Galvanized steel economics

Copper, aluminium, steel and galvanized iron are the most widely used metals for the purpose of grounding. Choice of any of them will depend upon availability and economics in addition to the climatic conditions (corrosion effect) at the site of installation. In Table 22.3 we provide a brief comparison of these metals for the most appropriate choice of the metal for the required application. 

The soluble copper ammonia ion passes through the condensate system and plates out as a cathode on steel surfaces in the deaerator, heaters, economizer, and the boiler itself. A secondary galvanic corrosion process is initiated that damages the surrounding steel by forming ferrous hydroxide and releasing copper and ammonia. The ammonia carries over into the steam, and the entire corrosion process repeats itself. 

Often the use of stainless-steel reinforcement is hmited to the outer part of the structure (skin reinforcement) or to its most critical parts for economical reasons. Furthermore, when stainless-steel bars are used in the rehabihtation of corroding structures, they are usually connected to the original carbon-steel rebars. Concern has been expressed with regard to the risk of galvanic corrosion of carbon steel induced by coupHng with stainless-steel bars. Actually, the galvanic corrosion that can arise when stainless steel is used in partial substitution of carbon steel has to be compared with that which takes place in the absence of stainless steels [30]. 

Magnesium and zinc are the predominantly used galvanic anodes for the cathodic protection of pipelines [13—16]. The corrosion potential difference of magnesium with respect to steel is 1 V, which Umits the length of the pipeline that can be protected by one anode. Economic considerations have led to the use of aluminum and its alloys as anodes. However, aluminum passivates easily, decreasing current output. To avoid passivation, aluminum is alloyed with tin, indium, mercury, or gallium. The electrochemical properties of these alloys, such as theoretical and actual output, consumption rate, efficiency, and open circuit (corrosion) potential, are given in Table 15.1. 

Wijngaard, B. H. (1971). Cathodic Protection of Steel Constructions in Seawater. Economic Choice of Galvanic Anodes. Dutch Institute for Applied Scientific Research (TNO), Metaalinst, Delft, Netherlands, Publication 196, 6 pp. 

The most frequently used materials for buried metal structures are the carbon steels. For prevention of their corrosion the most recommended, economical, and effective method is cathodic protection (CP). The use of CP is now standard procedm-e for long-term corrosion protection of imderground pipelines, oil and gasoline tanks, and other structures. With a shift of the metal potential to more of a negative value of -0.85 V versus a C11/CUSO4 reference electrode, it is possible to make the metal surface a cathode, which ensures an immune (no corrosion) state of the carbon steel. Cathodic polarization is achieved by direct current, which can be supplied either by sacrificial anodes in galvanic contact with the steel structure, or by impressed current from a rectifier. 

Galvanizing on the inside and outside of steel piping is more economic up to 30 cm (12 inches) OD - above this diameter hot metal spray, or spray and brush applied coatings are preferred. 

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