edited by ROBERTREEVE^
filtrate,& re~idue,
Marlbornugh School 250 S. Rosrmore Avenue Los Angeles, CA 90004
Determining the Zinc Coating Weight on Steel David E. Gillum Armco inc.-Research 8 Technology, 703 Curtis St., Middletown, OH 45043 Mary S. Henmann Miami University-Middletown. 4200 E. University Blvd.. Middletown, OH 45042 Introduce vour students to a chemical technique routinelv used by one-of the major industries in the united states. Allow them t o see the practical use of chemistry . by . applying - - .. i t to a finished product. This experiment descrihes the determination of the weight of the zinc coating that has been applied to steel by a steel mill or other steel finisher. Zinc is very widely used as a coatine on steel to orevent corrosion. Althoueh zinc is a reactiGe element, i t 'forms a tightly bonded oxide coating that prevents corrosion (rusting) of the steel. Zinc-coated steel produ& are&ed in automobiles, playground equipment, garbage cans, pipes, highway guard rails, metal silos, metal roofs, and galvanized nails. The heavier (or thicker) the zinc coating, the better the corrosion protection but the more expensive the steel. Specifications for coating weights have been set for many applications. All suppliers of zinc-coated material must have procedures for determining coating weights to insure that their products will meet specifications. Two different methods for determining the zinc coating weight are described, a gravimetric analysis and a volumetric analysis. Thus skills for both types of analyses can be developed. AIso a comparison can he made of the two techniques and their respective results.
0.075 M aqueous disodium EDTA 0.1% xylenol orange indicator solution distilled or deionized water balance pH meter or pH test paper flat pieces of zinc-coated steel (-2.5 X 2.5 em)-possible sources of the steel are metal shops at schools, metal specialty shops, steel processors or distributors, dealers in scrap steel, dealers in sheet metal 12.5 mg1mL standard zine solution (optional)-Dissolve 12.50 g reagent-grade zine in 300 mL 6 M HCI, and dilute to 1.00 L with distilled water. Procedure
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1. Weieh to the nearest millimam (to the nearest 0.1 me ifaoorooriate balances are availableja of zinc-coated steel. . s&le 2. Measureaccurately the lendh and widthofthe sample. 3. I'lare the sample in n 150-mL beaker containing 10 mL of fi M I I C I Leave in rhe solutwn until the zinc is removed. Note: This ~
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Calculations The coating weight expressed in gIcm2 can be determined from either the gravimetric or volumetric data. Gravimetric Method coating weight, g/m2 = lXw where A = weight of original sample (g), B = weight of stripped sample (g), 1 = length of sample (cm), and w = width of sample (em). Volumetric Method coating weight, glcm
Materials 6 M hydrochloric acid 8 M aqueous ammonia 3.2 M aqueous ammonium acetate
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Experimental
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will require only a few seconds; the removal of zinc is complete when the vigorous bubbling stops. Immediately remove the sample from the beaker, rinse, dry, and reweieh. Dilute thesolution from step 4 toabout 80mL.and adjust the pH to 6 r 0.5. Note:This should require 7-9 mL of 8 M YHdaq,. Add LO mL of 3.2 M ammonium acetate solutim and 5 drops of xylenol orange indicator solution. Titrate with 0.075 M EDTA solution to a dear yellow endpoint. Note: It is advisable to practice the titration (from step 5) using 10.0 mL of a 12.5 mgImL zinc standard solution.
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Journal of Chemical Education
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-- 65.38 glmol X V X M 1oOoXIXw
where V = volume of EDTA solution used (rnL), M = molarity of EDTA (0.075 MI, L = length of sample specimen (cm), and w = width of sample specimen (cm). American industrial engineers normally do not use the metric system! Coating weights are standardly reported in oz/ft2. SO, when the results are obtained in g/cm2, both should be converted to oz/ft2. Questions
1. What effect will leaving the sample in the acid too long have on your results? (Answer: The acid will dissolve some of the steel, which will make the coating weight results erroneouslv hieh.) 2. How could you check ;he corrosion resistance of the coat. ed and the uncoated samples? (Answer: Place each sample in a container of dilute sodium chloride.) 3. What methods are used to apply zinc coatings? (Answer: There are two methods most commonly used by industry: (1)electroplating, which in the case of zinc is called electrogalvanizing, and (2) dipping the steel in molten zinc, which is called hot-dipped galvanized.)
4. What other metal is used as a rust-resistant coating? (Answer: Aluminum is also used especially when the metal product must he heat resistant,-such as with automobile mufflers.) 5. What nonmetal coating is frequently applied to steel products in order to inhibit corrosion, and why is this used? (Answer: Paint is applied to many products such as automobiles and appliances primarily for appearance.) Results and Discussion The removal of the zinc coating from the steel sample by treatment with hydrochloric acid very quickly sets up the two analytical techniques. The stripped steel can he easily reweighed to provide the one result, and the zinc ion in solution is titrated with disodium EDTA to provide the other result. The two methods should give excellent agreement. All s u ~ ~ l i eofr szinc-coated steel have procedures for determining coating weights to insure that their products will meet specifications. Both of the procedures used in this experiment are used by industry. For material coated with only zinc, the gravimetric procedure is normally used hecause it is rapid and yields accurate results. The volumetric procedure is used for analyzing complex zinc coatings (e.g.,
Zn/Ni or ZnIFe). . . Caution must be exercised with the titration procedure because many elements may interfere if the titration conditions are not carsfully controlled. The procedures used in the experiment are for material (steel) with the same coating weight on both sides. Coating weights of products with a different coating thickness on opposite sides can he determined usingamodification of this procedure. Industries have a code to indicate the specifications for coating weights. For example, suppose that a G90 coating is specified for use in metal roofs. This means that the coating is zinc (galvanized) with a coating weight of 0.9 (901100) ounces per square foot. Conclusions The experiment provides students with an effective application of two important analytical techniques. I t is particularly appealing to students because they are performing the identical experiments done by chemists in industrial applications. A discussion of cost considerations will instill an appreciation for the importance of accuracy in measurements. Students also gain a practical knowledge of the nature and reactivity of some metals prevalent in everyday objects.
Volume 67
Number 1 January 1990
63
