Attack by Pure Water

The degree of the attack by pure water depends to a large extent on the perme-abihty of the concrete, but its Ca(OH)2 content also plays an important role. Concrete types with a low level of Ca(OH)2, Hke blast furnace slag cement concrete, have improved resistance with regard to this type of degradation. In addition to 

Usually concrete withstands normal potable water because the free Ume reacts with CO2 forming CaC03, which is not readily soluble and forms a protective skin on the concrete surface. Only in soft water with a high concentration of free CO2 is this protective layer dissolved by the formation of soluble Ca(HC03)2. A special case is the attack of concrete by very soft waters (e. g. condensation) that dissolve the free Hme (solubility 1.7 g/1). When free Hme is dissolved or leached out, the other constituents of the cement paste are attacked and the resistance of the concrete diminishes. The aggressiveness of waters to concrete can be classified, e. g. according to EN 206 [4], where pH, free CO2 content, ammonium, magnesium and sulfate ions are taken as criteria (Table 3.3). 

The calcium content in normal rain is not low enough to render it aggressive to concrete. Acid rain (with pH 3.5-4.S) in principle is aggressive, but the amount of precipitation is generally so low that the rate of attack is negligible to rather low. 

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CHEMICAL PROPERTIES noncombustible reacts with hot concentrated nitric acid reacts with boiling hydrochloric or sulfuric acid attacked by pure water attacked by weak organic acids in the presence of O2 resistant to tap water, hydrofluoric acid, brine, and solvents... 

CHEMICAL PROPERTIES stable to oxygen and water at ordinary temperatures attacked by pure water attacked by weak, organic acids in presence of oxygen resistant to tap water, hydrofluoric acid, brine, and solvent reacts with hot concentrated nitric acid reacts with boiling hydrochloric or sulfuric acid TC (0.083 at 50°C-0.077 at 225°C) RS (20.65 p-ohm-cm at 20°C) CLE (29 x 10 at 0-100°C, 31.3 x 1 O at 20-300°C). 

On the other hand, Greschuchna (38) reports that a carbonic acid solution saturated at 760 Torr (14.7 psi) and 25 C (770p) has a pH of 3.7, and that for pH >3 the corrosion rate should be hardly greater than that due to leaching by pure water, i.e. the acid effect becomes negligible. There is also evidence, however, that carbonic acid attack is enhanced by the presence of sulfates (39). 

The form of Figure 1.43 is common among many metals in solutions of acidic to neutral pH of non-complexing anions. Some metals such as aluminium and zinc, whose oxides are amphoteric, lose their passivity in alkaline solutions, a feature reflected in the potential/pH diagram. This is likely to arise from the rapid rate at which the oxide is attacked by the solution, rather than from direct attack on the metal, although at low potential, active dissolution is predicted thermodynamically The reader is referred to the classical work of Pourbaix for a full treatment of potential/pH diagrams of pure metals in equilibrium with water. 

Immersed aluminium and its alloys have excellent resistance to attack by distilled or pure condensate water, and are used in industry in condensing equipment and in containers for both distilled and deionised water, as well as in steam-heating systems... 

Addition of about 0 04% arsenic will inhibit dezincification of a brasses in most circumstances and arsenical a brasses can be considered immune to dezincification for most practical purposes . There are conditions of exposure in which dezincification of these materials has been observed, e.g. when exposed outdoors well away from the sea , or when immersed in pure water at high temperature and pressure, but trouble of this type rarely arises in practice. In other conditions, e.g. in polluted sea-water, corrosion can occur with copper redeposition away from the site of initial attack, but this is not truly dezincification, which, by definition, requires the metallic copper to be produced in situ. The work of Lucey goes far in explaining the mechanism by which arsenic prevents dezincification in a brasses, but not in a-/3 brasses (see also Section 1.6). An interesting observation is that the presence of a small impurity content of magnesium will prevent arsenic in a brass from having its usual inhibiting effect . 

The impurities likely to be present in nominally pure tin are unlikely to affect its corrosion resistance, except for minor effects on the rate of oxidation in air. Small aluminium contents, however, may result in a severely embrittling intercrystalline attack by water. The addition of antimony counteracts this effect. Although 0-1% magnesium appears to be tolerable, larger amounts produce effects similar to those of aluminium. 

C04-0037. Although aluminum cans are not attacked by water, strong acid oxidizes A1 to AL cations, liberating hydrogen gas in the process. How many moles of H2 gas will be liberated if a 2.43-g sample of pure A1 metal reacts completely with an excess of 3.00 M sulfuric acid solution ... 

Bender and Glasson (1959), in studies of alcoholysis and hydrolysis of idkyl esters in aqueous alcohol, found that the rate of disappearance of ester is decreased by increasing alcohol concentration. However, product analysis led to the conclusion that both methanolysis and ethanolysis are faster than hydrolysis in alcohol-water mixtures. It was calculated that in pure water attack by hydroxide, methoxide and ethoxide ions would occur at about the same rates. 

White metal with brdhant metaUic luster face-centered cubic crystals density 10.43 g/cm at 20°C, and 9.18 g/cm at 1,100°C melts at 961.8°C vaporizes at 2,162°C vapor pressure 5 torr at 1,500° C pure metal has the highest electrical and thermal conductive of aU metals, electrical resistivity of pure metal at 25°C 1.617x10 ohm-cm elastic modulus 71GPa (10.3x10 psi) Poisson s ratio 0.39 (hard drawn), 0.37 (annealed) viscosity of hquid silver 3.97 centipoise at 1,043°C thermal neutron absorption cross section 63 1 barns insoluble in water inert to most acids attacked by dilute HNO3 and concentrated H2SO4 soluble in fused caustic soda or caustic potash in the presence of air. 

The impure metal dissolves easily in mineral acids and in fluoroboric, sulfamic am trifluoromethylsulfonic acids to give Cr2+ solutions, but oxidation of Cr2+ by hydrogen ion (equation 6), °(Cr3+, Cr2+) = —0.41 V) even in an inert atmosphere is catalyzed by thi impurities and various ions.71 Indefinitely stable chromium(II) solutions can be obtained fron the pure (electrolytic) metal (99.5% or better), although the reaction with acid may need to b< initiated by heat and the inclusion of some metal previously attacked by acid. The use of ai excess of metal, which can be filtered off, ensures that little acid remains. In near neutra solution the hydrogen potential is lowered and the Cr2+ ion is stable. In alkaline condition brown Cr(OH)2, which slowly reduces water, precipitates.73,73... 

Tantalum is remarkably resistant to corrosion by acids, and is, in fact, referred to as a noble metal. It is not attacked by hydrochloric acid, nitric acid or aqua-regia, whether hot or cold, dilute or concentrated it is not attacked by hot dilute sulphuric acid, but boiling concentrated sulphuric acid dissolves it slowly. It dissolves in hydrofluoric acid, however, although when both metal and add are very pure, solution takes place only very slowly. A mixture of hydrofluoric add and nitric add attacks the metal rapidly, and in contact with platinum or carbon it is readily dissolved by hydrofluoric acid with evolution of hydrogen. Tantalum excellently withstands exposure to sea air, sea-water, sulphur dioxide, and mine effluents.1... 

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