Corrosion products, chemistry

Many of the measurements involved in corrosion tests are defined in standards to establish precision, accuracy, representativeness, and comparability [10-14], Analytical techniques and measurements used to characterize the environment and corrosion product chemistry, and to evaluate coatings properties and inhibitor performance, are largely covered by good laboratory practices" standards [45]. In addition, the requirements for certain analytical and environmental measurements used in regulatory efforts have been established in the Code of Federal Regulations (CFR) [47]. The corrosionist should become familiar with and use these sources of information as they apply to the goals and objectives of the study. Table 5 lists some of the ASTM standards that are more widely used by corrosionists. 

S. M. Ah, "An Updated Version of Computer Code CORA II for Estimation of Corrosion Product Mass and Activity Migration ia PWR Primary Circuits and Related Experimental Loops," Eourth International Conference on Water Chemistry of Nuclear Systems, Bournemouth, U.K., Oct. 1986, pp. 107-109. 

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. 

Oxygen corrosion involves many accelerating factors such as the concentration of aggressive anions beneath deposits, intermittent operation, and variable water chemistry. How each factor contributes to attack is often difficult to assess by visual inspection alone. Chemical analysis of corrosion products and deposits is often beneficial, as is more detailed microscopic examination of corrosion products and wasted regions. 

A third phase is sometimes identified in pitting corrosion, i.e. termination. Pits can become stifled by the build-up of insoluble corrosion products at their mouths. Removal of these mounds of corrosion products, either mechanically or through some change in the environmental chemistry, can allow the pits to restart growth. 

When mature concrete is contaminated by chloride, e.g. by contact with deicing salts, the cement chemistry is more complex, and less chloride is taken up by the cement hydrate minerals and a larger proportion is free in the pore solutions and can therefore pose a greater hazard. When embedded steel corrodes, the production of a more voluminous corrosion product pushes the concrete from the steel with resultant cracking and spalling of the concrete. 

Whenever corrosion resistance results from the formation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be to prevent their normal formation, to remove them after they have been formed, and/or to preclude their reformation. All metals that are protected by a film are sensitive to what is referred to as its critical velocity i.e., the velocity at which those conditions occur is referred to as the critical velocity of that chemistry/temperature/veloc-ity environmental corrosion mechanism. When the critical velocity of that specific system is exceeded, that effect allows corrosion to proceed unhindered. This occurs frequently in small-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of bends in pipelines, and on propellers, agitators, and centrifugal pumps. Similar effects are associated with cavitation and mechanical erosion. 

Thomas, R.G. (1990). Mineralogy of Copper Corrosion Products. Unpublished PhD Thesis, Department of Chemistry, University of Wales, Cardiff. 

The interfacial chemistry of corrosion-induced failure on galvanized steel has been investigated (2) adhesion of a polyurethane coating was not found to involve chemical transformations detectable by XPS, but exposure to Kesternich aging caused zinc diffusion into the coating. Similar results were obtained with an alkyd coating. Adhesion loss was proposed to be due to formation of a weak boundary layer of zinc soaps or water-soluble zinc corrosion products at the paint metal Interface. 

Radioactivity transport in reactor coolant circuits involves both surface corrosion and deposition. Several XPS studies(8,9) of reactor boiler alloys have been reported which show the very strong effect of coolant chemistry on the films deposited. The chemistry of corrosion products precipitated on ZrO and Al O surfaces has been studied using XPS.ly More recently, chemical decontamination of radioactive boiler circuits has been assisted by XPS analysis of the surface-active decontaminating agent.(1 ) Surface oxidation in gas-cooled reactor circuits has also been investigated. AES has been used to follow the CC>2 oxidation of a chromium steel(H) and some pure metals. (12)... 

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is a useful measure to know chemical environment of elements in material surface. The strength of XPS is its ability to identify different chemical states. This function is useful in physics, chemistry and material science, such as oxidation/corrosion products, adsorbed species or thin-film growth processes. Analysis of insulators is possible, and XPS is also capable of semiquan-titative analysis. 

K. Makela, T. Buddas, M. Zmitko, J. Kysela, The effect of hydrazine on high temperature water chemistry and corrosion product transport in primary circuit of a VVER 440 unit, Water Chemistry Conference, Nice, France, 1994. 

The corrosion of metallic materials in the atmosphere has been studied extensively (1). The majority of the work in this area has been to determine the performance of materials and to evaluate mitigation techniques in environments of interest. With only a few exceptions (see for example references 2, 3), attempts have not been made in studies conducted in the United States to fully characterize the environment and to determine the relationships between components of the environment and the performance of the material of interest (see reference 4 for a recent assessment of this area). Adherent corrosion products are often characterized, but no attempts have been made, except in laboratory studies ( ), to quantitatively relate the corrosion film chemistry to environmental parameters. 

Corrosion Film Chemistry. A linear relationship exists between the mass of corrosion product formed on carbon steel, Cor-Ten A, zinc, galvanized steel, and copper and the mass of metal in the corrosion film. This relationship is independent of site and the wide variation in environmental parameters between the sites in short-term exposures of 1 and 3 months. The ratio of the two masses is relatively sensitive to the composition of the corrosion film. The independence of this ratio from substantial variations in air quality, meteorology, and rain chemistry is interpreted as indicating, at least for the major constituents, that the composition of the corrosion film is independent of the environment in short-term exposures. 

Reynolds analogy allows estimates to be made of SO2 deposition velocity (V ) based on heat transfer or skin friction tests (or theory), of which the literature abounds. In so doing, one must realize that such a calculation deals only with the delivery of pollutant to the surface, through diffusion. If we assume that the concentration is zero at the surface (perfect absorption), we have tacitly assumed that the physical chemistry is not limiting, which will only be the case with reactive materials such as zinc or calcareous stones. For less reactive materials, the surface concentration in the pollutant profile may not be zero, leading to an interaction between physical and chemical processes. Such a situation may occur if the pH in the liquid film drops too low to permit additional SO2 dissolution, as given by Henry s law. Buffering of the film with corrosion products can prevent this from... 

The solubility of SO2 in water is strongly dependent on its pH, becoming limited below pH = 4. The presence of other pollutants can be important as they affect the pH of the liquid layer on the surface, which may also be buffered by corrosion products per se. Nitric acid deposits quite readily, for example, and could lower the pH and thus inhibit SO2 uptake. On the other hand, many atmospheric particulates are basic, and the limited literature on dew chemistry (Cadle and Goblicki) (22) does not indicate acidic dew composition, (it should be noted that these data were all taken in low SO2 environments). 

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