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414 Lubricant Additives: Chemistry and Applications
where
ΔG
o
= free energy of adsorption due to primary interaction in kilocalories per mole
R = 1.987 cal/K mol is the universal gas constant
T = absolute temperature in Kelvin
Figures 16.15 and 16.16 show analysis of boundary friction data of vegetable oil in steel-steel
and starch-steel contact using the Langmuir model. In all cases, plots of [O
b
]
−1
versus θ
−1
gave
FIGURE 16.15 Langmuir analysis of adsorption isotherms derived from steel/steel boundary friction of
vegetable oils in hexadecane. (Adhvaryu, A., Biresaw, G., Sharma, B.K., Erhan, S.Z., Ind. Eng. Chem. Res.,
45(10), 3735–3740, 2006. With permission.)
2.0
1.5
1.0
0.5
0.0
FractCov
−1
4003002001000
[Cottonseed]
−1
(M
−1
)
Cottonseed Langmuir analysis
'FractCov^-1'
'fit_FractCov^-1'
INT = 1.0075 ± 0.0139
SLOP = 0.002013 ± 0.000101
N = 13; CORR = 0.986447
FIGURE 16.16 Langmuir analysis of adsorption isotherms derived from starch/steel boundary friction of
vegetable oils in starch–oil composites.
6
5
4
3
2
1
0
θ
−1
35
0
300250200150100500
[
Meadowfoam
]
−1
(
k
g
/mol
)
pfg-meadowfoam
'theta^-1'
'fit_theta^-1'
INT = 0.9 ± 0.1 SLOP = 0.012 ± 0.001
N = 9; CORR = 0.984252
CRC_59645_Ch016.indd 414CRC_59645_Ch016.indd 414 12/6/2008 9:23:06 AM12/6/2008 9:23:06 AM
Surfactants in Lubrication 415
straight lines, with intercepts close to 1. From the slopes, K
o
values were obtained and used to cal-
culate ΔG
ads
using Equation 16.7.
Another model that has been used to analyze friction-derived adsorption isotherms is the Tem-
kin model [32]. Unlike the Langmuir model, the Temkin model assumes a net repulsive lateral
interaction, and the free energy of adsorption is given as follows:
∆G
ads
=G
o
+αθ (kcal/mol) (16.8)
where ∆G
o
is free energy of adsorption due to primary interaction calculated using Equation 16.7
and α the free energy of adsorption due to lateral interaction (α > 0).
Estimation of ∆G
ads
using the Temkin model (Equations 16.7 and 16.8) requires values of K
o
and α. For 0.2 ≤ θ ≤ 0.8, the Temkin model predicts the following relationship between solution
concentration, [O
b
], and surface concentration, θ, of vegetable oil [32]:
⫽
RT
K
ln
[]O
b
o
(16.9)
According to Equation 16.9, a plot of ln([O
b
]) versus θ in the speci ed θ range will result in a straight
line. K
o
and α are then obtained from the analysis of the slope and intercept of the line using Equa-
tion 16.9. The values of K
o
and α are then used to calculate ∆G
ads
using Equations 16.7 and 16.8.
Figure 16.17 shows the results of Temkin analysis of adsorption isotherms data derived from
boundary friction measurements of vegetable oils in steel-steel contact. As can be seen in Figure
16.17, the analysis gave a straight line in the range 0.2 ≤ θ ≤ 0.8, as predicted by the Temkin
model. From the slope and intercept of such analysis, K
o
and α are obtained and used to calculate
∆G
ads
using Equations 16.7 and 16.8.
1.0
0.8
0.6
0.4
FractCov
−7 −6 −5 −4 −3 −2 −1
[Cottonseed] (M)
Cottonseed Temkin analysis
FractCov
fit_FractCov
INT = 1.6513 ± 0.0916
SLOP = 0.17863 ± 0.0165
N = 6; CORR = 0.983439
FIGURE 16.17 Temkin analysis of adsorption isotherms derived from steel/steel boundary friction of
vegetable oils in hexadecane.
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416 Lubricant Additives: Chemistry and Applications
Table 16.2 summarizes ΔG
ads
values for various vegetable oils obtained from Langmuir and
Temkin analysis of steel-steel boundary friction data obtained using the ball-on-disk tribometer.
The data in Table 16.2 show that the Temkin model predicts a weaker adsorption (a smaller negative
number) than the Langmuir model. This observation is consistent with the fact that the Temkin models
assume a net repulsive lateral interaction (α > 0), which will counter the strong primary interaction.
Table 16.2 also shows that triglycerides generally adsorb stronger than the waxy ester jojoba, or the
simple fatty acid methyl esters. This has been attributed to the ability of the triglycerides to engage in
multiple binding with the surface, whereas the waxy esters and the methyl esters cannot do so since
they have only one functional group per molecule. The data in Table 16.2 also indicate that the adsorp-
tion of methyl esters weakens with decreasing fatty acid chain length. This may be an indication of
decreasing van der Waals interaction due to the decreasing molecular weight of the oil molecules.
TABLE 16.2
ΔG
ads
Values Obtained from Langmuir and Temkin Analyses
of Steel/Steel Boundary Friction Data Obtained on a Ball-
On-Disk Tribometer
Oil Friction Pairs Test Geometry
∆G
ads
(kcal/mol)
Langmuir Temkin
Cottonseed Steel/steel Ball-on-disk −3.68 −2.16
Canola Steel/steel Ball-on-disk −3.81 −2.04
Meadowfoam Steel/steel Ball-on-disk −3.66 −2.28
Jojoba Steel/steel Ball-on-disk −3.27 −1.31
Methyl oleate Steel/steel Ball-on-disk −2.91 −1.02
Methyl palmitate Steel/steel Ball-on-disk −2.7 −1.63
Methyl laurate Steel/steel Ball-on-disk −1.9 −0.6
TABLE 16.3
ΔG
ads
Values of Vegetable Oils Obtained from Langmuir Analysis of
Steel/Steel and Starch/Steel Boundary Friction Data
Vegetable Oil
Friction Pairs
Starch/Steel (Starch–Oil
Composite Dry Film Lube) Steel/Steel (Liquid Lube)
Starch Matrix ∆G
ads
(kcal/mol) Base Oil ∆G
ads
(kcal/mol)
Meadowfoam PFGS −2.41
a
Hexadecane −3.57
b
Jojoba PFGS −2.63
a
Hexadecane −3.27
c
Soybean PFGS −2.96
a
Hexadecane −3.6
d
Soybean Waxy −2.91
a
a
Biresaw, G., Kenar, J.A., Kurth. T.L., Felker, F.C., Erhan, S.M. Investigation of the mechanism
of lubrication in starch-oil composite dry lm lubricants, Lubr. Sci. 19(1): 41–55, 2007.
b
Adhvaryu, A., Biresaw, G., Sharma, B.K., Erhan, S.Z. Friction behavior of some seed oils:
Bio-based lubricant applications, Ind. Eng. Chem. Res. 45(10): 3735–3740, 2006.
c
Biresaw, G., Adharyu, A., Erhan, S.Z. Friction properties of vegetable oils, J. Am. Oil Chem.
Soc. 80(2): 697–704, 2003.
d
Biresaw, G., Adharyu, A., Erhan, S.Z., Carriere, C.J. Friction and adsorption properties of nor-
mal and high oleic soybean oils, J. Am. Oil Chem. Soc. 79(1): 53–58, 2002.
CRC_59645_Ch016.indd 416CRC_59645_Ch016.indd 416 12/6/2008 9:23:06 AM12/6/2008 9:23:06 AM
Surfactants in Lubrication 417
Table 16.3 compares ΔG
ads
data obtained from the Langmuir analysis of boundary friction data
from steel/steel and starch/steel contact. The data in Table 16.3 show that, in general, ΔG
ads
is smaller
for adsorption on steel than on starch. This means that vegetable oils will adsorb stronger on metal
than on starch. This observation is consistent with the fact that steel has much higher surface energy
than starch and will result in a stronger interaction with the polar ester groups of vegetable oils.
16.8 CONCLUSION
Surfactants are widely used in various consumer and industrial applications. They are invaluable in
various products ranging from simple cleaning to complex formulations of pharmaceuticals, foods,
lubricants, etc.
Surfactants owe their success to their unique chemical structure, which involves the presence
of two dissimilar segments in the same molecule. One segment is hydrophilic and is soluble only
in hydrophilic or polar solvents such as water, whereas the other is lipophilic and is soluble only in
lipophilic or nonpolar solvents such as hexanes. As a result, surfactants display various degrees of
solubility in both hydrophilic and lipophilic solvents. By varying the structures of the hydrophilic
and lipophilic segments, thousands of surfactants have been developed and used in thousands of
applications.
Depending on the nature of the hydrophilic component, surfactants can be broadly categorized
into anionic, cationic, zwitterionic, and nonionic. In general, ionic surfactants are more soluble in
hydrophilic solvents such as water than in lipophilic solvents. The reverse is true with nonionic
surfactants except for polyethoxylated alcohols. In the latter case, solubility depends on the rela-
tive “strength” of the hydrophilic and lipophilic segments, which can be systematically varied and
quanti ed using the HLB system. The HLB of polyethoxylated alcohols varies from 0 to 20, those
with low HLB (<7) are oil-soluble, whereas those with high HLB (>11) are water-soluble, and those
in between are soluble in both oil and water.
An important property of surfactants is their ability to spontaneously organize into various assem-
blies when in solution. The structures of these assemblies depend on the nature of the surfactant, the
nature of the solvent, the concentration of surfactant, the presence of cosolutes (e.g., salts and cosur-
factants), temperature, etc. At low concentrations, surfactants form monolayers. Progressive increase
of surfactant concentration leads to the formation of aggregates, micelles, lamellar, and hexagonal
liquid crystals. Other organized assemblies of surfactants in solution include vesicles and foams.
With proper selection of surfactant combinations, micelles can be used to solubilize large quan-
tities of oil in water, or large quantities of water in oil. The resulting single-phase solutions, which
are transparent due to the small size of the droplets, are called o/w or w/o microemulsions.
Surfactants perform a number of functions in lubrication so that the lubricant can meet its
lubricity, cooling, handling, waste treatment, disposal, and other requirements. Some requirements
of lubricants are contradictory and require proper selection of surfactant combinations to achieve
the desired objectives. An example of a contradictory requirement is that the lubricant possesses
both biostability and biodegradability. During use, the lubricant should be biostable, and the pur-
pose of the surfactant is to protect it from attack by microbes (bacteria and fungi). On the contrary,
used surfactants that are subjected to waste treatment and disposal processes should be biodegrad-
able, that is, easy to be broken down by composting microorganisms, and the purpose of the surfac-
tant is to enhance biodegradation.
Lubricant formulations can be broadly categorized into oil-based (straight oils) and water-
based. Formulation of water-based lubricants is one of the most important functions of surfactants
in lubrication. Water-based lubricants are used in processes that generate a lot of heat and must be
cooled to prevent tool wear, workpiece damage, re, and other undesirable outcomes. Surfactants
allow dispersion of the oil-phase lubricant formulation in water, without interfering with the lubri-
cation function of the oil phase and the cooling function of the water phase. Depending on the con-
centration of solubilized oil phase, water-based lubricants are classi ed into three groups: soluble
CRC_59645_Ch016.indd 417CRC_59645_Ch016.indd 417 12/6/2008 9:23:07 AM12/6/2008 9:23:07 AM
418 Lubricant Additives: Chemistry and Applications
oils, with up to 20% oil; semisynthetic, with up to 5% oil; and synthetic, with no solubilized oil.
Soluble oils are o/w emulsions, which, unlike o/w microemulsions, are milky and thermodynami-
cally unstable. Semisynthetic oils are o/w microemulsions, thermodynamically stable single-phase
clear solutions. Synthetic oils are micellar solutions with solubilized additives that provide lubricity
and other functions.
Another important function of surfactants in lubrication is adsorbing on solid and liquid sur-
faces, and interfaces, and modifying surface and interfacial properties. Agriculture-based surfac-
tants derived from farm products such as proteins, starches, and vegetable oils are effective at
reducing surface tension, interfacial tension, surface energy, and boundary friction. Boundary fric-
tion investigations in metal/metal and starch/metal contacts revealed that vegetable oils are effective
boundary additives. Free energy of adsorption values obtained from the analysis of friction-derived
adsorption isotherms was consistent with the expected effect of vegetable oil chemical structures
and friction surface properties on adsorption.
Surfactants have been known, investigated, and applied for centuries. Despite that, the current
rapid development of new and complex products, processes, and equipments have produced a great
deal of demand for new and improved surfactants and a deeper understanding of their performance.
As a result, surfactants will continue to be the subject of vigorous research and development effort
well into the future.
ACKNOWLEDGMENT
We are grateful to Danielle Wood for his help in preparing this manuscript.
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12. Byers, J.P. Metalworking Fluids, Marcel Dekker, New York, 1994.
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Metals Park, OH, 1983.
14. Mohamed, A.A., Peterson, S.C., Hojilla-Evangelista, M.P., Sessa, D.J., Rayas, D., Biresaw, G. Effect of
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17. Biresaw, G., Shogren, R. Friction properties of chemically modi ed starch, J. Synth. Lubr. 25: 17–30,
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18. Biresaw, G., Adharyu, A., Erhan, S.Z., Carriere, C.J. Friction and adsorption properties of normal and
high oleic soybean oils, J. Am. Oil Chem. Soc. 79(1): 53–58, 2002.
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19. Kurth, T.L., Byars, J.A., Cermak, S.C., Biresaw, G. Non-linear adsorption modeling of fatty esters and
oleic estolides via boundary lubrication coef cient of friction measurements, Wear 262(5–6): 536–544,
2007.
20. Biresaw, G., Adharyu, A., Erhan, S.Z. Friction properties of vegetable oils, J. Am. Oil Chem. Soc. 80(2):
697–704, 2003.
21. Adhvaryu, A., Biresaw, G., Sharma, B.K., Erhan, S.Z. Friction behavior of some seed oils: Bio-based
lubricant applications, Ind. Eng. Chem. Res. 45(10): 3735–3740, 2006.
22. Birsaw, G., Liu, Z.S., Erhan, S.Z. Investigation of the surface properties of polymeric soaps obtained by
ring opening polymerization of epoxidized soybean oil, J. Appl. Polym. Sci. 108: 1976–1985, 2008.
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pp. 110 –119.
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28. Biresaw, G., Kenar, J.A., Kurth. T.L., Felker, F.C., Erhan, S.M. Investigation of the mechanism of lubri-
cation in starch-oil composite dry lm lubricants, Lubr. Sci. 19(1): 41–55, 2007.
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423–430, 1986.
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Tribol. 108: 109–116, 1986.
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421
17
Corrosion Inhibitors and
Rust Preventatives
Michael T. Costello
CONTENTS
17.1 Introduction ......................................................................................................................... 421
17.2 Inhibitor Types.....................................................................................................................423
17.3 Mechanism ..........................................................................................................................425
17.4 Chemistry ............................................................................................................................426
17.4.1 Nitrites .....................................................................................................................426
17.4.2 Chromates ...............................................................................................................427
17.4.3 Hydrazines ..............................................................................................................427
17.4.4 Silicates ...................................................................................................................427
17.4.5 Oxidates ..................................................................................................................427
17.4.6 Sulfonates ................................................................................................................428
17.4.7 Carboxylates ............................................................................................................430
17.4.8 Alkyl Amines .......................................................................................................... 430
17.4.9 Alkyl Amine Salts................................................................................................... 431
17.4.10 Amine Carboxylate Salts ........................................................................................ 431
17.4.11 Amine Borate Salts ................................................................................................. 431
17.4.12 Phosphates ............................................................................................................... 433
17.4.13 Nitrogen Ring Structures ........................................................................................434
17.5 Corrosion Testing ................................................................................................................434
17.6 Future Requirements ........................................................................................................... 438
References ......................................................................................................................................440
17.1 INTRODUCTION
Since the observation by Plato that “things in the sea … are corroded by brine” in the fourth
century BC, the prevention of corrosion of metal parts has been a concern to manufacturers and is
the cause of serious economic loss estimated in billions of dollars per annum. The exact mecha-
nism of corrosion remained unclear for centuries after Plato. Then, in 1675, Robert Boyle wrote the
rst scienti c description of the mechanism of corrosion: “The Mechanical Origine or Production
of Corrosiveness and Corrosibility.” Subsequently, another century passed before Luigi Galvani,
in 1780, described the mechanism of bimetallic or Galvanic corrosion. After this advancement, the
study of corrosion progressed more rapidly, and W.H. Wollaston introduced the “electrochemical
theory of acid corrosion” around 1800 to explain the corrosion resistance of platinum boilers used
to make concentrated sulfuric acid [1]. Finally, in 1903, W. R. Whitney [2] published a complete
chemical description of corrosion in which he stated that oxygen and carbonic acid were neces-
sary for the formation of rust when iron was dissolved into solution and that the addition of base
inhibited corrosion. Whitney’s paper rmly placed corrosion as a chemical and electrochemical
phenomenon.
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422 Lubricant Additives: Chemistry and Applications
In the 1920s, methods to reduce corrosion involved the coating of a metal part with various
natural products such as petrolatum, wax, oxidized wax, fatty acid soaps, and rosin [3–5]. By the
1930s, petroleum sulfonates derived as by-products from oil-re ning processes were commonly used
and during World War II (WWII) the military demand expanded so rapidly that synthetic sulfonates
were produced to supplement the supply [6]. After the war, it was found that barium and overbased
barium sulfonates were very effective as rust preventatives (RPs) in slushing oils, and this became the
de facto standard [7,8]. In the 1940s and 1950s, it was discovered that corrosion could be prevented
in gas pipelines by injection of NaNO
2
, whereas in aqueous systems by using chromates and poly-
phosphates [9,10]. Subsequently, after WWII, the chemistry and various corrosion inhibitors that are
in current use have grown exponentially and expanded to a multitude of applications.
Corrosion inhibitors can be divided into several categories depending on their duration of
use, chemical composition, and end-use application. The common chemistries used for corro-
sion inhibitors include nitrites, chromates, hydrazines, carboxylates, silicates, oxidates, sulfonates,
amines, amine carboxylates, borates, amine borates, phosphates, amine phosphates, imidazoles,
imidazolines, thiazoles, triazoles, and benzotriazoles, and depending on the application different
product families are desired (Table 17.1). Primarily the performance property that determines the
type of chemistry desired for a corrosion inhibitor is the intended duration of use of the RP coating.
In particular, some coatings require long service periods due to outdoor exposure in extreme
climates where reapplication is dif cult, whereas other corrosion inhibitors are only intended for
short-term storage before a part is manufactured or painted. There are essentially three broad
classes of corrosion inhibitors, which can be divided into the following areas:
Oil coatings. These are temporary liquid coatings used to prevent corrosion during transport
of the metal part or for temporary indoor (months) or outdoor (weeks) storage. The RP
oil (or slushing oil) can be applied during the metal-forming operation or after the part is
completed.
Soft coatings. These are temporary soft solid coatings typically made of wax, petrolatum, or
grease. These coatings are used to coat structures exposed to the elements such as bridges,
cars, or trucks and last from a few months up to many years.
TABLE 17.1
Common Rust-Preventative Chemistries and Applications
Water Soluble Soluble Oil Oil Soluble
Oil coatings
Carboxylates
Sulfonates
Carboxylates Alkyl amines
Sulfonates Carboxylates Alkyl amine
Alkyl amines Sulfonates Phosphates
Alkyl amine Alkyl amines Alkyl amines
Phosphates Alkyl amines Borates
Alkyl amines Borates Imidazolines
Borates Thiazole
Benzotriazoles
Triazole
Soft coatings
Oxidates Oxidates
—
Sulfonates Sulfonates
Hard coatings
Nitrites Alkyl amines Alkyl amines
Chromates
Hydrazines
Phosphates — —
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Corrosion Inhibitors and Rust Preventatives 423
Hard coatings. These coatings are typically applied as an alkyd resin or as an inorganic coat-
ing (or galvanized coating) and are used to form a hard permanent barrier from corrosion.
These products are typically used as a pure barrier to prevent corrosion and do not possess
the RP additives used in lubricants.
Although the hard coatings are generally applied using aqueous solvents, the oil and soft coat-
ings are applied to the metal surface by various methods. Many oil and soft coatings are typically
applied using oil or solvent (naphtha)-based systems, but they can also be emulsi ed or solublized
in an aqueous system where the volatile solvent is allowed to dry and cast a lm of the RP oil. The
coatings can be classi ed into three different categories based on the method of application to the
metal surface as follows:
Water soluble. The additives are mainly inorganic materials (nitrite or arsenate) used for
aqueous systems such as water treatment or drilling muds.
Soluble oils. The additives used are mainly oil-soluble sulfonates and organic amines that can
be used in emulsions used for metal deformation and metal removal processes.
Oil soluble. The additives used are mainly oil-soluble additives that can be used in lubricant
oils for machinery or slushing oils.
In this chapter, the study of corrosion inhibitors will be restricted to the use of additives that are
used to prevent corrosion in lubricating oils or additives used to prevent corrosion in aqueous sys-
tems. In particular, although water treatment, re nery, drilling, and vapor-phase corrosion inhibi-
tors (VCIs) are not strictly lubricating oil applications, many of the additives used in these aqueous
systems have been adapted to oil-based systems and have been used as additives in lubricants.
17.2 INHIBITOR TYPES
Corrosion is a chemical or electrochemical reaction between a material, usually a metal, and its
environment, which produces a deterioration of the material and its properties. For our purposes,
it is instructive to envision corrosion as an electrochemical process in which the metal is oxidized
at the anode, and a reduction takes place at the cathode (Figure 17.1). Depending on the pH of the
system, the cathodic reaction can produce either H
2
in acidic media or OH
−
in neutral/alkaline
media.
The oxidized metal ions (Fe
2+
or Fe
3+
in the case of iron) formed at the anode then diffuse
through the system to react with either an inhibitor or the products of the reduction process at the
cathode. The end result is the formation of corrosion products (scale or rust) of poorly de ned stoi-
chiometry (Figure 17.2). In this standard model, inhibitors are used to prevent either the oxidation
of metal or the reduction of oxygen (or H
+
).
The mechanism of corrosion inhibition has been extensively investigated, and three basic
mechanisms are prevalent. The inhibitor disrupts the anodic process, cathodic process, or a combi-
nation of both processes (Figure 17.3). The mechanism of inhibition of the process is signi cantly
FIGURE 17.1 Corrosion reactions in acidic and neutral/alkaline media.
Acidic media
Anode: Fe Fe
2+
+ 2e
−
E
ox
= −(−0.44) V
Cathode: 2H
+
+ 2e H
2
E
red
= +0.00 V
Fe + 2H
+
Fe
2+
+ H
2
E
cell
= +0.44 V
Neutral or alkaline media
Anode: Fe Fe
2+
+ 2e
−
E
ox
= −(−0.44) V
Cathode: O
2
+ 2H
2
O +4e
−
→ 4OH
−
E
red
= +0.40 V
2Fe + O
2
+ 2H
2
O → 2Fe
2+
+ 4OH
−
E
cell
= +0.84 V
−
−
→
→
→
→
CRC_59645_Ch017.indd 423CRC_59645_Ch017.indd 423 10/31/2008 2:31:06 PM10/31/2008 2:31:06 PM