THE CORROSION OF IRON AND OTHER METALS 323 



the amount of oxygen present in solution ; the balance must 

 be of a very delicate character and easily disturbed in one or 

 the other direction. 



Certain salts, especially the chromates, have been found to 

 exercise a very marked protective effect and to prevent corro- 

 sion ; this has been attributed to the iron being rendered 

 " passive " by their action. It is by no means certain, how- 

 ever, that they do not act simply as salts of very weak acids — 

 in other words, by neutralising the traces of acid present in 

 the solution to which they are added. 



The action of alkaline solutions on iron has already been 

 referred to ; mention should be made, however, of experi- 

 ments which Friend has described on the action on iron, at 

 room temperatures, of solutions containing both caustic potash 

 and potassium chloride freely exposed to the air. His results 

 appear to show that in the case of solutions containing small 

 amounts of the salts, at first the corrosion is greater than in 

 distilled water but that the corrosive action decreases as the 

 amount of caustic potash is increased, becoming nil in the 

 presence of 56 grammes per litre. It is remarkable that 

 whereas in the absence of alkali corrosion proceeded uniformly, 

 the metal being covered with a layer of brown rust which 

 could be easily removed, when alkali was present the corrosion 

 was local in character, deep pits being formed in places and 

 the metal foil often eaten through. It would seem probable 

 that electro-negative impurities in the iron play a special part 

 in the latter case. 



Effect of Corrosion on the residual Iron x 



In his communication to the May meeting of the Iron and 

 Steel Institute, Mr. Longmuir states the opinion, under the title 

 " The Real Problem of Corrosion," that this " does not lie in the 

 amount of metal lost by rust or decay but on the influence 



1 The Corrosion of Metals. By Percy Longmuir, B.Met. — The Action of 

 Aqueous Solutions of Single and Mixed Electrolytes upon Iron. By J. Newton 

 Friend and Joseph H. Brown. — The Influence of Impurities on the Corrosion of 

 Iron. By John W. Cobb. — The Corrosion of Iron and Steel. By J. Newton 

 Friend, Ph.D., D.Sc. (Longmans, Green & Co.). Price 6s. net. — The Influence 

 of Carbon and other Elements on the Corrosion of Steel. Report of the Committee, 

 consisting of Prof. J. O. Arnold (Chairman), Dr. W. E. S Turner (Secretary), 

 Prof. W. P. Wynne, Prof. A. McWilliam, Mr. C. Chappell and Mr. F. Hohson. 

 (British Association Reports, Section B, 191 1.) 



