Preventing Corrosion of Iron

Objects made of iron are often covered with a coat of paint or another metal to protect against corrosion. Covering the surface with paint prevents oxygen and water from reaching the iron surface. If the coating is broken, however, and the iron exposed to oxygen and water, corrosion begins as the iron is oxidized. 

With galvanized iron, which is iron coated with a thin layer of zinc, the iron is protected from corrosion even after the surface coat is broken. The standard reduction potentials are 

Because E° for Fe is less negative (more positive) than for Zn, Zn(s) is more readily oxidized than Fe(s). Thus, even if the zinc coating is broken and the galvanized iron is exposed to oxygen and water, as in A FIGURE 20.23, the zinc serves as the anode and is corroded (oxidized) instead of the iron. The iron serves as the cathode at which O2 is reduced. 

Based on the values in Table 20.1, which of these metals could provide cathodic protection to iron Al, Cu, Ni, Zn  

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Red lead (Pb304) is manufactured on the 20000-tonne scale annually and is used primarily as a surface coating to prevent corrosion of iron and steel (check oxidation-reduction potentials). It is also used in the production of leaded glasses and ceramic glazes and. very substantially, as an activator, vulcanizing agent and pigment in natural and artificial rubbers and plastics. 

The calculation shows that zinc is oxidized preferentially over iron. Later in this chapter we describe the use of zinc as a sacrificial anode to prevent corrosion of iron. 

The use of various substances as additives to process streams to inhibit corrosion has found widespread use and is generally most economically attractive in recirculation systems, however, it has also been found to be attractive in some once-through systems such as those encountered in the petroleum industry. Typical inhibitors used to prevent corrosion of iron or steel in aqueous solutions are chromates, phosphates, and silicates. In acid solutions, organic sulphides and amides are effective. 

When it is desirable to prevent corrosion of iron, a new anode, such as a zinc (Zn) or magnesium (Mg) strip, is connected to the surface of the iron. These metals are stronger reducing agents than iron and will be more easily oxidized. As oxidation occurs, the zinc or magnesium, rather than the iron, will furnish electrons. These sacrificial anodes will erode instead of the iron. They must be replaced periodically, but the iron will remain intact. (See Figure 6.12.)... 

The following methods maybe adopted in order to prevent corrosion of iron or steel ... 

Aluminum is described in Group 13 of the Elements Handbook (Appendix A) as a self-protecting metal, and can be used in preventing corrosion of iron structures. Using electrochemical data, explain how aluminum protects iron strucmres. 

Cement-Lined Steel Cement-hned steel pipe is made by lining steel pipe with special cement. Its use prevents pickup of iron by the fluid handled, corrosion of the metal by brackish water, and growth of tuberculation. Threaded pipe in sizes from to 4 in is stocked however, cement-lined pipe in sizes smaller than in is not considered practical for common use. 

Figure 5.4 EfTect of temperature on corrosion of iron in water containing dissolved oxygen. (Courtesy of McGraw-Hill Inc., Corrosion, Causes and Prevention, by F. Speller, p. 168, McGraw-Hill, 1951)...

Corrosion of Iron and Steel by Industrial Waters and its Prevention, Special Report No. 41, I.S.I., London (1949)... 

The corrosion of iron (or steel) can be inhibited by the anions of most weak acids under suitable conditions " . However, other anions, particularly those of strong acids, tend to prevent the action of inhibitive anions and stimulate breakdown of the protective oxide film. Examples of such aggressive anions are the halides, sulphate, nitrate, etc. Brasher has shown that, in general, most anions exhibit some inhibitive and some aggressive behaviour towards iron. The balance between the inhibitive and aggressive properties of a specific anion depends on the following main factors (which are themselves interdependent). 

Where an alkaline EDTA cleaning program is employed, hydrazine sometimes is added to help keep the chelated iron in the ferrous state and prevent corrosion of exposed steel. Under very exacting circumstances, such as removing iron oxides from nuclear powered SGs, triammonium EDTA together with 0.5 M hydrazine may be employed, at a pH of 7 and a temperature of 200 °F (93 °C). 

Corrosion of iron has serious economic impacts. Approximately 20% of the iron and steel produced in the United States is used to replace rasted metal, and the cost of replacing corroded materials is several billion dollars per year. Consequently, the prevention of corrosion is a major business. 

After this primer is applied vinyl chloride-vinyl acetate copolymer is added in a series of thin films. The total thickness is usually 5 mils. Pigments like iron oxide, lead, or zinc chromate prevent corrosion of the metal substrate in acid environments and may also be included in the coating. The final coated metal has good resistance to water and many chemicals with about a ten-year lifetime. 

The enormous cost of corrosion of iron to society has prompted many efforts to devise ways of reducing or preventing it. Several electrochemical or chemical methods are available. One method is removal of the cathodic species (usually oxygen). Most methods are based on the principle of providing a barrier between the reacting species. The barrier may be physical, i. e. a metal or paint coating or a protective oxide film, or electronic, i. e. making the iron thermodynamically immune. Here, the em-... 

Corrosion inhibitor Two inhibitors could be used a. Copper corrosion inhibitor to help prevent sulfur, hydrogen sulfide, and mercaptan attack on copper b. Ferrous metal corrosion inhibitor to prevent water/oxygen initiated corrosion of iron and steel system components... 

Corrosion occurs more rapidly when there is a dissolved salt, higher acidity or air pollution, because dissolved CO2 produces and HCO3" ions. One way to prevent the corrosion of iron is to coat it with special paints containing potassium dichromate (K2Cr202) and trilead tetroxide (Pb304). 

Corrosion of iron (rusting) is an electrochemical process in which iron is oxidized in an anode region of the metal surface and oxygen is reduced in a cathode region. Corrosion can be prevented by covering iron with another metal, such as zinc, in the process called galvanizing, or simply by putting the iron in electrical contact with a second metal that is more easily oxidized, a process called cathodic protection. 

Describe the process of corrosion of iron and methods to prevent corrosion. 

Steel, stainless steel, and copper-free steel alloys are the preferred materials of construction for mercaptan service. In particular, stainless steel should be used for any vessel or line that is to be open and exposed to air frequently. Aluminum is also suitable for mercaptan use provided the pressure rating of aluminum equipment or piping is sufficient to meet the pressure requirements of the application. Iron or carbon steel is less acceptable than stainless steels or aluminum although it can be used if appropriate measures are taken to condition the iron or carbon steel equipment before putting it into service. Allowing a small amount of mercaptan to stand in it for a period of time and subsequently keeping the equipment under a dry, inert atmosphere can prevent corrosion of the equipment. The hazard in using iron or carbon steel is the formation of iron-sulfur complexes, which are pyrophoric... 

When iron corrodes, iron metai reacts with oxygen to form iron(lll) oxide—rust. Corrosion of iron can be prevented by covering the surface of exposed steei with paint or other coatings such as piastic. If the protective coating is damaged or cracked, rust forms quickiy. ... 

The main uses of tellurium are as semiconductors in the formation of alloys with lead (to prevent corrosion), cast iron (to improve machinabil-ity), copper, and stainless steel in ceramic materials tinting glass, see also Chalcogens. 

Q Deduce oxidation numbers and use them to balance redox equations. Recall the principles of manganate(Vll) and thiosulphate titrations. Recall the corrosion of iron and its prevention. 

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