ppt on corrosion

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Corrosion is the gradual destruction of material, usually metals, by chemical reaction with its environment. In the most common use of the word, this means electro-chemical oxidation of metals in reaction with an oxidant such as oxygen. rusting, the formation of iron oxides is a well-known example of electrochemical corrosion. This type of damage typically produces oxides or salts of the original metal. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term degradation is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and ability to contain a vessel's contents.

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Many structural alloys corrode merely from exposure to moisture in the air , but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate-conversion , can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable.

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GALVANIC CORROSION Galvanic corrosion occurs when two different metals have physical or electrical contact with each other and are immersed in a common electrolyte , or when the same metal is exposed to electrolyte with different concentrations. In a galvanic couple , the more active metal (the anode) corrodes at an accelerated rate and the more noble metal (the cathode) corrodes at a retarded (؟) rate. When immersed separately, each metal corrodes at its own rate. What type of metal(s) to use is readily determined by following the galvanic series . For example, zinc is often used as a sacrificial anode for steel structures. Galvanic corrosion is of major interest to the marine industry and also anywhere water (via impurities such as salt ) contacts pipes or metal structures.

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Galvanic corrosion of aluminum In this photo, a 67 cm thick aluminum alloy plate is physically (and hence, electrically) connected to a 10-mm thick mild steel structural support. Galvanic corrosion occurred on the aluminum plate along the joint with the mild steel. Perforation of aluminum plate occurred within 2 years due to the large acceleration factor in galvanic corrosion. [1] Factors such as relative size of anode , types of metal, and operating conditions ( temperature , humidity , salinity , etc.) affect galvanic corrosion. The surface area ratio of the anode and cathode directly affects the corrosion rates of the materials. Galvanic corrosion is often utilized in sacrificial anodes .

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Corrosion removal Often it is possible to chemically remove the products of corrosion to give a clean surface, but one that may exhibit artifacts of corrosion such as pitting. For example phosphoric acid in the form of naval jelly is often applied to ferrous tools or surfaces to remove rust. Corrosion removal should not be confused with Electro polishing which removes some layers of the underlying metal to make a smooth surface. For example phosphoric acid (again) may be used to electro polish copper but it does this by removing copper, not the products of copper corrosion.

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Corrosion in passivated materials Passivation is extremely useful in mitigating corrosion damage, however even a high-quality alloy will corrode if its ability to form a passivating film is hindered. Proper selection of the right grade of material for the specific environment is important for the long-lasting performance of this group of materials. If breakdown occurs in the massive film due to chemical or mechanical factors, the resulting major modes of corrosion may include pitting corrosion , crevice corrosion and stress corrosion cracking . Pitting corrosion Certain conditions, such as low concentrations of oxygen or high concentrations of species such as chloride which complete as anions , can interfere with a given alloy's ability to re-form a passivating film. In the worst case, almost all of the surface will remain protected, but tiny local fluctuations will degrade the oxide film in a few critical points.

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Corrosion at these points will be greatly amplified, and can cause corrosion pits of several types, depending upon conditions. While the corrosion pits only nucleate under fairly extreme circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is naturally deprived of oxygen and locally the pH decreases to very low values and the corrosion rate increases due to an auto-catalytic process. In extreme cases, the sharp tips of extremely long and narrow corrosion pits can cause stress concentration to the point that otherwise tough alloys can shatter; a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure fails . Pitting remains among the most common and damaging forms of corrosion in passivated alloys [ citation needed ] , but it can be prevented by control of the alloy's environment.

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Crevice corrosion Crevice corrosion is a localized form of corrosion occurring in confined spaces (crevices) to which the access of the working fluid from the environment is limited and a differential aeration cell is set up, leading to the active corrosion inside the crevices. Examples of crevices are gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles. This photo shows that corrosion occurred in the crevice between the tube and tube sheet (both made of type 316 stainless steel) of a heat exchanger in a sea water desalination plant. Crevice corrosion is influenced by the crevice type (metal-metal, metal-nonmetal), crevice geometry (size, surface finish), and metallurgical and environmental factors. The susceptibility to crevice corrosion can be evaluated with ASTM standard procedures. A critical crevice corrosion temperature (CCT) is commonly used to rank a material's resistance to crevice corrosion.



Microbial corrosion :

Microbial corrosion Microbial corrosion , or commonly known as microbiologically influenced corrosion (MIC), is a corrosion caused or promoted by microorganisms , usually chemoautotrophs . It can apply to both metallic and non-metallic materials, in the presence or absence of oxygen. Sulfate-reducing bacteria are active in the absence of oxygen (anaerobic); they produce hydrogen sulfide , causing sulfide stress cracking . In the presence of oxygen (aerobic), some bacteria may directly oxidize iron to iron oxides and hydroxides, other bacteria oxidize sulfur and produce sulfuric acid causing biogenic sulfide corrosion . Concentration cells can form in the deposits of corrosion products, leading to localized corrosion.

Methods of protection from corrosion :

Methods of protection from corrosion Surface treatments Applied coatings Galvanized surface Plating , painting , and the application of enamel are the most common anti-corrosion treatments. They work by providing a barrier of corrosion-resistant material between the damaging environment and the structural material. Aside from cosmetic and manufacturing issues, there are tradeoffs in mechanical flexibility versus resistance to abrasion and high temperature. Plantings usually fail only in small sections, and if the plating is more noble than the substrate (for example, chromium on steel), a galvanic couple will cause any exposed area to corrode much more rapidly than an unplanted surface would. For this reason, it is often wise to plate with active metal such as zinc or cadmium. Painting either by roller or brush is more desirable for tight spaces; spray would be better for larger coating areas such as steel decks and waterfront applications

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Flexible polyurethane coatings, like Durabak-M26 for example, can provide an anti-corrosive seal with a highly durable slip resistant membrane. Painted coatings are relatively easy to apply and have fast drying times although temperature and humidity may cause dry times to vary. Reactive coatings If the environment is controlled (especially in recirculation systems), corrosion inhibitors can often be added to it. These form an electrically insulating or chemically impermeable coating on exposed metal surfaces, to suppress electrochemical reactions. Such methods obviously make the system less sensitive to scratches or defects in the coating, since extra inhibitors can be made available wherever metal becomes exposed.

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Anodization This figure-8 descended is anodized with a yellow finish. Climbing equipment is available in a wide range of anodized colors. Aluminum alloys often undergo a surface treatment. Electrochemical conditions in the bath are carefully adjusted so that uniform pores several nanometers wide appear in the metal's oxide film. These pores allow the oxide to grow much thicker than passivating conditions would allow. At the end of the treatment, the pores are allowed to seal, forming a harder-than-usual surface layer. If this coating is scratched, normal passivation processes take over to protect the damaged area. Anodizing is very resilient to weathering and corrosion, so it is commonly used for building facades and other areas that the surface will come into regular contact with the elements. Whilst being resilient, it must be cleaned frequently. If left without cleaning, panel edge staining will naturally occur.

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Biofilm coatings A new form of protection has been developed by applying certain species of bacterial films to the surface of metals in highly corrosive environments. This process increases the corrosion resistance substantially. Alternatively, antimicrobial-producing biofilms can be used to inhibit mild steel corrosion from sulfate-reducing bacteria. Controlled permeability formwork Controlled permeability formwork (CPF) is a method of preventing the corrosion of reinforcement by naturally enhancing the durability of the cover during concrete placement. CPF has been used in environments to combat the effects of carbonation, chlorides, frost and abrasion.



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