Corrosion is a naturally occuring process, which is defi ned as the degradation or deterioration of a substance and/or its properties, usually a metal, over a period of time due to environmental exposure [1] . This is an exergonic process as the metal tends toward the lowest possible energy state. Therefore, metals such as aluminum and steel have a natural tendency to return to their lowest energy state when combined with oxygen and water to form hydrated aluminum and iron oxides (corrosion products). These corrosion products are the eventual fi nal state of processed metals which degrade over time when exposed to the elements. Thus the life cycle from mined and processed ores to industrial products and eventually back to their natural state is as shown in Fig. 1.1 .
The environment to which metals are exposed consists of the entire surrounding in contact with the metal. The major factors used to describe the environment are
• physical state of the environment either gas, liquid or solid;
• chemical composition which includes constituents and concentrations;
• temperature.
These three factors have a signifi cant infl uence on the rate of corrosion;
however, additional factors such as the velocity of a solution (fl ow rate) and mechanical stress and/or loads can also play an important role in the corrosion of metals. In order to understand and control corrosion, one must
DOI : 10.1533/9780857096883.1.3
take into account both the material and the environment. Identifying both of these components will determine effective strategies for combating these destructive processes. The corrosion of metals can be divided into three groups [2] :
• wet corrosion where the corrosive environment is aqueous with dissolved species, normally the electrolyte is a liquid and the process is electrochemical;
• corrosion in other fl uids such as fused salts and/or molten metals;
• dry corrosion where the corrosive environment is a high-temperature dry gas.
This chapter will focus exclusively on wet corrosion processes that affect metals and alloys and the methods available to control or inhibit their effects.
Corrosion is classifi ed into three groups: (a) nature of corrodent, (b) mechanism of corrosion and (c) appearance of corroded metal. In referring to group (a) by the ‘nature of the corrodent’ we mean either ‘wet’ or ‘dry’
corrosion. In order for ‘wet corrosion’ to occur this process requires a liquid, usually water (freshwater or saline). For group (b) the ‘mechanism of corrosion’ refers to how the corrosion occurs, which is via an electrochemical or a direct chemical reaction. Finally when referring to group (c) the
‘appearance of the corroded metal’ describes what type of corrosion is affecting the metal. This can be either a general uniform corrosion resulting in corrosion over the entire surface or localized corrosion in which only small areas are affected by corrosion processes. The appearance of the
1.1 Schematic representation of corrosion cycle of metal alloy [1] . Factory processing of
mined iron or aluminum oxide ore
Processed aluminum or steel alloy
Corrosion due to natural elements exposure
Iron or aluminum oxide degradation products
corroded metal is useful in identifying a specifi c type of corrosion and the methods by which corrosion can be minimized.
There are eight forms of wet corrosion: uniform or general; pitting;
crevice (including fi liform); galvanic; erosion (including cavitation and fretting corrosion); intergranular (including sensitization and exfoliation);
dealloying (including dezincifi cation and graphite); and environmentally assisted cracking (including stress-corrosion cracking (SCC), fatigue and hydrogen damage). Figure 1.2 shows schematically the types of corrosion listed above. In theory these forms are distinct but in reality most metals undergo a variety of corrosion processes.
The aqueous or, as commonly referred to, the wet corrosion process consists of three important elements which are necessary for the corrosion process to occur: anodic reaction, cathodic reaction and electrolyte solution or conducting liquid.
The anodic reaction or oxidation of the metal results in dissolution of the metal, which is transferred to the solution as M n + ions. The cathodic reaction or reduction involves oxygen. Reduction of oxygen is the dominant cathodic reaction in natural environments (seawater, freshwater, soil and atmos- phere). This process forms an electrical circuit without any accumulation of charges. The electrons are released by the anodic process and they are conducted through the metal to the cathode. The electrons released by the anodic process are consumed by the cathodic reaction. This electrochemical process requires an ionically conducting liquid, the ‘electrolyte’, which must be in contact with the metal. The electrochemical circuit is closed by ion
1.2 General scheme for various forms of corrosion on metals/alloys.
6 Intergranular corrosion 7 Dealloying
Tensile stress
8 Environmentally assisted cracking (stress-corrosion cracking) 1 General or
uniform corrosion
2 Pitting corrosion 3 Crevice corrosion
Metal or non metal
Flowing corrodent
5 Erosion corrosion 4 Galvanic corrosion
More noble metal
conduction through the electrolyte and all three elements must be present in order for wet corrosion to occur [2] . Typically the metal ions M n + are conducted towards OH − ions and together they normally produce a metal hydroxide, which is deposited on the surface of the metal. If, for example, the oxidizing metal is zinc and the liquid is water containing oxygen the Zn + 2 ions and OH − ions combine to form Zn(OH) 2 . Iron and copper metals also follow similar corrosion proccesses when the electrolyte is water in the presence of dissolved oxygen (Fig. 1.3 ).
As seen from Fig. 1.3 the two key features for corrosion are the availability of oxygen and the electrolyte. When corrosion products such as hydroxides are deposited onto the metal alloy, there is sometimes a subsequent reduction in the availability of oxygen for the corrosion process to continue.
This continuous layer of metal hydroxides will reduce the oxygen reduction reaction and therefore reduce the corrosion rate. Since both the metal dissolution and rate of oxygen reduction are equal, any decrease in one reaction will result in a decrease in the other reaction. In this system the corrosion rate is considered to be under cathodic control. This method is used extensively in the control of corrosion by nature and corrosion engineering. In several cases these corrosion products form a continuous layer of surface fi lm oxides that are similar in crystallographic composition to the oxidizing metal. Films that are generated from this process prevent the conduction of metal ions from the metal–oxide interface to the oxide–
liquid interface. By this process the corrosion rate is so low that the corrosion rate is considered to be under anodic control. This phenomenon is also known as ‘passivation’ and is typical for materials such as steel and aluminum in many natural environments.
1.1.1 Cost effects of the corrosion process
Just like other natural hazards (earthquakes, tornadoes or hurricanes), corrosion can cause severe and expensive damage to everything from automobiles to infrastructure (pipelines, buildings and bridges). Over the
1.3 Wet corrosion process of divalent metal (M) during the electrochemical corrosion cell.
Metal
Anodic reaction
Aqueous solution (electrolyte)
Cathodic reaction
Rust product (corrosion product) formed during corrosion electrochemical cell M M+2 + 2e– ½O2 + H2O + 2e– 2OH–
past 22 years the United States has experienced over 52 major weather- related disasters. This has resulted in losses of over US$17 billion annually ($280 billion total) [3] . Contrast this with the current costs of metallic corrosion on the US economy and estimates of over $276 billion annually which represents 3.1% of the US gross domestic product (GDP). The estimates for the US military are between $10 and 20 billion annually [4–7]
(Fig. 1.4 ).
Corrosion has a signifi cant impact both economically and environmentally on almost all the world ’ s infrastructure. This includes highways, bridges, autmobiles, pipelines, chemical processing, water/wastewater systems and military [8] . The annual costs of corrosion worldwide exceeds $US 1.8 trillon [9] . Studies done in China, Japan, United Kingdom, Europe and South America showed corrosion costs similar to the United States. This corrosion problem translates into 3–4% of the GDP of industrialized nations worldwide. Corrosion is so prevalent and manifests itself in such diverse forms in our industrialized society that its occurrence and associated costs can never be eliminated.