Cathodic protection mechanism

Hare, C, Steele, M, Collins, SP. 2001. Zinc Loadings, Cathodic Protection, and Post-Cathodic Protective Mechanisms in Organic Zinc-Rich Metal Primers. J. Protect. Coat. Linings, 54. 

The corrosion of metals results in significant costs worldwide associated with repair and replacement [1-4]. These costs are typically 2% to 5% of gross domestic product (GDP) [1,4]. Metal corrosion is predominantly an electrochemical process that needs four elements to take place an anode, a cathode, an electrolyte for ionic mobility, and an electrical connection between the anode and the cathode. If one or several of these elements is hindered, for example, through the use of corrosion inhibitors, cathodic protection mechanisms, or through the application of passive and/or active barrier systems such as paint, the electrochemical process will be slowed or, ideally, stopped. 

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

Cases (e), (g), and (h) are of interest in the cathodic protection of warm objects (e.g., district heating schemes [89] and high-pressure gas lines downstream from compressor stations [82]) because the media of concern can arise as products of cathodic polarization. The use of cathodic protection can be limited according to the temperature and the level of the mechanical stressing. The media in cases (a) and (f) are constituents of fertilizer salts in soil. Cathodic protection for group I is very effective [80]. 

The cathodic protection of pipelines is best monitored by an intensive measurement technique according to Section 3.7, by an off potential survey eveiy 3 years and by remote monitoring of pipe/soil potentials. After installation of parallel pipelines, it can be ascertained by intensive measurements whether new damage of the pipe coating has occurred. These measurements provide evidence of possible external actions that can cause mechanical damage. 

In the tidal zone and the spray zone (known as the splash zone), cathodic protection is generally not very effective. Here thick coatings or sheathing with corrosion-resistance materials (e.g., based on NiCu) are necessary to prevent corrosion attack [4]. The coatings are severely mechanically stressed and must be so formed that repair is possible even under spray conditions. Their stability against cathodic polarization (see Section 17.2), marine growths, UV rays and seawater must be ensured [4,5]. 

The conductivity of the soil i important as it is evident from the electrochemical mechanism of corrosion that this can be rate-controlling a high conductivity will be conducive of a high corrosion rate. In addition the conductivity of the soil is. important for stray-currenit corrosion (see Section 10.5), and for cathodic protection (Chapter 10). 

Evans, T. E., Mechanisms of Cathodic Protection in Seawater . In Cathodic Protection Theory and Practice, 2nd International Conference, Stratford-upon-Avon, June (1989) Choate, D. L., Kochanezyk, R. W. and Lunden, K. C., Developments in Cathodic Protection Design and Maintenance for Marine Struaures and Pipelines , NACE Conference on Engineering Solutions for Corrosion in Oil and Gas Applications, Milan, Italy, November (1989) not included in Proceedings... 

Numerous materials fall into the category of electronic conductors and hence may be utilised as impressed-current anode material. That only a small number of these materials have a practical application is a function of their cost per unit of energy emitted and their electrochemical inertness and mechanical durability. These major factors are interrelated and —as with any held of practical engineering—the choice of a particular material can only be related to total cost. Within this cost must be considered the initial cost of the cathodic protection system and maintenance, operation and refurbishment costs during the required life of both the structure to be protected and the cathodic protection system. 

Cast iron may be used under similar circumstances, but has inferior mechanical properties. It has been used, although not in current practice, for internal cathodic protection, where it has been demonstrated that the presence of ferrous ions in water is of benefit in reducing sulphide-induced attack on Cu alloy tube plate and tubesWater treatment has now been found to be a more practical method. 

These are iron alloys that contain 14-18% Si and are reported as first being developed in 1912 , although it was not until 1954 that they were first evaluated for use as impressed-current anode material in cathodic protection. Its major disadvantage is that it is a hard brittle material unable to sustain thermal or mechanical shock. 

Before considering the principles of this method, it is useful to distinguish between anodic protection and cathodic protection (when the latter is produced by an external e.m.f.). Both these techniques, which may be used to reduce the corrosion of metals in contact with electrolytes, depend upon the electrochemical mechanisms that result from changing the potential of a metal. The appropriate potential-pH diagram for the Fe-H20 system (Section 1.4) indicates the magnitude and direction of the changes in the potential of iron immersed in water (pH about 7) necessary to make it either passive or immune in the former case the stability of the metal depends on the formation of a protective film of metal oxide (passivation), whereas in the latter the metal itself is thermodynamically stable and egress of metal ions from the lattice into the solution is thus prevented. 

The determination of polarisation curves of metals by means of constant potential devices has contributed greatly to the knowledge of corrosion processes and passivity. In addition to the use of the potentiostat in studying a variety of mechanisms involved in corrosion and passivity, it has been applied to alloy development, since it is an important tool in the accelerated testing of corrosion resistance. Dissolution under controlled potentials can also be a precise method for metallographic etching or in studies of the selective corrosion of various phases. The technique can be used for establishing optimum conditions of anodic and cathodic protection. Two of the more recent papers have touched on limitations in its application and differences between potentiostatic tests and exposure to chemical solutions. ... 

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). 

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