Cathodic protection continued reducing

An alternative means of avoiding the hazard from fire is to bury the vessels or to employ the increasingly popular method of mounding. In either case, acknowledgment of the reduced hazard is indicated by the reduced separation distances (see Table 20.4). Since both burial and mounding preclude the possibility to monitor continuously the external condition of the vessels, very high-quality corrosion protection needs to be applied, often supplemented by cathodic protection, depending on soil conditions. 

In plain tinplate cans for acid foods, tin provides cathodic protection to steel (3,4). The slow dissolution of tin prevents steel corrosion. Many investigators (5-1I) have defined this mechanism in detail and have shown that the tin dissolution rate is a function of the cathodic activity of the base steel, the steel area exposed through the tin and the tin-iron alloy layers, and the stannous ion concentration. Kamm et al. showed that control of the growth of the tin—iron alloy layer provides a nearly continuous tin-iron alloy layer and improves the corrosion resistance of heavily coated (over 45 X 10"6 in. tin) ETP for mildly acid food products in which tin provides cathodic protection to steel (12). The controlled tin-iron alloy layer reduces the area of steel exposed to the product. ETP with the controlled alloy is designated type K, and since 1964, 75 type K ETP has been used to provide the same protection as 100 ETP provided previously (13). 

Hoffman and co-workers have carried out a series of studies on the passive films on iron, with particular attention to cell design. They have employed a so-called bag cell that allows for the in-situ passivation or cathodic protection of the iron films which were deposited onto gold films deposited on melinex (polymer film with excellent adhesive properties). In addition, they employed a setup in which the working electrode is partially immersed in solution and continuously rotated. In this way, they could expose the electrode to the x-ray beam with ostensibly only a very thin film of electrolyte. Under these conditions, they were able to obtain spectra of the film as prepared, a cathodically protected film as well as a film passivated in borate solution at 1.3 V. From an analysis of their data, they concluded that the passive film had an Fe—O coordination with 6 near neighbors at a distance of 2 + 0.1 A. The approach followed by these authors appears most appropriate since they were able to reduce the deposited films... 

The scope of application of CP is enormous and continuously increasing. It is possible to protect vessels and ships, docks, berths, pipelines, deep wells, tanks, chemical apparatus, underground and underwater municipal and industrial infrastructure, reinforced concrete structures exposed to the atmosphere, as well as underground parts, tunnels, and other metal equipments using cathodic protection. Apart from reduction of general corrosion, cathodic protection reduces SCC, pitting corrosion, corrosion fatigue, and erosion-corrosion of metallic materials. 

Observing the polarization diagram for the copper-zinc cell in Fig. 5.2, it is clear that, if polarization of the cathode is continued, using external current, beyond the corrosion potential to the thermodynamic potential of the anode, both electrodes attain the same potential and no corrosion of zinc can occur. This is the basis for cathodic protection of metals. Cathodic protection, discussed further in Chapter 13, is one of the most effective engineering means for reducing the corrosion rate to zero. Cathodic protection is accomplished by supplying an external current to the corroding metal that is to be protected, as shown in Fig. 5.14. Current leaves the auxiliary anode (composed of any metallic or nonmetaUic... 

The rate of attack can be appreciable in either dilute or concentrated alkalies. For this reason, when aluminum is cathodically protected, overprotection must be avoided in order to ensure against damage to the metal by accumulation of alkalies at the cathode surface. Lime, Ca(OH)2, and some of the strongly alkaline organic amines (but not NH4OH) are corrosive. Fresh Portland cement contains lime and is also corrosive hence, aluminum surfaces in contact with wet concrete may evolve hydrogen visibly. The corrosion rate is reduced when the cement sets, but continues if the concrete is kept moist or contains deliquescent salts (e.g., CaCb). 

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