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Mechanical Design and Operation of Alhnolamine Plants 203

H2S + R3N = R3NH+ + HS- (3-16)

COz + R3N + HzO = R3NH+ + HC03- (3-17)

COZ + 2RzNH = RzNHz' + RzNCOz (3-18)

Alkanolammonium ions (R2NHz+ and R3NH+) are acids in that they can provide protons for
the corrosion reaction. The corroding metal (iron in this case) will react with the most abun-
dant acid in the solution. In amine solutions, the alkanolammonium ion is much more concen-
trated than the hydrogen ion. Thus, reaction 3-3 can be rewritten in the following form:

Fe + 2R3NH+ = Fe2+ + 2H" + 2R3N (3-19)

Reaction 3-19 implies that corrosion rates should increase in proportion to the concentra-
tion of alkanolammonium ions, and generally this is true. Richer solutions are more corro-
sive than leaner, other things being equal. Also, undegraded lean MEA solutions are more
corrosive than undegraded lean diethanolamine @EA) solutions, because MEA, the stronger
base, retains a higher concentration of alkanolammonium cations and, therefore, forms a
more corrosive solution.
Reaction 3-19, like reaction 3-3, is irreversible; therefore, its rate cannot in principle be
affected by the concentrations of its products. In particular, heat stable anions such as the
oxalate ion and amine degradation products that form complexes with ferrous ions should
not affect the corrosion rate, yet experience shows that these contaminants often do aggra-
vate corrosion (Rooney et al., 1996). The reason probably is that the deposition of a protec-
tive film of iron compounds is prevented by removal of iron atoms from the carbon steel sur-
face by complexing agents. Furthermore, those complexing agents that associate with femc
as well as ferrous ions are very likely to inhibit the active-to-passive transition by preventing
the formation of an insoluble femc oxide passive film. For these reasons, certain heat stable
salt anions, amine carbamate anions, and amine degradation products may strongly affect
corrosion rates, although they may not appear in the overall corrosion reactions.
Uncontaminated solutions of tertiary amines such as MDEA are generally not corrosive
whatever the acid gas. According to API 945, solutions of most amines are not corrosive if
the ratio of hydrogen sulfide to carbon dioxide is above roughly 5/95, because the corrosion
reaction leads to formation of a protective sulfide film (MI, 1990). The most corrosive com-
binations appear to be those of primary or secondary amines with carbon dioxide.
Although it is widely believed that the acid gases are primarily responsible for the corro-
sion of carbon steel by amine solutions, the exact mechanisms have not been established
unequivocally. As indicated by reactions 3-2, 3-5, and 3-19, there are several possible
sources of protons that may accept electrons from elemental iron to cause corrosion and
release elemental hydrogen. It is possible, of course, that more than one of the proposed
species is active, depending on process conditions. Further fundamental work is needed to
firmly establish the primary mechanisms and to clarify the effects of other factors such as
solution contaminants, chelate formation, and scale deposition.

Effect of Amine Loadings and Temperature
Figure 3-6 shows the effect of COz loading on carbon steel corrosion (Fochtman, 1963).
At a given amine concentration, corrosion increases as the COz loading increases. Through