Iron, corrosion

Problem Students are aware of metal-oxygen reactions and are of the impression that rusting of iron is a pure iron-oxygen reaction. However, if one recalls from experience that, in the Californian or Sahara deserts oxygen exists but that cars never rust, then it can be assumed that the rusting of cars in Europe has to be connected to the damp air prevalent in the climate. The following experiment verifies the hypothesis that oxygen as well as water are necessary for iron corrosion. 

Material Test tubes, glass beaker iron wool, water. 

Procedure Fill one test tube to one-third with dry iron wool, and another test tube with damp iron wool. Place both test tubes upside down in a beaker which is half filled with water. Observe the water level in both test tubes after one or two days. 

Observation No changes are observed in the test tube with dry iron wool. In the test tube with damp iron wool, the water level rises approximately 2 cm in the test tube, iron wool turns black. 

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Muiier-Zuiow B, Kipp S, Lacmann R and Schneeweiss M A 1994 Topoiogicai aspects of iron corrosion in aikaiine soiution by means of scanning force microscopy (SFM) Surf. Sol. 311 153... 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

Reduction of oxygen is one of the predominant cathodic reactions contributing to corrosion. Awareness of the importance of the role of oxygen was developed in the 1920s (19). In classical drop experiments, the corrosion of iron or steel by drops of electrolytes was shown to depend on electrochemical action between the central relatively unaerated area, which becomes anodic and suffers attack, and the peripheral aerated portion, which becomes cathodic and remains unattacked. In 1945 the linear relationship between rate of iron corrosion and oxygen pressure from 0—2.5 MPa (0—25 atm) was shown (20). 

The concentration dependence of iron corrosion in potassium chloride [7447-40-7] sodium chloride [7647-14-5] and lithium chloride [7447-44-8] solutions is shown in Figure 5 (21). In all three cases there is a maximum in corrosion rate. For NaCl this maximum is at approximately 0.5 Ai (about 3 wt %). Oxygen solubiUty decreases with increasing salt concentration, thus the lower corrosion rate at higher salt concentrations. The initial iacrease in the iron corrosion rate is related to the action of the chloride ion in concert with oxygen. The corrosion rate of iron reaches a maximum at ca 70°C. As for salt concentration, the increased rate of chemical reaction achieved with increased temperature is balanced by a decrease in oxygen solubiUty. 

Graphitic Corrosion Graphitic corrosion usually involves gray cast iron in which metalhc iron is converted into corrosion products, leaving a residue of intact graphite mixed with iron-corrosion products and other insoluble constituents of cast iron. 

In oxygenated water of near neutral pH and at or slightly above room temperature, hydrous ferric oxide [FelOHla] forms on steel and cast irons. Corrosion products are orange, red, or brown and are the major constituent of rust. This layer shields the underl3dng metal surface from oxygenated water, so oxygen concentration decreases beneath the rust layer. 

When water pH is between about 4 and 10 near room temperature, iron corrosion rates are nearly constant (Fig. 5.5). Below a pH of 4, protective corrosion products are dissolved. A bare iron surface contacts water, and acid can react directly with steel. Hydrogen evolution (Reaction 5.3) becomes pronounced below a pH of 4. In conjunction with oxygen depolarization, the corrosion rate increases sharply (Fig. 5.5). 

Normal mill coolant pH was near 5. The upset caused large amounts of iron corrosion products to be swept into the coolant. Settling of iron oxides and hydroxides fouled many mill components. 

Dry abrasive blast cleaning should be used on new steelwork where the main contaminant is mill scale. For heavily rusted and pitted steelwork, increased durability can be obtained by the use of wet abrasive blasting where this is practicable. The water will be more effective in removing the potentially destructive and corrosive soluble iron-corrosion products that form at the bottom of corrosion pits. 

Ruther, W. E. and Hart, R. K., Influence of Oxygen on High Temperature Aqueous Corrosion of Iron , Corrosion, 19, 127t (1963)... 

Nickel in cast iron Corrosion rate (mm/year) % Nickel in cast iron Corrosion rate (mm/year)... 

Nickel-iron alloys are more resistant than iron to attack by solutions of various salts. In alternate immersion tests in 5% sodium chloride solution Fink and De Croly determined values of 2-8, 0-25 and 0-5 g m d for alloys containing 37, 80 and 100% nickel compared with 46 g m d for iron. Corrosion rates of about 0.4 g m d are reported by Hatfield for Fe-30Ni alloy exposed to solutions containing respectively 5 Vo magnesium sulphate, 10 Vo magnesium chloride and 10% sodium sulphate the same alloy corroded at a rate of about 1.2 g m d in 5% ammonium chloride. 

Those waters in which the carbon dioxide content is in excess of that required as bicarbonate ion to balance the bases present are among the most aggressive of the fresh waters. Hard waters usually, though not invariably, deposit a carbonate scale and are generally not appreciably corrosive to cast iron, corrosion rates of less than 0-02 mm/y being frequently encountered. Water-softening processes do not increase the corrosivity of the water provided that the process does not result in the development of an excess of dissolved carbon dioxide. 

Concentration Inhibition of iron corrosion in distilled water occurs only... 

The commonest staining trouble is iron stain —the blue-black stain caused by the interaction of soluble iron corrosion products and the natural tannins in wood. Hardwoods are generally more susceptible than softwoods. Steel wool should not be used for smoothing wood surfaces. Iron stains, if not too severe, can be removed with oxalic acid. Heavy contamination with soluble iron corrosion products usually results in migration and conversion to rust deposits in the wood. 

Nail sickness Nail sickness is chemical decay associated with corroded metals in marine situations. Chemical degradation of wood by the products of metal corrosion is brought about by bad workmanship or maintenance, or unsuitable (permeable) timber species, all of which permit electrolyte and oxygen access which promotes corrosion. Chemical decay of wood by alkali occurs in cathodic areas (metal exposed oxygen present). Softening and embrittlement of wood occurs in anodic areas (metal embedded oxygen absent) caused by mineral acid from hydrolysis of soluble iron corrosion products. 

Where serious problems develop, typically the waterside chemistry is poor and iron corrosion debris, sludges, and general deposition are evident. Perhaps there is no softener or the water treatment program is unsuitable for actual operating conditions. Possibly the protocol for BD is inappropriate (either too much or too little, or it is unrelated to steam demands) or flushing, cleaning, and boil-out programs have not been properly instituted. 

Low pH MU water sources are likely to adversely affect first preboiler equipment, such as economizers and other front-end components a severely low pH incursion (say, below pH of 5.0-5.5) also results in general corrosion of boiler section components, including the boiler itself. Under these conditions, iron corrosion debris may form composed of particulate magnetite needles. 

If the alkalinity is too low, iron corrosion may take place. If the alkalinity is too high, there is a further competing anion effect, with magnesium precipitated as Mg(OH)2, rather than chelated. 

If the suspended iron is high, there is a pH level and iron corrosion problem. 

Selective form of iron corrosion, primarily in gray cast iron but also less commonly in nodular cast iron, whereby the (anodic) iron matrix converts to iron oxide while the (cathodic) graphite remains intact. The casting retains it shape but loses all strength and can be cut with a knife. 

Fig. 11-2. Electron energy leveb for a mixed electrode reaction of iron corrosion in acidic solution = Fermi level of iron electrode Sfw/hj) = Fermi level of hydrogen redox...

Fig. 11-14. (a) Corrosion rate of metallic iron in nitric acid solution as a function of concentration of nitric add and (b) schematic polarization curves for mixed electrode reaction of a corroding iron in nitric add W p, = iron corrosion rate CHNO3 = concentration of nitric add t" (t ) = current of anodic iron dissolution (cathodic nitric add reduction) dashed curve 1= cathodic current of reduction of nitric add in dilute solution dashed ciuve 2 s cathodic current of reduction of nitric add in concentrated solution. [From Tomashov, 1966 for (a).]... 

Manson (72,) expanded the concept to the solid state by observing that the strength of composite materials also depended upon the acid-base interaction between continuous and dispersed phases. More directly, Vanderhoff et al. (21) addressed the issue of adhesion of polymeric materials to corroded steel. They synthesized eight corrosion products of iron, and used the interaction scheme developed by Fowkes and Manson first to characterize the iron corrosion products as Lewis acids or bases and then to select polymer vehicles for practical coating systems. Such results were employed to enhance the adhesion of epoxy systems to substrates which were predominantly iron oxide in nature. A good overview of these Issues was presented by Fowkes in 1983 (74). ... 

The first illustration is fashioned after Problem 29 [13] of iron corrosion in deoxygenated aqueous acid. It is assumed that four identically shaped iron-containing specimens labeled A, B, C, D are chosen randomly, and inserted in the acid carrying a corrosion inhibitor. The percentage reduction in the specific rate loss is 93.3(A), 97.3(B), 96.7(C) and 90.0(D). As shown in Table 1, the hypothesis H of no treatment effect (i.e. the hypothesis of all rankings being equally possible) can be stated as... 

Bauer, Ph. Genin, J.M. Rezel, D. (1986) Mossbauer effect evidence of chlorine environments in ferric oxyhydroxides from iron corrosion in chlorinated aqueous solution. Hyperfme Interactions 28 757-760... 

Fasiska, E.J. (1967) Structural aspects of the oxides and oxidehydrates of iron. Corrosion Sd. 7 833-839... 

Fryer, J.R. (1982) Electron microscope studies of iron corrosion in water at room temperature. Corrosion Sci. 22 147-154 Fiichtbauer, H. (1988) Sedimente und Sedi-mentgesteine. Schweizerbarf sche Verlagsan-stalt, Stuttgart, 1141 pp. 

Whenever deposits from fuel systems are analyzed and are found to contain high levels of iron, corrosion is probably occurring somewhere within the fuel system. 

When water pH is <6, iron corrosion and the formation of corrosion products such as colloidal ferric hydroxide can result. Colloidal ferric hydroxide, however, is difficult to detect and difficult to remove through filtration. Fuel containing these particles appears bright and clear. Only about 1 micron in diameter, colloidal ferric hydroxide compounds can pass through fuel filters and deposit onto fuel system components. Further system corrosion can follow. 

Ferrous Hydroxide A product of iron corrosion which is white in appearance, Fe(OH)2. When ferric ions enter, the product will appear black, brown, or green in color. 

Ferrous Oxide (Hydrous) A hydrated, jet black product of iron corrosion, FeO nH20. 

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