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atural corrosion inhibitors for steel reinforcement in concrete – a short
review
Pandian Bothi Raja, Seyedmojtaba Ghoreishiamiri, Mohammad Ismail*
Construction Research Centre, Faculty of Civil Engineering, University Teknologi Malaysia 81310, Johor Bahru, Malaysia
Abstract
Reinforced concrete is one of the widely used construction materials for bridges,
buildings, platforms and tunnels. Though, reinforced concrete is capable of withstanding a large
range of severe environments including marine, industrial and alpine conditions; there are still a
large number of failures of concrete structures for many reasons. Either carbonation or chloride
attack is the main culprit which owing depassivation of reinforced steel and subsequently leading
to rapid steel corrosion. Among many corrosion prevention measures application of corrosion,
inhibitors playing a vital role in metal protection. A numerous range of corrosion inhibitors were
reported for concrete protection that were also used commercially in industries. This review
summarizes the application of natural products as corrosion inhibitors for concrete protection and
also scrutinizes various factors influencing its applicability.
*
Corresponding author, Tel: +60 75531757, Fax: +60 75215615, Email: [email protected]
Keywords: Corrosion, Corrosion inhibitor, Natural products, Reinforcement steel, Concrete,
Review.
1. Introduction
Concrete is a composite material made of cement, water and aggregates which has been
used as the largest quantity for construction material in many decades. Cement is a major
component of concrete, when mixed with water forms a paste that sets and hardens due to
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hydration reactions. Usually, concrete is comparatively weak in tension, arrangements have to be
made for the tensile stresses in the structure to be transferred to another material that is strong in
tension. Hence, concrete structures are often strengthened by embedding steel ribs which is
known as reinforcement in concrete. Prime setback of reinforcement in concrete is corrosion.
Corrosion of reinforcement has huge economic implications as well as social issues in
which endangering safety of people who are working in industries. Federal highway
administration of USA has estimated the annual cost of corrosion damage of highway bridges is
$ 90 - 150 billion per year [1]. Corrosion accidents over concrete structure may cause loss of
human lives which has to be given priority than money.
Corrosion can be defined as an electrochemical / chemical interaction of a metal with its
surrounding environments subsequently results in deterioration of physical and chemical
properties of metal. In general, concrete provides reinforcing steel with excellent corrosion
protection. The high alkaline environment in concrete results in the formation of a tightly
adhering film, which passivates the steel and protects it from corrosion [2, 3]. Further, concrete
can be proportioned to have a low permeability, which minimizes the penetration of corrosion inducing substances and also increases the electrical resistivity of concrete, which impedes the
flow of electrochemical corrosion currents. Because of these inherent protective attributes,
corrosion of steel does not occur in the majority of concrete elements or structures.
Though, presence of porosity in concrete allows the oxygen to diffuse through it which
becomes dissolved in pore solution and at the end reaching the surface of the steel [4]. Further,
there are two more chemicals namely, chlorides and carbon dioxide can cause corrosion to the
steel bar. These hazardous species can penetrate through concrete cover without causing
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significant damage and then promote the corrosion of steel by removing the protective passive
oxide layer on the steel.
The passivity over the steel can be destroyed either by carbonation, in which case, the
concrete’s alkalinity is reduced due to neutralization with atmospheric carbon dioxide or by
depassivating anions such as chlorides which are able to reach the steel. Thus, reinforcing steel
in concrete corrodes readily in such environments which can be easily observed in marine
environment, bridge decks and in chemical manufacturing plants. Further, reinforcing steel can
easily be damaged by the acids, sulfates, ammonia, and other species produced by microorganisms as well. Following are the characteristic chemical reactions occurred during
carbonation process.
H2CO3
CO2 + H2O
H2CO3 + Ca(OH)2
CaCO3 + 2 H2O
Carbonation begins with chemical reaction between carbon dioxide (CO2) gas from the
atmosphere and the alkaline hydroxides from the concrete. Carbon dioxide readily dissolves in
water to form the carbonic acid which does not attack the cement paste, while neutralizes the
alkalis in the pore water and produces calcium carbonate that lines the pores [5]. Presence of
Calcium hydroxide in the concrete increases the alkalinity and maintains the pH level of 12 -13.
Further, the carbonates attack inside the concrete results in formation of Calcium carbonate
which reduced the pH (< 8) level and causes the corrosion of reinforcement.
Electrochemical reactions of chloride attack over reinforced steel in concrete are,
Fe 2+ + Cl -
[FeCl complex] +
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[FeCl complex] + + 2 OH -
Fe (OH)2 + Cl -
Chloride attack involves no drop in overall pH while it act as catalysts to corrosion when
there is sufficient concentration at the rebar surface to break down the passive layer. Chloride
ions not consumed in the process while they help to destroy the passive layer over steel surface,
allow the corrosion process to proceed quickly. When chloride ions appeared in solution around
iron, it reacts with Fe2+ of passive film over steel surface and forms an iron – chloride complex.
Subsequent hydrolyzes of iron – chloride complex results in ferrous hydroxide and also liberate
the chloride ions for further attack over iron surface.
Steel bar corrosion in concrete can be reduced by following well known methods [6];
selection of corrosion-resistant steel, use of coatings, addition of concrete sealers, use of
membranes, use of thicker concrete cover, addition of corrosion inhibitors and cathodic
protection. Corrosion inhibitors for reinforced concrete can be defined as the chemical
substances that when added in adequate amounts to concrete, can reduce the corrosion of
reinforcement, while adversely affect the nature and microstructure of the hydration products [7].
In general, traditional concrete corrosion inhibitors can be classified as [8]; inorganic corrosion
inhibitors (mainly nitrites) and organic corrosion inhibitors (alkanolamine and their inorganic,
organic acid salt mixtures).
The corrosion inhibitors can be classified based on their mechanisms of protection as;
anodic, cathodic, mixed and adsorption inhibitor [8]. Anodic inhibitors act on the dissolution of
the steel and reduce the corrosion rate by increase in the corrosion potential of the steel, cathodic
on the oxygen reaction on the steel surface and reduce the corrosion rate by a decrease in
corrosion potential, mixed act on the both anodic and cathodic sites and they reduce the
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corrosion rate without a significant change in the corrosion potential and adsorption inhibitor
(amines, alkanolamines) that are able to adsorbed over the metal surface and reduce the
corrosion rate.
Corrosion inhibitors can be introduced into reinforced concrete either as preventive
measures to new structure or as surface applied inhibitors for preventive and restorative purposes
[9]. Thus based on mode of applications inhibitors can be classified as; migrating inhibitors (can
penetrate into the hardened concrete) and admixed inhibitors (added to fresh concrete for new
structures).
Migrating corrosion inhibitors were in use for the last 20 – 25 years while admixed
inhibitors were in commercial use from 1970’s [10, 11]. Calcium nitrate was found as
commercialized concrete corrosion inhibitor during 1960 – 1970’s which was used for years in
Soviet Union, United States and Japan. Recently, several ranges of corrosion inhibitors including
nitrites, sodium mono fluoro phosphate, quarternary ammonium salts, alkanoamines, amines,
amino acids, unsaturated fatty acid ester of an aliphatic carboxylic acid and saturated fatty acid
are commercialized for concrete protection. Literature concerns with all the concrete corrosion
inhibitors were well reviewed by many authors [11 – 18]. Among many concrete corrosion
inhibitors most widely used in the construction field are calcium nitrite (CN), amine alkanolamine (AMA) based inhibitors and monofluorophosphate (MFP) [19, 20] which was
well reviewed by Söylev et al., [9].
Though all these corrosion inhibitors provide sufficient concrete protection, they have
negative impacts as well in the form of toxicity and hazard to the human beings and
environment. Toxic effects of these corrosion inhibitors may incurred either during the synthesis
of the compound or during its applications; which may cause reversible (temporary) or
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irreversible (permanent) damage to organ system namely, kidneys or liver, or to disturb a
biochemical process / an enzyme system at some site in the body [21]. All these factors enforce
many countries to frame environmental regulations for using synthetic corrosion inhibitors.
Nitrites were found to have excellent corrosion inhibition potential while its
carcinogenicity and biological toxicity forced banned in Germany and Switzerland [22].
Corrosion inhibitors containing vanadium, antimony, copper, and thiocyanate compounds were
classified as toxic pollutants by U. S. Environmental protection agency (EPA). Canada has
restrictions for the usage of toxic substances including several inorganic heavy metals is
restricted under the Canadian Environmental Protection Act (CEPA). Likewise, many
environmental regulations namely comprehensive environmental response compensation and
liability act (CERCLA), superfund reauthorization act (SARA), clean water act (CWA) were
implied in U. S. to restrict the usage of toxic chemicals as corrosion inhibitors [23]. Nevertheless
of all these concern, corrosion inhibitors still play a vital role in metal protection.
Researchers then focus more towards developing “green solution” for corrosion
problems. Their prime target is to find cheap, hazardless, eco – friendly and environmental
biocompatible corrosion inhibitors. Plant sources fulfill all these requirements since their
products namely; alkaloids, flavonoids, terpenes and polyphenolics are very rich in electron
donating hetero group (S, N, O and conjugated π – electrons). Thus, plant sources can be utilized
to synthesis “green” corrosion inhibitors while their recent development has been reviewed by
many authors [18, 24 – 26]. This short review summarizes the “green” corrosion inhibitors
developed for reinforced steel in concrete and also discusses various factors influencing its
applications.
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2. “Green” corrosion inhibitors for reinforcing steel in concrete
In general, corrosion inhibitors for concrete is added only once in the system and the
following are criteria for an effective inhibitor for reinforced concrete; efficient even at lower
concentrations, long term stability inside the concrete, homogeneous distribution and not readily
leachable from concrete and should not affect the properties of concrete.
Substitution of natural products as “green” inhibitors in place of synthetic compounds
may route to the development of less toxic, environmental benign, cheap and cost effective
corrosion inhibitors. Literature of natural corrosion inhibitors for reinforced steel concrete are
listed here.
Tannin - sugar fractions of vegetable extracts was also tested positively as “green”
inhibitors for reinforced steel in concrete [27]. Tantawi and Selim [28] have studied magrabe
banana’s stem juice in concrete admixtures to improve physiochemical and mechanical
properties of reinforced steel concrete in NaCl medium. This paper includes discussion on
admixture preparation of banana juice, components of reinforcing steel concrete and
measurement parameters. Further, admixture effect on physiochemical & mechanical properties
of concrete as well as corrosion behaviour of reinforcing steel were included in results and
discussion. White juice of banana stem found to show effective corrosion inhibition potential
when admixed at concentration 0.2 % (mL / 100 gm cement).
Acosta [29] had proven the corrosion inhibition potential of Opuntia ficus indica (Nopal)
for steel corrosion in alkaline media. He used half - cell potential and linear polarization
resistance method for corrosion analysis while microscopy analysis over steel bar was carried out
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at the end of measurement. The review concluded that addition of Nopal increases the
polarization resistance values 4 times than same steel in alkaline medium. Further, microscopy
analysis revealed that chemical reaction of Nopal with iron results in formation of denser oxide /
hydroxide film over the reinforcing steel surface which apparently owes the metal protection.
Arghel extract was tested as corrosion inhibitor for reinforced steel concrete in 0. 5 M
NaCl by Abdel – Gaber et al., [30]. He evaluated corrosion inhibition efficiency through
electrochemical techniques namely, DC method (potentiodynamic polarization) and AC method
(electrochemical impedance) as well as by visual inspection (after 18 months of sample
immersion). The review concluded that Arghel extract has high corrosion inhibition potential; it
acts by retarding the diffusion process rather than charge transfer or concrete resistance.
Abdulrahman et al., [31 – 33] had been extensively studied and reported the corrosion
inhibition potential of Bambusa arundinacea extract for reinforcement steel corrosion in
concrete. Techniques namely, linear polarization, electrochemical impedance spectroscopy and
FESEM – EDX techniques were used for corrosion analysis while the results obtained were
compared with calcium nitrite. Further, authors reported various concrete parameters namely,
concrete strength and water permeability as well. Mechanistic approach of corrosion inhibition
effect of Bambusa arundinacea extract was also proposed by the authors [33].
Okeniyi et al., [34] have tested Rhizophora mangle L extract as corrosion inhibitor
admixture for reinforcing steel in 0.5 M H2SO4. Authors analyzed the electrochemical results
through statistical distribution fitting models and analysis, and percentage of inhibition
efficiencies was also calculated. The results of correlation fitting model and the experimental
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model were found to be in good agreement; further, authors reported that Rhizophora mangle L
extract showing inhibition efficiency around 70 – 78 %.
Vernonia amygdalina extract was investigated as corrosion inhibitor for mild steel rebar
concrete in 3.5 M NaCl medium by Loto et al., [35]. The authors have tested corrosion inhibition
efficiency of Vernonia amygdalina by weight loss measurement, potential and pH measurements
and concrete parameter (compressive strength) was also reported. Vernonia amygdalina extract
was reported to show maximum inhibition efficiency of 90 % at concentration level 25 %.
Eyu et al., [36] have studied the corrosion inhibition property of Vernonia amygdalina
extract for carbon steel in concrete exposed to 3.5 M NaCl medium. They made corrosion
analysis through weight loss measurement, corrosion potential measurement, half - cell
measurement, concrete resistivity measurement and visual inspection. The results of weight loss
were compared with commercial inhibitors namely, sodium nitrite and calcium nitrites as well.
Vernonia amygdalina extract was shows inhibition efficiency of 75 % at concentration level 6 %
(v /v).
Thus, natural corrosion inhibitors have advantages in many ways. There are only few
reports of use of “green” inhibitors for corrosion protection of metals in reinforced concrete,
while “green” corrosion inhibitors for metals have enormous reports [18, 20 – 22]. The reasons
are that the corrosion analysis on metals in reinforced concrete needs; relatively high volume of
plant extract, longer time duration for corrosion assessment, sophisticated instruments and
expertise, to measure physiochemical, mechanical parameters and durability properties of
concrete and to test corrosion inhibitors blends with uncertain concrete composition.
Commercialization of “green” corrosion inhibitors for reinforced steel has not made the
breakthrough into mass use and the reasons may be; possible risk of microbiological corrosion,
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lower inhibition efficiency as compared with synthetic inhibitors, life time of “green” inhibitors
is uncertain inside the concrete, need of huge volume of raw materials (plant sources) for
manufacturing, immense use of herbs and rare species may ruin down the plant kingdom.
Nevertheless, all these drawbacks can be overcome by adopting suitable methodology.
Few recommendations are; agricultural wastes (coconut shell, palm oil fruit bunch, paddy husk,
sugarcane waste etc.,) can be developed as corrosion inhibitors, application of biocide materials
to increase the stability of inhibitors inside the concrete (biocides are substances added with
building materials to prevent microorganisms which includes fungi, algae and bacteria; most
popular biocides are Irgarol 1051, Sea Nine 211) [37, 38] and corrosion analysis in prototype
setup before used in industry / field.
3. Conclusion
Use of reinforced concrete is a common practice in many construction industries, since it
has superior physical properties and low cost. Corrosion of steel inside the concrete is main
setback of this; while corrosion inhibitors playing a vital role in metal protection. Synthetic
corrosion inhibitors were found to be toxic and hazard to the environment which encourage
researchers to develop “green” corrosion inhibitors. Literature review clearly evidenced that
corrosion inhibitors derived from natural products can very well be served as “green” inhibitors
for reinforced steel corrosion in concrete. Application of “green” corrosion inhibitors has few
drawbacks, which can be overcome by adopting suitable methodology.
Acknowledgment
The authors gratefully acknowledged the financial support provided by MOHE grant No
FRGS/1/2014/TK08/UTM/01/2, RMC grant No. QJ130000.21A2.01E65 (PDRU) and CRC,
Universiti Teknologi Malaysia.
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