thestability of thetriplebondand increasesthe probabilityofadsorptionand
polymerization.The effect of thepolar groups on thetriplebondreactivityis
maximumwhenthe substituentisinposition3relative to thetriplebond.
3.5I nhibitiono fN earN eutral Solutions
Because of the differencesi nt he mechanisms of the corrosion process
betweena cida nd near-neutral solutions,t he inhibitors used in acid
solutions usually have little or no inhibition effect in near-neutral solutions.
In acid solutions, the inhibitiona ction is due to adsorption on oxide-free
metal surfaces. In these media, the main cathodic process is hydrogen
evolution.
In almost neutral solutions, the corrosion process of metals results in the
formation of sparingly soluble surfacep roducts such as oxides, hydroxides, or salts. The cathodic partial reaction is oxygen reduction.
Inorganic or organic compoundsa sw ella sc helating agents are used as
inhibitors in near-neutral aqueous solutions. Inorganic inhibitors can be
classified according to their mechanisms of action:
1. Formation and maintenanceo fp rotective films can be accomplished by the addition of inorganic anions such as polyphosphates, phosphates,s ilicates,a nd borates.
2. Oxidizing inhibitors sucha sc hromates and nitrites cause selfpassivation of the metallic material. It is essential that the
concentration of these inhibitors be maintained above a“ safe”
level.I fn ot,s evere corrosion can occur as ar esult of pitting or
localized attack caused by the oxidizer.
3. Precipitation of carbonates on the metal surfaces forming a
protective film. This usually occurs because of the presence of
Ca
2 C
and Mg
2 C
ions usuallyp resent in industrial waters.
4. Modification of surface film protective properties is accomplished by the addition of Ni
C 2
,C o
2 C
,Z n
2 C
,o rF e
2 C
.
The sodiumsalts of organic acids suchasbenzoate, salicylate,cinnamate,
tartrate, and azelate can be used as alternatives to the inorganic inhibitors,
particularly in ferrous solutions. When using these particular compounds in
solutionsc ontaining certain anionss ucha sc hlorideso rs ulfates, the
inhibitor concentration necessary for effective protection willd epend on
the concentration of the aggressive anions. Therefore, the critical pH value
for inhibition must be considered rather than the critical concentration.
Other formulationsfor organic inhibition of near-neutral solutions are given
in Table 3.2.
Corrosion Inhibitors
63
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68
polymerization.The effect of thepolar groups on thetriplebondreactivityis
maximumwhenthe substituentisinposition3relative to thetriplebond.
3.5I nhibitiono fN earN eutral Solutions
Because of the differencesi nt he mechanisms of the corrosion process
betweena cida nd near-neutral solutions,t he inhibitors used in acid
solutions usually have little or no inhibition effect in near-neutral solutions.
In acid solutions, the inhibitiona ction is due to adsorption on oxide-free
metal surfaces. In these media, the main cathodic process is hydrogen
evolution.
In almost neutral solutions, the corrosion process of metals results in the
formation of sparingly soluble surfacep roducts such as oxides, hydroxides, or salts. The cathodic partial reaction is oxygen reduction.
Inorganic or organic compoundsa sw ella sc helating agents are used as
inhibitors in near-neutral aqueous solutions. Inorganic inhibitors can be
classified according to their mechanisms of action:
1. Formation and maintenanceo fp rotective films can be accomplished by the addition of inorganic anions such as polyphosphates, phosphates,s ilicates,a nd borates.
2. Oxidizing inhibitors sucha sc hromates and nitrites cause selfpassivation of the metallic material. It is essential that the
concentration of these inhibitors be maintained above a“ safe”
level.I fn ot,s evere corrosion can occur as ar esult of pitting or
localized attack caused by the oxidizer.
3. Precipitation of carbonates on the metal surfaces forming a
protective film. This usually occurs because of the presence of
Ca
2 C
and Mg
2 C
ions usuallyp resent in industrial waters.
4. Modification of surface film protective properties is accomplished by the addition of Ni
C 2
,C o
2 C
,Z n
2 C
,o rF e
2 C
.
The sodiumsalts of organic acids suchasbenzoate, salicylate,cinnamate,
tartrate, and azelate can be used as alternatives to the inorganic inhibitors,
particularly in ferrous solutions. When using these particular compounds in
solutionsc ontaining certain anionss ucha sc hlorideso rs ulfates, the
inhibitor concentration necessary for effective protection willd epend on
the concentration of the aggressive anions. Therefore, the critical pH value
for inhibition must be considered rather than the critical concentration.
Other formulationsfor organic inhibition of near-neutral solutions are given
in Table 3.2.
Corrosion Inhibitors
63
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68