Alkaline baths

An electrorefining plant may operate with either an acid or an alkaline bath. The acid bath contains stannous sulfate, cresolsulfonic or phenolsulfonic acids (to retard the oxidation of the stannous tin in the solution), and free sulfuric acid with P-naphthol and glue as addition agents to prevent tree-like deposits on the cathode which may short-circuit the cells. The concentration of these addition agents must be carefliUy controlled. The acid electrolyte operates at room temperature with a current density of ca 86—108 A/m, cell voltage of 0.3 V, and an efficiency of 85%. Anodes (95 wt % tin) have a life of 21 d, whereas the cathode sheets have a life of 7 d. Anode slimes may be a problem if the lead content of the anodes is high the anodes are removed at frequent intervals and scmbbed with revolving bmshes to remove the slime (7). 

Vat Dyes. These water-iasoluble dyes ate appHed mainly to ceUulosic fibers as soluble leuco-salts after teductioa ia an alkaline bath, usuaUy with sodium hydrosulfite. FoUowiag exhaustion onto the fiber, the leuco forms ate reoxidized to the iasoluble keto forms and aftertreated, usuaUy by soapiag, to redevelop the crystal stmcture. The principal chemical classes of vat dyes ate anthraquiaone and iadigoid. 

Pla.tinum, Platinum plating has found appHcation in the production of platinised titanium, niobium, or tantalum anodes which are used as insoluble anodes in many other plating solutions (see Metalanodes). Plating solutions were often based on platinum "P" salt, which is diamminedinitroplatiniim (IT). A dinitroplatinite sulfate—sulfuric acid bath has been used to plate direcdy onto titanium (129). This bath contains 5 g/L of the platinum salt, pH adjusted to 2.0 with sulfuric acid. The bath is operated at 40°C at 10—100 A/m. Other baths based on chloroplatinic acid have been used in both acid and alkaline formulations the acid bath uses 20 g/L of the platinum salt and 300 g/L hydrochloric acid at 65° C and 10—200 A/m. The alkaline bath uses 10 g/L of the platinum salt, 60 g/L of ammonium phosphate and ammonium hydroxide to give a pH of 2.5—9.0. The alkaline bath can be plated directly onto nickel-base alloys acid baths require a gold strike on most metals. 

Palladium and Palladium Alloys. Palladium is used in telephone equipment and in electronics appHcations as a substitute for gold in specific areas. Palladium is plated from ammoniacal and acid baths available along with chelated variations as proprietary processes. One typical alkaline bath uses 8 g/L diammine-dinitropalladium, 100 g/L ammonium nitrate, and 10 g/L sodium nitrite. The pH is adjusted to 9—10 using ammonium hydroxide, and the bath is operated at 100 A/m at 50° C. If ammonium sulfamate, 100 g/L, is used in some baths to replace the nitrate and sodium nitrite salts, the bath is mn at lower temperature, 25—35°C, and a pH of 7.5—8.5. A palladium—nickel alloy, 75% Pd, is plated from a bath having 6 g/L palladium from the same salt, 3 g/L nickel from nickel sulfamate concentrate, and 90 g/L ammonium hydroxide. The bath is operated at 20—40°C with 50-100 A/m/... 

Zinc—Iron. The Zn—Ee aHoy is plated from an alkaline bath. Deposits are 0.3—0.8% iron and can be given attractive, resistant, black, sHver-free chromate coatings. Corrosion protection requires the heavier, darker chromates. Zinc—iron baths are the most economical of the 2inc aHoys. 

Commercial processes Commercial electroless nickel plating stems from an accidental discovery by Brenner and Riddell made in 1944 during the electroplating of a tube, with sodium hypophosphite added to the solution to reduce anodic oxidation of other bath constituents. This led to a process available under licence from the National Bureau of Standards in the USA. Their solutions contain a nickel salt, sodium hypophosphite, a buffer and sometimes accelerators, inhibitors to limit random deposition and brighteners. The solutions are used as acid baths (pH 4-6) or, less commonly, as alkaline baths (pH 8-10). Some compositions and operating conditions are given in Table 13.17 . 

Electrolytic Tinplate. Much of the tin mill product is made into electrolytic tinplate (ETP). A schematic of an ETP cross section is given in Figure 1. The steel strip is cleaned electrolytically in an alkaline bath to remove rolling lubricants and dirt, pickled in dilute mineral acid, usually with electric current applied to remove oxides, and plated with tin. It is then passed through a melting tower to melt and reflow the tin coating to form the shiny tin surface and the tin-iron alloy layer, chemically treated to stabilize the surface to prevent growth of tin oxide, and lubricated with a thin layer of synthetic oil. 

Alkaline aqueous media containing selenosulfate ions (SeS03 ) and complexes of Cd with EDTA or nitrilotriacetate (NTA) have been successfully utilized to accomplish cathodic electrodeposition of nearly stoichiometric, wurtzite CdSe [62-64]. Similar results have been reported for seleno-cyanate (SeCN ) alkaline baths... 

Lokhande CD, Jadhav MS, Pawar SH (1989) Electrodeposition of ZnS films from an alkaline bath. J Electrochem Soc 136 2756-2757... 

If the process demands a gradual shift from about pH 9 to a slightly acidic pH, ammonium sulphate together with ammonia can be used. This gives a safer, more uniform development of acidity than can be achieved by making additions of acid to an alkaline bath, although the degree of acidity developed will clearly depend on the ease with which ammonia can escape from the system. In enclosed or partially enclosed machines this system does not function so efficiently [10-12]. 

The shaped pretzels are then given a short intermediate proof followed by passage through an alkaline bath of baking soda. Next the pretzels are covered with salt and baked at 230°C for 4—5 min. The effect of the alkaline treatment is to produce a crisp shiny crust on the baked piece as a result of the reactions of the starch reducing sugars and protein. 

Alkaline baths, in tin refining, 24 789 Alkaline batteries, 15 611 Alkaline catalysts, in phenolic resin polymerization, 18 762-765 Alkaline cleaners, for metal surfaces,... 

Dehairing is carried out using alkaline proteases such as subtilisin in a very alkaline bath. Alkaline conditions tend to swell the hair roots, so easing the removal of the hair by allowing the proteases to selectively attack protein in the hair follicle. Other specific enzymes are used for skins from particular species. 

Adjustment of pH by neutralization. Cellulose dyeing and finishing companies mainly release alkaline baths, which can be neutralized by introduction of C02-containing waste gas from the power/steam generation plant [76]. 

The effect of potential on the cathodic efficiency of a zinc electrodeposition in a cyanide-free alkaline bath was also investigated [416]. 

As has already been pointed out, in spite of the fact that the free sulphide mechanism is invariably assumed in thiosulphate depositions, there is no evidence up to now against the complex-decomposition mechanism. No thorough mechanistic or kinetic studies have been made on this system. Since the studies on CD using thiosulphate have not attempted to differentiate between these different mechanisms (and since such differentiation may be difficult, this is not surprising), we are left with the conclusion that there is no clear consensus on which mechanism is operative. Also, the mechanism may vary depending on conditions (as for the alkaline baths), and of course a combination of mechanisms may be operative in some cases. 

X-ray diffraction patterns, with peak widths characteristic of a crystal size of at least 10 nm and usually 20 nm or more, have been obtained from Bi2Ss fihns deposited from acidic solutions. In contrast, XRD has not shown any peaks for as-deposited films from alkaline baths, but annealing at low temperatures (150-200°C) was sufficient to give fairly sharp (equivalent to at least 20-mn crys-... 

The drag-out film on the workpieces coming from the alkaline bath is alkaline. Recycled water from acid bath rinses has an acid pH, and thus tends to neutralize the drag-out film. This reduces its viscosity, and accelerates die rinsing process (DHS 1988 USEPA 1982). 

Other rinsewater reuse opportunities are also available. Acid cleaning rinsewater effluent can be used as rinsewater for workpieces that have gone through a mild acid etch process. Effluent from a critical or final rinse operation, which is usually less contaminated than other rinse waters, can be used as influent for rinse operations that do not require high rinse efficiencies. Another option is using the same rinse tank to rinse parts after both acidic and alkaline baths. 

A large number of commercially important plating processes occur from complex ion baths in which the metal is a constituent of an anionic complex, e.g. copper, zinc, cadmium, silver and gold are all commonly plated from cyanide baths, and tin plates from a stannate bath in which [SnIV(OH)6]2 is present. Chromium is commonly plated from a chromate bath although in this case the background medium is acid rather than alkaline. Thus the mechanism of deposition of metals from anionic complexes is of particular interest. It will be instructive to comment on two situations, one occurring in alkaline baths, the other in acidic baths. 

Although, as explained above, zinc is commonly deposited from cyano baths, it may also be deposited from strongly alkaline baths ( alkaline non-cyanide bath ). Typically a zinc hydroxide ratio of 1 10 would be employed and the major solution species would be the tetrahedral zincate anion [Zn(OH)4]2" as evidenced from Raman studies.5 Careful studies have been carried out to investigate the electrode reaction.6 An electrokinetic study gave data consistent with a four-step mechanism for the overall process... 

Two common alternatives are available for electrodepositing tin 30 alkaline and acidic baths. The alkaline bath has good throwing power but consumes more power than the acid bath. Tin is present in the alkaline bath as stannate(TV), [Sn(OH)6]2, the bath being approximately 0.25 M in free hydroxide ion (pH 13.4). The hydroxide ion is the principal charge carrier. Potassium is superior to sodium as the counter ion (greater ionic mobility) but economic factors lead to the continued use of sodium in many plants. The hydroxide ions, acting as a sink for dissolved CO2, also prevent two undesirable reactions (equations 24 and 25). 

The deposits from the alkaline bath are fine grained, pure, but dull and somewhat porous. If bright non-porous films are required, the plated article is treated in a process known as flow brightening in which it is taken through a bath of palm oil operating at temperatures in excess of 232 °C, the melting point of tin. 

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