Chemistry in the Industries 281
portant to protect the boiler metal from corrosion as it is to
prevent the formation of scale. The mechanical engineer
looks at the boiler as a perfectly solid, stable piece of equip-
ment. The chemist considers it as something subject to con-
tinuous slow reaction between the water and the steel, the
rate of the reaction being fundamentally affected by the
composition of the water and materially increased by higher
temperatures. The entire region of chemical protection lies
on the alkaline side—in a comparatively narrow range at
high temperatures. Therefore one of the essentials for pro-
tecting the boiler metal from corrosion is that the water must
be maintained in this specific alkalinity range. Since the
corrosion rate is proportional to the oxygen content, the
oxygen must be completely removed. This can be done very
effectively by mechanical deaeration followed by chemical
treatment. Sodium sulphite is maintained in the boiler and
serves as the vigilant policeman. It is kept ever-present in
order to eliminate at once any oxygen which might slip into
the system. The deterioration of boiler metal, characterized
by cracking of plates and rivets along submerged riveted
seams, is referred to as caustic embrittlement. This type of
failure is caused by simultaneous chemical action and stress.
The disastrous effect of soluble silica in alkaline solution has
been conclusively demonstrated and seems to be the prin-
cipal cause of such failure. Its nearly complete removal can
be attained through the use of hydrous magnesium oxides.
Many of the operators of boilers in high-pressure ranges re-
gard one-piece forged or welded drums as an adequate guar-
antee against serious trouble caused by silica.
Many industries use distilled water as make-up for their
boilers. Due to condenser leakage and the addition of chem-
icals for corrosion control, treatment is still necessary. Quite
generally lime-soda softened water is most economical but