Corrosion is a gradual process of deterioration of a material, mainly surfaces of metallic objects through a chemical reaction facilitated by the direct exposure to the environment agents. Alternatively, corrosion is electrochemical oxidation where an oxidant reacts with a metallic surface. Moreover, corrosion is also pronounced among other non-metal surfaces like ceramics and polymers. However, this form of corrosion is differently termed degradation.
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Corrosion can pose dangerous risks and cause expensive costs across-the-board like on automobiles, household appliances, drainage and waterworks (such as pipe systems), bridges (case in point the Silver Bridge accident in December 15, 1967) and public buildings (like the Talese painting in the Brooklyn Navy Yard) (Pamela Talese, n.d; Nace International, n.d; Lonakers, 2006). Ridha et al. (2009, p.163) provides an assessment of the risks of corrosion on the built structures over a period of 5 years since the Tsunami occurred in the Aceh Province in December 2004.
The study showed that preventive measures were needed for some of the buildings with concrete reinforced steel to avert possible dangers of collapse due to corrosion agents. Eker and Yuksel (2005, p.2) explain a range of solutions for corrosion induced by agricultural chemicals. Agricultural chemicals dealt with are dairy equipment cleaning acid solutions, pesticides and herbicides, fertilizers, farm wastes, grain and silage preservatives as well as slurries.
The main reason behind the cause for corrosions is the fact that metals are derived from their ores. Corrosion-Doctors (2010) argue that during the ore formation, they absorb and store energy that enables the original compounds to release the metallic form. It is this energy that is later used to facilitate corrosion. The classified forms of corrosion are general and localized. The effects of general corrosion are widespread and affect the entire exposed area.
The general rusting of steel surfaces, which are exposed to environmental agents, is usually highly visual and predictable. Localized corrosion is less visual and predictable. Localized corrosion occurs mainly in four forms. Galvanic corrosion is a pronounced form of localized corrosion. Pitting corrosion is another popular form of localized corrosion. Other forms include stress and crevice corrosion.
Galvanic corrosion happens when different metal types experience electrical and/or physical contact. These metals are further exposed to an electrolyte of different concentrations. Thus, the electrolyte provides the corroding environment through electrolysis. Alloys of aluminum and copper are more prone to this form of corrosion.
Galvanic corrosion is a major concern within the marine industry, where the water is salty and has impurities that corrode metal surfaces when they come into contact. Rusted surfaces can be treated with naval jelly. Galvanic corrosion can further be understood through the galvanic series (Mansano et al. 2003, p.750).
Pitting corrosion is characterized by the formation of holes in metals. The depassivation on small areas that assume the anodic role, while the other larger area becomes the cathode, results in corrosion (Szklarska-Smialowska, 1999, p.1743). The diffusion process allows the metallic object to degrade. Acidity within the pit sustains the space separating the cathode and the anode half-reactions; this leads to a gradient difference and electro-migration that concentrates anions into the pit.
Corrosion-Doctors (n.d.) explain that low pH acid waters catalyze corrosion by releasing hydrogen ions to the corrosion process. Stainless steel is more prone to pitting corrosion. In seawater, pits are created through the autocatalytic process in the presence of chlorides. The Gasoline explosion in Guadalajara, Mexico in April 1992 that caused damage on drainage infrastructure is a pitting corrosion case-in-point.
The explosion resulted from vapors from a single hole-leak formed by corrosion, where zinc-galvanized water pipe and steel-plated gasoline were in contact. Pitting corrosion is also linked with the initiation of stress corrosion at the eye-bar of the Silver Bridge in West Virginia that led to fatalities in 1967.
Crevice corrosion happens on restricted spaces where working fluid media have limited access. Such restricted openings are generally referred to as the crevices. Crevices can be within cracks or seams, below seals or gaskets, beneath sludge piles as well as spaces that have deposits (Rashidi et al., 2007, p.216). Cracks that allow corrosion to progress have to allow the reacting agent to permeate through and then remain stagnant.
Thus, crevices should provide space enough for oxide films to break-down. This can happen by design faults such as incomplete weld penetration or overlapping surfaces. Problems associated with crevice corrosion could effectively be addressed at the design stage, by paying more attention to the formation of crevices.
However, in terms of localized severity, the pitting corrosion is more severe than the crevice one. The speed of propagation and depth intensity makes the pitting corrosion more severe vis-a-vis the crevice one. However, galvanic corrosion can lead to crevice corrosion (Rashidi et al., 2007, p.215). There are some similarities in how agents of corrosion will work in either crevice or pitting.
Microbial corrosion is a special form of corrosion resulting from activity of micro-organisms and affects both the metallic and non-metallic objects. The chemo-autotrophs are the microbial agents. Stress cracking can take place due to the activities of certain sulfate-reducing bacteria. Biogenic sulfide corrosion causes damage on sewer pipes due to the sulfuric acid produced by Acidithiobacillus present.
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Other microbial like the Ferrobacillus ferrooxidans can directly convert iron into its oxides and hydroxides through the oxidation process. Sulfate-reducing spore-forming bacteria such as the Desulfotomaculum orientis and Desulfotomaculum nigrificans cause corrosion in a reducing environment. Anaerobic corrosion can be sensed through the smell of hydrogen sulfide or the formation of layers of metal sulfides (Odokuma and Ugboma, 2012, p.42). Some of the inhibitors of microbial corrosion that have been used include the benzalkonium chloride formulae, which is common in the oilfields. Bacteria that degrade nylon and plastic is an example of how the microbial corrosion can occur on non-metallic objects.
High-temperature-induced corrosion occurs when materials are exposed to high temperature conditions (Uchida et al. 200, p.257). Metals can undergo corrosion (chemical deterioration) especially when subjected to the extremely high temperature atmosphere in the presence of reactants such as sulfur and oxygen, that catalyze through the oxidizing process. Machinery parts like power generators for car engines have to be reinforced from corrosion due to production of corrosives present during the combustion process.
The case of stress corrosion cracking occurs when cracks appear due to active corrosion agents in the environment. Structure exposed to high temperatures can fail, if metal materials are ductile and are subject to undue tensile stress. Particular chemical environments will allow alloys to undergo stress corrosion cracking (Schneider and Chen, 1998, p.662). Metal parts that have stress corrosion will appear shiny as microscopic cracks multiply. This form of corrosion can go undetected.
Stress corrosion cracking is pronounced among the alloys as compared to pure metals. In the presence of chlorides, alloys such as the austenitic stainless steels and aluminium develop cracks. Boiler cracking (for mild steel) appears when exposed to alkali. Season cracking characterize copper alloys in the presence of ammoniacal solutions. Stress corrosion cracking reduces the durability of objects made of these alloys. The Silver Bridge fell in 1967 because of stress corrosion that concentrated on stress points. Stress corrosion cracking leads to the crevice corrosion.
With regards to safety, concerns on corrosion can be focused based on risks exposed to components of the built structures as well as infrastructures. Such built structures and infrastructures include highway bridges, conveyor pipelines for gasoline and other fluids; railroads; air and marine ports; storage facilities for hazardous substances; sewer systems; industries and factories, inter alia. Farmers in the USA increasingly replace their machinery and equipments due to damage caused by wear and corrosion.
Nace international (n.d) approximates that corrosion cost up to 10 percent of replacement amounting to a countrywide US 100 Billion. In order to comply with the requirements of the Environmental Protection Agency for corrosion control protection, the USA government has to ensure safety for its aboveground and underground storage tanks containing hazardous materials.
In order to avert safety risks from flight failure, the USAF sponsored survey in 1997 that established that cost for prevention and repair of US aircrafts that year, which approximated US$795 million. Wallace et al. (1985) in a handbook manual provides comprehensive causes of the safety risks of corrosion on aircraft. Sarin et al. (2001, p.2962) delves on the impacts of corrosion on iron pipes, while giving insights to the safety risks of some water distribution system. Corrosion on bridges cost the USA government about $ 8.3 billion per year (World Corrosion Organization, 2009, p.32).
Environmental concerns on corrosion failure are widespread and interlaced with those of other sectors. Ambat (2012) reviews corrosion effects on electronics in relation to the environment. France has to spend on protection measures against corrosion for its Eiffel Tower in Paris. The Eiffel Tower is a monument of tourist value (World Corrosion Organization, 2009, p.39).
Health concerns arise when there is corrosion failure of facilities used for amenity and health services. Cases-in-point are running water from taps. In order, to ensure quality of water is not compromised by events of corrosion a lot factors have to be considered when selecting material for use in piping, pumps, fittings, taps and plumbing systems. World Corrosion Organization (2009, p.12) states that the metal ion concentration in the drinking water will depend materials selected. Corrosion failure will happen due to pH of water and the content of metal compounds in the water distribution system, through pitting. Neither metals nor plastic is the panacea to the problem of corrosion in piped water.
In terms of economic cost, the USA incurs a direct cost of $276 billion every year due to metallic corrosion (Nace International, n.d). This cost is equivalent to 3.1 percent of the countries Gross Domestic Product (GDP). The World Corrosion Organization (2009, p.5) estimates that global corrosion costs surpass $US 1.8 trillion, every year.
Materials have been made less affected by the environment agents of corrosion like water and air, through the process of passivation. Passivation provides a material with an outer coating or shield of corrosion, like a micro-coating, which is naturally occurring (Shoffner et al.1996, p.375). Passivation is important in reinforcing and lengthening the aesthetic value of metals. Based on this concept, micro-coats of metal oxides are used to protect materials from further adverse corrosion.
Nevertheless, passivation requires the conservation of certain conditions. Passivation has been used to improve silicon in microelectronics. In summary, passivation protects against deeper corrosion. Even exposed metals can develop shells through passivation that inhibit accelerated rates of corrosion. However, this will vary from one metal to another. Metal elements like Zinc, aluminium and titanium exhibit this. The Pourbaix diagram provides an elaboration of circumstances that allow for passivation to occur.
Problems arising from corrosion can be tackled by continuous innovation of technologies that improve capabilities and reduce limitations of existing technologies. This because corrosion is adverse in almost every sector thus challenge the efficiency of processes used. The problem of corrosion appears to mutate and pose risks with crop of every technology.
Innovative research can focus on providing solutions to corrosion like material stability when in a new environmental setting or material adjustments to the immediate environment. This will provide perspective for corrosion mitigation as well as ensure service integrity. Some of the creative ideas suggested include sensor devices for corrosion on superstructures and infrastructures.
There are no absolute preventive measures for corrosion other than avoid materials prone to corrosion. However, better options can be done by selecting materials that are less affected than others. While selecting the best material, some of the other guiding criteria to use include efficiency and durability. The range options for materials will depend on factors like economic, safety, health and environmental risks exposed to end-users of the final product.
There are no cardinal steps applicable across all industries, however preventive tips are available based on the properties of the material being handled. These tips are as diverse as the materials. Some of popular protective measures taken against corrosion include surface treatment (like applied coating, biofilm coating, reactive coating and anodization); cathodic protection (like the sacrificial anode protection) as well as the controlled permeability formwork.
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