Phosphoric aqueous corrosion

Niobium like tantalum relies for its corrosion resistance on a highly adherent passive oxide film it is however not as resistant as tantalum in the more aggressive media. In no case reported in the literature is niobium inert to corrosives that attack tantalum. Niobium has not therefore been used extensively for corrosion resistant applications and little information is available on its performance in service conditions. It is more susceptible than tantalum to embrittlement by hydrogen and to corrosion by many aqueous corrodants. Although it is possible to prevent hydrogen embrittlement of niobium under some conditions by contacting it with platinum the method does not seem to be broadly effective. Niobium is attacked at room temperature by hydrofluoric acid and at 100°C by concentrated hydrochloric, sulphuric and phosphoric acids. It is embrittled by sodium hydroxide presumably as the result of hydrogen absorption and it is not suited for use with sodium sulphide. 

In most cases, the separation of alcohols, usually methanol, ethanol, and glycerol, is carried out contemporaneously with the separation of sugars and organic acids, and almost always the desire is to quantify all these analytes. It is seen, therefore, that the mobile phase is often an aqueous acid solution, even though only water may be used (5,9). Sulphuric acid is the one most frequently used, although phosphoric acid is preferred by some, since it is less corrosive on the components of the HPLC system (10). The concentration of sulphuric acid normally varies between 0.004 N and 0.01 N or more. The choice of acid may, however, be dictated by other considerations. This is the case, for example, with the use of a conductivity detector, which requires an appropriate conductivity suppressor system. If such a device is not available for a particular... 

Fire-retardant chemicals used by the commercial wood-treating industry are limited almost exclusively to mono- and diammonium phosphate, ammonium sulfate, borax, boric acid, and zinc chloride (4,8). It is believed that some use is also made of the liquid ammonium polyphosphates (9). Some additives such as sodium dichromate as a corrosion inhibitor are also used. Aqueous fire-retardant treatment solutions are usually formulated from two or more of these chemicals to obtain the desired properties and cost advantages For leach-resistant type treatments, the literature shows that some or all of the following are used urea, melamine, dicyandiamide, phosphoric acid, and formaldehyde (10-12) ... 

Although a patent was issued as early as 1936 covering aqueous phosphoric acid- - the sulfuric acid ester process was unchallenged for nearly 20 years in spite of corrosion and pollution problems, but is no longer in much use. 

Then, in non-aqueous electrolytes, the corrosion of carbon is suppressed. Under conditions where the phosphoric acid concentration is very high, it might be supposed that the electrolyte could be considered as a non-aqueous solution, in which case the corrosion rate would be kinetically limited by the water activity. Arrhenius plots of corrosion currents at 1.0 V at various acid concentrations are shown in Fig. 13. It can be seen that there are two distinct slopes, one for acid concentrations greater than 97 wt% and one of acid concentrations lower than 93 wt%. 

Manganese(ll) phosphate is used to coat steel, aluminum, and other metals to prevent corrosion. It is produced in the reaction between solid manganese(ll) hydroxide and aqueous phosphoric acid. Write the complete equation for this reaction. 

Chapter 11 of Lustman and Kerze [LI] and Chap. 2 of ASTM Special Technical Publication 639 [S2] describe the generally excellent corrosion resistance of zirconium to most aqueous solutions. It is corroded, however, by hot concentrated sulfuric or phosphoric acids and is attacked by fluoride ion at concentrations as low as 0.001 percent [S2]. 

Reacts with water, forming phosphoric acid and flammable ethylene gas. Alkalis or elevated temperatures above 300°F/150°C can cause decomposition, forming flammable ethylene gas and phosphorus oxides. Aqueous solution is incompatible with sulfuric acid, alkalis, ammonia, aliphatic amines, alkanolamines, alkylene oxides, amides, epichlorohydrin, nitromethane, organic anhydrides, isocyanates, vinyl acetate. Attacks some plastics, rubber, and coatings. Corrosive to metals in the presence of moisture. 

There have been several new reports of corrosion studies of polyaniline or polyaniline composites on iron or steel [202—214]. In one study, PANI and PPy films were electropolymerized onto iron from aqueous oxalic acid and phosphoric acid solutions, respectively, and copolymers of PANI and PPy were formed on aluminum fi om a tosyhc acid solution [202]. Although all the polymers were reported to exhibit good corrosion protection, the polypyrrole-phosphate system exhibited superior corrosion protection of iron, compared to the polyanihne-oxalate system, attributed to the greater stability of the phosphate layer deposited at the iron substrate, again illustrating the importance of the dopant ion. 

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