Leonardo Electronic Journal of Practices and Technologies
Issue 11, July-December 2007
ISSN 1583-1078
p. 143-154
Synergistic Inhibition of Potassium Chromate and Sodium Nitrite on Mild
Steel in Chloride and Sulphide Media
Ayo Samuel AFOLABI
School of Chemical and Metallurgical Engineering, University of the Witwatersrand,
Johannesburg, South Africa & Metallurgical and Materials Engineering Department
Federal University of Technology, Akure, Nigeria
[email protected]
Abstract
The corrosion inhibition of mild steel in 3.5M sodium chloride and 0.3M
sodium sulphide media using varied concentrations of potassium chromate
and sodium nitrite was investigated in the study using the conventional weight
loss measurement method. The results obtained show that potassium chromate
produced a better inhibition performance than sodium nitrite in both media.
This is attributed to cathodic and anodic inhibitive effects produced by the
chromate ions as compared to only anodic inhibition in nitrite ions. Higher
concentrations of these reagents also produced more inhibition in both
environments under study. The combination of these inhibitors also produced
an enhanced inhibition on the mild steel in the two media. This is attributed to
synergistic action of these inhibitors which considerably reduced the cathodic
and anodic activities thus promoting a continuous and protective film on the
material within these media.
Keywords
Synergistic; Inhibition; Chloride and Sulphide Media
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Synergistic Inhibition of Potassium Chromate and Sodium Nitrite on Mild Steel in Chloride and Sulphide Media
Ayo S. AFOLABI
Introduction
Corrosion of metals is a major industrial problem that has attracted a lot of
investigations in recent years [1 - 4]. Most metals are inherently unstable and have the natural
tendency to react with their environments to obtain lower energy by forming a chemical
compound in a more stable state. Steel materials which are very susceptible to attack in
aggressive media are the commonly exposed metals in industrial environments [5]. Mild steel
is particularly used in most structural shapes such as beams, plates, bars, and pipes used in
seawater which contains chlorides and for conveying petroleum products that sometimes
contain sulphides that are detrimental to the steel.
The use of inhibitors is one method of corrosion prevention among others such as
cathodic protection [6, 7], anodic protection [8], and coating [9]. National Association of
Corrosion Engineers (NACE) editor described corrosion inhibitors as substances that when
added to an environment decrease or slow down the rate of attack on the metallic material
[10]. They can be divided into three: anodic, cathodic and mixed, depending on whether they
interfere with the corrosion reaction preferentially at the anodic or cathodic sites or both are
involved [11]. Inhibitors are being extensively employed to minimize corrosion of many
metallic structures in various media. Among common inhibitors are chromates, nitrates,
benzoates, phosphates and borates [12, 13]. Nitrite is being used as inhibition admixtures in
concrete reinforcement [14 - 16].
This work is set to study, compare the inhibition performance of potassium chromate
and sodium nitrite and investigate the synergistic effect of these inhibitors on mild steel
immersed in chloride and sulphide media. Synergistic effect of inhibitors is a combined action
of inhibitors greater in total effect than the sum of the individual effects [17, 18]. According
to Kalman et al, [19], cited by [18], synergism of corrosion inhibitors is either due to the
interaction between components of inhibitor composition or due to interaction between the
inhibitor and one of the ions present in the aqueous solution. Synergistic inhibition has been
proved by several authors as an effective means to improve the inhibitive force of inhibitor, to
decrease the amount of usage, and to diversify the application of inhibitor [20 - 23].
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Issue 11, July-December 2007
ISSN 1583-1078
p. 143-154
Material and Method
The chemical composition of the 15mm diameter mild steel used in this work is
presented in Table 1. Fourteen corrosion test samples of the steel were prepared as shown in
Figure 1. They were totally immersed (as shown in Figure 2) in various concentrations of
sodium chloride and sodium sulphide containing different concentrations of potassium
chromate and sodium nitrite inhibitors as shown in Table 2. Weight loss measurements were
conducted for all the samples in the media using a sensitive electronic weighing machine for
forty days following the procedures and precautions described by Bastidas et al, [24], Kolman
et al, [25], and Ashassi-Sorkhabi et al, [26].
Table1: Chemical composition of the mild steel sample
Element
%
Composition
C
0.140
Si
0.220
Mn
0.500
P
0.040
S
0.040
Cu
0.045
Fe
balance
Figure 1. Corrosion test sample
Figure 2. Experimental set-up showing immersion of the samples in corrosive media
145
Synergistic Inhibition of Potassium Chromate and Sodium Nitrite on Mild Steel in Chloride and Sulphide Media
Ayo S. AFOLABI
Table 2. Concentrations of corrosion media and/or inhibitors
Symbol
A
B
C
D
E
F
G
H
I
J
K
L
M
N
Concentration of Medium and/or Inhibitor
3.5M Sodium Chloride
0.3M Sodium Sulphide
3.5M Sodium Chloride and 0.5M Potassium Chromate.
3.5M Sodium Chloride and 1.0M Potassium Chromate
0.3M Sodium Sulphide and 0.5M Potassium Chromate
0.3M Sodium Sulphide and 1.0M Potassium Chromate
3.5M Sodium Chloride and 0.5M Sodium Nitrite
3.5M Sodium Chloride and 1.0M Sodium Nitrite
0.3M Sodium Sulphide and 0.5M Sodium Nitrite
0.3M Sodium Sulphide and 1.0M Sodium Nitrite
3.5M Sodium Chloride, 0.5M Potassium Chromate and 0.5M Sodium Nitrite.
3.5M Sodium Chloride, 1.0M Potassium Chromate and 1.0M Sodium Nitrite.
0.3M Sodium Sulphide, 0.5M Potassium Chromate and 0.5M Sodium Nitrite
0.3M Sodium Sulphide, 1.0M Potassium Chromate and 1.0M Sodium Nitrite
Results and Discussion
Effect of NaNO2 and K2CrO4 inhibitors on corrosion
The effects of sodium nitrite and potassium chromate inhibitors on corrosion
behaviour of mild steel in chloride and sulphide media are presented in Figures 3 and 4. It can
be seen from the Figures that the weight losses of the samples decrease with the additions of
sodium nitrite and potassium chromate inhibitors. Although corrosion occurred in all the
samples, it was to a lower extent in the inhibited ones. This further confirms the findings of
Fadayomi [27] that inhibitors do not totally stop corrosion but delays the onset of corrosion
thereby decreases its rate in a corrosive environment. It is noticeable that better inhibition was
obtained with potassium chromate than sodium nitrite inhibitor in the two media. It is also
conspicuous that higher concentrations of the inhibitors produced a better inhibition effect on
the steel in both media. This indicates that the higher the concentration of the inhibitor, the
higher the inhibition activity, which correlates with the findings of Trethewey and
Chamberlain [28] that at high concentration, anions may either become inhibitive or act in
such a way as to plug any pores in a passive film.
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0.1
3.5M NaCl
0.09
3.5M NaCl +0.5M NaNO2
3.5M NaCl + 1.0M NaNO2
0.08
3.5M NaCl + 0.5M K2CrO4
Weight loss (grammes)
0.07
3.5M NaCl + 1.0M K2CrO4
0.06
0.05
0.04
0.03
0.02
0.01
0
0
5
10
15
20
25
30
Exposure time (days)
35
40
45
Figure 3. Effect of NaNO2 and K2CrO4 inhibitors on corrosion behavior of mild steel in 3.5M
NaCl medium
0.09
0.3M Na2SO3
0.08
0.3M Na2SO3 + 0.5M NaNO2
0.3M Na2SO3 + 1.0M NaNO2
Weight loss (grammes)
0.07
0.3M Na2SO3 + 0.5M K2CrO4
0.3M Na2SO3 + 1.0M K2CrO4
0.06
0.05
0.04
0.03
0.02
0.01
0
0
5
10
15
20
25
Exposure time (days)
30
35
40
45
Figure 4. Effect of NaNO2 and K2CrO3 inhibitors on corrosion behavior of mild steel in 0.3M
Na2SO3 medium
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Synergistic Inhibition of Potassium Chromate and Sodium Nitrite on Mild Steel in Chloride and Sulphide Media
Ayo S. AFOLABI
A good comparison of the two figures shows that better inhibition of the material
occurred in sulphide medium than in chloride medium. This could be attributed to the
aggressiveness of chloride ion which continuously breaks down the passivity produced by the
inhibitors on the metal [28], as compared to the weak acidic nature of the sulphide medium.
The mild steel corrodes in air - saturated chloride and sulphide media as shown in
Figures 3 and 4. Chromate is known to be an effective oxidizing anodic inhibitor which
maintains the steel in the passive state thus preventing breakdown of the passive oxide which
can lead to localized and/or uniform corrosion in chloride and sulphide media respectively.
Chromate might have also acted as cathodic inhibitor which shows that acceleration of the
cathodic reaction could be due to some catalytic action leading to the possible evolution of
hydrogen, and not to the reduction of the chromate ion to the trivalent state CrO3-3. This
catalytic effect on hydrogen evolution might be explained on the basis that chromate ion is
absorbed on the surface of the steel and reduced to some intermediate valency e. g. tetravalent
state, CrO4-4 which acts as mediator between the metal and water molecules as follows:
CrO2-4 + 2e = CrO4-4
(i)
CrO4-4 + 2H2O = CrO2-4 + H2 + 2OH-
(ii)
[29, 30]
This double inhibitive effect of chromate on the steel is clearly evident in higher
inhibition obtained in chromate than nitrite inhibitor. This double protection action of
chromate ensures the gradual cover of the surface of the metal with a protective film of
chromium oxide. In the same vein, it can be observed that increase in concentration of
chromate produces better inhibition. This could be attributed to the occurrence of the
inhibition action at a certain concentration of chromate, and the attainment of a certain surface
coverage of the steel with chromium oxide which increases with concentration of the
inhibitor. It can be suggested that further reduction of the absorbed Cr4-4 to the trivalent state
does not take place electrochemically, but proceeds through disproportionate of the tetravalent
state. Also, at very low concentration the distances among the absorbed ions are too large to
permit any chemical reaction resulting to surface passivation hence metal surface remain
active.
Nitrite on the other hand, is an anodic inhibitor which functions at the anode and
oxidizes ferrous ions to ferric ions thereby offers only single action inhibitive effect on the
steel as reveals in the Figures [31]. The inhibitive effect also increases with increase in nitrite
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Leonardo Electronic Journal of Practices and Technologies
Issue 11, July-December 2007
ISSN 1583-1078
p. 143-154
concentration which indicates that a higher concentration of this reagent is required to
promote anodic passivity. This is in agreement with the findings of Rosenberg and Gaidis
[32] in which the presence of hydroxide ion in the concrete created more passivating medium
on the reinforced steel [16, 31].
Synergistic effect of sodium nitrite and potassium chromate
The effect of inhibitive action of sodium nitrite and potassium chromate mixture on
corrosion of mild steel in chloride medium is shown in Figures 5 and 6. This indicates that
better protection of the steel was obtained with the mixture of these reagents than with either
of the inhibitors was used. Comparing the two figures reveals that more protection is also
visible at higher concentration of the mixture of these inhibitors. The same features are
conspicuous in Figures 7 and 8 in which the mixture of the two reagents gives a better
inhibitive effect of mild steel in sulphide medium. However, better protection is evident in
sulphide medium than chloride medium in all the samples either inhibited with single or
mixture of the reagents.
0.1
3.5M NaCl
0.09
3.5M NaCl + 0.5M NaNO2
Weight loss (grammes)
0.08
3.5M NaCl + 0.5M K2CrO4
0.07
3.5M NaCl + 0.5M NaNO2 + 0.5M
K2CrO4
0.06
0.05
0.04
0.03
0.02
0.01
0
0
5
10
15
20
25
30
35
40
45
Exposure time (days)
Figure 5. Effect of 0.5M NaNO2 and 0.5M K2CrO4 mixture on corrosion behavior
of mild steel in 3.5M NaCl medium
149
Synergistic Inhibition of Potassium Chromate and Sodium Nitrite on Mild Steel in Chloride and Sulphide Media
Ayo S. AFOLABI
0.1
3.5M NaCl
0.09
Weight loss (grammes)
3.5M NaCl + 1.0M NaNO2
0.08
3.5M NaCl + 1.0M K2CrO4
0.07
3.5M NaCl + 1.0M NaNO2 + 1.0M K2CrO4
0.06
0.05
0.04
0.03
0.02
0.01
0
0
5
10
15
20
25
Exposure time (days)
30
35
40
45
Figure 6. Effect of 1.0M NaNO2 and 1.0M K2CrO4 mixture on corrosion behavior
of mild steel in 3.5M NaCl medium
0.09
0.3M Na2SO3
0.08
0.3M Na2SO3 + 0.5M NaNO2
0.3M Na2SO3 + 0.5M K2CrO4
0.07
Weight loss (days)
0.3M Na2SO3 + 0.5M NaNO2 + 0.5M K2CrO4
0.06
0.05
0.04
0.03
0.02
0.01
0
0
5
10
15
20
25
Exposure time (days)
30
35
40
45
Figure 7. Effect of 0.5M NaNO2 and 0.5M K2CrO4 mixture on corrosion behavior of mild
steel in 0.3M Na2SO3
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0.09
0.3M Na2SO3
0.08
0.3M Na2SO3 + 1.0M NaNO2
Weight loss (grammes)
0.07
0.3M Na2SO3 + 1.0M K2CrO4
0.3M Na2SO3 + 1.0M NaNO2 + 1.0M K2CrO4
0.06
0.05
0.04
0.03
0.02
0.01
0
0
5
10
15
20
25
30
35
40
45
Exposure time(days)
Figure 8. Effect of 1.0M NaNO2 and 1.0M K2CrO4 mixture on corrosion behavior of mild
steel in 0.3M Na2SO3
The possible explanation for the better protection observed with mixture of the nitrite
and chromate inhibitors can be attributed to synergistic action of these inhibitors on the steel.
Synergistic effect is believed to have occurred as a result of cathodic inhibition provided by
the chromate ion and anodic inhibition effected by the nitrite ion as previously discussed. The
combined cathodic and anodic protection offered by these inhibitors provides a uniform
protective film on the steel until there is film stability towards which is evident towards the
latter days of exposure period. This film stability was obtained mainly due to synergism of the
inhibitors as a result of continuous anodic and cathodic film repair effected by the inhibitive
power of these inhibitors. This behaviors is similar to the findings of Oni [17] where a
mixture of potassium chromate and sodium nitrate was used to obtain better inhibition of
stress corrosion cracking (SCC) of low carbon steel in sulphuric acid. Similar results were
also reported in recent research works of [20, 22, 23, 33, 34].
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Synergistic Inhibition of Potassium Chromate and Sodium Nitrite on Mild Steel in Chloride and Sulphide Media
Ayo S. AFOLABI
Conclusions
The following conclusions can be drawn from the investigation of the effect of
inhibition performance of potassium chromate and sodium nitrite on mild steel in chloride and
sulphide media.
1. Corrosion of mild steel in 3.5M sodium chloride is more than in 0.3M sodium sulphide.
This is as a result of aggressive chloride ions in the former and weak acidic nature of the
latter.
2. Potassium chromate produced a better inhibition performance of mild steel in 0.3M
sodium sulphide medium than in 3.5M sodium chloride medium.
3. The higher the concentration of sodium nitrite and potassium chromate inhibitors, the
more the inhibition performance on mild steel in the media studied.
4. The combination of sodium nitrite and potassium chromate inhibitors produced an
enhanced inhibition performance on the mild steel in both 3.5M sodium chloride and
0.3M sodium sulphide media. This is due to the synergistic effect of the inhibitors on the
steel within these media.
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