Page 667 - Corrosion Engineering Principles and Practice

620 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 621

(b)
FIGURE 14.9 (continued)

volatile basic compounds, such as morpholine or hydrazine, are
transported with steam to prevent corrosion in condenser tubes by
neutralizing acidic carbon dioxide or by shifting surface pH toward
less acidic and corrosive values. In closed vapor spaces, such as
shipping containers, volatile solids such as salts of dicyclohexylamine,
cyclohexylamine, and hexamethylene-amine are used.
On contact with the metal surface, the vapor of these salts
condenses and reacts with moisture to liberate protective ions. It is
desirable, for an efficient VCI, to provide inhibition rapidly and to
last for long periods. Both qualities depend on the volatility of
these compounds, fast action wanting high volatility, whereas
enduring protection requires low volatility. A convenient partial
vapor pressure for closed spaces VCIs will lie between 10 to 10 Pa
−3
(Table 14.5).
The first condition for good efficiency of a VCI is its capability to
reach the metallic surface to be protected. The second is that the rate
of transfer of the corrosion inhibiting molecules should not be too
slow to prevent an initial attack of the metal surface by the aggressive
environment before the inhibitor can act. These two conditions are
related partly to the vapor pressure of the inhibitor, partly to the
distance between the source(s) of the inhibitor and the metal surfaces,
and partly to the accessibility of the surfaces [14].
It is significant that the most effective volatile corrosion inhibitors
are the products of the reaction of a weak volatile base with a weak
volatile acid. Such substances, although ionized in aqueous solutions,
undergo substantial hydrolysis (reaction with water), the extent of