IX. A, 4 Cox et al. : Water Supplies in the Philippines 379 



were present did any iron go into solution. The action began at about 

 100°, and the evolution of hydrogen was most pronounced when calcium 

 and potassium chlorides and potassium and sodium sulphates were used, 

 which indicates that the rusting in the magnesium chloride solution was 

 not the result of splitting oflf hydrochloric acid. The magnesium salt 

 reacts with the oxidized iron to form a soluble salt of the latter, while 

 the magnesium is precipitated as hydroxide. Within certain limits this 

 reaction is reversible, and in no case will it proceed until all of the 

 magnesium salt is precipitated. When equilibrium is attained, no more 

 iron will be dissolved until a new supply of magnesium salt is introduced 

 or the equilibrium disturbed in some other manner. 



The view that magnesium chloride does not have the highly corrosive 

 action generally attributed to it is further corroborated by Bradbury," 

 who found that magnesium chloride, either in cold or hot solution, would 

 not attack iron at atmospheric pressure. 



Rohrig and Treumann °° have shown that at high pressures magnesium 

 salts and calcium carbonate interact to precipitate hydrated magnesium 

 oxide. When a 0.5 per cent water solution of magnesium chloride is 

 employed, it is possible to precipitate 63.3, 83.1, and 100 per cent of 

 the magnesium oxide present under a pressure of 5, 10, and 15 atmospheres, 

 respectively. 



Ost found that at a pressure of 10 atmospheres the equilibrium soon 

 established itself and even at lower pressure the carbon dioxide liberated 

 soon escaped with the steam. The interaction was not complete, but a 

 sufficient quantity of the magnesium is precipitated in the mud to stop 

 the solution of iron. 



The presence of free acid in hot water in more than 40 or 50 parts 

 per million is apt to cause serious corrosion of a boiler or any metal 

 parts with which it comes in contact. The action of acid due to grease 

 which, in spite of all precautions, finds its way into a boiler is sometimes 

 the cause of serious difficulty. The presence of acid-free volatile oil can 

 do no harm in a boiler. On the contrary, besides assisting in preventing 

 scale it is useful in reducing the rusting. For the treatment of acid 

 mine waters, Ba(0H)2 is the most serviceable reagent."^ 



Electrolytic action is sometimes the cause of corrosion. When there 

 is a difference of potential between different parts of a system immersed 

 in an electrolyte, electrochemical action is set up at the expense of the 

 more electropositive element; for example, if brass feed or internal pipes 

 were used in contact with iron, the iron would disintegrate rapidly. In 

 spite of this fact, it is common practice to use brass or copper piping for 

 feed pipes. Impurities in iron, as carbon or slag, would have the same 

 injurious effect. But the presence of a more electronegative substance is 

 not necessarily the only cause of electrochemical corrosive action. Strain, 

 distortion of any part, lack of homogeneity, or even a difference in temper- 

 ature between portions of the same piece of metal may cause differences 

 of potential and greatly accelerate corrosion. From electrochemical con- 

 siderations the corrosive action of acid waters is also readily explained. 

 Iron, which is electropositive to copper, will precipitate the latter from 



"Chem. News (1913), 108, 307. 



'■'• Zeitschr. f. off. Chew.. (1900), 6, 241-3. 



"Griffin, M. L., Journ. Am. Chem. Soc. (1899), 21, 665. 



