Review Questions
29.1 Why are metals coated?
Answer. (1) Corrosion protection, (2) enhance appearance, (3) provide a specified color, and (4) prepare surface for subsequent processing.
29.2 Identify the most common types of coating processes.
Answer. The common coating processes are: (1) plating, (2) chemical surface treatments, such as anodizing, (3) vapor deposition processes such as PVD and CVD, (4) organic coating - painting, (5) porcelain enameling, and (6) thermal and mechanical treatments.
29.3 What are the many reasons why a metallic surface is plated?
Answer. The reasons are: (1) corrosion protection of the substrate metal, (2) attractive
appearance, (3) wear resistance, (4) increased electrical conductivity, (5) improved solderability, and (6) enhanced lubricity of the surface.
29.4 What is meant by the term cathode efficiency in electroplating?
Answer. The cathode efficiency is the actual amount of metal deposited on the surface relative to the theoretical amount given by Eq. (33.1).
29.5 What are the two basic mechanisms of corrosion protection?
Answer. The mechanisms are (1) barrier protection, in which the coating simply covers the substrate to protect it, and (2) sacrificial protection, in which the coating metal corrodes sacrificially to protect the substrate.
29.6 What is the most commonly plated substrate metal?
Answer. Steel.
29.7 One of the mandrel types in electroforming is a solid mandrel. How is the part removed from a solid mandrel?
Answer. A solid mandrel has certain geometric features, such as a taper, that permit the part to be removed. Parts are also sometimes removed by taking advantage of a difference in coefficient of thermal expansion.
29.8 How does electroless plating differ from electrochemical plating?
Answer. Electroless plating uses only chemical reactions to form the plating; electroplating uses electrolysis.
29.9 What is a conversion coating?
Answer. A conversion coating is a thin coating produced by chemical reaction of the metallic surface. The most common conversion coatings are phosphates, chromates, and oxides.
29.10 How does anodizing differ from other conversion coatings?
Answer. Anodizing uses electrochemical processing methods to convert the metallic surface.
Best example is aluminum anodizing.
29.11 What is physical vapor deposition?
Answer. Physical vapor deposition (PVD) refers to a family of processes in which a material is converted to its vapor phase in a vacuum chamber and condensed onto a substrate surface as a very thin film.
29.12 What is the difference between physical vapor deposition (PVD) and chemical vapor deposition (CVD)?
Answer. In PVD, the coating vapors are synthesized by heating the coating material and allowing it to condense as a thin film on the surface of the workpart. In CVD a coating is formed on a heated substrate by the chemical reaction or dissociation of vapors and/or gases; the reaction product nucleates and grows on the substrate surface.
29.13 What are some of the applications of PVD?
Answer. PVC applications include: decorative coatings on trophies and automotive trim,
antireflection coatings on optical lenses, deposition of metal in electronic connections, and cutting tool coatings (e.g., TiN).
29.14 Name the three basic types of PVD.
Answer. The three types are: (1) vacuum evaporation, (2) sputtering, and (3) ion plating.
29.15 What is a commonly used coating material deposited by PVD onto cutting tools?
Answer. Titanium nitride (TiN). Titanium carbide (TiC) and aluminum oxide (Al2O3) might also be mentioned.
29.16 Define sputtering yield?
Answer. Sputtering yield is defined as the number of atoms ejected from the surface of a substance per ion bombarding the surface.
29.17 What are some of the advantages of chemical vapor deposition?
Answer. Advantages include: (1) capability to deposit refractory materials at temperatures below their melting or sintering temperatures; (2) grain size control; (3) process is performed at
atmospheric pressure; and (4) good bonding to substrate surface.
29.18 What are the two most common titanium compounds that are coated onto cutting tools by chemical vapor deposition?
Answer. TiC and TiN.
29.19 Identify the four major types of ingredients in organic coatings?
Answer. The major ingredients are: (1) binder, which are polymers, (2) dyes or pigments, which provide color, (3) solvents, and (4) additives such as surfactants and plasticizers.
29.20 What is meant by the term transfer efficiency in organic coating technology?
Answer. Transfer efficiency indicates how much of the organic coating liquid reaches the target surface.
29.21 Describe the principal methods by which organic coatings are applied to a surface.
Answer. The main methods include: brushing and rolling, spraying, immersion (dip coating), and flow coating. These methods are described in Article 33.4.2.
29.22 The terms drying and curing have different meanings; indicate the distinction.
Answer. Drying means evaporation of solvents in the organic coating liquid. Curing involves a chemical change in the organic resin (polymerization and/or cross-linking) which hardens the coating.
29.23 In porcelain enameling, what is frit?
Answer. Frit is glassy porcelain prepared as fine particles (powders) by crushing and milling.
29.24 What does the term mechanical galvanizing refer to?
Answer. Mechanical galvanizing refers to the mechanical plating of zinc onto a substrate, with thicknesses up to 0.003 in.
Multiple Choice Quiz
There are a total of 17 correct answers in the following multiple choice questions (some questions have multiple answers that are correct). To attain a perfect score on the quiz, all correct answers must be given, since each correct answer is worth 1 point. For each question, each omitted answer or wrong answer reduces the score by 1 point, and each additional answer beyond the number of answers required reduces the score by 1 point. Percentage score on the quiz is based on the total number of correct answers.
29.1 Which one of the following plate metals produces the hardest surface on a metallic substrate? (a) cadmium, (b) chromium, (c) copper, (d) nickel, or (e) tin.
Answer. (b)
29.2 Which one of the following terms is used in connection with dip coating of lead onto a substrate such as sheet steel? (a) aluminizing, (b) anodizing, (c) conversion coating, (d) galvanizing, or (e) terneplating.
Answer. (e)
29.3 Which one of the following plating metal is associated with the term galvanizing? (a) iron, (b) lead, (c) steel, (d) tin, or (e) zinc.
Answer. (e)
29.4 Which of the following is most typical of the thickness of an electroplated coating (choose either of two acceptable answers)? (a) 0.0001 in, (b) 0.001 in, (c) 0.010 in, or (d) 0.100 in.
Answer. either (a) or (b) is acceptable
29.5 Which of the following processes involves electrochemical reactions (more than one)? (a) anodizing, (b) chromate coatings, (c) electroless plating, (d) electroplating, or (e) phosphate coatings.
Answer. (a) and (d).
29.6 With which one of the following metals is anodizing most commonly associated (one answer)? (a) aluminum, (b) magnesium, (c) steel, (d) titanium, or (e) zinc.
Answer. (a)
29.7 Sputtering is a form of which one of the following? (a) chemical vapor deposition, (b) defect in arc welding, (c) diffusion, (d) ion implantation, or (e) physical vapor deposition.
Answer. (e)
29.8 Which of the following gases is the most commonly used in sputtering and ion plating? (a) argon, (b) chlorine, (c) neon, (d) nitrogen, or (e) oxygen.
Answer. (a)
29.9 The Mond process is used for which one of the following? (a) chemical vapor deposition of silicon nitride onto silicon, (b) an electroplating process, (c) physical vapor deposition for coating TiN onto cutting tools, or (d) reducing nickel carbonyl to metallic Ni.
Answer. (d)
29.10 Which of the following thin film processes is most common in semiconductor processing? (a) chemical vapor deposition or (b) physical vapor deposition.
Answer. (a)
29.11 The principal methods of applying powder coatings are which of the following (select two best answers)? (a) brushing, (b) electrostatic spraying, (c) fluidized bed, (d) immersion, and (e) roller coating.
Answer. (b) and (c).
29.12 Porcelain enamel is applied to a surface in which one of the following forms? (a) liquid emulsion, (b) liquid solution, (c) molten liquid, or (d) powders.
Answer. (d)
29.13 Which of the following are alternative names for thermal spraying (more than one answer)? (a) flexible overlay process, (b) hard facing, (c) metallizing, or (d) metal spraying.
Answer. (c) and (d).
29.14 Hard facing utilizes which one of the following basic processes? (a) arc welding, (b) brazing, (c) dip coating, (d) electroplating, (e) mechanical deformation to work harden the surface.
Answer. (a) Problems
Electroplating
29.1 What volume (cm3) and weight (g) of zinc will be deposited onto a cathodic workpart if 10 amps of current are applied for one hour?
Solution: From Table 29.1, C = 4.75 x 10-2 mm3/A-s, cathode efficiency E = 95%.
Volume V = ECIt = 0.95(4.75 x 10-2 mm3/A-s)(10 A)(1 hr)(3600 s/hr) = 1624.5 mm3
= 1.6245 cm3
Density of zinc from Table 4.1 ρ = 7.15 g/cm3. Weight W = 1.6245(7.15) = 11.615 g
29.2 A sheetmetal steel part with surface area A = 100 cm2 is to be zinc plated. What average plating thickness will result if 15 amps are applied for 12 minutes in a chloride electrolyte solution?
Solution: From Table 29.1, C = 4.75 x 10-2 mm3/A-s, cathode efficiency E = 95%.
Volume V = ECIt = 0.95(4.75 x 10-2 mm3/A-s)(15 A)(12 min)(60 s/min) = 487.35 mm3 Area A = 100 cm2 = 10,000 mm2
Plating thickness d = 487.35 mm3/10,000 mm2 = 0.049 mm
29.3 A sheetmetal steel part with surface area A = 15.0 in2 is to be chrome plated. What average plating thickness will result if 15 amps are applied for 10 minutes in a chromic acid-sulfate bath?
Solution: From Table 29.1, C = 0.92 x 10-4 in3/A-min, cathode efficiency E = 15%.
Volume V = ECIt = 0.15(0.92 x 10-4)(15)(10) = 0.00207 in3. Plating thickness d = 0.00207/15 = 0.000138 in.
29.4 Twenty-five jewelry pieces, each with a surface area = 0.5 in2 are to be gold plated in a batch plating operation. (a) What average plating thickness will result if 8 amps are applied for 10 min in a cyanide bath? (b) What is the value of the gold that will be plated if one ounce of gold is valued at $300? The density of gold = 0.698 lb/in3.
Solution: (a) From Table 29.1, C = 3.87 x 10-4 in3/A-min, cathode efficiency E = 80%.
Volume V = ECIt = 0.80(3.87 x 10-4)(8)(10) = 0.02477 in3.
With Q = 25 pieces and average area per piece = 0.5 in2, total area A = 25(0.5) = 12.5 in2 Plating thickness d = 0.02477/12.5 = 0.00198 in.
(b) Given density for gold ρ = 0.698 lb/in3
Weight of plated gold = (0.698 lb/in3)(0.02477 in3) = 0.01729 lb = 0.277 oz.
At $300/oz, value of plated gold = $300(0.277) = $82.99
29.5 A part made of sheet steel is to be nickel plated. The part is a rectangular flat plate which is 0.075 cm thick and whose face dimensions are 14 cm by 19 cm. The plating operation is carried out in an acid sulfate electrolyte, using a current I = 20 amps for a duration t = 30 min. Determine the average thickness of the plated metal resulting from this operation.
Solution: From Table 29.1, C = 3.42 x 10-2 mm3/A-s, cathode efficiency E = 95%.
Volume V = ECIt = 0.95(3.42 x 10-2 mm3/A-s)(20 A)(30 min)(60 s/min) = 1169.6 mm3. Area A = 2(19 x 14) + 0.075 x 2(19 + 14) = 536.95 cm2 = 53,695 mm2
Plating thickness d = 1169.6/53,695 = 0.022 mm
29.6 A steel sheetmetal part has total surface area A = 36 in2. How long will it take to deposit a copper plating (assume valence = +1) of thickness = 0.001 in onto the surface if 15 amps of current are applied?
Solution: From Table 29.1, C = 2.69 x 10-4 in3/A-min, cathode efficiency E = 98%.
Required volume of plate metal = 36(0.001) = 0.036 in3
Plated volume V = ECIt = 0.98(2.69 x 10-4 in3/A-min)(15 A) t = 0.003954 t in3. 0.003954 t = 0.036 t = 0.036/0.003954 = 9.1 min.
29.7 Increasing current is applied to a workpart surface in an electroplating process according to the relation I = 12.0 + 0.2t, where I = current, amps; and t = time, min. The plating metal is chromium, and the part is submersed in the plating solution for a duration of 20 min. What volume of coating will be applied in the process?
Solution: From Table 29.1, C = 0.92 x 10-4 in3/A-min, cathode efficiency E = 15%.
Plated volume V = EC∫Idt = EC∫(12 + 0.2t)dt = EC(12t + 0.1t2) over the range 0 to 20 min.
V = 0.15(0.92 x 10-4)(12 x 20 + 0.1(20)2) = 0.00386 in3
29.8 A batch of 100 parts are to be nickel plated in a barrel plating operation. The parts are identical, each with a surface area A = 7.8 in2. The plating process applies a current I = 120 amps, and the batch takes 40 minutes to complete. Determine the average plating thickness on the parts.
Solution: From Table 29.1, C = 1.25 x 10-4 in3/A-min, cathode efficiency E = 95%.
Volume V = ECIt = 0.95(1.25 x 10-4)(120)(40) = 0.57 in3.
Area A = 100(7.8) = 780 in2 Plating thickness d = 0.57/780 = 0.00073 in.