Before we can examine the methods available for corrosion scientists and engineers to inhibit corrosion on metals, one must fi rst understand the environment that one is working in. As stated in Section 1.1, this chapter will focus on wet corrosion processes. Wet corrosion occurs ‘when a liquid is present’ [1, 2, 23] . Typically this liquid is either aqueous solutions, electrolytes or small amounts of moisture. For this review we will focus exclusively on aqueous systems comprising either atmospheric, fresh- and/
or seawaters to which metals/alloys and composites are typically exposed.
1.3.1 Examples of wet corrosion environments
Atmospheric corrosion is an electrochemical process, whereby thin fi lms of water are deposited on a metallic surface leading to wet corrosion [32] . The process for this form of corrosion is cathodic resulting in reduction of oxygen. Several factors have a signifi cant infl uence on the rates of atmospheric corrosion: (a) fi lm thickness of water layer, (b) time of wetness, (c) composition of surface electrolyte and (d) temperature [33] . The rate of atmospheric corrosion also depends on the type of atmosphere the metal, alloy or composite is exposed to. Several atmospheric types listed will describe the environment: (a) rural or inland with a dry climate containing little or no pollution, (b) marine on or near sea with high humidity and
chlorides, (c) urban with pollution from exhaust, soot or smoke and (d) industrial with high pollution from smoke, soot and precipitate [34] . Depending on the atmospheric conditions, climate, pollutants and amount of wetness, corrosion rates for metals/alloys will vary widely. Corrosion rates for steel, stainless steel, aluminum alloys, copper/alloys, nickel/alloys, tin and zinc depend strongly on type of atmosphere, pollutant, climate and alloying elements.
Corrosion affecting metals/alloys in fresh waters such as found in lakes, rivers, brooks, streams, rain and ground water is normally dependent on oxygen concentration [35, 36] . The four factors affecting corrosion rates for metal/alloys in fresh waters are: (a) oxygen concentration, (b) temperature, (c) fl ow velocity and (d) chemical composition.
The oxygen concentration affects corrosion most directly due to the underlying oxygen reduction reaction (Equation 1.1) which predominates over the hydrogen reduction reaction (Equation 1.2) in most natural environments such as freshwaters. With access to air (e.g. freshwater environments), the oxygen reduction reaction is the crucial cathodic reaction, leading to corrosion in metal/alloys. The distribution and intensity of corrosion are related to the access of oxygen – a diffusion-controlled process to various parts of the metal/alloy surface.
Oxygen reduction reaction O2+4H++4e−→2H O2 [1.1]
Hydrogen reduction reaction 4H++4e−→2H2 [1.2]
Since corrosion in wet environments is a chemical reaction with an activation energy, higher temperatures will increase the rate of corrosion.
This increased corrosion rate is due to the increase in the number of molecules with enough energy to react in chemical and electrochemical reactions. This also increases diffusion rates, transport rates for the electrolyte, formation or breakdown of passive fi lms. Solubility of benefi cial gases/species can affect the corrosion rate of metals/alloys in freshwater environments.
Fluid velocity or fl ow velocity refers to the fl ow of a fl uid (aqueous or organic) over a solid surface; either laminar or turbulent fl ow occurs [37] . In most situations affecting corrosion, turbulent fl ow is the predominant method. This fl ow can either be single-phase or multiphase. High-velocity fl ow normally results in fl ow-induced corrosion, erosion-corrosion or cavitation. Low-turbulance fl ow results in corrosion that is usually found under deposits or in separated liquid phases. The infl uence of fl ow velocity- induced corrosion is greatly dependent on fl uid velocity, alloy composition, fl uid constituents, fl uid physical properties, geometry of the fl ow system and corrosion mechanism. In some instances the rate of fl uid fl ow can retard or inhibit corrosion.
1.3.2 Chemical composition in wet corrosive environments
Chemical composition also plays a signifi cant role in corrosion in freshwater systems. Freshwaters can be either hard or soft depending on dissolved minerals. Freshwaters that are hard have high levels of calcium (Ca 2 + ) and bicarbonate (HCO 3 − ) ions. In hard water environments under the right conditions, calcium carbonate (CaCO 3 ) fi lms may deposit, providing an effective diffusion barrier coating for decreasing corrosion rates of the underlying metal substrate [38] . However in soft waters the corrosion rate is greatly increased due to the lack of an effective diffusion barrier coating.
Besides calcium and bicarbonate ions, additional species play active roles in affecting the corrosion rate of metals. These other species include chloride (Cl − ), Mg 2 + , soluble silica (H 2 SiO 3 ), Na + , K + , SO 4 − ions and gases such as SO x , NO x , NH 3 and HCl. They are produced when rainwater comes into contact with the atmosphere and surface of the Earth. Each of these species and in combination has a profound effect on the saturation index which in turn determines CaCO 3 diffusion barrier fi lm formation on metal substrates when exposed to various compositions of freshwaters.
Finally we will briefl y review corrosion in seawater. Chlorinity in seawater is defi ned as the total amount (in kilograms (kg)) of halide ions (usually Cl − ) dissolved in 1 kg of seawater [39] . Salinity is defi ned as the total amount of salts dissolved in seawater expressed by the following equation:
Salinity=1 8 655 chlorinity. 0 × [1.3]
In seawater there are small differences in salinity across the oceans.
Seawater contains about 3.4% salt and is slightly alkaline, pH varying from 7.5 to 8.3 depending on CO 2 concentration [40] . The pH of surface seawater globally remains relatively constant at pH = 8.2. The major factors affecting the corrosion rate of metals in seawater are salinity, dissolved oxygen (DO) concentration, temperature, pH, carbonate, pollutants, and biological organisms. Corrosion of metal alloys (steels, aluminum) is controlled by the rate of DO to the metal surface in a similar fashion to freshwater corrosion.
This rate of DO to the metal surface is controlled by four factors: (a) oxygen concentration in the bulk seawater, (b) movement of seawater, (c) the diffusion coeffi cient for oxygen in seawater and (d) characteristics of the corrosion products formed on the metal surface (e.g. barrier to oxygen diffusion). Oxygen concentration in surface waters is near the equilibrium saturation value with the atmosphere. This oxygen concentration value varies inversely with the temperature of the seawater. Since salinity variations are very minor in seawater and do not affect oxygen solubility, temperature is the driving factor affecting oxygen concentration. Surface temperatures of the oceans varies with latitude from −2 °C (Arctic) to
∼ 35 °C in the tropics [40] . With an increase in temperature, the diffusion
coeffi cient for oxygen increases. The most corrosion-prone seawater environment is the splash or spray zone. The splash/spray zone is defi ned as the point above the mean tide level [41] . Corrosion depends on the amount of available oxygen. The splash zone is the most severely attacked corrosion area due to highly aerated seawater and the erosive effects of spray, wave and tidal action. A thin water layer or fi lm is present on the surface of the metal and corrosion products are constantly washed away, removing any barrier that might have built up, thererby increasing the corrosion attack. The splash zone is the most diffi cult in which to inhibit corrosion due to its unique environment.
1.3.3 Biologically infl uenced corrosion processes
Additionally, biological organisms are found in seawaters, such as in tidal bays, estuaries, habors, coastal and open ocean seawaters. Microbial adhesion and establishment of complex colonies and communities called microfouling are necessary for deterioration and corrosion of the underlying metal or substrate [42] . The attack of the micro-organisms can take place directly or indirectly on the substrate with attachment of the micro-organisms and subsequent formation of the biological fi lm or ‘biofi lm’ [43] . Several factors infl uence the formation of the biofi lm on the substrate: indigenous microfl ora, material composition, nature of surface, and environmental conditions [44] . The corrosion of metals due to the formation of biofi lms is called
‘microbiologically infl uenced corrosion’ (MIC). The biofi lms that form on the surface of metals infl uence corrosion via the ability of the micro-organisms to change the environment, including oxidizing power, temperature, velocity and concentration. Both aerobic and anaerobic colonies can form, depending on oxygen concentration within and near the colonies. With the formation of anaerobic colonies, such as sulfate-reducing bacteria, rapid corrosion of the underlying metal can occur. Corrosion of metals results in loss of tensile strength and subsequent failure of the system. MIC can affect iron, steel, stainless steels, copper, aluminum and zinc metals, producing pitting, degradation and slime formation of the underlying metal substrate.