Water aqueous corrosion

The special case of the bimetallic effect between a zinc coating and the substrate that it is protecting is discussed under hot water aqueous corrosion resistance as the normal bimetallic effect whereby zinc protects steel is reversed in some waters, usually at 60-90°C. Bimetallic corrosion of zinc occurs mainly when zinc or zinc-coated steel is protecting uncoated steel or other base metals such as copper. Many of the uses of zinc deliberately invoke this principle, but in other cases an unwanted effect arises as a result of constructional requirements, and avoidance of bimetallic corrosion is needed. 

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 ... 

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. 

Only a small amount of the metal used in underground service is present in the ground water zone. Such structures as well casings and under-river pipelines are surrounded by ground water. The corrosion conditions in such a situation are essentially those of an aqueous environment. 

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. 

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)... 

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... 

We saw in Section 5.6 that the dry oxidation of metals by oxygen or air can be viewed as an electrochemical process in which the electrolyte of the cell is the developing solid oxide layer itself. If liquid water is present, diffusion of the ions and molecules involved in the electrochemical corrosion process is greatly facilitated, and consequently aqueous corrosion of metals is much more important than dry oxidation at near-ambient temperatures. Although most corrosion problems encountered in practice involve only a single metal, aqueous electrochemical corrosion can be especially severe, and its principles most clearly illustrated, in cases where two different metals are in electrical contact with one another. 

Even single metals, however, are subject to aqueous corrosion by essentially the same electrochemical process as for bimetallic corrosion. The metal surface is virtually never completely uniform even if there is no preexisting oxide film, there will be lattice defects (Chapter 5), local concentrations of impurities, and, often, stress-induced imperfections or cracks, any of which could create a local region of abnormally high (or low) free energy that could serve as an anodic (or cathodic) spot. This electrochemical differentiation of the surface means that local galvanic corrosion cells will develop when the metal is immersed in water, especially aerated water. 

At the other extreme, the oxide layers on aluminum, beryllium, titanium, vanadium, chromium, nickel, and tantalum are very insoluble in water at intermediate pH values and do not have easily accessible reduced states with higher solubility. The oxide films on those metals are therefore highly protective against aqueous corrosion. 

In general, the susceptibility of metals M to aqueous corrosion is expected to correlate inversely with the E° values for the reduction of Mm+(aq) to M(s) the less positive E° is, the greater is the tendency of M to corrode in aerated water. Factors that can upset predictions based on E° include the presence of protective films (passivation), overpotential effects (Sections 15.4 and 16.6), effects of complexing agents, and incursion of a cathode reaction other than O2 reduction or H2 evolution. 

Aqueous corrosion can occur even when the metallic object to be protected is ostensibly not immersed in water, if the relative humidity of the atmosphere exceeds 60%. In that case, a film of water will in fact be present on the metal surface. Further, if sulfur dioxide is present in the air, corrosion in the thin film of water will be greatly accelerated, partly because the acidity of the dissolved SO2 facilitates the oxygen absorption reaction... 

Since nearly all environments that involve burial or deposition on soil-vegetation surfaces involve some water, dry corrosion is usually superseded by aqueous corrosion. However, many metal objects will have undergone dry corrosion prior to deposition. When a freshly polished, bright metal is left exposed to a dry atmosphere, it may become dull and tarnished. For instance, a new copper alloy coin will form a layer largely composed of red-brown copper (I) oxide, cuprite (Cu20). 

Aluminum pistons in an engine that bums H2 will be exposed to not only H2 but also H2O at temperatures of 80 to 120°C. Aluminum alloys can be totally immune to H2 embrittlement and H2-induced crack growth if the natural AI2O3 oxide is intact. However, there are processes that can disrupt this film, and it is known that aluminum alloys will absorb H2 when exposed to H2O vapor at 70°C. There will also be periods when the engine is cool and condensed water will be present so that aqueous corrosion could occur, but this is not expected to be any different than with an engine with cast aluminum pistons that bums gasoline. 

Silicate glasses resist corrosive elfcets of atmosphere, water, aqueous solutions as well as other reagents. They generally resist acidic media better than alkaline media. However, chemical durability depends considerably on glass composition glass of unsuitable composition will lose its gloss and become coated with a white layer under the effect of atmosphere containing moisture. In many instances, chemical durability is the main criterion in the choice of a suitable material. 

Properties (free acid) Liquid. Bp 185-190C (760 mm Hg), 90-92C (14 mm Hg), d 1.389 (22.8 4C). Very soluble in water and alcohol soluble in ether, (sodium salt) Crystals salty taste. Decomposes 174—176C. Corrosive to iron. Soluble in water aqueous solutions hydrolyze above 70C. 

The aqueous corrosion of ceramics may involve a charge-transfer or electrochemical dissolution process. However, in many cases, dissolution or corrosion may take place with no charge transfer yet may be determined by one or more electrochemical factors such as absorbed surface charge or electronic band bending at the surface of narrow-band-gap semiconducting ceramics. The aqueous corrosion of ceramics is important in a number of areas. One of the most important is the stability of passive oxide films on metals. The stability of ceramics is a critical aspect in some aqueous photoelectrochemical applications (12), an example being the photoelectrolytic decomposition of water. Structural, nonoxide ceramics such as SiC or Si3N4 are unstable in both aqueous acid and alkaline environments the latter is virtually unstudied, however. 

ALUM (10043-01-3) Al2(SOj3 Noncombustible solid. Forms sulfuric acid with water. Aqueous solution has a violent reaction with bases, amines, amides, inorganic hydroxides, and many other materials. See also sulfuric acid. Dry material is weakly corrosive to carbon steel aqueous solution attacks aluminum and other metals, forming hydrogen gas. Hydrogen gas can accumulate to explosive concentrations within enclosed or confined spaces. 

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