Alkaline solutions, lead corrosion

Vitreous silica is susceptible to attack by alkaline solutions, especially at higher concentrations and temperatures. For 5% NaOH at 95°C, although craving may be evident, surface corrosion is only 10 p.m after 24 h (87). For 45 wt % NaOH at 200°C, dissolution proceeds at 0.54 mm /h (88). The corrosion rates in other alkaline solutions are Hsted in Table 3. Alkaline-earth ions inhibit alkaline solution attack on vitreous siUca. Their presence leads to the formation of hydrated metal siUcate films which protect the glass surface (90). 

Zinc is attacked at high pH. However, in weakly alkaline solutions near room temperature, corrosion is actually very slight, being less than 1 mil/y (0.0254 mm/y) at a pH of 12. The corrosion rate increases rapidly at higher pH, approaching 70 mil/y (1.8 mm/y) at a pH near 14. Just as in aluminum corrosion, protection is due primarily to a stable oxide film that forms spontaneously on exposure to water. High alkalinity dissolves the oxide film, leading to rapid attack. 

Some metals are amphoteric. That is, they form simple cations (in acid solutions) and soluble oxyanions (in alkaline solution) only in the mid-pH range is a protective film stable. Since cathodic protection produces alkali at the structure s surface, it is important to restrict the polarisation, and thereby the amount of hydroxyl ion produced, in these cases. Thus both lead and aluminium will suffer cathodic corrosion under cathodic protection if the potential is made excessively electro negative. 

The pH of a metalworking fluid must be kept above neutrality in order to prevent acid corrosion of the metal In vitro, acid catalyzed nitrosation is optimized at pH 3.5 (4 0) however, it has been shown that In the presence of other catalysts, aqueous solutions of amines and nitrite leads to significant yields of nitrosamines at room temperature over the pH range of 6.4 to 11.0 (41). Furthermore, C-nitro-containing, formaldehyde-releasing biocides, such as bronopol or tris nitro, exert their potential catalytic effect in alkaline solution. It would thus be desirable to determine the optimum pH for a metalworking fluid that would lead to the lowest concentration of nitrosamine possible. 

Another example for the HMRRD electrode is given in Fig. 9 for Fe in alkaline solutions [12, 27]. The square wave modulation of the rotation frequency co causes the simultaneous oscillation of the analytical ring currents. They are caused by species of the bulk solution. Additional spikes refer to corrosion products dissolved at the Fe disc. This is a consequence of the change of the Nemst diffusion layer due to the changes of co. This pumping effect leads to transient analytical ring currents. Besides qualitative information, also quantitative information on soluble corrosion products may be obtained. The size of the spikes is proportional to the dissolution rate at the disc, as has been shown by a close relation of experimental results and calculations [28-30]. As seen in Fig. 7, soluble Fe(II) species are formed in the po-... 

Alkalis Most metals are protected by a passive oxide in mildly alkaline solutions, but the protective oxide will redissolve in strong alkali to form oxy-anions of the metal, allowing corrosion to occur. For carbon steels, the region of corrosion in alkali is very limited, but it can lead to the serious problem of caustic stress corrosion cracking (SCC). 

The theoretical potential-pH domains for corrosion, immunity, and passivation are shown in Figure 28.1 [1]. As shown in the diagram, in acid or neutral solution, lead can be cathodicaUy protected by controlling the potential to less than -0.3 V. In alkaline solution, potential control for cathodic protection is below -0.4 to -0.8 V, depending on the pH [1]. 

Zinc ions inhibit corrosion by a cathodic polarization mechanism based on the precipitation of a zinc hydroxide film at cathodic sites on the metal surface. Zinc in combination with phosphates will lead to a protective film containing zinc phosphate. Film formation is usually rapid due to the low solubility of the zinc compounds at an alkaline pH. The low solubility of zinc in alkaline solutions requires the incorporation of dispersants. The rate of film formation with cathodic inorganic inhibitors should be carefully controlled, as dangerous fouling may occur. Protective films caused by cathodic inhibition are macroscopic and often easily visible, whereas anodic inhibitors generally from very thin, hardly detectable passive films. 

A unique application for zirconium is in processes that cycle between HCl or H2SO4 and alkaline solutions. One company replaced a lead-and-brick-lined carbon steel reactor vessel with zirconium because the reaction alternated between hot H2SO4 and caustic. The vessel has been in use for several years with no corrosion problems. 

In alkaline solutions, corrosion of steel is controlled by the rate of oxygen diffusion through the precipitated corrosion product (usually ferrous hydroxide, Fe(OH)2), so corrosion rates are low. Steel is easily passivated in alkaline solutions. Amphoteric metals such as aluminum, zinc, and lead corrode slowly at low alkali concentrations, but above pH 9.0 their rates are very high and inhibitors are required. 

Effect of Temperature. The effect of temperature on the corrosion of aluminum by high-purity water has already been mentioned. In general, an increase in temperature leads to a higher corrosion rate in many chemicals such as mineral acids, organic acids, and alkaline solutions. However, the relationship might not be simple, as shown in Fig. 11 for sulfuric acid. In other chemicals and in waters, the accelerating effect can be counteracted by the formation of a protective film. For exan le, in monoethanolamine, increasing temperature reduces the rate of corrosion as a result of surface film formation. 

Deposition corrosion is a special case of galvanic corrosion that takes the form of pitting. It occurs when particles of a more cathodic metal in solution plate out on an aluminum surface to set up local galvanic cells. The ions aggressive to aluminiim ate copper, lead, mercury, nickel, and tin, often referred to as heavy metals. The effect of heavy metals is greato in acidic solutions. In alkaline solutions, their solubility is much lower, resulting in less severe effects (Ref 27). 

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