Nature of Corrosion Products

The atmospheric corrosion rate is influenced by many parameters, one of the more important being the formation and protective ability of the corrosion 

Depending on the rate of crystallation and the rate of formation, the corrosion products may be amorphous or crystalline. If the former is ratedetermining, one expects amorphous phases to form. From colloid chemistry, it is known that aging, or slow growth, of amorphous phases may result in a transition from the amorphous to the crystalline state, a process that may occur through slow transformation in the solid state or through the dissolution-reprecipitation process. 

The corrosion products formed are the composition of the metal or alloy and the contaminants present in the atmosphere. For example, carbon steel does not have the ability to form its own protective coating except in a dry, clean atmosphere. Under these conditions, a thick oxide film will form that prevents further oxidation. Initiation of corrosion is the result of the presence of solid particles on the surface. This settled airborne dust promotes corrosion by absorbing SO2 and water vapor from the air. Even greater corrosive effects result when particles of hygroscopic salts, such as chlorides or sulfates, settle on the surface and form a corrosive electrolyte. 

Additional information regarding the weathering steels and other metals or alloys as to their resistance to atmospheric corrosion will be found in the chapter dealing with the specific material. 

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Bicarbonate ions concentration at Viriato is higher as the value of pH increases. Thus, the corrosion process and the corrosion products transformation occur to a higher pH and this can lead to changes in the corrosion mechanism and in the nature of corrosion products formed. 

Water adsorption should be caused by salts content but, perhaps, the presence of high levels of contaminants can determine the formation of a corrosion product having a more hygroscopic nature. To make clear this possibility, adsorption isotherm of corrosion products from Viriato corrosion station after elimination of salts were determined. The values of adsorption are lower after salt elimination, but higher than those corresponding to Quivican. It could be due to a more hygroscopic nature of corrosion products formed in coastal stations. 

Other factors of importance in atmospheric corrosion of zinc are (i) the distance from the ground (ii) orientation of the samples (iii) wind or rain shielding (iv) distance to the local contaminant sources (v) wind, radiation (vi) condensation and drying rate (vii) amount of contaminants and nature of corrosion products and (viii) seasonal variation of factors also should be considered. This shows the complexity of the problem of determining the atmospheric corrosion rates to a high degree of certainty. This uncertainty is exemplified by the observed corrosion rate of 0.6-3.8 pm/yr at 26 sites in rural area in Spain.95 The corrosion rate of 8.5 pm/yr observed on the zinc coating in an under-vehicle situation is comparable to severe marine atmospheric conditions.96... 

The relative influence of those three effects depends i.a. on the degree of pollution, on the corrosion mechanism and on the nature of corrosion products of the metal. 

The physical nature of corrosion products often has a major influence on the rate of corrosion. We can divide products into three basic tyqjes ... 

The same factors that promote atmospheric corrosion of metals, time of wetness, atmospheric types, initial exposure conditions, sheltering, wind velocity, and the nature of corrosion products also affect the atmospheric corrosion of ceramics. 

Nature of corrosion products Can be protective Rarely protective... 

In the deposition zone, corrosion products will accumulate, following their nucleation on the substrate. The corrosion products formed under thin-film atmospheric conditions are closely related to the formation of naturally occurring minerals. Over long periods of time, the most thermodynamically stable species will tend to dominate. The nature of corrosion products foimd on different metals exposed to the atmosphere is shown in Fig. 2.6. The solution known as the inner electrolyte can be trapped inside or under the corrosion products formed. The deposited corrosion product layers can thus be viewed as membranes, with varying degrees of resistance to ionic transport. Passivating films tend to represent strong barriers to ionic transport. 

Figure 2.6 Nature of corrosion products formed on four metals.

Economizers are fitted to most watertube boilers. An exception is on a waste-heat application, where it may be desirable, due to the nature of the products being burned, to maintain a relatively high gas outlet temperature to prevent corrosive damage to the boiler outlet, ductwork and chimney. 

Metal/environment interface—V ne cs of metal oxidation and dissolution, kinetics of reduction of species in solution nature and location of corrosion products him growth and him dissolution, etc. 

Severe attack frequently occurs at a water-line, which in practice can range from structural steel partly immersed in a natural water to a lacquered tin can used for containing emulsion paint. This can be illustrated by adding increeising amounts of sodium carbonate to a sodium chloride solution in which a steel plate is partly immersed (Fig. 1.48c, d and e). With increase in concentration of the inhibitor, attack decreases and becomes confined to the water-line. The attack at the water-line is intense and is characterised by a triangular pasty mass of corrosion products bounded on the upper surface by a dark-brown membrane that follows the contour of the water-line. The mechanism of water-line attack is not clear, but it is likely that the membrane of corrosion products results in the formation of an occluded cell, in which the anolyte and catholyte are prevented from mixing. These occluded cells are discussed in more detail subsequently. 

The corrosivity of a salt solution depends upon the nature of the ions present in the solution. Those salts which give an alkaline reaction will retard the corrosion of the iron as compared with the action of pure water, and those which give a neutral reaction will not normally accelerate the corrosion rate appreciably except in so far as the increased conductivity of the solution in comparison with water permits galvanic effects to assume greater importance. Chlorides are dangerous because of the ability of the anions to penetrate otherwise impervious barriers of corrosion products. 

The general corrosion rate of zinc and zinc alloys in practice often have been shown to be much less than in simulated conditions this is because many naturally occurring substances act as inhibitors. Figure 4.42 is a good example of this. The diagram is valuable for the qualitative relationship between acid, neutral and alkaline conditions but, in practice, the corrosion rates are usually very much lower than indicated by the pH because of the effect of other dissolved constituents and the barrier effect of corrosion products. Seawater around the British Isles is much less corrosive to zinc than tropical seawater. 

Materials may be subject to intense localised attack at the liquid level when they are partially immersed in a solution under conditions where the water line remains at a fixed position for long periods. This attack may be the result of concentration cell effects complicated by differences in the nature and adherence of corrosion-product films as they form in the water-line region as compared with those that form above or below this region. 

The presence of corrosion products is not always a negative event some small degree of surface corrosion of all steel heat exchanger surfaces is generally beneficial. Under the reducing conditions normally found on the surfaces of pre-boiler FW heaters, FW lines, and boiler surfaces, black magnetite naturally forms by the direct thermal reaction of water with steel. The development of this self-limited magnetite film is most desirable, and optimum formation is achieved at pH levels of 10.5 to 11.5. 

Additionally, as a result of the turbulent nature of the flow of FW as it passes through the system, both new and old debris also may continue to pass through and eventually lodge in the boiler. As a consequence, the transport of corrosion products and debris from various parts of the overall boiler plant system through the pre-boiler section (where additional debris may originate) and on to the boiler continues to be a... 

FIGURE 40 Patina. Patina is a colored (usually green) layer of corrosion products that frequently develops naturally on the surface of copper and copper alloys exposed to the environment. Since it is sometimes appreciated aesthetically and as a proof of age, patina is also developed artificially, by chemical means, as a simulated product of aging. Copper patina generally includes such compounds as copper oxides, carbonates, and chlorides. In bronze and brass patinas, these compounds are mixed with the oxides of tin and lead resulting from the corrosion of the other components of the alloys. In any particular patina there may be many layers, not necessarily in the order shown in the illustration. 

Almost all tests carried out to study the starting process of atmospheric corrosion have been performed in a surface without corrosion products however, in real conditions, the metal is covered with corrosion products after a given time and these products begin to play its role as retarders of the corrosion process in almost all cases. Corrosion products acts as a barrier for oxygen and contaminants diffusion, the free area for the occurrence of the corrosion is lower however, the formation of the surface electrolyte is enhanced. Only in very polluted areas the corrosion products accelerate the corrosion process. Water adsorption isoterms were determined to corrosion products formed in Cuban natural atmospheres[21]. Sorption properties of corrosion products (taking into account their salt content-usually hygroscopics) determine the possibilities of surface adsorption and the possibility of development of corrosion process... 

A third mechanism by which the structural bonds between Fe atoms in iron oxides may be weakened involves reduction of structural Fe to Fe". In natural environments, reductive dissolution is by far the most important dissolution mechanism. It is mediated both biotically and abiotically. The most important electron donors, particularly in near surface ecosystems result from metabolic oxidation of organic compounds under O2 deficient conditions. In anaerobic systems, therefore, the availability of Fe oxides i. e. the electron sink, may control the degradation of dead biomass and organic pollutants in the ground water zone (see chap. 21). Reductive dissolution is also often applied to the removal of corrosion products from piping in industrial equipment and the bleaching of kaolin. 

Raman, A. Kuban, B. Razvan, A. (1991) The application of infrared spectroscopy to the study of atmospheric rust systems. I. Standard spectra and illustrative applications to identify rust phases in natural atmospheric corrosion products. Corrosion Sci. 32 1295-1306... 

The composition of bronze artifact corrosion products excavated from several lacustrine sites could be explained in relation to some characteristics of the environment. In one study, a combination of potential pH diagrams for copper and those defining different types of natural soil was used to explain the composition of corrosion products of copper artifacts excavated from different locations. While sulphides are predominant in anaerobic conditions, mostly carbonates are... 

The EHD method with a RDE has been applied to the characterisation of porous layers of corrosion products formed on carbon steel [110], for the characterisation of salt films formed on copper [90, 111, 112] and iron [113], and for biofilms developed in natural seawater [114]. Corrosion inhibition films formed by an organic surfactant acting on the surface of pure iron have been characterised in this way, too [115]. An effect of a... 

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