Alkaline aggression

The composition of the builders in an alkaline cleaner is dependent on the metal substrate from which the soil is to be removed. For steel (qv) or stainless steel aggressive, ie, high pH, alkaline salts such as sodium or potassium hydroxide can be used as the main alkaline builder. For aluminum, zinc, brass, or tin plate, less aggressive (lower pH) builders such as sodium or potassium siUcates, mono- and diphosphates, borates, and bicarbonates are used. 

Tubercles are mounds of corrosion product and deposit that cap localized regions of metal loss. Tubercles can choke pipes, leading to diminished flow and increased pumping costs (Fig. 3.1). Tubercles form on steel and cast iron when surfaces are exposed to oxygenated waters. Soft waters with high bicarbonate alkalinity stimulate tubercle formation, as do high concentrations of sulfate, chloride, and other aggressive anions. 

It is worth emphasising too, that the position of those lines representing equilibria with the dissolved species, M, depend critically on the solubility of the ion, which is a continuous function of pH. For example, iron in moderately alkaline solution is expected to be very passive and so it is in borate solutions (in the absence of aggressive ions). However, the anodic polarization curve still shows a small active loop at low potential. 

Zinc coatings on steel (galvanised) are attacked in the same way Jis iron, but usually more slowly. Very alkaline waters are usually aggressive to zinc and will often remove galvanised coatings the corrosion products consist of basic zinc carbonate or other basic compounds and may take the form of a thick creamy deposit or hard abrjisive particles. 

In practice, pitting of nickel and nickel alloys may be encountered if the corrosive environment contains chloride or other aggressive ions and is more liable to develop in acidic than in neutral or alkaline solutions. In acidic solutions containing high concentrations of chloride, however, passivity is likely to break down completely and corrosion to proceed more or less uniformly over the surface. For this reason nickel and those nickel alloys which rely on passivity for their corrosion resistance are not resistant to HCl. 

In short, the current demand for cathodic protection varies according to the aggressiveness of the corrosive environment. It is for this reason that cathodic protection finds its greatest application where the pH is close to neutral. The more acid environments entail a current output that rapidly becomes uneconomic. The more alkaline environments prove less aggressive to the structure and therefore often do not justify cathodic protection. Table 10.5 provides some estimated current densities for cathodic protection that illustrate the point. 

Coatings of tin produced from tin-containing aqueous solutions by chemical replacement may be used to provide special surface properties such as appearance or low friction, but protect from corrosion only in non-aggressive environments. Copper and brass may be tinned in alkaline cyanide solutions or in acid solutions containing organic addition agents such as thiourea. Steel may be first coated with copper and then treated... 

The reasons why some anions exhibit strong inhibitive properties while others exhibit strong aggressive properties are not entirely clear. The principal distinction seems to be that inhibitive anions are generally anions of weak acids whereas aggressive anions are anions of strong acids. Due to hydrolysis, solutions of inhibitive anions have rather alkaline pH values and buffer capacities to resist pH displacement to more acid values. As discussed... 

The use of CA inhibitors as diuretics is limited by their propensity to cause metabolic acidosis and hypokalemia. Their use can be indicated in patients with metabolic alkalosis and secondary hyperaldosteronism resulting for example from aggressive use of loop diuretics. Furthermore, CA inhibitors are effective dtugs to produce a relatively alkaline urine for the treatment of cysteine and uric acid stones as well as for the accelerated excretion of salicylates. Perhaps the most common use of CA inhibitors is in the treatment of glaucoma. 

The conventional technology of water electrolysis makes use of alkaline solutions [7]. In particular, about 30% KOH is used at about 80 °C. The use of KOH, although more expensive than NaOH, is dictated by two reasons (1) KOH is more conductive (about 1.3 times) than NaOH and (2) KOH is chemically less aggressive than NaOH. A 30% concentration is used because the conductivity exhibits a maximum there. 

The SPE technology solves some problems but it poses others. In particular, the strong acid environment developed on the membrane calls for a complete change of electrode materials from those used in the conventional alkaline electrolysis. More specifically, especially the requirements for electrode materials for O2 evolution are stringent since the anodic conditions are especially aggressive for corrosion problems. 

Water electrolysis is usually carried out at constant current so that the performance of electrocatalysts should be constantly stable. Nevertheless, it has been shown that shutdown of cells in alkaline electrolysis results in the temporary dissolution of Ni cathodes. This implies that any variation of the electrolysis regime results in weakening of the resistance of materials to aggressive conditions. In other words, perturbations in the conditions of electrolysis are responsible for the appearance and the growth of the instability [cf AVt in Equation (7.16)]. 

An alkaline marine environment high in H2S and HS" is favorable for incorporation of sulfur into organic matter HS" is an aggressive nucleophile. Such conditions also would have been favorable for the formation of polysulfides and elemental sulfur. All of these species, either alone or in combination, are expected to have played a role in the incorporation of sulfur into the vegetable debris that ultimately formed Rasa coal. Many of these species are known to be reactive with hydrocarbons at mild temperatures. Elemental sulfur reacts with hydrocarbons to form organosulfur compounds, including thiophenes at mild temperatures (28,29). Polysulfides react with conjugated ene carbonyls at room temperature to form thiophenes and other sulfur heterocycles (30). ... 

Rainwater is essentially free of mineral solutes. It is usually slightly acidic due to the presence of dissolved carbon dioxide, or more highly acidic because of acid rain-forming constituents. As a result of its slight acidity and lack of alkalinity and dissolved calcium salts, rainwater is chemically aggressive toward some kinds of mineral matter, which it breaks down by a process called chemical weathering. 

The active phase of the Deacon catalyst is usually assumed to be a complex melt of copper or chromium and alkaline metal chlorides under reaction conditions, which is distributed within the pore network of an inert carrier [42]. Such supported liquid-phase catalysts (SLPC) are eminently suitable for adsorbing large amounts of the reacting components as sorption takes place in a bulk phase and is not restricted to only a limited number of suitable surface sites. The periodic expansion and contraction of the melt as a result of (de) sorption imposes considerable strains on the carrier structure hence, special mechanically robust support materials are needed to withstand such strains and prevent the catalyst crumbling away and disintegrating after a few cycles. In addition, even when it is immobilized on the carrier, the melt is extremely aggressive and resistant materials must be used for reactor construction. 

Obtained carbon materials are characterized by high stability at increased temperatures (temperature of testing should not exceed the temperature at which the material is produced, though), as well as in humid and aggressive (acidic and alkaline) media and under hyperbaric conditions. 

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