Corrosion Inhibitors
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10.3.3 interactions between adsorbed inhibitors
When the coverage of the metal surface by the adsorbed inhibitor species
increases, lateral reactions between inhibitor molecules may arise, thereby
influencing efficiency.
Attractive lateral interactions usually give rise to stronger adsorption and
higher inhibition efficiency. This effect has been shown in the case of compounds containing long hydrocarbon chains, because of attractive van der
Waals forces. In the presence of ions or molecules containing dipoles, repulsive attractions may occur, thus weakening the adsorption and diminishing
the inhibition efficiency.
10.3.4 relationships between inhibitor reactivity and Efficiency
The nature of the inhibitor initially present in acid solutions may change
with time and/or electrode potential as a consequence of reduction reactions,
polymerization reactions, or the formation of surface products. The inhibition because of the reaction products is usually called secondary inhibition,
whereas primary inhibition is attributed to the compound initially added
to the solution. Secondary inhibition may be higher or lower than primary
inhibition, depending on the effectiveness of the reaction products.
An example of inhibitors undergoing electrochemical reduction is that of
sulfoxides, the most important being dibenzyl sulfoxide, whose reduction
gives rise to a sulfide that is more effective than the primary compound.
On the contrary, the reduction of thiourea and its alkyl derivatives gives
rise to HS – ions, whose accelerating effect is known. In some instances, the
reduction reaction may be followed by polymerization reactions at the metal/
electrolyte interface. This mechanism of action is generally accepted for acetylenic derivatives. Electrochemical measurements on iron electrodes in sulfuric acid solutions inhibited by alkynes showed that acetylenic compounds act
as cathodic inhibitors, giving rise to a surface barrier phenomenon. Duwell
et al. 24 found hydrogenation and dehydration reaction products in heptane
extracts of acid/iron powder/ethynylcyclohexan-1-ol. According to Duwell et
al., the efficiency of ethynylcyclohexan-1-ol as a corrosion inhibitor apparently
depends on the properties and rates of formation of the reaction products.
10.4 Inhibition of Near-Neutral Solutions
Because of the differences in the mechanisms of the corrosion process
between acid and near-neutral solutions, the inhibitors used in acid solutions usually have little or no inhibition effect in near-neutral solutions. In
acid solutions, the inhibition action is due to adsorption on oxide-free metal
surfaces. In these media, the main cathodic process is hydrogen evolution.
321
10.3.3 interactions between adsorbed inhibitors
When the coverage of the metal surface by the adsorbed inhibitor species
increases, lateral reactions between inhibitor molecules may arise, thereby
influencing efficiency.
Attractive lateral interactions usually give rise to stronger adsorption and
higher inhibition efficiency. This effect has been shown in the case of compounds containing long hydrocarbon chains, because of attractive van der
Waals forces. In the presence of ions or molecules containing dipoles, repulsive attractions may occur, thus weakening the adsorption and diminishing
the inhibition efficiency.
10.3.4 relationships between inhibitor reactivity and Efficiency
The nature of the inhibitor initially present in acid solutions may change
with time and/or electrode potential as a consequence of reduction reactions,
polymerization reactions, or the formation of surface products. The inhibition because of the reaction products is usually called secondary inhibition,
whereas primary inhibition is attributed to the compound initially added
to the solution. Secondary inhibition may be higher or lower than primary
inhibition, depending on the effectiveness of the reaction products.
An example of inhibitors undergoing electrochemical reduction is that of
sulfoxides, the most important being dibenzyl sulfoxide, whose reduction
gives rise to a sulfide that is more effective than the primary compound.
On the contrary, the reduction of thiourea and its alkyl derivatives gives
rise to HS – ions, whose accelerating effect is known. In some instances, the
reduction reaction may be followed by polymerization reactions at the metal/
electrolyte interface. This mechanism of action is generally accepted for acetylenic derivatives. Electrochemical measurements on iron electrodes in sulfuric acid solutions inhibited by alkynes showed that acetylenic compounds act
as cathodic inhibitors, giving rise to a surface barrier phenomenon. Duwell
et al. 24 found hydrogenation and dehydration reaction products in heptane
extracts of acid/iron powder/ethynylcyclohexan-1-ol. According to Duwell et
al., the efficiency of ethynylcyclohexan-1-ol as a corrosion inhibitor apparently
depends on the properties and rates of formation of the reaction products.
10.4 Inhibition of Near-Neutral Solutions
Because of the differences in the mechanisms of the corrosion process
between acid and near-neutral solutions, the inhibitors used in acid solutions usually have little or no inhibition effect in near-neutral solutions. In
acid solutions, the inhibition action is due to adsorption on oxide-free metal
surfaces. In these media, the main cathodic process is hydrogen evolution.