Cathodic protection continued mechanism

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). 

Adsorption-induced brittle fracture. This model is based on the hypothesis that adsorption of environmental species lowers the interatomic bond strength and the stress required for cleavage. This model of chemical adsorption can explain the fact that a certain alloy is susceptible to specific ions. An important factor in support of this mechanism is the existence of a critical potential below which the SCC does not occur in some systems, and this model underlines the relation between the potential value and the capacity of adsorption of the aggressive ion. It also explains the preventive action of SCC for some systems by cathodic protection. This model may interpret the rupture of plastic materials or glass. It is referred to as the stress-sorption model, and similar mechanisms have been proposed for HE and LME. In this model, the crack should propagate in a continuous way at a rate determined by the arrival of the embrittling species at the crack tip. The model does not explain how the crack maintains a sharp tip in a normally ductile material.156... 

In recent years, continuous zinc ribbon anodes have been used in a variety of underground applications (Kurr, 1973 Peabody, 1976 O Connell, 1977). This type of product has broadened the applications for zinc anodes, for it provides small increments of current continuously along the entire length of a cathode. Its uses are generally considered to lie in specialty applications, where other methods of cathodic protection are either impractical or extremely costly (see later section on induced ac on pipelines). Bagnulo (1973, 1984) has developed a tape with an electrically conducting adhesive as described in the Mechanical Coatings part of Chapter 1. 

For the third step, if the determined total wall thickness (mechanical + CA) is not acceptable, great effort should be exercised to select and evaluate a suitable corrosion prevention measure to lower the required corrosion allowance. Some of these measures cannot withstand the process conditions, e.g. temperatures too high for polymer lining, no facilities for chemical injection, no continuous electrolyte for cathodic protection, and so on. 

SCC starts by an electrochemical mechanism. A pit, scratch, or rupture in a protective film can act as the starting point for corrosion. Anodic and cathodic areas form on the metal surface, with the weakly film-covered region and the tip of the crack acting as an anode and the oxide-covered region acting as a cathode. Once corrosion starts, the stresses tend to concentrate at the tip of the crack, which remains active. At some critical stress value, deformation results in the formation of a fresh surface (at tips where all the stresses are relieved). The electrochemical mechanism takes over on the fresh surface, building up stress at the tip of the crack. This sequence of events repeats continuously. 

The anodic reaction corresponds to the oxidation of zinc particles (loss of electrons) while the cathodic one usually involves oxygen reduction (gain of electrons) on the surface of iron or steel the "pressure" of electrons released by zinc prevents or controls the oxidation of the metal substrate. Theoretically, the protective mechanism is similar to a continuous layer of zinc applied by galvanizing with some differences because the coating film initially presents in general a considerable porosity (Jegannathan et al., 2006). 

Electrodiemical cycle This cycle takes place several times in an hour. The anodic reaction is balanced by the cathodic reduction ofhydrated ferric oxide (rust) which forms magnetite under wet conditions. During the dry period, the magnetite is converted to freshly formed rust (FeOOH) by an electrochemical mechanism. This rust is not protective and corrosion continues. This cycle based on electrochemical reactions is called the electrochemical cyde. 

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