Scales aqueous corrosion

Organophosphonates are similar to polyphosphates in chelation properties, but they are stable to hydrolysis and replace the phosphates where persistence in aqueous solution is necessary. They are used as scale and corrosion inhibitors (52) where they function via the threshold effect, a mechanism requiring far less than the stoichiometric amounts for chelation of the detrimental ions present. Threshold inhibition in cooling water treatment is the largest market for organophosphonates, but there is a wide variety of other uses (50). 

Boiler tubes are often cleaned with EDTA or NTA solutions to remove both CaC03 scale and corrosion products. In pressurized heavy water nuclear power reactors, radioactive corrosion deposits (in effect, magnetite in which some of the Fe has been replaced by radioactive Co ) can be removed from the coolant water circuits with an aqueous mixtiu e of oxalic and citric acids (both good chelators for Fe " ") and EDTA. In home laundry operations, bloodstains on clothing can be removed by treatment with oxalic acid, which takes up the iron from the hemoglobin (Section 8.2) as Fe(ox)3 . By the same token, oxalates axe toxic when taken internally, as are many other complexing agents. For example, EDTA is used as a means... 

There is a lack of fundamental understanding of the effect of elastic tensile stress or strain and plastic strain on dissolution by either thermodynamic or kinetic interpretations. The roles of stress and/or strain in aqueous corrosion reactions occurring at the atomistic level close to room temperature have been modeled for micrometer-scale descriptions of stress corrosion cracking (SCC) and hydrogen... 

While some of the effects of, and issues with, stress under aqueous corrosion conditions are fairly well recognized, those associated with high temperatures have received less attention. However, these stress effects play a critical role in determining whether a scale is truly protective with respect to the substrate on which it has grown. 

In aqueous corrosion, raising the temperature increases the dissolution of zinc in water. A marked increase occurs up to around 60°C followed, by a decrease at higher temperatures due to the decrease in oxygen supply and the formation of more compact and adherent scale. Intergranular corrosion of zinc casting alloys is a risk above 70 C in wet or humid conditions, such as in steam, when no protective layer can form and selective dissolution of the structure occurs. In hot hard waters, scale forms at water temperatures above 55 C. This scale has a coarse grain structure and does not adhere well to the zinc surfece. Corrosion of the zinc occurs locally because of the discontinuities in the scale or local electrochemical action. Above 60°C, zinc usually becomes cathodic to steel therefore, the steel will corrode to protect the zinc coating. 

One of the key issues of high temperature corrosion is that as a rule all metallic materials are unstable in their service environment. This means that in all cases corrosion products are formed and that corrosion itself cannot be avoided at all. In order to achieve corrosion resistance, therefore, the corrosion reaction has to be guided in such a way that the corrosion product itself offers protection against the environment. This is the case if the corrosion product forms a protective surface scale. Such a scale can be compared with passive layers in aqueous corrosion, which prevent increased attack of the underlying metal. 

The methacrylamide-MA copolymer has also been studied for purification of industrial waste water/ The disodium salt of MA has been copolymerized with the sodium salt of 2-acrylamido-2-methyl-propanesulfonic acid, using peroxide initiators, to give useful dispersant and deflocculants for water-insoluble compounds of Fe, Ca, Al, and for silt and clay particles/" The copolymers also prevent boiler scale formation, corrosion, etc., without being affected by the hardness of the water. Copolymers prepared in aqueous solutions are claimed to be useful in well wall materials, coatings for microcapsule production, and for paper dry-strength agents. 

G. E. Moore and E. L. Compere, Small-scale Dynamic Corrosion Studies in Toroids, Aqueous Thorium Oxide Slurries, USAEC Report ORNL-2502, Oak Ridge National Laboratory, to be issued. 

The processes of cathodic protection can be scientifically explained far more concisely than many other protective systems. Corrosion of metals in aqueous solutions or in the soil is principally an electrolytic process controlled by an electric tension, i.e., the potential of a metal in an electrolytic solution. According to the laws of electrochemistry, the reaction tendency and the rate of reaction will decrease with reducing potential. Although these relationships have been known for more than a century and although cathodic protection has been practiced in isolated cases for a long time, it required an extended period for its technical application on a wider scale. This may have been because cathodic protection used to appear curious and strange, and the electrical engineering requirements hindered its practical application. The practice of cathodic protection is indeed more complex than its theoretical base. 

Nitric acid is one of the three major acids of the modem chemical industiy and has been known as a corrosive solvent for metals since alchemical times in the thirteenth centuiy. " " It is now invariably made by the catalytic oxidation of ammonia under conditions which promote the formation of NO rather than the thermodynamically more favoured products N2 or N2O (p. 423). The NO is then further oxidized to NO2 and the gases absorbed in water to yield a concentrated aqueous solution of the acid. The vast scale of production requires the optimization of all the reaction conditions and present-day operations are based on the intricate interaction of fundamental thermodynamics, modem catalyst technology, advanced reactor design, and chemical engineering aspects of process control (see Panel). Production in the USA alone now exceeds 7 million tonnes annually, of which the greater part is used to produce nitrates for fertilizers, explosives and other purposes (see Panel). 

Immersion in aqueous media open to air Solutions in which tin is cathodic to steel cause corrosion at pores, with the possibility of serious pitting in electrolytes of high conductivity. Porous coatings may give satisfactory service when the corrosive medium deposits protective scale, as in hard waters, or when use is intermittent and is followed by cleaning, as for kitchen equipment, but otherwise coatings electrodeposited or sprayed to a sufficient thickness to be pore-free are usually required. 

R. P. Kreh. Method of inhibiting corrosion and scale formation in aqueous systems. Patent US 5073339, 1991. 

Titanium as a carrier metal Titanium (or a similar metal such as tantalum, etc.) cannot work directly as anode because a semiconducting oxide layer inhibits any electron transport in anodic direction ( valve metal ). But coated with an electrocatalytic layer, for example, of platinum or of metal oxides (see below), it is an interesting carrier metal due to the excellent corrosion stability in aqueous media, caused by the self-healing passivation layer (e.g. stability against chlorine in the large scale industrial application of Dimension Stable Anodes DSA , see below). 

We can estimate the pH of an aqueous solution very quickly by using a strip of universal indicator paper, which turns different colors at different pH values. More precise measurements are made with a pH meter (Fig. 10.10). This instrument consists of a voltmeter connected to two electrodes that dip into the solution. The difference in electrical potential across the electrodes is proportional to the pH (as will be explained in Section 12.11) so once the scale on the meter has been calibrated, the pH can be read directly. In the United States, the Environmental Protection Agency (EPA) defines waste as corrosive if its pH is either lower than... 

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