VCI mechanism of action

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LITERATURE REVIEW

Atmospheric corrosion is an electrochemical process that involves acid-base interactions between the metal ions formed and other species present in the electrolyte (Ramachandran et al., 1996; Roberge et al., 2002). (An electrolyte is a solution containing free ions capable of conducting electrical current, such as moisture.) The rate of any given electrochemical corrosion process depends largely on the rate of an anodic and a cathodic reaction at the metal surface (Rozenfeld, 1981). Anodic reactions entail oxidation and consist of transferring a metal ion from the lattice to the solution, with the liberation of electrons (Rozenfeld, 1981).
Atmospheric corrosion is divided into two types: dry and wet corrosion (Du Preez, 1998). Dry atmospheric corrosion is a result of the direct chemical reaction of the metal with the environment, thereby forming the oxide layer on the surface of the metal. Wet corrosion is an electrochemical process and it takes place in neutral or acidic environments (Myles & Associates, 1995). The presence of an electrolyte is essential in wet corrosion (Du Preez, 1998).
Atmospheric corrosion occurs due to the high concentrations of oxygen in air, water or moisture droplets (from air) and of dissolved ions present on the surface of the metal (Kui et al., 2008). If one considers metallic iron as an example (see Figure 1), iron is oxidised in the anodic region, resulting in the formation of soluble ferrous ions (Fe²⁺), while the dissolved oxygen is reduced in the cathodic region to form OH^– ions. The ferrous ions and OH^– ions then combine to form a solid deposit Fe(OH)₂ on the metal surface. As the solubility of this solid deposit is relatively high, the dissolved Fe(OH)₂ can be further oxidised by access to dissolved atmospheric oxygen to form ferric ions (Fe³⁺). These unstable ferric ions later convert to form an insoluble ferric oxide (Fe₂O₃) layer on the surface of the corroding iron metal (Roberge et al, 2002; Kui et al., 2008; Tamura, 2008). This layer is commonly known as rust. The ferric ions in rust can be further reduced in the cathodic region to form FeO-Fe₂O₃ product, which contains both ferrous and ferric ions. The reduction of ferric ions is a cyclical process that can be speeded up by variations in the temperature and in the moisture coverage (Roberge et al., 2002; Kui et al., 2008). This results in the metal having an uneven surface and holes, which makes it difficult to stop the atmospheric corrosion process. The main factors that accelerate the atmospheric corrosion of metals are relative humidity, acid rain and trace volatile contaminants (pollutants), such as hydrochloric acid (Roberge et al., 2002).

Relative humidity

Atmospheric corrosion requires a critical relative humidity level, below which corrosion does not occur (Syed, 2006). This level varies, depending largely on the nature of the corroding material and the tendency of the corrosion products and surface deposits to absorb moisture (Sastri, 1998; Roberge et al., 2002). Values for the critical humidity level also differ for various pollutants. For example, the value is 60% for sulphur dioxide but higher for chloride pollutants (Sastri, 1998). The thin electrolyte layer that forms on the metallic surface is an essential requirement for the atmospheric corrosion process to occur when the metal is exposed to a critical level of humidity (Roberge et al., 2002; Hœrlé et al., 2004). The corrosion rate of the metal increases sharply beyond the threshold level of a critical relative humidity (Hœrlé et al., 2004; Ahmad, 2006). In the presence of thin-film electrolytes, atmospheric corrosion proceeds by balanced anodic and cathodic reactions, where the anodic reaction involves attack of the metal and the cathodic reaction is oxygen reduction (Roberge et al., 2002; Hœrlé et al., 2004).

Acid rain

Acid rain is a rain that possesses elevated levels of hydrogen ions (has low pH values). In the presence of atmospheric gaseous pollutants such as sulphur dioxide, atmospheric corrosion increases considerably. Atmospheric pollutants such as sulphur dioxide dissolve in water and upon oxidation it produces sulphuric acid by the following reaction sequence (Sastri, 1998; Du Preez, 1998; Kui et al., 2008):
SO₂ + H₂O ® H₂SO₃ (sulphurous acid)
SO₂ + 2Fe₂O₃ ® FeSO₄ + Fe₃O₄
The acids that form further facilitate the atmospheric corrosion process of the metal; this phenomenon is known as the acid-rain effect (Sastri, 1998). Acid rain increases the rate of atmospheric corrosion of metals considerably because it promotes the formation of a thicker electrolyte layer on the metal surface (Ahmad, 2006).

Contaminants (pollutants)

Among the various contaminants present in salt, chloride has the greatest effect on atmospheric corrosion. Once chloride is adsorbed, tenacity of adherence is so great that it cannot be desorbed by using only simple surface-cleaning techniques (Sastri, 1998). Chlorides accelerate atmospheric corrosion by being oxidised in the metallic anode region due to the presence of atmospheric oxygen in air (Tamura, 2008):
Fe + 2HCl ® FeCl₂ + H₂O
FeCl₂ + (1/4)O₂ + (5/2)H₂O ® Fe(OH)_3(s) + 2HCl
In general, a combination of high relative humidity, high temperature and high levels of pollutants increases the rate of atmospheric corrosion of metals.
Atmospheric corrosion can be prevented by using VCIs, as mentioned in the previous chapter. VCIs function by changing the kinetics of the electrochemical reactions causing the corrosion process (Rozenfeld, 1981). In general, corrosion inhibitors restrict the anodic and or cathodic processes by blocking the active sites on the metal surface. Alternatively, an inhibitor may work by increasing the potential of the metal surface so that the metal enters the passivation region where a protective oxide film forms. Some corrosion inhibitors inhibit corrosion by contributing to the formation of a thin protective film on the metallic surface. In summary, the performance of corrosion inhibitors is often related to (Sanyal, 1981):

• The chemical structure and physicochemical properties of the compound that is used as an inhibitor
• The adsorption of molecules or their ions on anodic and or cathodic sites
• An increase in cathodic and/or anodic over-voltage
• The formation of protective barrier films, consisting of complexes or films produced as a result of the interaction between the metal, its ions and ions available in the surroundings.

VCI formulations

VCIs are commercially available in different forms, i.e. liquids, powders, sachets, tablets, films and emitters (Bastidas et al., 2005). Some available VCI forms are briefly summarised below:
VCI powders
Powders are the cheapest form of VCI among all the available forms. The powder forms of VCIs are easily transported. The disadvantage of some VCI powders is that they are flammable and the suspension of flammable powder in air during usage can be explosive (Myles & Associates, 1995). While the vapours preventing corrosion may be non-toxic, powders may be toxic when inhaled in dust form. Difficulties are also experienced with the removal of VCI dust after usage.
VCI tablets
VCI tablets are made by compressing the VCI powder together with a polymer binder material (Myles & Associates, 1995). The use of a polymer binder is reported to add strength and form a thin-walled matrix in the tablet to regulate the evaporation of a volatile corrosion inhibitor (Goldade et al., 2005). Unlike powder, tablets do not generate any dust and can be easily removed after usage (Myles & Associates, 1995).

Abstract 
Dedication 
Acknowledgements 
Declaration
List of Figures
List of Tables
List of Schemes 
List of Abbreviations 
List of Symbols
1 Introduction and Aim of Study
1.1 Introduction.
1.2 Brief history of VCIs
1.3 Problem statement.
1.4 Aims and objectives
1.5 Dissertation outline
2 Literature review 
2.1 VCI formulations
2.2 VCI mechanism of action
2.3 Compounds used as VCIs
2.4 Review of the methods used to test the effectiveness of VCIs
2.5 Factors affecting the effectiveness of VCIs
2.6 Amine-carboxylic acid interactions
2.7 The effect of water
2.8 Review of amine-carboxylic acid characterisation methods
3 Experimental
3.1 Reagents and suppliers
3.2 Experimental methods
3.3 Instrumentation
4 Results 
Part 1: Method Development
4.1 Liquid phase FTIR
4.2 DSC.
4.3 TGA-FTIR
Part 2: Method Application
4.4 Liquid phase FTIR and refractive index .
4.5 Liquid phase DSC..
4.6 TGA
4.7 Vapour phase FTIR.
4.8 Corrosion tests
5 Discussion.
6 Conclusion 
References
Publications
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