Environmental Engineering Reference
In-Depth Information
•
Gasoline (Pitman, 1933)
•
Camphor and turpentine (Dinley, 1937)
1.2.6.6 Metal Inhibitors
Metal inhibitors deactivate the catalytic properties of metal surfaces and complex any metal salts
that might form. Metals and their salts promote solvent breakdown through catalysis. Metal inhibi-
tors may also terminate free radicals through hydrogen donation (Archer, 1984; Howell and Tarrer,
1994). Metal inhibitors are Lewis bases that inhibit solvent-degradation reactions in the presence of
a metal and its chloride (e.g., aluminum and aluminum chloride).
*
1,4-Dioxane is a Lewis base
because the oxygen molecules in 1,4-dioxane have electrons available for sharing (a base is a proton
acceptor; a Lewis base is an electron-pair donor). Other Lewis bases used as metal inhibitors include
1,3-dioxolane,
tert
-amyl alcohol, methyl ethyl ketone, isopropyl nitrate, and nitromethane (van
Gemert, 1982).
Metal inhibitors compete with the solvent for the aluminum chloride produced at microcorrosion
sites on the aluminum surface. They either react with the active aluminum site, forming an insoluble
deposit, or complex with aluminum chloride, preventing degradation of the solvent. Successful inhi-
bition involves complexing of the chemisorbed aluminum chloride product with electronegative
chemical groups in the molecular structure of the stabilizer (Archer, 1982). The solubility of the
complex formed by the stabilizer and the metal chloride in the solvent determines the effectiveness
of the stabilizer. Highly soluble stabilizer-metal chloride complexes are undesirable, because
instead of forming a protective coating over the active reaction sites on the metal surface, the com-
plexes are removed by dissolution (Archer, 1982). Excess water promotes corrosion at the solvent-
water interface because the metal chloride reaction product dissolves easily from the metal surface
into the water phase (Archer, 1982).
The main function of a metal stabilizer is to compete with the solvent for electron-dei cient sites
on aluminum chloride adsorbed to the metal or metal oxide surface. The Lewis base stabilizer
converts aluminum chloride into an insoluble coating on the metal surface, following the series of
steps presented by Archer (1982):
Al(OH)
3
↔
Al(OH)
+
+
OH
−
, (1.17)
Al(OH)
+
+
Cl
−
→
Al(OH)
2
Cl (soluble aluminum hydroxychloride salt),
(1.18)
2[Al
3+
]•Cl
adsorbed
+
3CH
3
Cl
3
→
2[Al
3+
]
+
4Cl
−
+
3(CH
3
Cl
2
C•),
(1.19)
3
__
Al
surface
+
3(CH
3
Cl
2
C•)
→
2
(CH
3
CCl
2
CCl
2
CH
3
).
(1.20)
Methyl chloroform
2,2,3,3-Tetrachlorobutane
In addition to the radical dimer product 2,2,3,3-tetrachlorobutane, an equimolar amount of
1,1-dichloroethane and a minor amount of chloroethane have been identii ed as products of the
aluminum
methyl chloroform reaction, which suggests a reductive reaction pathway coni rmed by
an absence of vinyl chloride (van Gemert, 1982).
+
*
A Lewis base is any molecule or ion that can form a new coordinate covalent bond, by donating a pair of electrons. Lewis
bases are preferred as metal inhibitors because they have available electrons capable of complexing as electron donors
with an electron-dei cient atom such aluminum. In this context, aluminum chloride is a Lewis acid.
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