VAPOR-PHASE INHIBITORS
17.5 VAPOR-PHASE INHIBITORS
Substances of low but signifi cant vapor pressure, the vapor of which has corrosion - inhibiting properties, are called vapor - phase inhibitors. They are used to protect critical machine parts (e.g., ball bearings or other manufactured steel articles) temporarily against rusting by moisture during shipping or storage. They have the advantage over slushing compounds of easy application, with the pos- sibility of immediate use of the protected article without fi rst removing a residual oil or grease fi lm. They have the disadvantgage of accelerating the corrosion of some nonferrous metals, discoloring some plastics, and requiring relatively effec- tive sealing of a package against loss of the inhibiting vapor. The latter require- ment is relatively easily achieved, however, by using wrapping paper impregnated
314 INHIBITORS AND PASSIVATORS
on the inside surface with the inhibitor and incorporating a vapor - barrier coating on the outside.
The mechanism of inhibition has not been studied in detail, but it appears to be one of adsorbed fi lm formation on the metal surface that provides protec- tion against water or oxygen, or both. In the case of volatile nitrites, the inhibitor
may also supply a certain amount of NO − 2 that passivates the surface. Detailed data have been presented for dicyclohexylammonium nitrite [31] , which is one of the most effective of the vapor - phase inhibitors. This substance is white, crystalline, almost odorless, and relatively nontoxic. It has a vapor pres- sure of 0.0001 mmHg at 21 ° C (70 ° F), which is about one - tenth the vapor pressure
of mercury itself. * One gram saturates about 550 m 3 (20,000 ft 3 ) of air, rendering the air relatively noncorrosive to steel. The compound decomposes slowly; nevertheless, in properly packaged paper containers at room temperature, it effectively inhibits corrosion of steel over a period of years. However, it should
be used with caution in contact with nonferrous metals. In particular, corrosion of zinc, magnesium, and cadmium is accelerated. Cyclohexylamine carbonate has the somewhat higher vapor pressure of
0.4 mmHg at 25 ° C and its vapor also effectively inhibits steel [32] . The higher vapor pressure provides more rapid inhibition of steel surfaces either during packaging or on opening and again on closing a package, during which time the concentration of vapor may fall below that required for protection. The vapor is stated to reduce corrosion of aluminum, solder, and zinc, but it has no inhibit- ing effect on cadmium, and it increases corrosion of copper, brass, and magnesium.
Ethanolamine carbonate and various other compounds have also been described as vapor - phase inhibitors [23, 32] . A combination of urea and sodium nitrite has found practical application, including use in impregnated paper. The mixture probably reacts in the presence of moisture to form ammonium nitrite, which is volatile, although unstable, and conveys inhibiting nitrite ions to the metal surface.
17.5.1 Inhibitor to Reduce Tarnishing of Copper
Benzotriazole (BTA), C 6 H 5 N 3 , is a corrosion inhibitor widely used for copper and its alloys. Typical protective fi lms formed on copper are reported to consist of various multilayers. Film thickness is normally controlled by the pH of the solu- tion, ranging from less than 0.01
μ m in near - neutral and mildly basic solutions to
0.5 μ m in acidic solutions [33] . For a wide variety of conditions [34] , this inhibitor
is reported to form a Cu + surface complex following its adsorption on Cu 2 O that is present on copper surfaces.
* I. Rosenfeld et al. ( Symposium sur les Inhibiteurs de Corrosion , University of Ferrara, Italy, 1961, p. 344) report the lower value of 0.00001 mmHg obtained for dicyclohexylammonium nitrite carefully purifi ed by multiple recrystallization from alcohol.
REFERENCES