Zinc bath

Examples of plating solutions having good throwing power include cyanide plating baths such as copper, zinc, cadmium, silver, and gold, and noncyanide alkaline zinc baths. Examples of poorer throwing power baths are acid baths such as copper, nickel, zinc, and hexavalent chromium. 

Use only zinc not lower in quality than high grade per ASTM Specification B-6. Heat zinc to a range allowing pouring at 950°F (510°C) to 975°F (524°C). Skim off any dross accumulated on the surface of the zinc bath. Pour molten zinc into the socket basket in one continuous pour if possible. Tap socket basket while pouring. 

Zinc antimonide, 3 44, 53—54 Zinc atomizing process, 26 598 Zinc baths, 9 828-829, 830t Zincblende semiconductors, 22 141 band structure of, 22 142-144 transport properties of, 22 148, 149t Zinc borates, 4 282-283 Zinc brass... 

A great number of measurements have been reported for articles electroplated with zinc. The various aims have been evaluation of the corrosion rate of zinc that had been plated in a number of commercial cyanide-free zinc baths," comparison of the corrosion rate of a composite material (zinc with codeposits of various oxides) and of pure zinc deposits," corrosion testing of various alloyed zinc platings (Zn-Ni, Zn-Co, Zn-Fe), with or without subsequent post-treatment. Most of the work in the last category was only recorded in internal reports. The published work consists of an examination of the corrosion behavior of a ctoomated Zn-Fe... 

Today about one-third of all the zinc metal is used for the process known as galvanization. This process provides a protective coating of zinc on other metals. A thin layer of zinc oxidizes in air, thus providing a galvanic corrosion protection to the iron or steel item that it coats. Several processes are used to galvanize other metals. One is the hot dip method wherein the outer surface of the item to be galvanized is pickled and then immersed into a molten zinc bath. A... 

Zinc ash as a solid waste at the surface of the zinc bath... 

This can have some interesting effects. For example, in a mixed copper—zinc bath containing cyanide, since Astab(Cu) > Mstab(Zn), it is possible for the two metals to be deposited at almost the same potential and, indeed, the alloy brass may be plated from such solutions (vide infra). 

Tin—Zinc, Baths for tin—zinc alloys stem from work done after World War II in efforts to find a substitute for cadmium. Although alloys of all concentrations are possible, 80% tin—20% zinc gives the best combination of properties. This alloy has a low coefficient of friction, low electrical contact resistance, is solderable, slightly anodic to steel, and does not form voluminous corrosion products. In addition, the tin—zinc alloy has good paint adhesion qualities, good ductility, and is easily spotwelded. 

For general zinc plating in the United States, estimates of the zinc baths in use in 1970 showed cyanide at over 90%, chloride at 3%, and noncyanide alkaline (zincate) baths at 4%. By 1990, cyanide zinc was 20%, chloride 50%, and zincate 30% (138). In 1992 (139) the cyanide was 16%, chloride 48%, and zincate 36%. Moreover, the cyanide zinc baths of 1992 were more likely to be run at 15—30 g/L sodium cyanide in contrast to the 100 g/L normal in 1970. Each of the three zinc systems uses proprietary brighteners. 

Muffle Furnaces or Retorts of Graphite or Silicon Carbide. The metal is fed into the furnace either batchwise as a solid or continuously as a liquid. The heat of vaporization is supplied by heating the outside of the retort with a burner. In a muffle furnace the vaporizing section is separated by a silicon carbide arch from the heating chamber. The heat from burning gas or oil in the heating chamber is transferred to the zinc bath from the arch by radiation. 

To Obtain Byrogallic Add. It may be prepared by boating goUio acid (previously dried at 212 Fahr.) m a glass retort, by means of a chloride of zinc bath, to 419 , when the pure acid sublimes, and forms in crystals on the neck of the retort, and in the receiver, which should be kept well cooled. 

The final solution for corrosion protection is the coating of the surface with a film of an inert substance. This can be a layer of a metal with better protection properties than the bulk material. One example is the covering of iron or unalloyed steel by a zinc layer with a thickness of some micrometers. Zinc coating is possible by dipping into a hot zinc bath or by electroplating. The zinc layer must be further treated for sufficient corrosion protection. This can be chromatizing as described in the previous chapter or by a newly developed process free of chromate. 

Coating structure can also be greatly influenced by the composition of the molten zinc bath for example, small aluminum additions reduce or... 

Materials of widely differing sectional thickness must not be combined in the same stmcture otherwise heating in the zinc bath will take place... 

The source of the iron that oxidizes to give the brown stain has been studied by Sjoukes (1991), who concluded it is free iron in the galvanized layer and arises from (a) the release of iron from the phase during the 8- conversion, (b) iron present in the liquid zinc bath, and (c) the consumption of the zinc outer layer by the I phase, thus releasing iron during the continued growth of the alloy layer while the component is hot. 

Fig. 3.16 Corrosion in flowing seawater for various types of galvanized coating, showing the effect of diffusion annealing on the corrosion resistance in flowing seawater for coatings obtained in a zinc bath with 0.04% (I, 111, IV) and 0.12% Al (II, V-X). Length of test, 3600 hours. Coatings I and II had not been heat-treated. Coatings I V-X had been heat-treated (1) in a furnace, and (2) in an inductor (Pros-kurkin, 1973b).

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