Corrosion amphoteric metals

FIG. 28-2 Effect of pH on the corrosion rate, a) Iron, (h) Amphoteric metals (aluminum, zinc), (c) Noble metals. 

Variable pH of returned condensate, which affects FW quality BW is always alkaline (often strongly caustic) and highly alkaline condensate may cause amphoteric corrosion to metal components in the condensate system... 

NOTE Although zinc is commonly used as an example of an amphoteric metal, it should be considered that iron and steel are also amphoteric, albeit wider swings in pH are required for corrosion to occur. 

FIGURE 10.3. Corrosion rate at different pH for amphoteric metals (or oxides) /, Zn, Sn... 

Mild steel may be suitable as a material of construction for handling caustic soda at ambient temperature. At elevated temperatures, >60°C (140°F), corrosion may occur. Nickel and/or its alloys are most suitable for caustic handling at all temperatures and concentrations, including anhydrous molten caustic up to 480°C (896°F) (Leddy et al. 1978). Polypropylene, fluorocarbon plastics, and flberglass/vinyl ester resins are being used for many applications. Aluminum, tin, zinc, and other amphoteric metals should not be used in construction materials. 

CORROSION, CATHODIC - Corrosion resulting resulting from a cathodic condition of a structure usually caused by the reaction of an amphoteric metal with the alkaline products of electrolysis. 

The concept of corrosive carbonic acid is in fact non-specific. It would be more correct to refer to the corrosiveness of the water which can be caused by inorganic or organic acids giving the water a pH in the acid range, or for example for amphoteric metals to a corrosiveness which is caused by a... 

These equations represent oxidation in the chemical sense. Corrosion of metals usually occurs at the anode. Nevertheless, alkahne reaction products forming at the cathode can sometimes cause secondary corrosion of amphoteric metals, such as Al, Zn, Pb, and Sn, which corrode rapidly on exposure to either acids or alkalies. 

If the metal is polarized slightly beyond the open-circuit potential, of the anode, the corrosion rate remains zero. Net current flows from the electrolyte to the metal hence, metal ions cannot enter the solution. Current in excess of the required does no good, however, and may damage amphoteric metals and coatings. In practice, therefore, the impressed current is kept close to the theoretical minimum. Should the applied current fall below that required for complete protection, some degree of protection nevertheless occurs. For example, if the corrosion potential is moved from to a in Fig. 5.15, by appUed current e-b, the corrosion current decreases from Icon to b. As the applied current e-b is increased, the potential a moves to more active values, and the corrosion current b becomes smaller. When a coincides with < )x, the corrosion current becomes zero, and the applied current for complete cathodic protection equals /appi. 

Figure 2-10 shows weight loss rate-potential curves for zinc in artificial soil solutions and tapwater [36,37]. Sufficiently low corrosion rates are only to be expected in water containing HCO 3. According to Eq. (2-53 ), the protection potential is t/jj = -0.96 V. Since zinc is an amphoteric metal, its susceptibility to corrosion increases in caustic solutions or in saline solutions with strong cathodic polarization. The system zinc/water belongs to group II, with a critical potential at... 

The Zn-HjO (Fig. 1.17) diagramshows that extensive corrosion zones exist at both low and hi pH values (compare the very restricted corrosion zone in the Fe-H20 diagram at high pHs) similar zones in the region of low and high pH are obtained with other amphoteric metals such as aluminium, lead and tin. The diagram for Zn-H20 predicts with some accuracy the behaviour of the metal in practice, where it has been established that zinc corrodes rapidly outside the range pH 6-12 5 but is passive within... 

The effect of oxygen and pH on the corrosion rate of steel at two temperatures is shown in Fig. 8.7 [11]. In a broad range of about pH 5 to 9, the corrosion rate can be expressed simply in terms of the amount of DO present (e.g., micrometer per year per milliliter DO per liter of water). At about pH 4.5, acid corrosion is initiated, overwhelming the corrosion rate by DO. At about pH 9.5 and above, deposition of insoluble ferric hydroxide, FelOHlj, or magnetite, FejO, tends to slow down the corrosion attack. Amphoteric metals such as aluminum, zinc, and lead, are however additionally sensitive to high pH situations and show a corrosion rate increase in alkaline enviromnents. Figure 8.8 compares the behavior of steel and aluminum as a function of pH. 

Anhydrous NaOH reacts very slowly with most substances, at room temperature it attacks most metals only slowly (iron, magnesium, calcium). The corrosion rate increases rapidly with increasing temperature. Amphoteric metals such as zinc, aluminum, tin and lead are attacked by dilute NaOH solutions at room temperature, iron,... 

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. 

Acid-soluble metals such as iron have a relationship as shown in Fig. 28-2 7, In the middle pH range ( 4 to 10), the corrosion rate is controlled by the rate of transport of oxidizer (usually dissolved O9) to the metal surface. Iron is weakly amphoteric. At very high temperatures such as those encountered in boilers, the corrosion rate increases with increasing basicity, as shown by the dashed line. 

Pitting is also promoted by low pH. Thus, acidic deposits contribute to attack on stainless steels. Amphoteric alloys such as aluminum are harmed by both acidic and alkaline deposits (Fig. 4.4). Other passive metals (those that form protective corrosion product layers spontaneously) are similarly affected. 

The most harmful deposits are those that are water permeable. Truly water-impermeable material is protective, since without water contacting metal surfaces corrosion cannot occur. Innately acidic or alkaline deposits are troublesome on amphoteric alloys (those attacked at high and low pH—e.g., aluminum and zinc). 

Certain alloys frequently used in cooling water environments, notably aluminum and zinc, can be attacked vigorously at high pH. These metals are also significantly corroded at low pH and thus are said to be amphoteric. A plot of the corrosion behavior of aluminum as a function of pH when exposed to various compounds is shown in Fig. 8.1. The influence of various ions is often more important than solution pH in determining corrosion on aluminum. 

The primary corrosion product PbHj is unstable and decomposes in a subsequent reaction into lead powder and hydrogen gas. Figures 2-11 and 2-12 are typical examples of cathodic corrosion of amphoteric and hydride-forming metals. 

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