Hydroxide suppression mechanism

Zinc-nickel Zn-Ni alloys with 5 to 15 wt% Ni offer excellent corrosion resistance and are mainly used in the automotive, aerospace, and electronics industries. Above 15% Ni, the alloy coating becomes more noble than steel, and the corrosion-protection mechanism changes from a sacrificial to a pure physical one (comparable to pure Ni coatings, see Sect. 5.5.4.2.2). They can be electrode-posited from acid or alkaline baths. The acid baths are usually based on sulfate, chloride, sulfate-chloride, pyrophosphate, or acetate (Table 15). The system shows anomalous codeposition (see Sect. 5.5.1.2), which has been explained by a hydroxide suppression mechanism [47]. As in the case of Ni-Fe, the alkaline baths must contain complexing agents (see Sect. 5.5.4.6.2). The alloys electroplated from add haths contain approximately 10 to 14% Ni, whereas the alkaline Zn-Ni... 

Different explanations for such behavior are offered in the literature [3]. The most likely one appears to be the hydroxide suppression mechanism [20- 23]. According to this concept, coevolution of hydrogen during the electrodeposition causes an increase of pH at the electrode/solution interface, producing hydrolysis of less noble metal species and their precipitation as a layer of solid hydroxide. Formed hydroxide layer provides a good supply of ions of the less noble metal for their discharge and electrodeposition but suppresses the transport of species of the more noble metal to the cathode surface, causing anomalous codeposition. 

The dependence of at. % of both metals as a function of the log (Ni/Fe) ratio is presented in Fig. 5.27a. As it could be expected the content of Ni increases, while the content of Fe decreases with the increase of the Ni/Fe ratio. According to Fig. 5.27a almost linear dependence of the percentage of both metals in the powder as a function of the logarithm of their concentration ratios indicates that their content in the powder depends exponentially on the Ni/Fe ratio. Considering Fig. 5.27b it could be concluded that the anomalous character is more pronounced in the electrolytes containing Fe(III) salt. This is in accordance with the statement that the formation of some amount of [Fe(OH)2] is possible in this electrolyte and assuming hydroxide suppression mechanism [125, 126] the anomalous character of Fe and Ni deposition should be more pronounced in this electrolyte. 

A cation exchange membrane was developed to overcome these disadvantages. This membrane allows continuous regeneration and consists of a sulfonated polyethylene derivative. It is resistant to water-miscible organic solvents and exhibits a high permeability for quaternary ammonium bases such as tetrabutylammonium hydroxide. Hence, the suppression mechanism differs substantially from the process for anion exchange chromatography described in Section 3.3.3. 

Hornsby, P.R. and Watson, C.L., Aspects of the mechanism of fire retardancy and smoke suppression in metal hydroxide filled thermoplastics, IOP Short Meetings Series No 4, Institute of Physics, London, U.K., April 1997, p. 17. 

The afterglow of open-cell foams (punking) can be suppressed by the addition of flame-retardants such as boron trioxide or aluminum hydroxide. Hybrid foams with good mechanical properties and low combustibility can be produced from PF resins and polyisocyanates.104... 

Modem suppressors for cation chromatography are both efficient and self-regenerating. The principles are similar to the suppressors for anion chromatography, described in Chapter 6. The mechanism of suppression for a cation self-regenerating suppressor is illustrated in Fig. 7.1 and described in some detail by Rabin et al. [3]. Suppressors for cation chromatography are limited to those cations that do not form precipitates with the hydroxide ions from the suppressor. 

In assessing the potential anti-inflammatory activities of drugs against this type of reaction it must be remembered that a number of non-specific effects reduce the oedema . Thus Lorenz , and Biich and Wagner-Jauregg showed that hydrogen peroxide, sodium hydroxide, kaolin and talc have a nonspecific anti-inflammatory effect when administered intra-peritoneally. These substances suppress oedema produced by egg-white, dextran, 5-hydroxytryptamine and formalin, probably as the result of an obscure reflex mechanism caused by irritation of the tissues at the site of... 

At a pH above roughly 3, cobalt is deposited through a pH-dependent mechanism by the formation of a cobalt hydroxide complex (Co(OH)2 or CoOH [9, 10]. At a constant potential, the cobalt deposition current decreases with decreasing pH, indicating an increase in the inhibition of cobalt deposit. Therefore, dendrites should be suppressed at a lower pH. At a pH below roughly 3, cobalt electrodeposition occurs through the direct reduction of Co and is practically independent of pH [10]. 

Hornsby PR, Watson CL. A study of the mechanism of flame retardance and smoke suppression in polymers filled with magnesium hydroxide. Polym Degrad Stab, 1990 30 pp.73-87. 

Hornsby PR, Watson CL. Mechanism of smoke suppression and fire retardancy in polymers containing magnesium hydroxide filler. PlastRub Proc Appl, 1989 11 pp. 45-51. 

The use of plasticizers that are themselves flame retardants is discussed in Chapter 9, as well as formulating so as to optimize their effect and to minimize their tendency to be less efficient than flammable plasticizers. Chapter 9 also considers combustion mechanisms. A useful review of the latter has been given by Green. This chapter presents the use of inorganic additives as flame retardants and smoke suppressants as a basis for formulation for specific applications. These additives can be divided into antimony compounds and derivatives the class of metal hydroxides, carbonates, and basic carbonates and a further range of molybdenum, zinc, and iron compounds. 

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