Corrosion process modes

All areas of the cooling water system where a specific form of damage is likely to be found are described. The corrosion or failure causes and mechanisms are also described. Especially important factors influencing the corrosion process are listed. Detailed descriptions of each failure mode are given, along with many common, and some not-so-common, case histories. Descriptions of closely related and similarly appearing damage mechanisms allow discrimination between failure modes and avoidance of common mistakes and misconceptions. 

Fig. 2 Schematic illustration of the modes of charge-transfer process (a) polarized electrode with a net external current, (b) corrosion process with no net external current, and (c) chemical process with no local and external currents.

From studies of service behavior and from extensive laboratory investigations, the well-established terms stress-corrosion cracking (SCC) and corrosion fatigue have been shown to relate to a continuum of failure modes classified as environment-sensitive fracture. In many environments, the addition of stress, with associated strains, introduces a variable that can result in brittle failure in the sense of very limited plastic flow in otherwise ductile materials such as the stainless steels. Environment-sensitive fractures propagate at an advancing crack tip at which, simultaneously, the local stresses can influence the corrosion processes, and the corrosion can influence the crack-opening processes. Since these processes proceed by kinetic mechanisms, they are time and stress dependent with the result that the crack propagation rate can become very sensitive to the stress application rates. Conventional SCC usually has been associated with static stress, but this is seldom realized... 

Despite the fact that the peculiarities of metal wearing in contact with polymers were determined more than 30 years ago [35], the serviceability of metal-polymer friction joints has resisted estimates for a long time. The origin of this corrosion wear mode was considered only for plastics processing equipment, in which metals are in contact with the moving polymer melt [36]. 

FYom the multitude of intricate corrosion processes in the presence of mechanical action (friction, erosion, vibration, cavitation, fretting and so on) it is justified to touch upon corrosion types joined under a single failure mode induced by mechanical stresses. These are the stresses that govern the corrosion wear rate of metals during friction. Such processes are usually called corrosion stress-induced cracking in the case that the mechanical action is effective only in one definite direction, or otherwise termed corrosion fatigue in the case that compressive and tensile stresses alternate within cycles. In spite of the differences between the appearance of these corrosion types, they have much in common, e.g. fundamental mechanisms, the causes, and they overlap to a certain degree [19]. 

The related add-on challenge is to optimize materials for conjoint failure modes when conjoint, nonlinear and coupled corrosion processes occur, including mechanically induced modes (wear, fretting, fatigue, and creep). Another need is the ability to handle or anticipate changes in solution or processes with time and transitions in corrosion modes. 

Erosion is one of several wear modes involved in tribocorrosion. Solid particle erosion is a process by which discrete small solid particles, with inertia, strike the surface of a material, causing damage or material loss to its surface. This is often accompanied by corrosion due to the environment. A major environmental factor with significant influence on erosion-corrosion rates is that of flow velocity, but this should be set in the context of the overall flow field as other parameters such as wall shear stress, wall surface roughness, turbulent flow intensity and mass transport coefficient (this determines the rate of movement of reactant species to reaction sites and thus can relate to corrosion wall wastage rates). For example, a single value of flow velocity, referred to as the critical velocity, is often quoted to represent a transition from flow-induced corrosion to enhanced mechanical-corrosion interactive erosion-corrosion processes. It is also used to indicate the resistance of the passive and protective films to mechanical breakdown [5]. 

AVhile the Series is primarily classified on the basis of types of materials and their processing modes, some volumes will focus on particular groups of applications (Nuclear Materials, Biomedical Materials), and others on specific categories of properties (Phase Transformations, Characterization, Plastic Deformation and Fracture). Different aspects of the same topic are often treated in two or more volumes, and certain topics are treated in connection with a particular material (e.g., corrosion in one of the chapters on steel, and adhesion in one of the polymer volumes). Note, however, that corrosion is now to receive its own dedicated volume, number 19. Special care has been taken by the Editors to ensure extensive cross-references both within and between volumes, insofar as is feasible. A Cumulative Index volume will be published upon completion of the Series to enhance its usefulness as a whole. 

In most SECM experiments, the tip is held at a constant potential in an amperometric mode or scanned in the cyclic voltammetry (CV) mode. The substrate can also be subjected to various potential treatments. Studies involving transients or time-dependent signals are especially useful in obtaining information about adsorbed intermediates or products, as discussed in Chapter 16. Another time-dependent technique involves an AC signal applied to the tip, a form of electrochemical spectroscopy impedance (EIS). Examples of this approach have been discussed in Chapter 14 as applied to studies of the mechanism of corrosion processes. For example, in the... 

In more recent work embrittlement in water vapour-saturated air and in various aqueous solutions has been systematically examined together with the influence of strain rate, alloy composition and loading mode, all in conjunction with various metallographic techniques. The general conclusion is that stress-corrosion crack propagation in aluminium alloys under open circuit conditions is mainly caused by hydrogen embrittlement, but that there is a component of the fracture process that is caused by dissolution. The relative importance of these two processes may well vary between alloys of different composition or even between specimens of an alloy that have been heat treated differently. 

The majority of phosphate processes in use today are accelerated to obtain shorter treatment times and lower processing temperatures. The most common mode of acceleration is by the addition of oxidising agents such as nitrate, nitrite, chlorate and hydrogen peroxide. By this means, a processing time of 1 to 5 min can be obtained at temperatures of 43-71 °C. The resultant coatings are much smoother and thinner than those from unaccelerated processes, and, while the corrosion resistance is lower, they cause less reduction of paint gloss and are more suited to mass-production requirements. 

In a similar study, Gray et al. (60) investigated the possible formation of N-nitrosamines in heated chicken frankfurters which been prepared with various levels of nitrite (0-156 mg/kg). As expected, apparent N-nitrosamine levels increased with increasing concentrations of nitrite, but did not exceed 4 yg/kg except for two samples which contained 8 and II yg/kg of NMOR. The presence of these relatively high levels of NMOR was confirmed by mass spectrometry and raised the question as to its mode of formation. It was shown to be due to the morpholine present in the steam entering the smokehouse, as this amine is commonly used as a corrosion inhibitor in steam process equipment ( ). The detectable levels of NMOR in the Canadian study ( ) were also attributed in part to the use of morpholine as an anti-corrosion agent in the steam supply (62). 

It is of primary interest to avoid corrosive mineral acids in synthetic processes. This can easily be achieved by use of acidic solid supports coupled with microwave irradiation. This has been applied to the preparation of quinolines [53] (Scheme 8.35). This procedure is a safe, green alternative to the use of H2S04 at more than 150 °C. In the same way, quinoxaline-2,3-diones were prepared [54] by use of single-mode irradiation. Previous attempts in solution led to explosions, but the authors successfully used solvent-free conditions with acidic supports or catalysts (the best being p-toluenesulfonic acid) and irradiation times of 3 min (Scheme 8.36). 

Plastics. Part of the trend to substitute plastic and composite substrates for metals can be attributed to a desire to avoid the process of metallic corrosion and subsequent failure. Relatively little attention has been called to the possible failure modes of plastics under environments considered corrosive to metals. More extensive work should be conducted on the durability and life expectancy of plastic and composite materials under end-use environments. A further consideration is the potential for polymer degradation by the products of metal corrosion in hybrid structures comprising metal and polymer components. Since it is expected that coatings will continue to be used to protect plastic and composite substrates, ancillary programs need to be conducted on the mechanisms by which coatings can protect such substrates. 

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