Identification markings

(3)  Markings. A further test for identifying metals is to check for markings such as the manufacturer's part or specification numbers. Check such data against part numbers or the group numbers shown in the TO 00-25-113 series, or the alloy group and material specifications.

(4) Common use. When a specimen cannot be identified by part or specification numbers, examine its physical appearance and determine its possible common use. Check the color of the metal. Is the color silver, like polished aluminum or magnesium; yellow, like brass or gold; gray, like zinc or lead? The metal color may indicate which alloys and elements are present.

    b. Mechanical Testing. If a metal cannot be positively identified by visual examination, see if it is attracted by a magnet. Perform a spark test if the metal is magnetic.

(1) Magnetic testing. To determine whether the specimen is attracted by a magnet, the magnet should be free-swinging from a chain, ring, or string. Metals commonly attracted by a magnet are iron, steel, or iron-based alloys containing nickel, cobalt, or chromium. However, there are exceptions to this general rule. This test can serve only as an initial step in identifying a specimen and never as a final test. Strong-magnetic metals include pure iron, nickel, and cobalt; iron-nickel-cobalt alloys; and alnico which consists of aluminum, nickel, and cobalt. Light-magnetic alloys include stainless steel and Monel Metal (nickel and copper). All other metals and alloys are nonmagnetic.

(2) Spark testing. This test identifies some metals by characteristic sparks that are thrown off when the specimen is held against a high-speed grinding wheel. The spark streams may vary from a few tiny sparks to a shower of sparks. Skill in spark testing takes practice. When possible, compare the sparks thrown off by the unknown specimen with spark streams from known samples. Standard samples of known specifications should be maintained for comparison purposes. When testing, hold the specimen with a firm, even pressure against the top of the grinding wheel. To free a wheel of metal particles retained during previous uses, clean the grinding wheel surface frequently.

(3)  A high-speed bench grinder is recommended for spark testing. Use a 6- to 8-inch wheel, medium-grit composition, 1/2 to 3/4 horsepower, 110 or 220 volts, and 3,400 to 4,000 revolutions per minute. Always wear goggles when spark testing. Metals and alloys that produce a spark on the grinder include aluminum, brass, cadmium, copper, gold, lead, zinc, and antimony. Stainless steels and high-temperature alloys with iron- and nickel-based compositions will produce characteristic sparks (Figure 5-13). As a general rule, the more iron in a specimen, the lighter the spark. As the percent of iron decreases and the percent of nickel increases, the spark will darken.

Figure 5-13. Typical spark streams

     c. Chemical (Acid) Spot Testing. When metals cannot be identified by visual examination or mechanical testing use an acid spot test. Acid spot testing is done by placing one or more drops of acid on the metal surface of a specimen, observing the surface's reaction to the acid, and comparing the color reaction to a specific table of acid spot-test reactions. Clean a small surface of the metal with an emery cloth, file, or grinding wheel before spot testing. Some of the acids needed for testing are—

(1)  Nitric-acid testing. There are two forms of nitric-acid testing; they are concentrated and diluted.

              (a) For concentrated nitric-acid testing, place one drop of concentrated nitric acid on a clean metal surface. Table 5-6 shows the possible test reactions.

Table 5-6. Concentrated nitric-acid test reactions

              (b) For diluted nitric-acid testing, place one drop of diluted nitric acid (50 percent acid and 50 percent water) on a clean metal surface. Table 5-7 shows the possible test reactions.

Table 5-7. Diluted nitric-acid test reactions

              (c) Aluminum, antimony, cobalt, gold, high-temperature alloys, tungsten, lead, platinum, stainless steels, tantalum, and titanium do not react to nitric-acid (concentrated or diluted) testing.

(2) Hydrochloric-acid testing. Aqua regia (nitrohydrochloric) acid consists of one part nitric acid mixed with three parts of hydrochloric acid. Since this mixture breaks down after 24 hours, perform the test by placing one drop of nitric acid and three drops of hydrochloric acid directly on the spot being tested. This solution should turn blue-green for cobalt-based alloys and green for nickel-based alloys.

(3) Potassium-ferricyanide testing. Use potassium ferricyanide (10 percent solution) to determine the iron content of nickel-based alloys since there is no simple spot-testing for determining the percent of nickel content. Perform this test by adding a drop of potassium ferricyanide to the spot tested by aqua regia. The color reaction will be very dark blue-black for high iron content and light blue for low iron content.

     d.  Complex Testing. Complex tests are qualitative tests consisting of a spectrographic or chemical analysis. Use them only when a specimen cannot be identified by other tests or when the quantity involved warrants it.