Water, corrosion importance

Oxygen Dissolved oxygen is probably the most significant constituent affecting corrosion, its importance lying in the fact that it is the most important cathodic depolariser in neutral solutions. Other depolarisers also occur, but as oxygen is an almost universal constituent of natural waters its importance will readily be understood. 

Corrosion> A process of gradual disintegration or decomposition of a chem nature. The most common example of corrosion is the rusting of iron in air or acidic water. Corrosion of metals is electrochemical in nature. Extensive info is available in the literature regarding measures that have been and can be taken to minimize or prevent various forms of corrosion which occur in industries. It is important to recognize the various forms of corrosion and to effectively apply the available info in or der to overcome corrosion problems. See the Refs given below... 

Because pilot plant work indicated that deaeration of the sea water was important for corrosion prevention, a deaerator has been incorporated in the flowsheet. This deaerator will use a small part of the vapor from the eleventh effect to strip out oxygen from sea water feed that has been partially preheated, so that it will be at its boiling point under conditions in the deaerator. 

Oxygen in water is important when evaluating aggressivity of water towards metals (so-called oxygen corrosion), and it is an important index for checking the operation of biological wastewater treatment plants. 

Advice on eye contact with irritant or corrosive substances, e. g., at least 10-15 min eye washing with pure water, is important. 

This chapter covers information applicable to zinc corrosion behavior in general. Chapter 2 covers corrosion in the atmosphere—which is the most important group of environments in which zinc is used. Attack is usually approximately linear with time, but often with some reduction of rate as protective films form. Many results are available, and tables have been prepared for the guidance of designers. Water corrosion follows in Chapter 3, with distinctions between hard and soft tap water (hot and cold), temperate and tropical seawater, and tidal and splash zones. Buried structures—together with a section on earth reinforcement—follow in Chapter 4, and conditions appropriate for zinc sacrificial anodes are included in both Chapters 3 and 4. 

Deminerahzed water also may be used sometimes as makeup. Characteristics of makeup water are important with respect to corrosion in high-temperature hot-water systems. If the circulating water pH is properly adjusted, much of the corrosion potential can be minimized. In all-steel systems, the pH can be adjusted to 11.0 to minimize corrosion. However, in bimetallic systems, pH values should not be allowed to reach this level because of possible alkaline reaction with brass, bronze, copper, and/or aluminum. 

Some metals, such as aluminum, form corrosion products that adhere tightly to the underlying metal and protect it from further corrosion. Iron oxide (rust), however, flakes off and constantly exposes fresh surface. This difference in corrosion behavior explains why cans made of iron deteriorate rapidly in the environment, whereas aluminum cans have an almost unlimited lifetime. The simplest method of protecting a metal from corrosion is to cover it with paint or some other protective coating impervious to water, an important reactant and solvent in corrosion processes. 

Phosphorus is also important in the production of steels, phosphor bronze, and many other products. Trisodium phosphate is important as a cleaning agent, as a water softener, and for preventing boiler scale and corrosion of pipes and boiler tubes. 

Reactor-grade zirconium is essentially free of hafnium. Zircaloy(R) is an important alloy developed specifically for nuclear applications. Zirconium is exceptionally resistant to corrosion by many common acids and alkalis, by sea water, and by other agents. Alloyed with zinc, zirconium becomes magnetic at temperatures below 35oK. 

In unalloyed steel containers formamide discolors slowly during shipment and storage. Both copper and brass are also subject to corrosion, particularly in the presence of water. Lead is less readily attacked. Aluminum and stainless steel are resistant to attack by formamide and should be used for shipping and storage containers where the color of the product is important or when metallic impurities must be minimized. Formamide attacks natural mbber but not neoprene. As a result of the solvent action of formamide, most protective paints and finishes are unsatisfactory when in contact with formamide. Therefore, formamide is best shipped in containers made of stainless steel or in dmms made of, or coated with, polyethylene. Formamide supphed by BASF is packed in Lupolen dmms (230 kg) or Lupolen canisters (60 kg) both in continental Europe and overseas. 

Niobium is also important in nonferrous metallurgy. Addition of niobium to tirconium reduces the corrosion resistance somewhat but increases the mechanical strength. Because niobium has a low thermal-neutron cross section, it can be alloyed with tirconium for use in the cladding of nuclear fuel rods. A Zr—l%Nb [11107-78-1] alloy has been used as primary cladding in the countries of the former USSR and in Canada. A Zr—2.5 wt % Nb alloy has been used to replace Zircaloy-2 as the cladding in Candu-PHW (pressurized hot water) reactors and has resulted in a 20% reduction in wall thickness of cladding (63) (see Nuclear reactors). 

Table 8 summarizes domestic consumption by use for amyl alcohols. About 55% of the total 1-pentanol and 2-methyl-1-butanol production is used for zinc diamyldithiophosphate lubrication oil additives (150) as important corrosion inhibitors and antiwear additives. Amyl xanthate salts are useful as frothers in the flotation of metal ores because of their low water solubiUty and miscibility with phenoHcs and natural oils. Potassium amyl xanthate, a collector in flotation of copper, lead, and zinc ores, is no longer produced in the United States, but imports from Germany and Yugoslavia were 910 —1100 t in 1989 (150). 

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