Galvanic corrosion application

Bimetallic corrosion in atmospheres is confined to the area of the less noble metal in the vicinity of the bimetallic joint, owing to the high electrolytic resistance of the condensed electrolyte film. Electrolytic resistance considerations limit the effective anodic and cathodic areas to approximately equal size and therefore prevent alleviation of atmospheric galvanic corrosion through strict application of the catchment area principle. 

There is some evidence that nickel slag can be involved in the corrosion of iron and steel in the presence of moisture (probably galvanic corrosion). In Canada, where nickel slag is used in fill applications, it is common practice to provide a layer [typically 150 mm (6 in.) thick] of natural aggregate between ferrous materials and the slag.21... 

The basic objective of the conversion coating process is to provide a corrosion-resistant film that is integrally bonded chemically and physically to the base metal and that provides a smooth and chemically inert surface for subsequent application of a variety of paint films. The conversion coating processes effectively render the surface of the basis material electrically neutral and immune to galvanic corrosion. Conversion coating on basis material coils does not involve the use of applied electric current to coat the basis material. The coating mechanisms are chemical reactions that occur between solution and basis material.1-4... 

This technique is referred to by a variety of terms, depending on the application immersion deposition, galvanic deposition (galvanic corrosion), conversion, cementation (in the metal recovery industry), and so on. 

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. 

The geometries discussed above are relevant to laboratory scale experimental cells. The geometries discussed below are more appropriate for situations encountered in industry or practical application involving galvanic corrosion, cathodic protection, or field corrosion probes. 

In the traditional enamelling methods, the adhesion is aided by the ground coat enamel and by adhesive oxides which arc responsible for galvanic corrosion of the metal and thus for coarsening the surface. More recent advances in technology allow direct application of the cover coat enamel onto the metal surface. This method is possible in the case of titania enamels and requires steel of special composition, either alloyed with titanium or carbon-free. Another possibility is based on the deposition of Ni on the metal surface. It is also necessary to adjust the slip composition and the coating thickness suitably the fired enamel thickness is only 0.1 —0.2 mm, compared with the usual 0.4 mm. 

Special considerations in conductive applications, special conductive blacks must be employed in silver containing gaskets, galvanic corrosion is a problem, attention should be given to material with which diield is connected (potential difference), with zinc or aluminum casing nickel filled materials are preferred... 

External corrosion of water systems may be caused by general corrosion, stray current corrosion MIC, and/or galvanic corrosion. Corrosion mitigation techniques include the application of protective coatings, wrapping pipe in a plastic cover, and the application of CP. The areas of major external corrosion impact are generally those where localized attack may take place, such as in the proximity of other systems like galvanic corrosion or in areas where stray currents may occur. 

Corrosion resistance in subsea environments is of prime importance, as FPSO s are constructed in open water to facilitate mooring tankers at offshore oil and gas exploration stations without jetties and breakwaters. Unlike bronze components, which can develop severe galvanic corrosion, PEEK bearings and thrust washers provide longterm corrosion resistance and superior wear resistance. PEEK compoimd requires no external lubrication, another key advantage in this application. Bronze bearings and thrust washers are subject to premature wear and reduced service life should they run dry. (Victrex pic, Victrex Tech. Ctr., Hillhouse Int L, Thornton Cleveleys, Lancashire FY5 4QD, UK Phone +44—1253-897700 URL www. victrex.com). 

Plastics and rubbers are used as insulating and separating materials between different metallic materials to prevent galvanic corrosion. For this application specific requirements must be fulfilled, such as the necessary mechanical strength and thickness they must be fireproof, and resistant to the environment (including heat-resistant to a specified extent). In addition the following are important ... 

Example Problem 13.1a, in which copper and iron are used to construct an electrochemical cell, points to the origin of galvanic corrosion. Thus far we have implied that the separation of the electrodes is something that is carefully constructed and achieved through a salt bridge. Deciding which electrode is the anode and which is the cathode and identifying the cell reactions are relatively simple tasks. But in real world applications, the separation may be much more subtle. 

Hydrogen embrittlement of steel is not strictly related to corrosion of zinc, but it is briefly mentioned here because in hot dip galvanized steel applications it is often confused by users with corrosive attack. 

---