Galvanized pipe

Aqueous media corrosion. Natural water is widely distributed and stored in steel pipe, galvanized steel pipe, and steel tanks. Natural waters, so long as they are reasonably free from aggressive ions, such as chloride and acidic species, are noncorrosive and have been handled satisfactorily by mild steel pipes and tanks for many years. The primary impurities in these waters are calcium and magnesium salts. These salts can form a hard carbonate protective scale on the surface of steel exposed to hard water. Chemically pure, distilled water is, in fact, corrosive, and when the concentration of these salts is low, the corrosion of steel must be controlled by reducing the oxygen present in the water by chemical treatment or by cathodic protection. 

Handling Temperatures. Optimum temperature for pumping is in 37—48°C range. Piping should be stainless steel, aluminum, or galvanized iron. Valves and pumps should be bronze, cast-iron with bronze trim, or stainless steel. A pump of 3.15-L/s (50-gal/min) capacity unloads a tank car of warm glycerol in ca 4 h. 

The vessel, as weU as the wick, must be compatible with the working fluid. Where possible, the wick and vessel are made of the same material to avoid the formation of galvanic corrosion ceUs in which the working fluid can serve as the electrolyte. In addition to its role within the heat pipe, the vessel also serves as the interface with the heat source and the heat sink. 

Ziac foil coated with a conductive, pressure-sensitive adhesive is used for repair of other ziac coatings or for imparting corrosion resistance at field sites. The 0.08-mm ziac tape or sheet has a 0.025-mm coaductive adhesive. The laminate is cut to size and pressed tightly to activate the adhesive. Conductive tape can be wrapped around pipe, especially around welds or connections. The corrosion resistance of this material is iatermediate between galvanized or thermally sprayed coatings and zinc-filled paints (21,50). 

Tungsten has Htde effect on recrystallization temperature or the high temperature properties of molybdenum. However, the Mo—30% W alloy is recognized as a standard commercial alloy for stirrers, pipes, and other equipment that is required to be in contact with molten zinc during processing of the metal and in galvanizing and die casting operations. 

Ak ammonia piping should be standard (Schedule 40) or extra heavy (Schedule 80) steel having welded or screwed joints, respectively. Galvanized piping or brazed joints should never be used. Ammonia accepted for pipeline transportation must meet the fokowing specifications NH, 99.5 vol % min dissolved inerts, 0.16% max, ok, 5 ppm max and water, in the form of steam condensate, 0.2% min, or distiked water, 0.5% max. 

Plastic Pipe In contrast to other piping materials, plastic pipe is free from internal and external corrosion, is easily cut and joined, and does not cause galvanic corrosion when coupled to other materials. Allowable stresses and upper temperature limits are low. Normal operation is in the creep range. Fluids for which a plastic is not suited penetrate and soften it rather than dissolve surface layers. Coefficients... 

Liquid-Metal Corrosion Liquid metals can also cause corrosion failures. The most damaging are liqmd metals which penetrate the metal along grain boundaries to cause catastrophic failure. Examples include mercury attack on aluminum alloys and attack of stainless steels by molten zinc or aluminum. A fairly common problem occurs when galvanized-structural-steel attachments are welded to stainless piping or eqmpment. In such cases it is mandatoty to remove the galvanizing completely from the area which will be heated above 260°C (500°F). 

Galvanized steel Cooling tower components, fan blades and shrouds, transfer pipes, plumbing fixtures... 

The principal metal used in galvanizing coatings is zinc. In cooling water systems, use of galvanized panels and steel pipe is common. 

Carbon steel heat exchangers, cast iron water boxes, screens, pump components, service water system piping, standpipes, fire protection systems, galvanized steel, engine components, and virtually all non-stainless ferrous components are subject to significant corrosion in oxygenated water. 

Figure 5.12 Severely corroded galvanized steel pipe supplying water to a cooling tower fan-bearing system. Where the galvanized metal has been consumed, only a brown rust patch is visible.

Figure 5.13 As in Fig. 5.12 but with pipe in cross section, showing severe wastage where the galvanized layer is absent. 

Figure 5.18 Heavily fouled and corroded internal surface of a galvanized carbon steel pipe.

Copper is a galvanic metal and causes corrosion, in the presence of moisture, in nearby metals, such as cable sheathes, steel structure and water, gas or drain pipes, buried in its vicinity. With all such metafs. it forms a complete electrolytic circuit and corrodes them. Tinning may give protection against its galvanic effects but this is ati expensive proposition... 

Cast-iron pipes were used for the mains in the early stages of town gas supply, their sockets being sealed with tarred rope, oakum, or lead. Originally the connection pipes were lead and later of galvanized or coal-tarred forged iron. After the defeat of Napoleon in 1815, there was a surplus of cheap musket barrels, and these... 

Fig. 13-6 Potential variation of a galvanized steel easing pipe ehannel eathodi-cally protected with a flexible polymer cable anode.

The electrolysis protection process using impressed current aluminum anodes allows uncoated and hot-dipped galvanized ferrous materials in domestic installations to be protected from corrosion. If impressed current aluminum anodes are installed in water tanks, the pipework is protected by the formation of a film without affecting the potability of the water. With domestic galvanized steel pipes, a marked retardation of the cathodic partial reaction occurs [15]. Electrolytic treatment alters the electrolytic characteristics of the water, as well as internal cathodic protection of the tank and its inserts (e.g., heating elements). The pipe protection relies on colloidal chemical processes and is applied only to new installations and not to old ones already attacked by corrosion. 

At elevated temperature and pressure hydrogen embrittlement can result Most metals when gas is moist. Galvanized pipe or brass or bronze fittings... 

Avoid galvanized pipe and brass or bronze fittings. 

Cadmium 0.005 0.005 Kidney damage Corrosion of galvanized pipes erosion of natural deposits discharge from metal refineries runoff fiom waste batteries and paints... 

Figure 2.5. Gasket insertion between pipe llangcs for sealing purposes and to minimize galvanic corrosion between dissimilar piping metals.

In situations in which piping protrudes partitions or bulkheads of dissimilar metals, proper precautions should be taken against galvanic corrosion. Possible solutions include the use of dielectric gaskets or sleeves and the use of plastic adhesive tapes. Examples are illustrated in Figure 2.6. 

Tinning of copper piping or components is a good approach toward minimizing galvanic action between dissimilar metals. 

Figure 2.6. Examples of minimizing galvanic corrosion when piping penetrates partitions and bulkheads.

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