Salt solutions cast iron

With some important exceptions, gray-iron castings generally have corrosion resistance similar to that of carbon steel. They do resist atmospheric corrosion as well as attack by natural or neutral waters and neutral soils. However, dilute acids and acid-salt solutions will attack this material. 

Waters of pH less than 6 may be expected to be corrosive, but, because any weak acids present in the solution may not be fully ionised, it does not follow that water of pH greater than 7 will not be corrosive. Mine waters are particularly corrosive to cast iron, often to such an extent as to preclude its use with them, because of their relatively high acid content, derived from the hydrolysis of ferric salts of the strong acids, mainly sulphate, and because the ferric ion can act as a powerful cathodic depolariser. 

Those salts which hydrolyse to give an acid solution, e.g. the strong acid salts of aluminium, iron and, to a lesser extent, calcium, give solutions which may be very corrosive to cast iron, particularly if they are well aerated. When oxidising salts are also present in these acid solutions, a particularly dangerous system may be created. It is owing to this combination of oxidising and acidic character that mine waters are so corrosive. 

Table 3.54 Corrosion of Type 1 Ni-Resist and cast iron in various inorganic salt solutions ...

Dicyclohexylammonium nitrite s (DCHN) has a solubility of 3-9g in 100 g of aqueous solution at 25°C, giving a solution pH of about 6-8. Its vapour pressure at 25°C appears to be about 1-3 x 10 N/m but the value for commercial materials depends markedly on purity. It may attack lead, magnesium, copper and their alloys and may discolour some dyes and plastics. Cyclohexylammonium cyclohexyl carbamate (the reaction product of cyclohexylamine and carbon dioxide, usually described as cyclo-hexylamine carbonate or CHC)" is much more volatile than DCHN (vapour pressure 53 N/m at 25°C), and much more soluble in water (55 g in 100cm of solution at 25°C, giving a pH of 10-2). It may attack magnesium, copper, and their alloys, discolour plastics, and attack nitrocellulose and cork. It is said to protect cast iron better than DCHN, and to protect rather better in the presence of moderate concentrations of aggressive salts. 

The purer forms of iron (wrought iron and steel) appear to be much more susceptible to this kind of reaction than cast iron.3 If the attacking acid is readily reducible by hydrogen, many secondary reactions may take place. Thus with nitric acid, oxides of nitrogen and ammonia may be evolved, whilst with selenic acid a deposit of elementary selenium is obtained (see below). When iron is exposed to the action of acids that are also powerful oxidisers—such as, for example, fairly concentrated solutions of nitric and chromic acids,—it is frequently rendered inert or passive.4 Its surface may remain perfectly bright, but the metal does not show any appreciable solution in the acid, and if removed and immersed in dilute solutions of such salts as copper and silver sulphates, no reaction takes place, although ordinary active iron would cause an immediate precipitation of the more electronegative metal. 

Potassium Carbonate.—Density of weak liquor will vary greatly, dependins on the source of the product. Solutions are concentrated until the salt crystal, separate in a vertical-tube evaporator with a steam pressure of from 5 to 10 lb and a vacuum of 26 in., at a capacity of 1 gal. per square foot. The carbonate crystals are recovered in salt filters. Evaporators are made of cast iron or steel, and tubes of steel or charcoal iron. 

Table 9-9, from Rowe and Dickert s investigation of solutions of diisopropyl dithiophosphates in n-hexadecane [19], lists the rates of wear in parallel with the coefficients of friction for steel against steel and copper against steel. A strong effect from the presence of dithiophosphate is seen for steel on steel in the sharp decrease in both the wear rates and the coefficients of friction relative to that with uncompounded hexadecane as the lubricant. For copper against steel, the wear rate decreases significantly, particularly with the metal salts of diisopropyl dithiophosphate, but the coefficient of friction is not altered systematically. Table 9-10 shows the frictional behavior of hardened alloy cast iron in the presence of decalin solutions of triphenyl phosphate and diphenyl phosphate [15]. The mechanisms governing the action of phosphates and dithiophosphates are discussed in Chapter 11. 

What became known as the Leblanc process was actually several interrelated processes. Salt was first reacted with sulphuric acid in a cast-iron pan, then in a reverberator furnace (in which heat was apphed from a flame blown from a separate chamber, not in direct contact with the salt), to produce saltcake (sodium sulphate), with hydrochloric acid released as a waste gas. Saltcake was used to make sodium carbonate, or roasted with limestone (calcium carbonate) and coal or coke to produce black ash. This mixture of sodium carbonate, calcium sulphide, sodium sulphide, hme, salt, carbon, and ash could be treated further with hot water to produce impure sodium carbonate in solution, evaporated into soda crystals (washing soda), or heated to yield anhydrous sodium carbonate. The latter, in turn, could be reacted with lime to made caustic soda (sodium hydroxide), the strongest commercial alkali then available. 

N,A-Dimethylaniline 355 cf-1044 A mixture of aniline (93 g), pure methanol (105 g), and 94% sulfuric acid (9.4 g) is heated at 215° for 6 h in an autoclave with a cast-iron lining. Some of the amine is alkylated thereby to the quaternary salt, and to cleave this to the tertiary base the reaction product is treated, after cooling, with 30% sodium hydroxide solution (25 g) and heated again in the autoclave for 5 h at 170°. The autoclave contents are then distilled in steam, and the amine is salted out of the distillate, separated in a separatory funnel, and distilled through a column. This gives almost pure A,jV-dimethylaniline (117 g, 96%), b.p. 192°. 

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