Corrosion aqueous

Aqueous corrosion of tin results in production of oxides of tin. The oxides and hydroxides of tin are stable in the pH range 3-10. The amphoteric nature of the oxides results in attack by mineral acids to produce stannous or stannic salts, stannites and stannates in basic solutions. While borates and bicarbonate render stability to the oxide 

Solution Temperature (°C) Duration of test (days) Average corrosion rate (mils/yr)b  

Lead-tin solders have been replaced by high-tin solders such as Sn-Ag, Sn-Sb and Sn-Sb-Ag in domestic heating and water distribution systems. High-tin alloys are used in food packaging, and failure of a can through dissolution of solder is rare. Inhibitors used in automotive antifreeze have been found to attack the solder side seams in tin plate 

Metals corrode in aqueous environments by an electrochemical mechanism in which an anodic and a cathodic reaction occur simultaneously. The anodic reaction is an oxidation process. The metal loses electrons and dissolves into the solution, Fe — Fe + + 2e . The excess electrons generated in the electrolyte are usually consumed in two ways at a cathodic site where a 

they might create hydroxyl ions by the reduction of dissolved oxygen, according to Equation 4.24  

One of the key factors in any corrosion situation is the environment. The definition and characteristics of this variable can be quite complex. One can use thermodynamics, e.g., Pourbaix or -pH diagrams, to evaluate the theoretical activity of a given metal or alloy provided the chemical makeup of the environment is known. But for practical situations, it is important to realize that the environment is a variable that can change with time and conditions. It is also important to realize that the environment that actually affects a metal corresponds to the microenvironmental conditions that this metal really sees, i.e., the local environment at the surface of the metal. It is indeed the reactivity of this local environment that will determine the real corrosion damage. Thus, an experiment that investigates only the nominal environmental condition without consideration of local effects such as flow, pH cells, deposits, and galvanic effects is useless for lifetime prediction. 

In our societies, water is used for a wide variety of purposes, from supporting life as potable water to performing a multitude of industrial tasks such as heat exchange and waste transport. The impact of water on the integrity of materials is thus an important aspect of system management. Since steels and other iron-based alloys are the metalhc materials most commonly exposed to water, aqueous corrosion will be discussed with a special focus on the reactions of iron (Fe) with water (H2O). Metal ions go into solution at anodic areas in an amount chemically equivalent to the reaction at cathodic areas (Fig. 1.1). In the cases of iron-based alloys, the following reaction usually takes place at anodic areas  

This reaction is rapid in most media, as shown by the lack of pronounced polarization when iron is made an anode employing an external current. When iron corrodes, the rate is usually controlled by the 

This reaction proceeds rapidly in acids, but only slowly in alkaline or neutral aqueous media. The corrosion rate of iron in deaerated neutral water at room temperature, for example, is less than 5 p,m/year. The rate of hydrogen evolution at a specific pH depends on the presence or absence of low-hydrogen overvoltage impurities in the metal. For pure iron, the metal surface itself provides sites for H2 evolution hence, high-purity iron continues to corrode in acids, but at a measurably lower rate than does commercial iron. 

The cathodic reaction can be accelerated by the reduction of dissolved oxygen in accordance with the following reaction, a process called depolarization  

The resistivity of soil is an important characteristic which often determines the rate of corrosion— low resistivity is usually associated with high rates of corrosion. This is shown in Table 10.4. Soluble salts and high moisture content account for low resistivity-high conductivity. The density and particle size can control the moisture level and permeability of the soil to water and oxygen. 

A low pH of soil (pH 3.5-4.5)—high acid level—contributes to the corrosion rate. Soil of pH 5 is much less corrosive. Alkaline soil, pH 7, can be corrosive to aluminum, and if ammonia is formed by bacterial activity, then even copper will be attached. The weak organic acids present in humic acid can solubihze surface oxides and lead to corrosion of metals by complexation processes. Anaerobic bacterial action in soil can lead to H2S (and CH4 plus CO2) which, though a weak acid, will form insoluble metallic sulfides, reducing the free metal ions in the soil and shifting the equilibrium toward metal dissolution. 

As indicated previously, the corrosion of metals in aqueous environments is determined by the Nemst equation in terms of the electrode potential and pH—called a Pourbaix diagram. This is shown in Fig. 10.5 for iron where the vertical axis is the redox potential of the corroding system and the pH 

Iron will corrode in acids except H2Cr04, cone. HNO3, H2SO4 70 %, and HF 90 %. Pourbaix diagrams are available for most metals and help define the corrosion-free conditions. 

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Aqueous Corrosion. Several studies have demonstrated that ion implantation may be used to modify either the local or generalized aqueous corrosion behavior of metals and alloys (119,121). In these early studies metallic systems have been doped with suitable elements in order to systematically modify the nature and rate of the anodic and/or cathodic half-ceU reactions which control the rate of corrosion. 

R. A. Buchanan and E. E. Stansbury, "Aqueous Corrosion", in R. Kossowsky, ed., Suface Modfcation Engineering Fundamental Aspects, CRC Press, Boca Raton, Fla., 1989. 

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. 

Additionally, crevice corrosion can be reduced by two techniques used successfully on most aqueous corrosion—chemical inhibition and cathodic protection. However, both these techniques may be cost prohibitive. 

Removing suspended solids, decreasing cycles of concentration, and clarification all may be beneficial in reducing deposits. Biodispersants and biocides should be used in biofouled systems. Simple pH adjustment may lessen precipitation of certain chemical species. The judicious use of chemical corrosion inhibitors has reduced virtually all forms of aqueous corrosion, including underdeposit corrosion. Of course, the cleaner the metal surface, the more effective most chemical inhibition will be. Process leaks must be identified and eliminated. 

A further serious limitation is that diagrams evaluated from thermodynamic data at 25°C have little relevance in high-temperature aqueous corrosion, but it is now possible to construct diagrams that are applicable at elevated temperatures from data obtained at 25° C see Section 2.1). 

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

Consideration will also be given to attack arising from contact with solids such as refractories, and with molten materials such as salts, glasses, and lower-melting-point metals and alloys. On a fundamental basis, the distinction between some of these latter reactions and normal-temperature aqueous corrosion is not always clear, since galvanic effects may be of significance in both cases, but for practical purposes a distinction can be made on the basis of the temperature involved. 

A few cases occur in which hot-corrosion and wet corrosion are interdependent, the wet corrosion arising from the condensation of liquids generated during a period at elevated temperatures. The formation of condensates of hydrobromic acid in engines burning anti-knock fuels containing ethylene dibromide is important in this context. Such cases are properly considered as aqueous corrosion. 

In principle, cathodic protection can be applied to all the so-called engineering metals. In practice, it is most commonly used to protect ferrous materials and predominantly carbon steel. It is possible to apply cathodic protection in most aqueous corrosive environments, although its use is largely restricted to natural near-neutral environments (soils, sands and waters, each with air access). Thus, although the general principles outlined here apply to virtually all metals in aqueous environments, it is appropriate that the emphasis, and the illustrations, relate to steel in aerated natural environments. 

The aqueous corrosion of iron under conditions of air access can be written ... 

Whilst cathodic protection can be used to protect most metals from aqueous corrosion, it is most commonly applied to carbon steel in natural environments (waters, soils and sands). In a cathodic protection system the sacrificial anode must be more electronegative than the structure. There is, therefore, a limited range of suitable materials available to protect carbon steel. The range is further restricted by the fact that the most electronegative metals (Li, Na and K) corrode extremely rapidly in aqueous environments. Thus, only magnesium, aluminium and zinc are viable possibilities. These metals form the basis of the three generic types of sacrificial anode. 

Klein, H. A., Corrosion of Fossil Fuelled Steam Generators , Conference on Water Chemistry and Aqueous Corrosion of Steam Generators, Ermenonville, France (1972)... 

Corrosion by liquid metals is usually controlled by diffusion processes in the solid and liquid phases and, unlike aqueous corrosion, does not generally involve galvanic effects, and, even where electrochemical phenomena are known to occur, it has not, in general, been demonstrated that they have been responsible for a significant portion of the corrosion observed . In... 

Test method for sandwich corrosion test Recommended practice for preparing, cleaning, and evaluating corrosion test specimens Practice for aqueous corrosion testing of samples of zirconium and zirconium alloys Test method for corrosion testing of products of zirconium, hafnium and their alloys in water at 633 K or in steam at 673 K [metric] Recommended practice for conventions applicable to electrochemical measurements in corrosion testing... 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

Iron and Stainless Steel. The purpose of XPS investigations on typical corrosion systems like iron or stainless steel, is the determination of the composition of the passive surface layer, if possible, as a function of depth. As a consequence of the technical and economic relevance of corrosion reactions, XPS investigations on corrosion systems are numerous. With respect to the application of XPS, there is no difference between corrosion systems and any other electrochemical surface reaction like oxide formation on noble metals. Therefore, in this paragraph only a few recent typical results of such studies, using XPS, will be mentioned. For a detailed collection of XPS corrosion studies the reader is referred to references [43,104], A review of aqueous corrosion studies, using XPS, was given by McIntyre for the elements O, Cr, Mn, Fe, Co, Ni, Cu and Mo [105], The book edited by M. Froment [111] gives an impression of the research achieved on passivity of metals up to 1983. 

Aqueous cleaners, for electroplating, 9 781 Aqueous corrosion, ion implantation and, 14 451-452... 

Aqueous corrosion resistance, 13 513 Aqueous dispersion polymerization, 18 291 of acrylonitrile, 11 197-200 Aqueous dispersions, 13 292. See also Aqueous polytetrafluoroethylene dispersions... 

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