What environment. Corrosion creates a natural voltaic cell
What is corrosion?Corrosion is the process through which metals in manufactured states deteriorate to their natural oxidation states as a result of reactions between the metals and their surrounding environment. Corrosion creates a natural voltaic cell in which the metal acts as an anode through the addition of moisture. This process often results in loss of the metal’s functional stability. The rate at which the metal in question corrodes is based on both the type of metal involved in the reaction and the metal’s environmental conditions.Corrosion consists of four different categories:General Attack Corrosion:This is a common type of corrosion during which the entire surface of the metal is attacked. This form of corrosion happens often, however, it is predictable and thus possible to plan for in advance.Localized Corrosion:This form of corrosion, as opposed to ‘general attack’, attacks only portions of a metal’s surface. Within this category of corrosion there are three subcategories:Pitting – creates small holes in the surface of the metalCrevice corrosion – occurs in stagnant locations of the metalFiliform corrosion – occurs when water disrupts a corrosion-preventive coatingGalvanic Corrosion:This type of corrosion occurs when two different metals are together in a liquid electrolyte. During this process, one metal’s molecules are drawn to those of the other metal, leading to corrosion in only one of them (the stronger reducing agent).Environmental Cracking:During stressful environmental conditions, some metals crack or weaken, becoming brittle. Which metals are prone to corrosion?All metals can corrode. However, metals differ in their corrosion potential. Some, like pure iron, corrode very quickly in the proper conditions, while others, like the Noble Metals (which includes copper, palladium, silver, platinum, and gold) are much less reactive; thus they corrode rarely but are consequently more precious and therefore expensive. Alloys -the combination of two or more metals- are often manufactured to be stronger than pure metal and to hinder corrosion, which is why alloys are used more frequently in structures.What electrochemical processes take place during corrosion?Similarly to a battery, metals corrode through a series of oxidation-reduction reactions. When in contact with an electrolyte (water), a natural voltaic cell is created in which the metal behaves as an anode. To begin the process, the metal is oxidized, forming a metal ion (cation) and free electrons. Iron (Fe) is used as an example:Fe(s) ? Fe2+(aq) + 2e-The free electrons then move through the metal to the outside of the droplet of water, and reduce the oxygen, forming hydroxide:O2(g) + 2H2O(l) + 4e- ? 4OH-(aq)Within the droplet, the hydroxide ions can react with the metal ions, precipitating on the surface of the metal:Fe2+(aq) + 2OH?(aq) ? Fe(OH)2(s)Rust is often produced as a result of the oxidation of the precipitate:4Fe(OH)2(s) + O2(g) ? 2Fe2O3 •H2O(s) + 2H2O(l)Under what conditions is the process of corrosion promoted?There are three main components necessary for corrosion to occur:Metal:Where the metal falls in the Galvanic Series determines how likely that metal is to corrode. This means that it is less likely for a metal near the top of the series to corrode, especially when in contact with a metal near the bottom: in this case the resistance of the higher metal will be reinforced while the lower metal will corrode more (as in Sacrificial Anodes).Stressed areas of the metal’s surface will corrode more quickly because they are more active than the unstrained metal.Oxygen:Environmental factors such as oxygen concentration and temperature contribute to a metal’s likelihood of corroding.Electrolyte:Exposure to moisture, especially salt water, hastens the corrosion of metal.Other factors also accelerate the rate at which a metal will corrode. These include:pH;salt concentration;and velocity of waterWhy is corrosion a problem?Corrosion is a natural process, however, it becomes a problem in industry when the metal used in a particular product corrodes. The process causes the deterioration of a number of different metal products such as machinery, generally damaging their framework and ultimately rendering them useless. Allowing this process to occur is economically inefficient, can inhibit productivity, and may even be dangerous. In itself, corrosion is already problematic, but this issue is amplified when the metal is used in a product that forces it to undergo great amounts of stress. The corrosion of the steel reinforcing bar in a block of concrete for example, is unseen and therefore overlooked; until it gives out. The resulting damage can include: failure of sections of highways, collapse of electrical towers, and damage to buildings, parking structures, and bridges. This damage generates heavy repair costs and makes the surrounding areas unsafe to the public. Power plants are especially prone to corrosion (because of their water-steam circuits — the metal components are constantly in contact with water) so when it occurs, it is harmful to not only workers, but surrounding homes and civilians of the plant. An event of this caliber may be so dangerous that the plants are forced to shut down.What are the costs and consequences of corrosion?In 2016, the World Corrosion Organization estimated that the global cost of corrosion was about CAD$ 3.11 trillion annually: the equivalent of 3.4% of the global GDP (Gross Domestic Product). An earlier 2003 study estimated that the annual impact of corrosion on the Canadian economy cost about $46.4 billion. This is roughly 2.5 % of Canada’s GDP. To add to it, NACE International alleged that almost 50% of those costs could have been avoided using proper corrosion prevention techniques. Manufacturing and buying corrosion-preventive metals or materials can be costly and seem like a burden, but allowing corrosion to occur can damage metal products, which is even more economically straining and can put lives in danger: Insufficient corrosion prevention at a thermal power plant could result in fatal repercussions if a worker is exposed to steam, gas, ash, or radioactive materials from a leaky component.There are many negative consequences of corrosion in general, but are there any positive effects of this process? Ironically, what good does come from corrosion is in fact a method to prevent further corrosion: the attachment of a highly active metal to a less active metal is often used to protect the it from corroding by letting the active metal corrode in its place and form a protective layer (hence “sacrificial anode”).How does corrosion affect the environment?Corrosion does not directly affect the environment, but some prevention methods have been harmful. A handful of different Canadian government regulations were issued in the late 1900s indirectly prohibiting the use certain methods of corrosion protection because of the impact they had on the environment. Lead-based paints on houses and bridges, chromate inhibiting paints on aircraft, and oil-based paints throughout industry were restricted in attempt to reduce smog from the atmosphere.What corrosion-prevention methods exist?To prevent corrosion, one must think ahead. Corrosion protection must then begin in the design stage of construction. A good understanding of environmental conditions, metal properties, and their chemical interactions is necessary to construct a strong, stable, rust-free metallic structure. Engineers work together with metallurgical experts to select the proper metals/alloys for any situation. After that, special prevention techniques are used to protect the chosen metals:Painting: Coating the metal with a protective layer forms a barrier between the metal and the moisture that would corrode it. The downside to this method is that if at any location on the metal’s surface the protective layer is breached, that pinhole can become an immediate corrosion site, eventually spreading to the rest of the metal’s surface area.This type of protection can be used for structures and bridges.Plating: Plating is a more heavy-duty and long-lasting version of painting. In this method, a thin layer of metal, such as tin, nickel, or chromium is deposited onto the surface of the substrate metal using a number of different techniques including:Electroplating: (electrolytic bath) Ex. Ag+(aq) + e- ? Ag(s)Mechanical plating: (welding – metal powder + aqueous solution)Electroless: (chemical reaction)E.g. Zinc metal is immersed in copper(II) sulfate solution; zinc atoms dissolve and are spontaneously replaced by copper atoms from solution:Oxidation: Zn(s) ? Zn+2 + 2e- E°(anode) = 0.76VReduction: Cu+2(aq) + 2e- ? Cu(s) E°(cathode) = 0.34VOverall reaction: Zn(s) + Cu+2(aq) ? Zn+2(aq) + Cu(s) E°(cell) = 1.1V Hot dipping: (molten bath of coating metal; used for fittings, fasteners, small items)Galvanization: an oxide-carbonate coating formed using zinc: Fe+(aq) – 2e- ? Fe(s) E°r = -0.44V Zn2+(aq) + 2e- ? Zn(s) E°r = -0.76VBecause zinc is a stronger reducing agent, it will be the metal that is oxidized, producing a coating that clings to its substrate metal, and, as opposed to the painting method, will continue to protect the metal despite a scratch on the surface.Alloying: The combination of two or more metals that form their own oxide coatings is also used to prevent corrosion.This type of protection is used in a variety of products including appliances and surgical implants.Cathodic Protection: Cathodic protection is the most common form of prevention for steel in buried fuel tanks and pipelines. The idea is that the metal is protected by electrons, causing the metal to become the cathode of the cell. There are two ways to approach this method:Sacrificial Anodes: by attaching a more active metal (and better reducing agent) to the metal being protected, the electrons of the voltaic cell will be supplied by the sacrificial anode and therefore keep the substrate metal from being oxidized (corroding). Common metals used as sacrificial anodes are: magnesium (used for onshore pipelines because of higher resistance to electrolytes), zinc and aluminum (used on structures generally present in salt-water conditions, such as hulls of ships, boat propellers, and production platforms because of lower electrolyte resistivity) Ex. If zinc is used as a sacrificial anode for copperOxidation: Zn(s) ? Zn2+(aq) + 2e- Reduction: Cu2+(aq) + 2e- ? Cu(s)Impressed current: by attaching the metal to the negative terminal of a DC power supply and an inert electrode to the positive terminal, the electrons will be continuously pumped to the metal being protected, thus rendering it unable to be oxidized. This cathodic protection approach is often used when structures, such as oil/gas pipelines, are too large to use the sacrificial anode method cost-efficiently.Corrosion: SummaryThere are multiple forms of corrosion, but its purpose is always the same: to oxidize a manufactured metal back to its natural state using electrochemical equations. Any metal can corrode, however, some metals are more prone to the process than others. A metal’s likelihood of reacting can be determined using its placement on the Galvanic Series (its electrode potential) – the higher the metal, the less likely it is to corrode. A metal is also less likely to corrode in a drier environment, as moisture is what causes the metal to become an anode in the natural voltaic cell created. Although this process is natural, it poses a problem in our industry because so many of our structures are made of metal. Corrosion of these products can create a dangerous environment for the public as well as cost millions in repairs. To solve this, a number of corrosion prevention methods have been devised. Whether blocking moisture from reaching the metal at all, or stopping the electrochemical equation responsible for the oxidation from taking place, these methods have proven effective for (sometimes) centuries, and have been able to protect the destruction of our structures.