CHAPTER II

PERfORMANCE AND IMPACT OF

SCHIFF BASES AS METALLIC

CORROSION INHIBITORS

•!• Organic compounds as Corrosion Inhibitors

•!• Effect of Functional Groups on Inhibitors

•!• Nitrogen containing Organic Inhibitors

•!• Classification of Inhibitors

•!• Phenomenon of Adsorption

•!• Structure of Organic Inhibitors and their

Inhibitive Action

•!• Influence of Inhibitor Concentration on Corrosion

Rate

•!• Effect of Inhibitors on Electrochemical Corrosion

Process

•!• Thermodynamics and Corrosion Kinetics in the

presence of Inhibitors

•!• Effect of Temperature on Cathodic and Anodic

Reactions

•:• Organic compounds as corrosion inhibitors

Many organic compounds have been investigated as corrosion

inhibitors but Schiff bases, i.e. azomethines have received lesser

attention so far. Due to the presence of the >C = N- group, such

compounds should be good metallic corrosion inhibitors [ 1].

Different types of organic compounds have been used as

corrosion inhibitors and the reviews by Putilova and others are worth

mentioning [2, 3, 4, 5]. Other reviews on Corrosion Inhibitors that are of

importance are due to Hackermann [6, 7], Rosenfeld [8], Fischer [9, 1 0],

Hoar [11], Elze [12], Heusler [13], Mercer [14], Thomas [15],

Papavinasam [ 16].

Organic compounds containing nitrogen, sulphur, oxygen etc are

known to act as corrosion inhibitors for acids corrosion of metals. Thus

thioureas, mercaptans, sulphoxides, sulphides, amines, aldehydes, etc.,

have long been used as inhibitors of acid corrosion of metals and alloys

such as iron, steel, copper, aluminium, zinc and their alloys. High

molecular weight compounds such as polysaccharides, fatty acids and

proteins have also been shown to possess inhibitive properties against

acid corrosion of metals [ 17, 18, 19].

Heterocyclic compounds containing delocalized 7t-electrons and

hetero-atoms in a ring system have also been shown to inhibit metallic

corrosion by a number of researchers [20, 21, 22].

•:• Effect of Functional Groups on Inhibitors

Inhibitors can bind to metal surfaces not only by electrostatic

interaction but also by electron transfer to the metal to from a coordinate

type of linkage. This type of interaction is favored by the presence in the

- 16-

metal of vacant electronic orbitals of low energy as they occur m

transition metals.

Electron transfer from the adsorbed species is favored by the

presence of relatively loosely bound electrons, such as may be found in

anions and neutral organic molecules containing lone pair of electrons or

n-electron systems associated with multiple bonds or aromatic rings. In

organic compounds suitable lone pair of electrons for coordinate bonding

occurs in functional groups containing elements of group V and VI of

the periodic table. The tendency to stronger coordination bond formation

and hence stronger adsorption by these elements increases with

decreasing electro-negativities in the order 0 < N < S < Se [23, 24] and

also depends on the nature of the functional groups containing these

elements.

•:• Nitrogen containing Organic Inhibitors

Finley and Hackermann [25] investigated the inhibitor

effectiveness ofthe first few members of the series cyclic polymethylene

imines whose structure may be represented as

Their study included compounds with 'n' ranging from four to

seven and showed an increase in inhibitor efficiency with the increase in

the ring size by about 20% per >CH2 group. In view of the high

efficiency of large rings, the authors synthesized even larger rings [7].

Further, since the compounds appeared to be quite stable, their

effectiveness was tested under more rigorous conditions. These authors

studied the structural effects of organic nitrogen containing compounds

on corrosion inhibition. They compared the inhibitor efficiencies within

- 17-

a senes of cyclic imines and secondary amines and showed that the

differences were clearly related to the differences in their molecular

structure. It was observed that there is a little direct dependence of

inhibitor efficiency on the surface area covered by the amines.

Hackermann and Hurd [6] studied a series of N-methylanilines and

compared their inhibition efficiencies with the cyclic imines under

identical conditions. They showed that there was no direct dependence of

inhibition efficiencies with their base strengths. According to the

authors, n-orbital character of the free electrons on the nitrogen atom

requires to be taken into consideration.

The n-orbitals in aromatic compounds are perpendicular to the

bonds of nitrogen-carbon and nitrogen-hydrogen, and the more the

n-orbital character present for the bond between metal-substrate and

adsorbed amines, the better the inhibition. Hine [26] has shown for the

aniline structure that the electronic configuration of the amine nitrogen

atom may conjugate with the n-orbital system of the benzene ring and

thereby assume some n-orbital character. The degree of participation of

the unshared electron pair of nitrogen with the n- ring system may be

altered by the substitution in the para position and either electron

repelling or electron attracting groups may cause increase in their

n- orbital character. For several substituent, the electron density on the

amine nitrogen increases in the order,

- CH3CO >- COOH >- Cl >None >- CH3 >- OCH3

Hackermann [6] has concluded that n-character of the electrons

must be included with the properties of nitrogen-containing orgamc

molecules known to contribute to inhibition by adsorption.

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•:• Classification of Inhibitors

Classification of corrosion inhibitors is somewhat a subjective

exercise. Some of the more commonly encountered descriptives are

anodic, cathodic, passivating, oxidizing, film-forming, orgamc, vapor

phase, volatile and 'safe' or 'dangerous' inhibitors.

Mixed potential theory of an electrochemical corrosion process

expressed graphically in the form of an Evans type polarization diagram

is an extremely useful way to picture how an inhibitor works. Figure II-I

shows a simple corrosion reaction described in this manner defining the

steady state free corrosion potential and corrosion current (rate). These

values result from the mutually polarizing partial reactions, anodic and

cathodic, which combine to constitute the overall corrosion.

E

Ec CATHODIC

Ecorr

Ea

'----------....1.---- I

Figure II-1 : Evans type polarization diagram describing

a simple corrosion process.

Anodic M ~ M"+ + ne-

Cathodic : Acid

2H+ + 2e- ~ H2

0 2 + 4H+ + 4e- ~ 2H 20

Neutral to Alkaline

2H 20 + 2e- ~ 20H- + H 2

0 2 + 2H 20 + 4e- ~ 40H-

- 19-

Figure II-2 given below shows six basic changes that can result from the

situation on adding a corrosion inhibitor.

Einh

Ecorr

Einh

Ecorr

Einh

E

ANODIC POLARI.S'AT!ON L..-----'---L----1

linh lcorr

E

E

~------------~,~in_h ________ ~lc-o-rr-- 1

Einh

Ecorr

E

/ /

~NOD!C PASSIVATION L..-----~_. _____ ,

linh lcorr E

(.'ATIJODIC PASSIVATION

" " " Ecorr

Einh

linh lcorr

E

L.._ _____ _._~--1

linh lcorr

Figure II-2 : Evans type polarization diagram describing how a

chemical inhibitor can effect corrosion inhibition.

It can be seen that a corrosion inhibitor can either affect or

interfere with the anodic partial reaction, cathodic partial reaction or

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both. One can now appreciate how the classifications of anodic and

cathodic inhibitors have been derived. In reducing the corrosion current,

the corrosion potential is shifted anodically (positively) for an anodic

inhibitor and cathodically (negatively) for a cathodic inhibitor. In the

case, where the inhibitor affects both partial reactions (a common feature

in practice), the shift in corrosion potential will depend on which is more

dominant effect and little or no potential shift may occur. The presence

of an ohmic resistance has also been depicted in the above figure. Ohmic

resistance may result from the presence of a potential drop in solution or

in the presence of poorly conducting film, a feature associated with

certain types of inhibitor films (say with benzotriazole on copper) and

paint films.

From what has been stated above, it is now possible to describe

more specific inhibitor types. Passivating inhibitors are a specific type of

anodic inhibitors. They cause a large shift in corrosion potential to

render a metal passive, and if present in sufficient concentration (and

maintained) are generally the most effective of all inhibitors because

they can stifle corrosion almost completely. If the concentration is not

sufficient, they can promote localized corrosion (combination of a large

cathodic area and a small anodic area). Thus they are often classified as

dangerous inhibitors. Cathodic inhibitors are often classified as safe

inhibitors as they do not interact directly with the anodic reaction. There

are however exceptions, e.g., cathodic poisons which interfere with the

cathodic hydrogen recombination so reducing general corrosion, but can

promote hydrogen blistering and embitterment due to absorption of

atomic hydrogen by the metal.

- 21-

The action of a passivating inhibitor may be described as shown

in the Figure II-3 below:

Ep

Ecorr

E

ANODIC

CATHODIC PASSIVITY

ANODIC

INCREASING INHIBITOR

CONCENTRATION

--------------------------1 lp lcorr

Figure II-3 : Evans type polarization describing the action of a

passivating anodic corrosion inhibitor.

If the inhibitor is below a critical concentration, the corrosion

potential can sit either in the active or passive region, an unstable

situation generally resulting in pitting. The behavior of a passivating

inhibitor is somewhat different compared to a simple anodic inhibitor.

The anodic shift in corrosion potential certainly applies to passivating

inhibitors but how is this achieved?

The above polarization diagram shows no basic change in the

shape I position of the overall anodic polarization curve, whereas that for

the cathodic reaction changes significantly. The inhibitor is causing the

potential of the corroding metal to be shifted sufficiently anodic for it to

sit within the thermodynamically defined passive region inherent to that

metal. The inhibitor is thus oxidizing in its action on the metal and the

degree of this effect will be dependent on inhibitor concentration.

-22-

Typical oxidizing inhibitors are chromate and nitrite, which are

themselves easily reduced and so have the effect of depolarizing the

cathodic reaction as is shown in Figure II-3. However, in the case of

chromate, adsorption of chromate at anodic areas also appears to play a

part in the inhibition process- polarizing the anodic reaction and so

decreasing the anodic current required to move the potential in the

passtve regiOn.

Certain passivating inhibitors such as phosphate, silicates and

molybdates are classified as non-oxidizing as they require the presence

of oxygen to induce passivity. They can be dangerous if present in

insufficient concentration. Thus we have seen that in reviewing this

example, the inhibitor classification moved through passivating to

oxidizing, dangerous and non-oxidizing with some help from cathodic

depolarization and adsorption while still fulfilling the basic classification

of anodic. Additionally the inhibitor anion usually gets incorporated into

the passive oxide film formed further helping to stabilize it. Such is the

difficulty of unambiguous classification.

Organic compounds constitute broad class of corrosion inhibitors,

which cannot be designated specifically as anodic, cathodic or ohmic.

Anodic and Cathodic effects alone are sometimes observed in the

presence of organic inhibitors, but as a general rule, organic inhibitors

affect the entire surface of a corroding metal when preserit in sufficient

concentration. Both anodic and cathodic areas probably are inhibited, but

to varying degrees, depending on the potential of the metal, chemical

structure of the inhibitor molecule and size of the molecule. The

corrosion inhibition increases with concentration of inhibitor suggesting

that inhibition is the result of adsorption of inhibitor on the metal

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surface. Organic inhibitors are adsorbed according to the ionic change of

the inhibitor and the change on the metal surface.

•:• Phenomenon of Adsorption

Adsorption is an important step in inhibition by organic-based

film forming corrosion inhibitors. Adsorption results from the polar or

charged nature of the organic molecule I ionic species first establishing a

physisorbed surface film (through vander Waals Forces) which may

further stabilize through chemisorption to form a donor type bond. The

donor type bond has been considered in tenns of Lewis acid and base

theory applied to the stability of the donor (adsorption) bond; the Hard

and Soft Acids and Bases (HASB) principle [27], where a hard acid will

bond with a hard base and vice versa.

Hard Low Polarizabili ty

High electronegativity

Soft High polarizability

Low electronegativity

Hard Acid Hard base

Iron oxide 0, N

Soft acid Soft base

Active iron S, P, Se, As

Adsorption is the primary step m achieving inhibition in acid

solutions. This is a consequence of the fact that the corroding metal

surface to be inhibited is usually oxide-free allowing the inhibitor a

ready access to retard the cathodic and or the anodic electrochemical

process of corrosion. Once the inhibitor has been adsorbed on the metal

surface it can then affect the corrosion reaction in a number of

ways [28].

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/3~13

The inhibitive action of organic compounds is due to adsorption

of the inhibitor over the metal surface. The relationship between

inhibition coefficient and inhibitor concentration at any temperature

obeys the adsorption isothenn. Direct physical methods like radio-tracer

techniques [29, 30, 31], solution depletion measurements [32, 33] have

provided evidence of adsorption of an inhibitor over the metal surface

before or during the corrosion process.

It has been observed that adsorption on iron, nickel, etc., and their

alloys occur from aqueous solutions of inhibitors such as mercaptans

[33], iodide ions [34], carbon-monoxide [35] and organic amines [36],

thioureas [3 7] and sulphoxides [3 8, 39]. These studies have shown that

efficiency of inhibitors can be quantitatively related to the amount of

adsorbed inhibitor on the metal surface. In general, the area of the metal

surface covered by the adsorbed inhibitor is prevented from undergoing

corrosion whereas inhibitor free areas become more susceptible to

corrosion. The inhibitive efficiency is then directly proportional to the

fraction of the surface covered by the inhibitor molecules. This

hypothesis has been applied successfully in deducing the effect of

concentration of an inhibitor on its adsorption.

Iofa and Tomashova [ 40] showed that inhibitive properties of

some thioureas are due to their adsorption on the metal surface. At low

surface coverage the effectiveness of some inhibitors in retarding the

corrosion reaction may be greater than that at high coverage [3 7, 41, 42].

While some inhibitors such as thioureas [43, 44] and amines [45, 46]

stimulate corrosion at low surface coverage. The studies of adsorption

from solution indicate [ 4 7, 48, 49] that inhibitor adsorption on metals is

influenced by many factors such as (a) surface charge on the metal (b)

the functional group and structure of the inhibitor (c) the interaction of

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the inhibitor molecule with water molecules (d) the interaction of

adsorbed inhibitor species with each other and (e) the action of adsorbed

inhibitors on the surface metal atoms to fonn complexes of the type

(M-OH-Inad) and (M-Inad).

Adsorption of inhibitors during the corrosiOn process can be

quantitatively represented by the coverage of a metal surface by the

adsorbed inhibitor as

e = 1-5_ k

(1)

where 8 is the fraction of the metal surface covered by the

adsorbed inhibitor and k1 and k are the corrosion rates with and without

inhibitor respectively. The variation in coverage with concentration of

inhibitor frequently follows the Frumkin isothenn [50, 51],

_!!_eueJ = K.C. 1-8

(2)

where K is the equilibrium constant for the adsorption-desorption

equilibrium, C is the concentration of the inhibitor and f is a parameter

connected with variation of adsorption energy with 8 and is positive

when the free energy of adsorption decreases with surface coverage and

negative when it increases.

Three other important adsorption equations have also been used to

study the adsorption of inhibitors on the corroding metal surface. These

equations are known as (i) Langmuir adsorption isotherm, (ii) Freundlich

adsorption isotherm and (iii) Temkin adsorption isothenn.

=> Langmuir adsorption isotherm

Langmuir proposed a quantitative theory of the adsorption of gases

and assumed that a gas molecule condensing from the gaseous phase

-26-

would adhere to the surface for a short time before evaporating and the

condensed layer is one atom or molecule thick.

If 8 is the fraction of the surface covered by adsorbed molecules at

any time, the desorption is proportional to 8 and equal to ~8, where~ is

a constant at the given temperature. Similarly, the rate of adsorption will

be proportional to the area of the bare surface, (1-8), and to the rate at

which the molecule strikes the surface, which is proportional to the

pressure of the gas 'p'. ka is constant at the given temperature. At

equilibrium, the rate of adsorption equals the rate of desorption, i.e.,

~8 = ka (1-8) p

The Langmuir adsorption isotherm can be written as

e ka 1 - e = k p = ap, where

d

(3)

(4)

In the case of adsorption of a species from solution, the equation may be

represented as

a' c. 8= I

1 + a'c; (5)

where a' Is constant and c; IS the concentration of species i in the

solution.

The Langmuir adsorption isotherm is obeyed by a number of

systems at low and high coverage but not in the intermediate range of

coverage. Determination of the heat of adsorption on a clean metal

surface shows that it frequently decreases markedly with increasing

surface coverage. This indicates non-uniformity of the surface due to

intrinsic heterogeneity of the surface and to the repulsive forces between

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adsorbed atoms or molecules. Berzins and Delahay [52] during their

study of the kinetics of adsorption of organic compounds confirm to the

Langmuir aqsorption isotherm and rate constants of the adsorption and

desorption processes are exponential functions of the electrode

potentials. Laitinen and Mosier [53] studied the adsorption isotherms of

thirty organic compounds on mercury and found that they follow

Langmuir adsorption isotherm. Lorenz et al. [54, 55] detennined

adsorption isotherms of various amines, alcohols and organic acids on

mercury from differential capacity data. They showed that if the

attractive forces predominate between the adsorbed particles, the

adsorption isotherm is s-shaped and if the repulsive forces predominate

as in the case of (CH3) 4N+, (C2H5) 3NH+, and the adsorption isotherms lay

below the Langmuir isotherm. It was found that [56, 57] the dependence

of surface coverage on aniline concentration and on phenol concentration

in the case of copper powder at the open circuit potential was in

satisfactory agreement with the Langmuir isothenn. The coverage of a

heterogeneous surface by an organic substance following the Langmuir

isotherm may be explained as a result of two mutually compensating

factors. The free energy of adsorption decreases with the increasing

coverage and the free energy adsorption increases due to attractive forces

between the adsorbed molecules.

Langmuir isotherms were obtained in the studies of adsorption of

stearic acid and some aliphatic amines from organic solvents on iron and

steel, and in the studies of adsorption on iron and steel of

o-phenonthroline and phenyl-thiourea from aqueous hydrochloric acid

solutions [58, 59]. Studies by tracer-atom method on the adsorption of

decylamine [8] and phenyl thiourea [60] also confinn that Langmuir

isotherm may be applicable despite the heterogeneity of the surface.

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~ Freundlich isotherm

A fairly satisfactory empirical isotherm, which can be applied to

adsorptions of gases with considerable success but has been principally

for adsorption from solution, has been discussed by H. Freundlich. If y

is the weight of solute adsorbed per gram of adsorbent and c is the

concentration of the solute in the solution, this empirical relation is,

y = Xc 1111 _______ (6)

where K and n are empirical constants. The equation is conveniently

used in the logarithmic form,

log y = log X+ 1/n log c _______ (7)

when applied to gases, y is the amount of gas adsorbed and c is a

replaced by the pressure of the gas. Experimental results confonn to the

Freundlich expression if a plot of log y against log c, or log 'P, yield a

straight line. The constants can then be determined from the slope and

intercept [ 61].

~ Temkin isotherm

Temkin suggested that deviation from Langmuir adsorption isotherm

at high coverage could be accounted for by regarding the surface of the

metal as being composed of small patches of equal size, at each of which

Langmuir isotherm held independently with a characteristic local

standard free energy of adsorption that depended on the patch

distribution. The standard free energy of adsorption for each patch was

assumed to change by equal small decrements over successively covered

patches with increase in coverage and the variation is expressed by

-29-

(8)

where 11Gt and 11Gt is the standard free energy of adsorption

corresponding to 8 = 0 and finite adsorption respectively. Temkin

derived the isotherm,

---------------· (9)

where ao is the value of Langmuir constant for the first micro-adsorption

patch covered (8=0) and f is defined by the equation

f =_I_ d(11Gt) RT d()

---------------· (10)

As stated earlier, the phenomenon of adsorption is important in

inhibition action. The literature reveals various types of adsorption

isotherms being followed. A few references are cited here:

Abdel-Hamid [62] while studying some cationic corrosion

inhibitors mentioned that the Frumkin [63] adsorption equation could

explain the inhibition mechanism. Mohammad Asmal, Rawat and

Quraishi [64] showed that the adsorption of macrocyclic compounds on

mild steel/acidic solution interface obeys Temkin's adsorption isotherm.

Quraishi and his associates [65] suggested that in the case of inhibitors

such as Cordia latifolia and Curcumin for cooling systems obey Temkin

adsorption equation.

Mohamed [66, 67, 68] and his research workers have shown that

dimethyl-4-methyl benzyl dodecyl ammonium chloride is a good

inhibitor for pickling of steel in sulphuric acid. They showed that the

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degree of surface coverage (steel) varied linearly with the logarithm of

inhibitor concentration fitting a Temkin isotherm.

Quraishi and associates [69] while investigating fatty acid

oxadiazoles as acid corrosion inhibitors for mild steel showed that the

adsorption of these inhibitors on steel surface in 1.0 N HCl and 1.0 N

H2S04 was found to obey Temkin's adsorption isotherm. Quraishi and

his colleagues [70] while studying the influence of some condensation

products (various aldehydes with thiosemicarbazide) as corrosion

inhibitors for mild steel in acidic solution found that the adsorption of all

these condensation products on mild steel surface in 1.0 N H2S04 and

1.0 N HCl has been found to obey Temkin's adsorption isotherm.

Quraishi [71] and his colleagues studied the influence of

4-amino-5-mercapto-3-n-propyl-1-2-4 triazole on the corrosiOn and

permeation of hydrogen through mild steel in acidic solutions. The

adsorption of this inhibitor on mild steel surface obeys Temkin's

adsorption isotherm.

Ramdas [72] and his colleagues studied the synergistic effect of

thiourea derivatives and non-ionic surfactants on the inhibition of

corrosion of carbon steel in acid environments. They observed that

Langmuir adsorption isotherm has been used to interpret the mono-layer

adsorption of the inhibitor molecules.

Mansri and his colleagues [73] studied poly (4-vinyl pyridine) and

poly ( 4-vinyl pyridine poly-3-oxide ethylene) as corrosion inhibitors for

brass (Cu 60% - Zn 40%) in 0.5 M HN03• The dependence of the

fraction of the surface covered e with log c, where c is the ratio

Eoy{00 and e is the inhibitor concentration. The plot obtained is

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consistent with an S-shaped adsorbed isotherm for the inhibitors

showing adsorption on 60/40 brass surface electrode according to

Frumkin [74] isothenn.

Shah and co-workers [75, 76, 77, 78, 79, 80] have shown with a

number of Schiff bases as corrosion inhibitors for zinc in sulphuric acid

that the adsorption obeys Langmuir adsorption isothenn.

•:• Structure of Organic Inhibitors and their Inhibitive Action

When the adsorption process is considered in relation to corrosion

inhibition, it seems logical to presume that there must exist an optimum

type of adsorption bond. As a first approximation, the chemisorption

bond between a surface atom and an inhibitor molecule could be

considered as Lewis acid- Lewis base type of interaction, which would

require primarily electron availability and vacant orbital character

[81' 82, 83, 84, 85].

It was proposed m the above-mentioned references that the

inhibitor can act either as an electron donor and the metal as an electron

acceptor or as an electron acceptor with the metal acting as a donor,

however, for the most practical purposes inhibitor acts as a donor.

Hackermann and Hurd [86] pointed out that systematic and

regular changes in molecular structure could be correlated to inhibitor

action. They also showed that a relationship exists between the

instability constants of certain complexes and inhibition efficiency. Since

the type of bonding in these complexes was of the Lewis acid - Lewis

base type, it should therefore have many of the characteristics of the

chemisorption bond. The work of Hayakawa and Ida [87], with

substituted hydrazo compounds on aluminium and of Keelen and

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Anderson [88], with EDTA and citric acid on zirconium and iron support

the observed correlation of Hackermann and Cook [89].

A systematic investigation of the effects of molecular structure on

inhibitor efficiency has been carried out by many workers. It was

observed that adsorption of surface-active organic compounds increased

with their molecular weight and dipole moment. [83, 89, 90, 91, 92, 93,

94, 95, 96, 97, 98]. According to the work reported in these references,

the different classes of aliphatic compounds fonn the following series in

the order of their decreasing adsorbabilities : acids > amines > alcohols >

esters. Organic molecules containing active electron-donor groups (-CN,

-CNS, -CNO, >CO, -CHO, -NH2) are chemisorbed on the surface of

metals having incompletely filled electronic orbitals [99]. Substituents in

the hydrocarbon chain have been found to influence the adsorptive

capacity and inhibitive action of organic compounds. The influence

diminishes with increasing length of the hydrocarbon chain between the

substituents and the atom, which is the adsorption site of the molecule.

Derivatives of ethylene [ 1 00] and especially of acetylene

[ 101, 1 02] series have higher adsorptive capacity owing to the

interaction of n-electrons of these molecules with the surface atoms of

the adsorbent.

Attempts have been made to establish quantitative relationship

between the structure of an organic compound and its adsorptivity with

the inhibitive property. This was based on the direct relation between the

electron density of the adsorption site and the adsorbability of the

organic compound at the metal solution interface.

Many investigators have used Hammett relation for this purpose

[ 103, 104, 105, 1 06]. This relationship has been used widely in organic

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chemistry for correlation of chemical reaction rates [ 1 07] and may be

expressed as,

(11)

where KR and Ko are the rate constants for the reactions of the

substituted and unsubstituted compounds respectively and p and cr are

the reaction and substituent constants respectively. The constant cr,

characterizes the influence of the substituent group on the electron

density of the reaction centre of the molecule and is independent of the

nature of that centre and of the structure of the molecule. For

electrophilic substituents (-F, -Cl, -Br, -COCH3, -COOH etc.), cr is

positive and for nucleophilic substituents, it has negative values. The

constant p represents the influence of structure on the electron density at

the reaction centre.

Donahue and Nobe regard adsorption as a reversible chemical

reaction, __i

RY+M -r-- RY-M

where RY is an organic compound with adsorption centre Y and

substituent R. M is the metal and RY-M is the chemisorption compound

on the metal surface. Then, at C= 1 we can write for the adsorption

equilibrium [1 08],

e K =­

R I +8 (12)

where 8 is the surface coverage. In the case of an unsubstituted

organic compound, the surface coverage is 80 and the corresponding K0

can be represented as,

-34-

K =~ 0 1-8 0

(13)

Donahue and No be [ 1 08] correlated the Hammett equation with

the corrosion characteristics of metals. They obtained the following

expression,

log R = pcr (14)

where, R is the ratio of the corrosion rates of the metal in the

presence and absence of the inhibitor. This equation was valid for the

same concentration of organic compounds of a given series and was

found to hold for methyl substituted pyridines inhibiting the dissolution

of iron in aqueous solution [ 1 09]. However, the inhibitive action of

aniline and its derivatives on the corrosion of iron and nickel increased

with their increasing electro-negativities as well as . . mcreasmg

electro-positivi ties of the substituents [ 6]. This fact was explained by

presuming that the transition from electronegative to electropositive

substituents is accompanied by reorientation of the substituted aniline

molecules in the double layer. Grigorev and Ekilik [ 1 1 0] attributed this

type of dependence of the inhibitive action of organic substances on the

nature of the substituents, with a minimum corresponding to the

unsubstituted organic compound to participation of two forms of the

organic compounds, molecules (RNH2) and the cation (RNH3 +) in the

adsorption process. Amines are adsorbed in either of these fonns,

depending on the influence of the substituents on the electron density at

the nitrogen atom. On the one hand, chemisorption of RNH2 molecules

and the strength of their bonds with the surface metal atoms increased

with increasing electron-donor properties. On the other hand increase in

the electron density at the nitrogen atom lowered the effective density of

-35-

the positive charge on the nitrogen atom in RNH 3 +ion and diminished its

adsorbability .

.:• Influence of Inhibitor Concentration on Corrosion Rate

Sieverts and Lueg [Ill] observed that the rate of corrosion of

steel in acidic solution depends upon the concentration of the inhibitor.

They found that at constant temperature, the relationship between the

concentration of inhibitor and inhibition co-efficient followed the

adsorption isotherm and they suggested that inhibition of corrosion was

probably due to adsorption of the inhibitor on the metal surface. To be

fully effective, all inhibitors were required to be present above a certain

minimum concentration. However, the direct relationship between

inhibitor concentration and inhibition efficiency may not be applicable to

all cases. It was further found that at low inhibitor concentration, the

effectiveness of some inhibitors in retarding corrosion was greater than

at high inhibitor concentration whereas in other cases, low concentration

of the inhibitor stimulated the corrosion process and inhibition was

observed only at higher concentration of inhibitor [ 112].

With the inhibitors investigated in this thesis, it has been observed

generally that at low concentrations of the inhibitors, the corrosion of

zinc is accelerated in hydrochloric acid. The effect of inhibitor

concentration is very interesting. On going through the work carried out

in our laboratory it has been observed that sometimes at low

concentrations of the inhibitor, the corrosion was found to be protected

and as the inhibitor concentration increased, the efficiency decreased or

even the corrosion was accelerated.

It has been observed with 63/37 brass in 1.0 N NaOH (duration 5

days) that cupferron [113] inhibits the corrosion of 63/37 brass at lower

-36-

concentrations (upto 0.1 %) to an extent of 89% but the corrosion of

63/37 brass accelerated to an extent of 25 % as the concentration of the

inhibitor was increased to 0.5 %.

With Al-65 Sin hydrochloric acid solutions (1.0 N-120 minutes,

2.0 N-30 minutes and 3.0 N-10 minutes), the efficiency of

furfuraldehyde [ 114] decreased as its concentration was increased.

The efficiency of salicylaldehyde [ 115] for AI-57 S in 0.5 N

hydrochloric acid (duration 24 hours) at a concentration of 0.09 mllliter

was found to be around 82% and the efficiency decreased to 43% at the

concentration of 8. 7 ml/liter. Similarly, with thiourea, the efficiency was

found to be 82% for Al-57S in 0.5 N hydrochloric acid (duration 24

hours), which decreased to around 41% at 2% concentration of thiourea.

Also, with paraldehyde and salicylaldehyde, the efficiency was found to

decrease with increase in the concentration of these inhibitors. Thus, in

1.0 N hydrochloric acid, the efficiency of paraldehyde for Al-57S for

120 minutes duration decreased from 85 % to 8% as the inhibitor

concentration was increased. In the case of salicylaldehyde at

0.22 ml/liter, the corrosion of Al-57S in 1.0 N hydrochloric acid (120

minutes) was protected to an extent of 80% and interestingly the

corrosion was accelerated to an extent of 2% at 8. 7 mllliter concentration

of salicylaldehyde.

The corrosion of 60/40 brass in 0.2 N NaOH (duration 5 days) in

the presence of p-amino phenol [ 116] at the concentration of 0.001%,

was inhibited to an extent of about 82% and as the concentration of the

inhibitor was increased to 0.05%, the efficiency decreased to about 18%.

It is, however, interesting to note that the corrosion of 60/40 brass in 1.0

N NaOH (duration 5 days) in the presence of 0.01 % a-Naphthol, was

-37-

accelerated to an extent of about 82% and at 0.05% concentration of the

inhibitor, the corrosion was protected to an extent of about 60%.

It has been observed that the corrosion is accelerated at low

concentrations of the inhibitor. Thus, m-hydroxybenzaldehyde [117]

protected Al-3S in 0.5 N HCl (duration 24 hours) to an extent of 74%

and at 0.2% concentration of the inhibitor, the corrosion was accelerated

to an extent of 29%. Similarly, with p-hydroxybenzaldehyde, the

corrosion of Al-3S in 0.5 N HCl (duration 24 hours) was accelerated at

low concentrations of the inhibitor and again the efficiency decreased

with the higher concentrations of the inhibitor. With veratraldehyde also,

in the above system, the corrosion decreased as the concentration was

increased. With salicylaldehyde also, in the above corrosion system, the

corrosion increased with the inhibitor concentration.

It is interesting to note that the corrosion of 63/37 brass in 1.0 N

hydrochloric acid (duration 5 days) in the presence of 0.1% thiourea

[118] was inhibited to an extent of 66% but the corrosion was found to

be accelerated to an extent of about 460% at 2% concentration of the

inhibitor.

At 0.01 % concentration of ethylenediamine-N,N'-disalicylidene

[119], the corrosion of AI-SIS in 1.0 N hydrochloric acid (120 minutes)

was inhibited to an extent of about 87% but at 0.5% concentration, the

corrosion was found to be accelerated to an extent of about 155%.

•:• Effect of Inhibitors on Electrochemical Corrosion Process

In electrochemical studies, the inhibition may be defined as

decrease in the rate of electrode reaction by one or more different

components of the electrolytic solution accumulating at or near the

metal-solution interface. Inhibition may be defined more precisely by a

-38-

corresponding decrease of the rate of one or more partial steps of the

total electrode reaction. In case of electrochemical corrosion, anodic,

cathodic or both types of reactions may be inhibited simultaneously.

It is interesting to record the ISO [120] definition of an inhibitor

that is stated as "a chemical substance, which decreases corrosion rate

when present in corrosive system at a suitable concentration without

significantly changing the concentration of any other corrosive agent".

This definition thus excludes chemicals, which can reduce the corrosion

rate by affecting the composition of the environment, e.g. oxygen

absorbers, chemicals that alter the pH and chemicals that change

hardness or scale fonning properties.

Nature of metal is very important, as inhibitors are specific in

their action towards metals. While chromates are universal in their

action, however, the same is not true of nitrites, which protect ferrous

metals but not zinc or aluminum. Thus variation in alloy composition

can affect inhibitor perfonnance. In neutral solutions, for example, it is

found that sodium benzoate will protect mild steel but not cast iron,

whereas the related compound sodium cinnamate will protect both

materials [121]. Chemicals, which in other circumstances may protect

steel, can fail to do so when it is in contact with zinc. Similarly it cannot

be assumed that inhibitors that function satisfactorily with clean surfaces

will necessarily be effective with rusting surfaces. This has been

demonstrated with the example of borax as an inhibitor for corroding

zinc [ 122].

In the majority of cases, inhibition is achieved through interaction

or reaction between the corrosion inhibitor and the metal surface,

resulting in the formulation of an inhibitive surface film; this may occur

-39-

directly at the metal/environment interface or the environment may be

modified to render it less corrosive.

In the case of electrochemical corrosiOn under specified

conditions, a finite rate of charge-transfer at any electrode involves

activation overpotential (11), which provides the activation energy

required for the reactant to overcome the energy barrier that exists

between the energy states of the reactant and product. Most electrode

processes involve more than one step and the slowest one is rate

detennining.

The activation energy is therefore required to maintain the rate

detennining step (r.d.s). The activation energy 'E' is given by

E =- ZF11 ---------------. (15)

where E is in joules per mole and Z is the number of electrons

involved in one act of the r.d.s.

The activation overpotential, and hence the activation energy,

varies exponentially with the rate of charge-transfer per unit area of the

electrode surface by the well-known Tafel equation. The Tafel equation

for a cathodic process can be expressed as

RT RT . n =--lni ---lnl 'tc aZF o aZF c

---------------. (16)

and for the anodic process, the activation overpotential is given by

RT l . RT ln. n=--nl--- l 'ta aZF o aZF n

- ----- --- ------. (17)

where a. is the transfer coefficient and i0 is the equilibrium

exchange current density. Now putting

-40-

RTlni=a aZF o c

---------------· (18)

and -2.3RT=b

oZF c ---------------· (19)

in equation (2), the Tafel equation for cathodic process may be written as

Similarly lla = aa + ba log ia

(20)

(21)

Thus by plotting current density-potential relationship on a

semilogarithmic graph paper one can obtain from the resulting Evans'

diagram, the Tafel slopes be and ba respectively for cathodic and anodic

processes from which the nature and mechanism of the respective

electrode reactions may be deduced.

During corrosion of metals, the metallic electrode undergoes a

number of simultaneous processes, which are known as cathodic and

anodic. Anodic process is mainly connected with the metal dissolution

reaction, whereas cathodic process involves a number of reactions. These

are hydrogen evolution reaction, cathodic reduction of dissolved oxygen

or reduction of any other reducible species present in the medium. In

deaerated neutral aqueous media, the main reaction is hydrogen

evolution.

An inhibitor may decrease the rate of the anodic process, the

cathodic process or both of them. The change in the corrosion potential

on addition of the inhibitor is often a useful indication of the process

being retarded [ 43, 123]. Displacement of the corrosion potential in the

positive direction indicates mainly retardation of the anodic process,

whereas displacement in the negative direction indicates retardation of

-41-

the cathodic process, a little change in the corrosion potential suggests

both anodic and cathodic processes are being retarded.

It is however to be noted that the nature of the controlling reaction

also affects the duration in which the corrosion potential would change

in the presence of an inhibitor. In an anodically controlled reaction, the

corrosion potential is displaced to a more electronegative value if both

the electrode reactions are inhibited to the same extent. When the

corrosion process is under mixed control, the corrosion potential is not

affected by equal polarization of both electrode reactions. It is likely that

equal inhibition of both electrode reactions in a completely cathodically

controlled reaction causes the corrosion potential to become more

electropositive. The trend in change of corrosion potential in the

presence of inhibitors has been discussed in chapter IV. Two factors

appear to be important in detennining the dimension in which the

corrosion potential will move upon the addition of the inhibitor. These

are the nature of the controlling electrode reaction and polarity of the

electrode reaction, which is inhibited to the greater extent.

The anodic and cathodic polarization curves of the corroding

metal with and without inhibitor show how the adsorbed inhibitors

influence the individual electrode reactions involved in corrosion [ 43,

124, 125, 126].

A displacement of the polarization curve without change in the

Tafel slope in the presence of the inhibitor indicates that the adsorbed

inhibitor acts by blocking active sites so that reaction cannot occur,

rather than by affecting the mechanism of the reaction. A change in the

Tafel slope due to the presence of inhibitor indicates that the inhibitor

acts by affecting the mechanism of the reaction. However it is to be

-42-

noted that the determination of the Tafel slope often requires the metal to

be polarized under conditions of sufficient current density and of

potential, which are far removed from those of nonnal corrosion. This

may result in differences in the adsorption and mechanistic effects of

inhibitors at polarized metals compared to naturally corroding metals

[43, 127, 128]. Thus the interpretation of the effect of an inhibitor on the

corrosion potential as found from the current potential polarization

curves might not be conclusive.

Electrochemical studies have shown that inhibitors in aqueous

solution may affect the corrosion reaction of metals in a number of ways:

(a) Formation of a diffusion barrier : The adsorbed inhibitor

may form a surface film which acts as physical barrier for the

diffusion of ions or molecules to or from the metal surface

and hence retard the corrosion reaction [38, 123, 129, 130].

(b) Blocking of reaction sites : The interaction of adsorbed

inhibitor with surface atoms may prevent these metal atoms

from participating in either the anodic or the cathodic

reactions of corrosion. This blocking effect decreases the

number of active sites at which these reactions can occur and

hence the rates of these reactions exhibit mverse

proportionality to the extent of adsorption. However, the

mechanism of the reactions is not affected and the Tafel slopes

ofthe polarization curves remain unchanged [43, 126, 131].

(c) Participation in the electrode reaction : The electrode

reaction of corrosion may form adsorbed intermediate species

with surface metal atoms e.g. adsorbed hydrogen atoms in

hydrogen evolution reaction and adsorbed (FeOH) in the

anodic dissolution of iron. The presence of adsorbed inhibitors

-43-

will interfere with the fonnation of these adsorbed

intennediates, but the electrode reaction may then proceed by

alternative paths through the intermediates containing the

inhibitor [34, 43, 126, 128, 132, 133, 134, 135]. Inhibitors

may also retard the rate of hydrogen evolution on metals by

affecting the mechanism of the reaction, as indicated by the

increase in the cathodic Tafel slope values. This effect has

been observed for iron in the presence of inhibitors such as

phenylthiourea [126, 135], acetylenic hydrocarbons [136,

137], benzaldehyde derivatives [138] and aniline derivatives

[139]. Inhibitors like amines [45, 46, 90] and sulphoxides

[ 140] which can take up hydrogen ions from aqueous solutions

to form protonated species, may accelerate the rate of cathodic

evolution of hydrogen on metals, due to participation of the

protonated inhibitor species in the electrode reaction. This

effect increases significantly as the hydrogen overvoltage of

the metal increases and hence is found to a greater extent on

zinc than on iron [90].

(d) Alteration of the electrical double layer : The ions or

protonated species adsorbed on metal surface change the

electrical double-layer at the metal solution interface. The

adsorption of cations, e.g., quaternary ammonium ions and

protonated species makes the potential of the metal more

positive and hence retards the discharge of hydrogen ions.

Conversely, the adsorption of anions makes the potential more

negative on the metal side of the electrical double layer and

will accelerate the discharge of hydrogen ions. Such

observations have been made by Iofa [141] and Kelly [128]

for sulphosalicylate and benzoate ions respectively.

-44-

Thus, in general, inhibitors can affect the corrosion reaction m a

number of ways, some of which may occur simultaneously. It is often

not possible to suggest a single mechanism of inhibitor-action, because

the reaction path may change with experimental conditions such as,

inhibitor concentration, pH, nature of anion of the electrolytic solution,

and the nature of the metal.

•:• Thermodynamics and Corrosion Kinetics in the presence of

Inhibitors

According to the nature of the inhibitor, increase in temperature

may have different effects. Balezin [142] stated that the rate of corrosion

of a metal could be accelerated by increasing temperature particularly in

media in which evolution of hydrogen is the cathodic reaction. When the

cathodic process involves the reduction of dissolved oxygen, the

relationship between corrosion rate and temperature is complex due to

the decreased solubility of oxygen at higher temperatures. Speller [ 143]

found that in closed system the corrosion rate was linearly related to

temperature and in system open to atmosphere, the linear relation was

limited upto a certain temperature above which the corrosion rate fell

with the rise of temperature owing to decreased solubility of oxygen.

The effect of temperature on reaction between metals and acids

has been studied in connection with heterogeneous reaction kinetics. It

was shown by Calcott and Whetzel [144] that between 20° and 100°C,

the logarithm of the corrosion rate, y, was a function of temperature 't' as

shown below:

log y = a + bt -------- · (22)

where, a and b were empirical constants and 't' was in °C.

Putilova et al. [ 145] established a relationship between the logarithm of

-45-

the corrosion rate, y and the value of 1/T (T=K) and obtained the linear

relationship,

A logy=-+B

T --------· (23)

The above relationship resembled the Arrhenius equation.

Gorbachev et al. [ 146] proposed a similar relationship between corrosion

current density and temperature in the electrochemical reaction as,

A logK=-+B

T --------· (24)

where, A and B were characteristic constants for a given reaction.

Thus comparing the above equation (24) with equation (23), it will be

evident that y is proportional to the rate constant of the reaction.

Gorbachev et al. [ 146] also showed that the constant A had the value,

Eetr A - .. ----

2.3R (25)

Here, Eerr is the effective activation energy given by the slope of

the function y = t( ~) and R is the universal gas constant. For corrosion

process, the value of Eeff can be written as,

d log r -I ( 26) E=-2.303 x1.987 cal mol --------· d(Yr)

The values of effective activation energy for vanous metals

corroding in acids in the presence and absence of inhibitors have been

calculated by many authors [ 14 7].

The corrosion rates of metals m acid solutions containing

inhibitors were shown by them to rise rapidly with increasing

temperature. Sieverts and Lueg [148] were the first to explain the

protective action of inhibitors in terms of adsorption on metal surfaces

-46-

and were of the opinion that when the temperature was raised, desorption

of the inhibitor occurred which led to the loss of protective action. On

the other hand Machu [147], who studied the influence of temperature on

the action of dibenzyl sulphide, dibenzyl sulphoxide, aniline, gelatin and

other corrosion inhibitors for steel at 15°C, 4 7°C and 75°C, concluded

that in the presence Qf powerful inhibitors, the temperature coefficient as

well as the corrosion rate is lowered. Putilova [ 14 7] suggested that the

relationship logy = 1( ~) is not always linear in the presence of

inhibitors. Three different types of behaviors have been observed. In the

first case, the activity of the inhibitor that retards corrosion at lower

temperature decreases at higher temperatures. In this case activation

energy, E, of the reaction rate is higher in the presence of the inhibitor

than in its absence. The behavior of such inhibitors which include

thiourea in sulphuric acid can be compared with the behavior of unstable

catalytic poisons. The second group of inhibitors does not change the

effective activation energy of the process when the temperature is raised.

In this case, corrosion rate remains unchanged at low as well as at

elevated temperatures in the presence of inhibitors. The behavior of these

inhibitors resembles the behavior of stable poisons in heterogeneous

catalysis which do not affect the temperature coefficient of the reaction.

According to Putilova [ 14 7], the following inhibitors belong to this

group: thiodiglycol and many alkaloids in sulphuric acid, formaldehyde,

hexamine, diethylaniline and a number of other amines in hydrochloric

acid solutions. Finally there are compounds, belonging to the third group

of inhibitors for which the effective activation energy of the process is

lower in the presence of the inhibitor than in its absence. Such inhibitors

are of great interest from the practical point of view, when the reduction

of corrosion at elevated temperature is desired. Such substances are

-47-

finnly held on the metallic surface. It may be presumed that they are

bound to the surface by specific adsorption forces or by chemisorption,

as a result of which a surface film of the reaction product is fonned. It

may be presumed that in such cases specific adsorption of the products

of reaction between the inhibitors and the metal salt formed in the acid

may occur, as well as adsorption of the inhibitor itself, e.g., dibenzyl

sulphide [147], dibenzyl sulphoxide [147] and iodides [149].

•:• Effect of Temperature on Cathodic and Anodic Reactions

Cathodic reactions are (i) hydrogen evolution processes

(ii) reduction of dissolved oxygen and (iii) reduction of any other

oxidizing species present in the solution.

In deaerated solutions, when hydrogen evolution process is under

activation control, the main effect of increasing the temperature is the

increase in the exchange current density. Conway et al. [150] have

observed that for nickel, the exchange current density increases from

approximately 1 o-2 A/m2 to 1.0 A/m2 as the temperature is changed from

10° to 75° C and the activation energy is about 59kJ/mol. Thus the

corrosion rate increases by atleast 100 times if the anode process is

unaffected by the temperature increase, whilst for control by

concentration polarization, the diffusion coefficient for hydrogen ions

increases only twice for the same temperature range. Corrosion process

involving the reduction of dissolved oxygen is expected to be perfectly

based on concentration polarization due to low solubility of oxygen. The

increase of temperature has a complex effect as though the diffusivity of

oxygen molecule increases, its solubility decreases. The net mass

transport of oxygen should increase with temperature [ 151] until a

maximum is reached (at about 80°C) when the concentration falls as the

boiling point is approached. Zembura [ 14 7] has found that for copper in

-48-

aerated 0.1 N H2S04, the controlling process is the oxygen reduction

reaction and that upto 50°C, the slow step is the activation process for

that reaction. At 75°C, the process is controlled by diffusion and

increasing solution velocity has a large effect on the corrosion rate. Thus

not only temperature but also other factors like concentration and

solution velocity have also additional effects on corrosion.

The effect of temperature on anodic polarization should be

discussed with reference to film-free condition, film forming condition

and active-passive transition. For many metals in film-free condition, it

has been observed that the magnitude of the critical current density Cicrit)

increases with temperature and the activation energy is low. This

suggests diffusion limited anodic process when migration of corrosion

products away from the metal surface is rate controlling. The

relationship between icrit and temperature can be expressed by an

Arrhenius-type equation.

log i . =: A *- E (2 7) em 2.303RT --------·

where, A* is constant and E is the activation energy.

A more useful form is,

I . I .r E A' ogzc,.;, =: ogzc,.;, - 2.303RT + --------· (28)

where, logi~it is the value at room temperature (25°C) and A' is a

constant characteristic of the metal.

In film-forming condition, in a few instances, the passive current

(ip) has been found to increase with temperature and high activation

energies (46-84 kJ/mole) have been reported [152]. This indicates a large

increase in the rate as the temperature increases. Thus the rate of change

of ip under activation control is much greater than that of icrit which is

-49-

under diffusion control, and for the same condition of solution velocity

the two rates become equal at some common temperature i.e., icrit = ip and

there is no active-passive transition. Above this temperature, the

activation energy is lower and is found to be diffusion controlled and

anodic protection by passive films cannot be effective. A majority of the

anodic protective films are oxides and hydroxides whose dissolution

depends upon the H+ ion concentration. At some temperature, ip exceeds

icrit when no active-passive transition can be observed and no protection

of the metal by passive films is possible because of the high rate of

dissolution.

-50-

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