Corrosion prevention anodic

In most cylindrical carbon—zinc cells, the zinc anode also serves as the container for the cell. The zinc can is made by drawing or extmsion. Mercury [7439-97-6J has traditionally been incorporated in the cell to improve the corrosion resistance of the anode, but the industry is in the process of removing this material because of environmental concerns. Corrosion prevention is especially important in cylindrical cells because of the tendency toward pitting of the zinc can which leads to perforation and electrolyte leakage. Other cell types, such as flat cells, do not suffer as much from this problem. 

Corrosion prevention is achieved by correct choice of material of construction, by physical means (e.g. paints or metallic, porcelain, plastic or enamel linings or coatings) or by chemical means (e.g. alloying or coating). Some metals, e.g. aluminium, are rendered passive by the formation of an inert protective film. Alternatively a metal to be protected may be linked electrically to a more easily corroded metal, e.g. magnesium, to serve as a sacrificial anode. 

Light anodic treatment and various corrosion preventive treatments have been developed by magnesium producers. 

The selection of an alloy for a specific application is based on the cost and the corrosion resistance of the alloy in the environment of interest. It is also possible to subject the chosen alloy to a process by which the corrosion resistance of the selected material can be improved within the acceptable limits. Some of the corrosion prevention and protection strategies with respect to the aluminum-based alloys are (i) design (ii) alloy selection and joint sealants (iii) aluminum thermal spraying anodic coatings (iv) inhibitors (v) conversion and organic coatings and (vi) cathodic protection. 

The corrosion prevention strategy consisted of wrapping the valve with wax tape, along with use of magnesium anodes. The chamber was thoroughly cleaned and valves wrapped in wax tape, as shown in Figure 7.3. Since the chamber was entered for limited number of times the corroded ladder was left intact and enter with the restraining device and/or portable ladder. 

The corrosion, or rusting, of iron is an example of a naturally occurring voltaic cell. To prevent corrosion, sacrificial anodes are sometimes attached to rust-susceptible iron. Sacrificial anodes must... 

Electrolytic dissolution in nitric acid has been used at the Savannah River [B22] and Idaho Qiemical Processing plants [AlO, All] to dissolve a wide variety of fuels and cladding materials, including uranium alloys, stainless steel, aluminum, zircaloy, and nichrome. The electrolytic dissolver developed by du Pont [B22], pictured in Fig. 10.4, uses niobium anodes and cathodes, with the former coated with 0.25 mm of platinum to prevent anodic corrosion. Metallic fuel to be dissolved is held in an alundum insulating frame supported by a niobium basket placed between anode and cathode and electrically insulated from them. Fuel surfaces facing the cathode undergo anodic dissolution in a reaction such as... 

CP or other protection systems should be incorporated, if necessary, in the design phase of the ship. CP system is a secondary defense against corrosion when holidays or cracks form in the coating. CP systems use either sacrificial zinc anodes or impressed-current systems to mitigate corrosion. Other corrosion prevention equipment and materials are inert gases to drive out corrosive gases. Corrosion inhibitors are also used. 

Inhibitors are used in severe and harsh environments encountered in rotary cookers and hydrostatic sterilizers. The medium consists of hot water, steam, and cooling water. A single approach to this problem may not provide the solution. A combination of anodic, cathodic, and filming inhibitors was selected for corrosion prevention depending on the water composition and equipment material. 

Sometimes the need to be environmentally acceptable may lead to new problems. For instance, ozone was suggested to replace biocides with no data available on the performance in the chlorination of water (60). Corrosion control techniques can have both favorable as well as ill effects and hence one has to exert balanced judgment before embarking on a corrosion prevention method. Organotin antifouling coatings on ships were effective, but they polluted the seawater and hence were banned from further use. The use of cadmium as a sacrificial anode is restricted because of its toxicity. Large amounts of zinc are used to protect steel platforms in the sheltered and shallow waters of the sea, and the effects of zinc on the contamination of waters are not known. 

The anodic and cathodic reactions ((2.1) and (2.2)) are only the first steps in the process of creating rust. However, this pair of reactions is critical to the understanding of corrosion and is widely quoted in any discussion on corrosion and corrosion prevention for steel in concrete. The reactions will be referred to often in this book. 

Figure 13.23 I Sacrificial anodes are one effective method of corrosion prevention. An unprotected iron or steel pipe buried in the ground would be at high risk for corrosion. By connecting the buried pipe to a metal such as magnesium, which is more easily oxidized, a galvanic cell is created with the pipe as the cathode. In the case of a buried pipe, the soil itself serves as the electrolyte. The anode is called sacrificial because it will be eaten away over time by oxidation. But replacing the anode—which may be nothing more than a metal block or stake—is much easier than replacing the buried pipe.

Solvent-free electrochemical fluorination is an alternative method for preventing anode passivation and acetoamidation [18, 19]. As already mentioned, handling extremely corrosive and poisonous anhydrous HF in a laboratory setting is accompanied by serious hazards and experimental difficulties. Molten salts such as 70 % HF/pyridine (Olah s reagent) and commercially available EtaN-SHF [20] are often used to replace anhydrous HF. Other molten salts with the general formula R4NF-nHF (n > 3.5, R = Me, Et, and n-Pr) are useful in selective electrochemical fluorination. These electrolytes... 

As water penetrates through the coating, slight hydrolysis of zinc phosphate occurs, resulting in secondary phosphate ions. These phosphate ions in turn form a protective passive layer [53,54] that, when sufficiently thick, prevents anodic corrosion [55]. Porosity of the phosphate coatings is closely related to the coating protective performance [37]. The approximate formula for the phosphatized metallic compound is ... 

Figure 13.16 Ductile iron municipal watermain protected with tape wrap and sacrificial magnesium anodes. (Courtesy of Dean Rookes, West Coast Corrosion Prevention Ltd.)...

Light anodic treatment and various corrosion-preventive treatments have been developed by magnesium producers (Dow 17 and Dow 7, Dow Chemical Co.)... 

The polyphosphate molecule bonds with divalent calcium and other ions to form positively charged colloidal particles which are attracted to the cathode and form a protective film. As some metal ions, such as iron, may also be adsorbed on the film, polyphosphate also shows a partial anodic behavior although basically they are cathodic inhibitors. The mechanism of corrosion prevention by polyphosphates is shown in Fig. 6.12. 

They are effective inhibitors for iron and a number of metals in a wide variety of waters. Like chromates, nitrites are anodic inhibitors and they inhibit the system by forming a passive film with ferric oxide. These are environmentally-ffiendly inhibitors. Besides steel, nitrites also inhibit the corrosion of copper, tin and nickel alloys at pH levels 9-10. Chromate is an extremely effective inhibitor for corrosion prevention of aluminum alloys. Nitrites should not be used in open systems as they would oxidize to nitrates in the presence of oxygen. 

Mg anodes can deliver sufficient current density 21.5 to 32.28 mA/nF to protect the inner wall of a galvanized steel hot water tank. It is reported that galvanized hot water tanks either corrode within few years or, if they last longer than this, remain sound for 15 to 20 years. This is supported by the fact that scale formation has a preservative or corrosion preventive effect of galvanized steel in hard water. Soft water can prevent the formation of this scale due to localized galvanic corrosion that can lead to perforation. It is suggested that if a tank is cathodically protected for the first year or... 

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