Zinc surfaces

Fig. 4. Example.s of rough surfaces pretreated for adhesive bonding (a) microtibrous oxide on copper (cf. 29J) (b) a dendritic zinc surface (cf. [30J).

The Clemmensen reduction can be formulated to proceed by a sequence of one-electron and proton transfer reactions. It is a heterogenous reaction, taking place at the zinc surface. Initially an electron is transferred from zinc to the carbonyl group of ketone 1, leading to a radical species 3, which is presumed to react further to a zinc-carbenoid species 4 ... 

Polarization can be divided into activation polarization and concentration polarization , Activation polarization is an electrochemical reaction that is controlled by the reaction occurring on the metal-electrolyte interface. Figure 4-418 illustrates the concept of activation polarization where hydrogen is being reduced over a zinc surface. Hydrogen ions are adsorbed on the metal surface they pick up electrons from the metal and are reduced to atoms. The atoms combine to... 

White rust If a fresh zinc surface is allowed to stand with large drops of dew on it, as may easily happen if it is stored in a closed place in which the temperature varies periodically, it is attacked by the oxygen dissolved in the water, owing to differential aeration between the edges and the centres of the drops. A porous form of zinc oxide builds up away from the surface and quickly takes up carbon dioxide from the air to form the basic carbonate known as white rust or wet storage stain. 

Zinc Zinc surfaces corrode more slowly in the country than in either marine atmospheres or in industrial areas where sulphur pollution constitutes the main danger both to them and to many other building materials. 

Zinc in contact with wood Zinc is not generally affected by contact with seasoned wood, but oak and, more particularly, western red cedar can prove corrosive, and waters from these timbers should not drain onto zinc surfaces. Exudations from knots in unseasoned soft woods can also affect zinc while the timber is drying out. Care should be exercised when using zinc or galvanised steel in contact with preservative or fire-retardant-treated timber. Solvent-based preservatives are normally not corrosive to zinc but water-based preservatives, such as salt formulated copper-chrome-arsenic (CCA), can accelerate the rate of corrosion of zinc under moist conditions. Such preservatives are formulated from copper sulphate and sodium dichromate and when the copper chromium and arsenic are absorbed into the timber sodium sulphate remains free and under moist conditions provides an electrolyte for corrosion of the zinc. Flame retardants are frequently based on halogens which are hygroscopic and can be aggressive to zinc (see also Section 18.10). 

Zinc will initially react with cement-based materials with the evolution of hydrogen. This reaction can be controlled by the presence of soluble chromate either in the cement (over 70 ppm) or as a chromate passivation treatment to the zinc surface. Zinc can therefore be used to provide additional protection to steel in concrete. It is more effective in cmbonated concrete than in chloride-contaminated concrete. 

Chromating Chromating is considered in Section IS.3. The chromate film on zinc is adherent and can be drab, yellow-green or colourless in appearance the colour varies considerably with the method of application. It retards white rust , the white deposit which sometimes forms on fresh zinc surfaces which are kept under humid conditions (see Section 4.7). A chromate film is damaged by heat and if used as a basis for paint adhesion, should preferably not be heated above 10°C, nor for longer than 1 h. 

Paint applied to a suitably prepared zinc coating will last longer than would be the case if it were applied direct to iron or steel, and the need for repainting thus becomes less frequent. With hot-dip galvanised or zinc-plated coatings, however, it is necessary either to use special primers or to prepare the surface before painting. This is primarily because most oil-based paints react with the unprepared zinc surface to form zinc soaps resulting in poor adhesion. 

Schmid, E. V., Painting of Zinc Surfaces and Zinc-Containing Anticorrosive Primers, Monograph No. 3, OCCA, Wembley, UK (1986)... 

There is no intermetallic compound formation and the electrodeposit behaves as a simple mixture of the two metals. It can be considered as basically a stable wick of tin through which zinc is fed to be consumed at a rate lower than its consumption from a wholly zinc surface. If the conditions are such that zinc is rapidly consumed, and no protective layer of corrosion products is formed, the coating may break down, but in mildly corrosive conditions some of the benefits of a zinc coating, without some of its disadvantages, are obtained. 

In condensed moisture, there is sufficient corrosion of zinc to give protection at pores in a coating on steel without the formation of as much zinc corrosion product as would develop on a wholly zinc surface. In solder-ability the coating is tin-like when new or stored dry, but the selective corrosion of zinc in humid conditions may produce a layer obstructive to easy soldering. 

The quantitative results quoted above all refer to zinc surface. It is likely that the behaviour of cadmium would be similar in view of the fact that the equivalent weight of cadmium is double that of zinc, it is even more important that the passivation solution shall not attack and dissolve the metal to any appreciable extent. 

The consequences of shape change are densification and loss of electrode porosity, increased current density caused by loss of zinc surface area, and finally earlier passivation. Two different forms of pasi-vation can stop the discharge of a zinc electrode before the active material is exhausted. "Spontaneous" passivation occurs... 

At high temperatures (> 170 K), the water desorbs and so the autocatalytic reaction cannot be sustained and is an explanation for why the H2 + 02 reaction slows, the formation of OH species now being solely dependent on the H(a) + O(a) reaction, which is the slowest step in the above scheme. That the water + oxygen reaction was fast and facile was evident from the spectroscopic studies at both nickel and zinc surfaces, when the oxygen surface coverage was low and involving isolated oxygen adatoms. 

As demonstrated in Section 5.2, the electrode potential is determined by the rates of two opposing electrode reactions. The reactant in one of these reactions is always identical with the product of the other. However, the electrode potential can be determined by two electrode reactions that have nothing in common. For example, the dissolution of zinc in a mineral acid involves the evolution of hydrogen on the zinc surface with simultaneous ionization of zinc, where the divalent zinc ions diffuse away from the electrode. The sum of the partial currents corresponding to these two processes must equal zero (if the charging current for a change in the electrode potential is neglected). The potential attained by the metal under these conditions is termed the mixed potential Emix. If the polarization curves for both processes are known, then conditions can be determined such that the absolute values of the cathodic and anodic currents are identical (see Fig. 5.54A). The rate of dissolution of zinc is proportional to the partial anodic current. 

Two of the zinc surface samples show results that might intuitively be expected for metal coated surfaces. The polycarbonate and RIM-polyurethane substrates show much lower Fs values with the zinc coating as expected, but Q values are very similar to that of the uncoated substrates. For modified-polyphenylene oxide, however, a very low Q value is obtained suggesting flame... 

Half-time persistence of zinc in the prawn (Palaemon elegans) is about 17 days (Nugegoda and Rainbow 1988b), and between 30 and 270 days for five other crustacean species (NAS 1979). Differences in half-time persistence are finked to differences in excretion rates of ionic zinc and complexed zinc. In general, ionic zinc in crustaceans is excreted first, then complexed zinc surface-adsorbed zinc is turned over faster than internally adsorbed zinc molting accounts for 33 to 50% loss of the total body burden in crabs (Eisler 1981). 

Addition of sodium dodecyl benzene sulfonate to dilute alkaline electrolyte depresses the passivation of zinc surface [275]. Owing to the dodecyl benzene sulfonate adsorption, the passive layer on zinc has a loose and porous structure. Zinc electrodissolution was inhibited by the presence of sodium metasdicate [276] and some acridines [277]. The protection effect was described by a two-parameter equation. 

An important group of zinc corrosion inhibitors are compounds that are adsorbed on the metal surface and/or that form precipitate with Zn(II) on the zinc surface together with zinc hydroxide and oxide [301-307]. 

The electrochemical way can also be used to activate massive zinc being under plates or rod forms. First, the electrooxidation of the zinc anode by a catalytic amount of electricity allows one to strip the surface similarly to an acidic treatment. Second, the electrolytic deposit of a small amount of zinc on a massive zinc surface also leads to the activation of the metal according to a process which is not understood yet. 

Other electrochemical ways have been attempted to generate an activated zinc surface from massive zinc (rod or plate). Two types of activation have been developed. In the first one, a low current density is applied on the massive zinc, which is used as the anode. The electroscoring of the zinc surface makes it reactive towards organic halides that are easily reducible. These reactions are catalytic in electricity. This electroscoring, which is all the more efficient as the applied current density is low, is equivalent to an acidic treatment of the surface. 

In the second one, a catalytic amount of ZnX2 (X = Cl, Br) is reduced at the zinc surface to be activated. As described below, this activation gives excellent and reproducible results with organic halides that are relatively easily reducible. 

However, the mechanisms involved in these reactions have not been elucidated yet. Nevertheless, we can advance the formation of an organozinc compound able to reduce oxide-type impurities being at the zinc surface. 

In a divided cell and under low current intensity, the organozinc compounds are almost only produced in the anodic compartment. This can be explained by an attack process of the zinc anode by anodic electroscoring of the latter. This electroscoring generates an active zinc surface which chemically reacts with CF3Br (equation 10). 

In the typical old-fashioned Reformatsky protocol12a d, a mixture of a-bromoester, carbonyl compound and zinc powder is heated in a solvent, generally benzene, for several hours. Under these conditions, the chemical yields often suffer from the concurrence of side-reactions, such as self-condensation of enolizable aldehydes, Claisen condensation of bromoesters or crotonization of the Reformatsky products. However, ever since the outset of Reformatsky studies, chemists have been aware about the need to activate the zinc surface in order to get higher reaction rates and shorter induction times before the process starts, with lower by-product formation. Thus, it became common practice to... 

In general for a highly reflective sample, 50 to 60 percent of the energy is lost through the reflection optics. If the optical constants of the film are known, the thickness of the film can be calculated based on optical theory 134-144>. Studies have been made of thin films of poly(methyl methacrylate) depolymerization on gold, nickel and zinc surfaces using ERS 135) and FT-IR. 

The most important use is in primers for zinc-coated substrates. The pH change occurring on hydrolysis of the calcium plumbate etches the zinc surface which improves adhesion of primers, especially on hot-dip galvanized steel [5.151],... 

This can be substantially improved by high black and high sulphur compounds. The only bottleneck is that not all metals are conducive to rubber bonding. Carbon steels are better than cast iron for bonding with rubber. Copper and zinc surfaces can be bonded with much ease. 

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