Protection of Iron by Corrosion Inhibitors: Comparison
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Iron is a widely used metal due to its low cost and availability, but it is susceptible to corrosion in many circumstances. This corrosion can result in economic and environmental losses, and negatively affect the physical and chemical properties of the metal.

  • archeological iron
  • corrosion
  • mechanism
  • green inhibitors

1. Introduction

The high cost of annual material degradation has prompted the use of protection methods to save materials and energy, and to meet new requirements, such as the use of non-toxic products.
The protection of materials from corrosive environments can be achieved through various actions at the metallurgical, structural, electrochemical, and environmental levels. Three degrees of action are identified [42][1]:
  • Actions on the material, such as modifying its composition or microstructure or isolating it from its environment through a metallic or organic coating or anodization.
  • Actions on the environment, such as incorporating corrosion inhibitors or avoiding moisture accumulation in the structure.
  • Actions on the electrochemical corrosion process, such as cathodic protection.
The protection of metal heritage requires consideration of both preservation of surface information and protection against corrosion. Organic coatings, such as varnishes and waxes, can provide protection. Anodic, cathodic, and galvanic protection, as well as the use of inhibitors, are common and effective methods of protection.

2. Background

The Romans were aware of corrosion and its effects on metal objects. Pliny the Elder, a Roman naturalist and historian, mentioned in the first century A.D. the use of oil or bitumen for protecting bronze and pitch, gypsum or ceruse for iron from corrosion.
This shows that even in ancient times, people were aware of the need for protection against corrosion and sought methods to preserve their metal objects.
The study of corrosion has a long history, dating back to the 17th century. However, it was not until the 19th century that the means of protecting against corrosion were studied scientifically. The number of references dealing with corrosion inhibitors increased rapidly after 1945, with numerous articles written on the subject in various fields such as aviation, oil refining, and diesel engines. In recent years, there has been a significant increase in works on corrosion inhibition, reflecting technological advancements in the field [43][2].

3. Definition

A corrosion inhibitor is a substance added to a corrosion system to slow down the corrosion rate of a metal without significantly altering the concentration of corrosive agents in the environment. The definition used by the National Association of Corrosion Engineers (NACE) states that an inhibitor is a substance that retards corrosion in low concentration. The international standard ISO 8044 defines an inhibitor as a chemical substance added to the corrosion system in a chosen concentration to decrease the corrosion rate of the metal. The properties of an effective inhibitor include lowering the corrosion rate, stability in the presence of other constituents, stability at temperatures of use, effectiveness at low concentrations, compatibility with non-toxicity standards, and cost-effectiveness [44,45][3][4].

4. Classification

Inhibitors can be classified based on different criteria such as mechanism of inhibition, application, or chemical nature. This classification helps in better understanding the working of inhibitors and selecting the right inhibitor for a particular corrosion problem. The different classifications of inhibitors provide a comprehensive understanding of the different types of inhibitors and their uses in various corrosion scenarios [46][5].
The classification of inhibitors based on their field of application is a useful way to differentiate between inhibitors used in different environments. For example, inhibitors used in acidic media are mainly used to prevent electrochemical attack during pickling processes and in drilling fluids in the oil industry. Inhibitors for neutral media are mainly used to protect cooling circuits. In organic media, a large number of inhibitors are used in engine lubricants and gasoline to protect against corrosion caused by the presence of water and ionic species. This classification helps in selecting the appropriate inhibitor for a specific corrosion problem and ensures maximum protection against corrosion [47][6].
Inhibitors for paints and gas phases are used to temporarily protect various packaged objects from corrosion during transport and storage. Electrochemically, inhibitors are classified as anodic, cathodic, or mixed inhibitors, depending on their effect on the rate of oxidation and reduction reactions. Anodic inhibitors slow down the oxidation of the metal, cathodic inhibitors slow down the reduction of the oxidant, while mixed inhibitors affect both anodic and cathodic domains. The electrochemical effect of inhibitors on the surface can be explained by various physico-chemical mechanisms. In some cases, the inhibitor forms a physical barrier between the metal and the corrosive medium, as in the case of thick coatings such as waxes and varnishes. In other cases, the inhibitor operates through a pH or redox buffer effect, which can passivate the metal and reduce the corrosion rate. The mode of action of an inhibitor can also involve the formation of surface films due to the precipitation of inorganic salts or poorly soluble organic complexes. These films reduce the accessibility of oxygen to the metal surface and partially block the anodic dissolution [48][7].
The technique of removing the corrosive agent from the medium is only applicable in closed systems, such as in the closed hot water circuits of thermal power stations. Additionally, it is important to note that many inhibitors act through multiple mechanisms simultaneously, which can increase their effectiveness in preventing corrosion [49][8].

5. Inhibitors Specific to Ferrous Metals in an Acid Medium

Different corrosion inhibitors can effectively protect ferrous metals from corrosion in acidic environments, either individually or in combination. The selection of the appropriate inhibitor depends on various factors such as the type of metal, environment, and specific conditions [50][9].

5.1. Synthetic Inhibitors

Organic inhibitors contain elements such as nitrogen, sulfur, and oxygen that can exchange electrons with the metal and protect it from corrosion. These inhibitors work through a chemical process called spontaneous adsorption and provide good results in inhibiting steel corrosion in acidic medium [51][10]. Organic inhibitors have several advantages over inorganic inhibitors, as they provide uniform passivation on the metal surface for maximum protection, while inorganic inhibitors form brittle and porous films that can lead to localized corrosion [52][11].
The addition of organic compounds to the acid solution generally reduces its aggressiveness, however, these compounds are toxic and harmful to the environment, leading to the need for alternative, eco-friendly, and efficient inhibitors. One such alternative is the use of natural substances, including vegetable oils [58][12].

5.2. Corrosion Inhibitors Based on Natural Substances

The use of natural substances such as vegetable oils is attractive due to their low cost and abundant availability as environmentally friendly and biodegradable compounds. The use of plant extracts as corrosion inhibitors dates back to 1930, with the use of Chelidonium majus and other plants in a pickling bath of H2SO4. The first patent on corrosion inhibition was granted to Baldwin, who used molasses and vegetable oils for pickling steel sheets in acidic media [59][13].
Currently, many research groups around the world are exploring the use of plant products as corrosion inhibitors for metals and alloys in various corrosive environments [60][14]. There is an increasing number of publications addressing this topic, as shown in Table 1, which details some of the main green inhibitors for the corrosion of ferrous metals in acidic medium.
Table 1. Literature review on the use of natural substances as corrosion inhibitors.
Non-Toxic Inhibitors Type of

Inhibitor		 IE (%) Environment Substrate Adsorption

Mechanisms		 References
Henna, L. inermis Cathodic 92.1 HCl (1 M) Mild

steel		 Chemisorption [61][15]
N. fruticans Wurmb Mixed 75.1 HCl Mild

steel		 Physisorption [62][16]
Eugenol derivatives Mixed 91 HCl (1 M) Steel Chemisorption [63][17]
Khillah extract (seeds (A. visnaga)) Mixed 99.3 HCl (2 M) Steel SX 316 Chemical

adsorption		 [64][18]
Natural oil extracted from pennyroyal mint (Mentha pulegium, PM) Cathodic 80 HCl (1 M) Steel Simple blocking of the available surface, intermolecular synergistic Active molecules of this oil [65][19]
Plant extract of Z. alatum Cathodic 95 HCl (5%) Mild

steel		 Chemisorption [63][17]
Flavonoids (Monomers) Cathodic >70 HCl

(0.5 M)

aerate d		 Steel Chemisorption [66][20]
Succinic acid (SA) Anodic 97.5 HCl (1.0 M)

aerate d

unstirred		 Low

carbon

steel		 Film of inhibitor adsorbed on electrode surface [67][21]
Aqueous extract of olive leaves (O. europaea L.) Mixed 91 HCl (2 M) Carbon

steel		 Physical adsorption [68][22]
T. occidentalis, (TO) Cathodic 91–97 HCl (1 M) Mild

steel		 Physisorption [69][23]
A. indica, (AI)
H. sabdariffa, (HS)
G. kola (GK) seed extract
Extract from J. gendarussa (JGPE) Mixed 93 HCl (1 M) Mild

steel		 Physisorption [70][24]
Extracts of leaves and seeds of P. amarus Mixed 80.1–94.1 HCl (2 M) Mild

steel		 Chemisorption [71][25]
L. albus L. Mixed 77.6 HCl (2 M) Steel Chemisorption [72][26]
85 H2SO4 (1 M) Physisorption
Pennyroyal oil from M. pulegium Cathodic 80 HCl (1 M) Steel Chemisorption [65][19]
Zest of (Mango, Orange, Passion, Cashew) Mixed 80–95 HCl (1 M) Carbon

steel		 Adsorption of organic

compounds present in the extracts on the active sites of the electrode surface		 [73][27]
Juniperus phoenicea (Cupressaceae) essential oil Mixed 83 HCl (1 M) Mild

steel		 adsorption of

aromatic

compounds on the metal surface		 [74][28]
Methanolic extract of A. Pallens Mixed 96.5 HCl (4 N) Mild steel Formation of a very tightly

adhering adsorbent film on the metal surface		 [75][29]
Guar gum Mixed 93.6 H2SO4 (1 M) Carbon

steel		 Formation of

passive, active and continuously propagating centers.		 [76][30]
Chamomile (C. mixtum L.) Mixed 90.2 H2SO4 (1 M) Steel Adsorption of the stable complex to the steel surface [77][31]
Halfab ar (C. proximus)
Black cumin (N. sativa L.)
Kidney bean (P. vulgaris L.)
Berberine Mixed 98 H2SO4 (1 M) Mild steel Chemical

adsorption		 [78][32]
Fenugreek leaves (AEFL) Mixed 88.3 H2SO4 Mild steel Chemical

adsorption of

inhibitor molecules on mild steel		 [79][33]
Black pepper extract 90 [80][34]
Saffron-o (SO) 65 [81][35]
Alizarin Yellow (GG) Mixed 85 H2SO4 (2 M) Mild steel Physisorption [82][36]
Caffeic acid 83.9 H2 SO4

(0.1 M)		 [83][37]
Lignin extracted from black liquor of the pulp and paper industry Cathodic 95 H2SO4

(0.5 M)		 Mild steel Adsorption of more lignin molecules on the metal surface, preventing the

electrochemical

corrosion process		 [84][38]
Galactomannan extracted from Carob seeds (Ceratonia Siliqua) Mixed 86.6 HCl (1 M) Archaeological iron Establishment of inhibitor film on iron substrate

surface		 [85][39]
Formulations based on oils Opuntia ficus

indica (OTH)		 Mixed 99.6 Acid rain-

simula ted

enviro nment pH = 3.6		 Archaeological iron Establishment of

inhibitor film on

iron substrate

surface		 [86][40]
extracted from

the seeds of		 Nigella sativa

(FBN)		 99.3 [87][41]
Jatropha Curcas (JAC) 97 [88][42]
Ceratonia Siliqua L., (FCSL) 98.6 [89][43]
Aleurites

moluccana (ALM) 97 [90][44]
Opuntia Dillenii (FOD) 99 [91][45]

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