Aqueous corrosion processes

Before examining in detail the theories of aqueous corrosion processes and the bases for making quantitative calculations of corrosion rates, it will be useful to develop qualitatively the major phenomena involved. The following sections review several general types of metal/corrosive-environment combinations, the chemical reactions involved, idealized mechanisms for the transfer of metal ions to the environment, and the electrochemical processes occurring at the interface between the metal and the aqueous environment. 

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. 

The corrosion conditions can be different at the fluid line from the bulk condition. Aqueous liquids have a concave meniscus, which creates a thin film of liquid on the vessel wall immediately above the liquid line. Some corrosion processes, particularly the diffusion of dissolved gases, are more rapid in these conditions. Additionally, the concentration of dissolved gases is highest near the liquid surface, especially when agitation is poor. Locally high corrosion rates can therefore occur at the liquid line, leading to thinning in a line around the vessel. This effect is reduced if the liquid level in the vessel varies with time. Any corrosion tests undertaken as part of the materials selection procedure should take this effect into account. 

The majority of metals and alloys available depend for their resistance to corrosion on the properties of an oxide film or corrosion product which is formed initially by the corrosion process. In many cases the protectiveness of the oxide film is determined by its stability in aqueous solutions in a specific pH range, either chemically dissolving to form aquocations at lower pH values or complex anions (aluminate, ferroate, plumbate, zincate, etc.) at higher pH values (Fig. 2.22). An important property of the chemical is therefore the pH value that it develops when dissolved in water. For many materials and many chemicals this is the overriding factor and in many cases... 

The need for temperature cycling should be taken into account when designing or conducting tests. The nature of the test vessel should be considered for tests in aqueous solutions at temperatures above about 60°C since soluble constituents of the test vessel material can inhibit or accelerate the corrosion process. An inhibiting effect of soluble species from glass, notably silica, on the behaviour of steel in hot water has been shown . Pure quartz or polymeric materials are often more appropriate for test vessel construction. 

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... 

Impedance analyses of the Al under corrosion were conducted via EIS. On the basis of the models previously established for the corrosion of other metals in both aqueous and nonaqueous electrolytes,the corrosion process was proposed as a two-step adsorption/oxidation/desorption process (Scheme 19). ... 

Genin, J.M., Bauer, P., Olowe, A.A. Rezel. D. (1986) Mossbauer study of the kinetics of simulated corrosion process of iron in chlorinated aqueous solution around room temperature. The hyperfme structure of ferrous hydroxide and green rust. Hyp. Interact. 29 1355-1360... 

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. 

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. 

As was noted above, one parameter that has received relatively little attention is fluid flow rate. This situation no doubt reflects the difficulties inherent in carrying out see and ep studies in aqueous systems under well-controlled hydrodynamic conditions. Intuitively, we expect fluid flow to affect all phases of the development of damage (particularly the initiation and growth phases) because of the known sensitivities of many corrosion processes to mass transport. These effects may be direct, as in the case of the transport of oxygen to the metal surface, or they may be indirect, through the effect of flow rate on the corrosion potential. 

If one wants to obtain a comprehensive understanding of the interaction between a metal (or metal alloy) and a hydrothermal solution, then electrochemical kinetics and/or corrosion studies must be carried out. In particular, an electrochemical system capable of reliably operating at temperatures above 300 °C should be developed. It is a matter of fact that there are almost no data on the exchange current densities and the anodic and cathodic transfer coefficients for even the most fundamental electrochemical reaction in high-temperature subcritical and supercritical aqueous systems. Even the primary HERs and OERs have been poorly studied at temperatures above 100 °C. Therefore, the creation of a well-established method for measuring electrochemical kinetics and corrosion processes over a wide range... 

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