Sacrificial anode anodic dissolution

The most significant chemical property of zinc is its high reduction potential. Zinc, which is above iron in the electromotive series, displaces iron ions from solution and prevents dissolution of the iron. For this reason, zinc is used extensively in coating steel, eg, by galvanizing and in zinc dust paints, and as a sacrificial anode in protecting pipelines, ship hulls, etc. 

Naturally, because the protection depends on the dissolution of the anodes, these require replacement from time to time (hence the term sacrificial anodes). In order to minimise the loss of anode metal, it is important to have as good a barrier layer around the pipe as possible, even though the pipe would still be protected with no barrier layer at all. 

Contact with steel, though less harmful, may accelerate attack on aluminium, but in some natural waters and other special cases aluminium can be protected at the expense of ferrous materials. Stainless steels may increase attack on aluminium, notably in sea-water or marine atmospheres, but the high electrical resistance of the two surface oxide films minimises bimetallic effects in less aggressive environments. Titanium appears to behave in a similar manner to steel. Aluminium-zinc alloys are used as sacrificial anodes for steel structures, usually with trace additions of tin, indium or mercury to enhance dissolution characteristics and render the operating potential more electronegative. 

For use in high resistivity soils, the most common mixture is 75% gypsum, 20% bentonite and 5% sodium sulphate. This has a resistivity of approximately 50 ohm cm when saturated with moisture. It is important to realise that carbonaceous backfills are relevant to impressed current anode systems and must not be used with sacrificial anodes. A carbonaceous backfill is an electronic conductor and noble to both sacrificial anodes and steel. A galvanic cell would therefore be created causing enhanced dissolution of the anode, and eventually corrosion of the structure. 

Anode Materials General Requirements A major problem and thus a decisive factor for the choice of anode materials is corrosion, except when the dissolution of a metal is the desired reaction ( sacrificial anodes , see Sect. 2.4.1.2.4). The stability of anode materials is extremely dependent on the composition of the anolyte (e.g. pH value, aqueous or non-aqueous medium, temperature, presence of halogenides, etc.). 

An electroreductive Barbier-type allyla-tion of imines (434) with allyl bromide (429) also occurs inaTHF-PbBr2/Bu4NBr-(Al/Pt) system to give homoallyl amine (436) (Scheme 151) [533]. The combination of Pb(II)/Pb(0) redox and a sacrificial metal anode in the electrolysis system plays a role as a mediator for both cathodic and anodic electron-transfer processes. The metals used in the anode must have a less positive anodic dissolution potential than the oxidation potentials of the organic materials in order to be present or to be formed in situ. In addition, the metal ion plays the role of a Lewis acid to form the iminium ion (437) by associating with imine (435) (Scheme 151). 

If possible, the cell should be undivided to minimize the construction cost and also the energy consumption (see goal 1). The application of a controlled reaction at the auxiliary electrode taking place at low potential allows for the use of undivided cells in many cases. For oxidations, the cathodic process at the auxiliary electrode may be a proton reduction under formation of hydrogen. For reductions, the anodic process may be the oxidation of formate or oxalate under production of carbon dioxide [68] or the dissolution of sacrificial anodes [69] (see also Sec. V.B). 

Anode material In aqueous solutions the anodic processes are either breakdown of the electrolyte solution (with oxygen evolution at an inert anode being favored) or the use of soluble anodes. The use of soluble anodes is limited by the passivation of many metals in aqueous solutions. In ionic liquids, however, the first option is not viable due to the cost and the nature of the anodic breakdown products. New strategies will therefore have to be developed to use soluble anodes where possible or add a sacrificial species that is oxidized to give a benign gaseous product. Preliminary data have shown that for some metals the anodic dissolution process is rate limiting and this affects the current distribution around the cathode and the current density that can be applied. 

Nanocrystalline copper with an average crystallite size of about 50nm can be obtained without additives in the ionic liquid 1-butyl-l-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]Tf2N) [92], Because of the limited solubility of the tested copper compounds in this ionic liquid, copper cations were introduced into the ionic liquid by anodic dissolution of a sacrificial copper electrode. The electrodeposition of copper was also investigated in the ionic liquid... 

One example of the application of polarization curves in a predictive manner involves their use in galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals are in electrical and ionic contact as is schematically shown in Fig. 29. Galvanic corrosion is used to advantage in sacrificial anodes of zinc in seawater and magnesium in home water heaters. It slows corrosion of millions of tons of structural materials. The darker side of galvanic corrosion is that it also causes major failures by the accelerated dissolution of materials that are accidentally linked electrically to more noble materials. 

FIGURE 26.35 Current-voltage Tafel plot showing cathodic protection by nse of a sacrificial anode. = equilibrium potential of the redaction reaction = equilibrium potential of the primary metal dissolution reaction E a = equilibrium potential of the sacrificial anode oxidation reaction. 

Until recently, CE always involved impressed current For a few years now, sacrificial anodes have been used as weU [31,66]. With sacrificial systems, a zinc or aluminium anode is connected to the reinforcement, providing current by galvanic dissolution. No power source is needed, but the treatment time is longer than with impressed current CE. 

Galvanized steel is a common example of galvanic coupling where steel (Fe) with a standard electrode potential of—0.440 Vvs. SHE is cathodicaUy protected by a coating of zinc with a more active standard electrode potential of—0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects the steel from corrosion because of its sacrificial properties. Because zinc is less noble than steel, it acts as the anode. The sacrificial anode is continuously consumed by anodic dissolution and protects the more positive metal from corrosion. In practice, sacrificial anodes are... 

When more and less noble materials are placed in contact, the more noble material offers an extra area for the cathodic reaction. Therefore flie total rate of the cathodie reaction is increased, and this is balanced with an increased anodic reaction, i.e. increased dissolution of the less noble material (galvanic corrosion. Section 7.3). If the more noble material (the cathodic material) has a large surface area and the less noble metal (the anodic metal) has a relatively small area, a large cathodic reaction must be balanced by a correspondingly large anodic reaction concentrated in a small area. The intensity of the anodic reaction, i.e. the corrosion rate (material loss per area unit and time unit) becomes high. Thus, the area ratio between the cathodic and the anodic materials is very important and should be kept as low as possible. It should be mentioned that in a galvanic corrosion process, the more noble material is more or less protected. This is an example of cathodic protection, by which the less noble material acts as a sacrificial anode (see next section). 

The two principal alloys of aluminum usually employed as sacrificial anodes are Al-Zn-Hg and Al-Zn-I. These are exclusively used in seawater, which is a major disadvantage. As well, they spark when struck with rusting iron and therefore may not be very useful in the petroleum industries. In addition, these alloys passivate when operated in the cold. This problem is worse in muddy environments. Another limitation to their usage is that they produce poisonous dissolution products. 

As described already, not only the property of corrosion resistance, but other desired functions are required for materials. Therefore, the best surface finishing processes need to be selected for specific materials in certain environments. Figure 8 shows the concept to design the surface finishing for corrosion control. In addition to the conventional concept, we have to take the possibility of environmental harmfulness into consideration. Corrosion often leads to the contamination of our environment, since the anodic dissolution is the essence in most cases. Therefore, sacrificial corrosion protection might be prohibited in some cases. From that perspective, the appropriate surface coating will be more versatile in the future. Chapter The Application of Corrosion Protection presents and discusses surface finishing processes. 

In these conditions, the possibility for the onset of corrosion processes is high, and these wiU occur preferentially in places in contact with cathodic species, such as stainless steel or iron rich dust particles, and inside the artificial crevices. In this last case, when a region of a sample is undergoing a massive dissolution process, it tends to act as a sacrificial anode because the rest of the surface is needed for the cathodic reaction. As all the central portions of the coupons have undergone this type of attack, owing to the crevices formed with the ceramic separators, etc., it is possible that corrosion of the free portions has to some extent been inhibited by this effect. 

Figure 63 Schematic of sacrificial anode cathodic protection. Diis makes all the steel negative by dissolution of anode metal t( generate electri>ns. Kesistance must be li>vv for ent>ugh current to pass.

There are two basic approaches to the synthesis of nanosized materials top-down or bottom-up methodologies (Table 6.2). Top-down refers to pulverization of bulk material into fine particles that can be collected as solid powder, suspended in a liquid, or deposited directly on the electrode surface. The particles are usually obtained by physical methods, such as thermal evaporation, sputtering, or laser ablation. In addition, metallic nanosized particles can also be obtained via electrochemical synthesis, exploiting the dissolution of a sacrificial anode [59]. 

The first is a passive protection that consists in connecting electrically the metal to a less noble material that will result in a galvanic coupling of the two materials, which leads to the anodic dissolution of the sacrificial anode. The second method is an active protection that consists in using an impressed current power supply in order to polarize cathodically the workpiece versus a nonconsumable or inert anode. 

Electrochemical dissolution of sacrificial anodes, for example, iron, aluminum, or magnesium, has been proposed for phosphate removal from urine. Ikematsu et al. [15] used an electrochemical reactor craisisting of two DSA and one iron electrode for combined nitrogen oxidation and phosphate precipitatirai. First, urea was oxidized at the DSA, then the current direction was changed, and phosphate was precipitated by dissolving the iron electrode. Zheng et al. [29, 30] used synthetic and real fresh urine for their experiments with iron and aluminum electrodes. With both types of electrodes, complete phosphate removal was achieved. At 40 mA cm and a gap width of 5 mm, 1.3 mol Fe. mol had to be dosed to remove 98 % of the phosphate (calculated by assuming a current efficiency... 

Intergranular dissolution of zinc alloy sacrificial anodes in seawater at elevated temperatures... 

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