380 ^^^6 Philippine Journal of Science 1914 



solutions of its salts, the iron going into solution at the same time. In 

 the same manner, acid hydrogen, being negative to iron, is liberated and 

 iron is dissolved (corroded) . 



Stray currents are another important factor influencing corrosion. At 

 best proper insulation is a difficult problem in any city, but it is especially 

 difficult in Manila, where the heavy rains keep the ground saturated 

 four or five months in the year. Here many cases have been found 

 in which excessive corrosion of boilers has been accompanied by appreciable 

 differences of potential between boiler parts and surrounding objects. 



Corrosion may be avoided in a variety of ways. Corrosive ingredients 

 may be eliminated by proper chemical treatment. Iron does not corrode 

 readily in the presence of alkalies, hence the addition of soda ash or 

 lime to water is beneficial. Since iron is attacked electrolytically only 

 when it is the anode, any method whereby it is made the cathode will 

 prevent corrosion. Zinc is electropositive to iron, and hence when con- 

 nected with it will make the iron the cathode of an electrolytic cell. It 

 corrodes very easily, and is now very satisfactorily in use to protect 

 the internal parts of boilers and condenser tubes on steamers from electro- 

 lytic corrosion." 



Rather recently the suggestion has repeatedly appeared " to make 

 iron the cathode by impressing an electromotive force from the outside 

 counteracting many internal cui-rents which might tend to destroy the 

 iron. Another method of inhibiting corrosion takes advantage of the 

 passive state of iron. As is well known, iron, under the influence of certain 

 chemicals, notably oxidizing agents, becomes passive and resistant to 

 ordinary forms of attack. The addition of 1 kilogram of potassium 

 dichromate (KjCrjO;) to 12.5 metric tons of water should prevent corrosion." 



FOAMING AND PRIMING 



Foaming is the formation of an aggregation of bubbles on the surface 

 of the water of a boiler. Sometimes the bubbles which constitute the foam 

 are durable and remain for a long time without breaking; when formed 

 in rapid succession, they entirely fill the steam space and pass along with 

 the steam. When this is the case, a boiler is said to prime. In passing- 

 through the steam pipe the bubbles become broken up and are carried into 

 the cylinder as hot water. 



The priming tendency is influenced by the steam space and the design of 

 the boiler, and in general increases as the steam space diminishes. A boiler 

 ordinarily supplying dry steam may prime heavily under an overload. 



Some of the causes of foaming are the presence of sodium and potassium 

 salts in quantity, mud, organic and suspended matter, or any material 



" "It may perhaps pay to prevent the corrosion of steel docks by con- 

 necting them with a plate or block of zinc. In this way the oxidation 

 of the iron can be prevented, although the zinc will of course be destroyed. 

 Aluminium theoretically should be better than zinc, but has not proved 

 so satisfactory in practice probably on account of the oxide film which 

 is always formed on an aluminium surface." — Electrochem. & Met. Ind. 

 (1903), 1, 318. 



"Cf. Clement and Walker, Tech. Paper, U. S. Bur. Mines (1913), 

 No. 15. 



'* Electrochem. & Met. Ind. (1907) 5, 363. 



