critical. For any specifici nhibitor in any given medium there is an optimal
concentration.F or example,aconcentration of 0.05% sodiumb enzoate,
or 0.2% sodiumc innamate,i se ffective in water that has ap Ho f7 .5 and
contains1 7ppm sodiumchloride or 0.5% by weight of ethyl octanol.
The corrosion because of ethylene glycol cooling water systems can be
controlled by the use of ethanolaminea sa ninhibitor.
3.3.3 Precipitation Inhibitors
Precipitationi nhibitors arec ompoundst hatc ause the formationo f
precipitates on the surfaceo ft he metal, thereby,p roviding ap rotective
film. Hard water that is highi nc alciuma nd magnesium is less corrosive
than soft water because of the tendencyo ft he salts in the hard water to
precipitate on the surfaceo fthe metal and form ap rotective film.
If the water pH is adjusted in the range of 5–6, ac oncentration of
10–100 ppm of sodiump yrophosphate will cause ap recipitate of calcium
or magnesiumo rthophosphate to form on the metal surfacep roviding
aprotective film. The inhibition can be improved by the addition of zinc salts.
3.4I nhibitiono fA cidS olution
The inhibition of corrosioni na cid solutions can be accomplished by the
use of avariety of organic compounds. Among those used for this purpose
are triple-bonded hydrocarbons; acetylenic alcohols, sulfoxides, sulfides,
and mercaptans; aliphatic, aromatic, or heterocyclic compounds containing
nitrogen; and many other families of simple organic compounds and of
condensation products formed by the reaction betweentwo different species
such as amines and aldehydes.
Incorrect choice or use of organic inhibitors in acid solutions can lead
to corrosions timulation and/or hydrogen penetration into the metal.I n
general, stimulation of corrosion is not relatedt ot he type and structure of
the organic molecule. Stimulation of iron’sa cid corrosion has been found
with mercaptans, sulfoxides, azole, and triazole derivatives, nitrites, and
quinoline. This adverse actiond epends on the typeo fa cid.F or example,
bis(4-dimethylamino-phenyl) antipyrilcarbinola nd itsd erivatives at
a1 0
K 4
Mc oncentration inhibited attacko fs teel in hydrochloric acid
solutions but stimulated attack in sulfuric solutions. Much work has been
done studyingt he inhibiting and/or stimulating phenomena of organic
compounds on ferrous as well as nonferrous metals. Organic inhibitors have
acritical concentration value, below which inhibition ceases and stimulation
begins. Therefore, it is essential that when organic inhibitors are used,
constantm onitoring of the solutions hould take place to ensure that the
inhibitor concentration does not fall below the critical value.
Corrosion Inhibitors
59
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68
concentration.F or example,aconcentration of 0.05% sodiumb enzoate,
or 0.2% sodiumc innamate,i se ffective in water that has ap Ho f7 .5 and
contains1 7ppm sodiumchloride or 0.5% by weight of ethyl octanol.
The corrosion because of ethylene glycol cooling water systems can be
controlled by the use of ethanolaminea sa ninhibitor.
3.3.3 Precipitation Inhibitors
Precipitationi nhibitors arec ompoundst hatc ause the formationo f
precipitates on the surfaceo ft he metal, thereby,p roviding ap rotective
film. Hard water that is highi nc alciuma nd magnesium is less corrosive
than soft water because of the tendencyo ft he salts in the hard water to
precipitate on the surfaceo fthe metal and form ap rotective film.
If the water pH is adjusted in the range of 5–6, ac oncentration of
10–100 ppm of sodiump yrophosphate will cause ap recipitate of calcium
or magnesiumo rthophosphate to form on the metal surfacep roviding
aprotective film. The inhibition can be improved by the addition of zinc salts.
3.4I nhibitiono fA cidS olution
The inhibition of corrosioni na cid solutions can be accomplished by the
use of avariety of organic compounds. Among those used for this purpose
are triple-bonded hydrocarbons; acetylenic alcohols, sulfoxides, sulfides,
and mercaptans; aliphatic, aromatic, or heterocyclic compounds containing
nitrogen; and many other families of simple organic compounds and of
condensation products formed by the reaction betweentwo different species
such as amines and aldehydes.
Incorrect choice or use of organic inhibitors in acid solutions can lead
to corrosions timulation and/or hydrogen penetration into the metal.I n
general, stimulation of corrosion is not relatedt ot he type and structure of
the organic molecule. Stimulation of iron’sa cid corrosion has been found
with mercaptans, sulfoxides, azole, and triazole derivatives, nitrites, and
quinoline. This adverse actiond epends on the typeo fa cid.F or example,
bis(4-dimethylamino-phenyl) antipyrilcarbinola nd itsd erivatives at
a1 0
K 4
Mc oncentration inhibited attacko fs teel in hydrochloric acid
solutions but stimulated attack in sulfuric solutions. Much work has been
done studyingt he inhibiting and/or stimulating phenomena of organic
compounds on ferrous as well as nonferrous metals. Organic inhibitors have
acritical concentration value, below which inhibition ceases and stimulation
begins. Therefore, it is essential that when organic inhibitors are used,
constantm onitoring of the solutions hould take place to ensure that the
inhibitor concentration does not fall below the critical value.
Corrosion Inhibitors
59
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68