Corrosion is a natural process that converts a refined metal into a more chemically-stable form such as oxide , hydroxide , or sulfide. Corrosion engineering is the field dedicated to controlling and preventing corrosion. In the most common use of the word, this means electrochemical oxidation of metal in reaction with an oxidant such as oxygen or sulfates. Rusting , the formation of iron oxides, is a well-known example of electrochemical corrosion.
This type of damage typically produces oxide s or salt s of the original metal and results in a distinctive orange colouration. 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 permeability to liquids and gases.
Many structural alloys corrode merely from exposure to moisture in 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. 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 slower 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.
There are numerous ways to slow or prevent it
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 containing salts contacts pipes or metal structures.
Factors such as relative size of anode , types of metal, and operating conditions temperature , humidity , salinity , etc. The surface area ratio of the anode and cathode directly affects the corrosion rates of the materials. Galvanic corrosion is often prevented by the use of sacrificial anodes. In any given environment one standard medium is aerated, room-temperature seawater , one metal will be either more noble or more active than others, based on how strongly its ions are bound to the surface.
Two metals in electrical contact share the same electrons, so that the "tug-of-war" at each surface is analogous to competition for free electrons between the two materials. Using the electrolyte as a host for the flow of ions in the same direction, the noble metal will take electrons from the active one. The resulting mass flow or electric current can be measured to establish a hierarchy of materials in the medium of interest.
This hierarchy is called a galvanic series and is useful in predicting and understanding corrosion. Often it is possible to chemically remove the products of corrosion. 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 electropolishing , which removes some layers of the underlying metal to make a smooth surface. For example, phosphoric acid may also be used to electropolish copper but it does this by removing copper, not the products of copper corrosion. Some metals are more intrinsically resistant to corrosion than others for some examples, see galvanic series.
There are various ways of protecting metals from corrosion oxidation including painting, hot dip galvanizing , cathodic protection, and combinations of these. The materials most resistant to corrosion are those for which corrosion is thermodynamically unfavorable. Any corrosion products of gold or platinum tend to decompose spontaneously into pure metal, which is why these elements can be found in metallic form on Earth and have long been valued.
More common "base" metals can only be protected by more temporary means.
Some metals have naturally slow reaction kinetics , even though their corrosion is thermodynamically favorable. These include such metals as zinc , magnesium , and cadmium. While corrosion of these metals is continuous and ongoing, it happens at an acceptably slow rate.
An extreme example is graphite , which releases large amounts of energy upon oxidation , but has such slow kinetics that it is effectively immune to electrochemical corrosion under normal conditions. Passivation refers to the spontaneous formation of an ultrathin film of corrosion products, known as a passive film, on the metal's surface that act as a barrier to further oxidation.
The chemical composition and microstructure of a passive film are different from the underlying metal. Typical passive film thickness on aluminium, stainless steels, and alloys is within 10 nanometers. The passive film is different from oxide layers that are formed upon heating and are in the micrometer thickness range — the passive film recovers if removed or damaged whereas the oxide layer does not.
Passivation in natural environments such as air, water and soil at moderate pH is seen in such materials as aluminium , stainless steel , titanium , and silicon. Passivation is primarily determined by metallurgical and environmental factors. The effect of pH is summarized using Pourbaix diagrams , but many other factors are influential. Some conditions that inhibit passivation include high pH for aluminium and zinc, low pH or the presence of chloride ions for stainless steel, high temperature for titanium in which case the oxide dissolves into the metal, rather than the electrolyte and fluoride ions for silicon.
On the other hand, unusual conditions may result in passivation of materials that are normally unprotected, as the alkaline environment of concrete does for steel rebar. Exposure to a liquid metal such as mercury or hot solder can often circumvent passivation mechanisms. 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.
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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 passive film due to chemical or mechanical factors, the resulting major modes of corrosion may include pitting corrosion , crevice corrosion , and stress corrosion cracking.
Certain conditions, such as low concentrations of oxygen or high concentrations of species such as chloride which compete 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 autocatalytic 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. Pitting results when a small hole, or cavity, forms in the metal, usually as a result of de-passivation of a small area.
This area becomes anodic, while part of the remaining metal becomes cathodic, producing a localized galvanic reaction. The deterioration of this small area penetrates the metal and can lead to failure.
This form of corrosion is often difficult to detect due to the fact that it is usually relatively small and may be covered and hidden by corrosion-produced compounds.
Stainless steel can pose special corrosion challenges, since its passivating behavior relies on the presence of a major alloying component chromium , at least Because of the elevated temperatures of welding and heat treatment, chromium carbides can form in the grain boundaries of stainless alloys.
corrosive damage in metals and its prevention.ppt
This chemical reaction robs the material of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a galvanic couple with the well-protected alloy nearby, which leads to "weld decay" corrosion of the grain boundaries in the heat affected zones in highly corrosive environments. This process can seriously reduce the mechanical strength of welded joints over time. A stainless steel is said to be "sensitized" if chromium carbides are formed in the microstructure.
A typical microstructure of a normalized type stainless steel shows no signs of sensitization, while a heavily sensitized steel shows the presence of grain boundary precipitates. The dark lines in the sensitized microstructure are networks of chromium carbides formed along the grain boundaries. Special alloys, either with low carbon content or with added carbon " getters " such as titanium and niobium in types and , respectively , can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of "knifeline attack".
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As its name implies, corrosion is limited to a very narrow zone adjacent to the weld, often only a few micrometers across, making it even less noticeable. 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.
Formation of a differential aeration cell leads to 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.
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 is commonly used to rank a material's resistance to crevice corrosion.
In the chemical industry , hydrogen grooving is the corrosion of piping by grooves created by the interaction of a corrosive agent, corroded pipe constituents, and hydrogen gas bubbles.
The iron sulfate coating will protect the steel from further reaction; however, if hydrogen bubbles contact this coating, it will be removed. Thus, a groove will be formed by a traveling bubble, exposing more steel to the acid: a vicious cycle.
The grooving is exacerbated by the tendency of subsequent bubbles to follow the same path. High-temperature corrosion is chemical deterioration of a material typically a metal as a result of heating. This non-galvanic form of corrosion can occur when a metal is subjected to a hot atmosphere containing oxygen, sulfur, or other compounds capable of oxidizing or assisting the oxidation of the material concerned.
For example, materials used in aerospace, power generation and even in car engines have to resist sustained periods at high temperature in which they may be exposed to an atmosphere containing potentially highly corrosive products of combustion.
The products of high-temperature corrosion can potentially be turned to the advantage of the engineer. The formation of oxides on stainless steels, for example, can provide a protective layer preventing further atmospheric attack, allowing for a material to be used for sustained periods at both room and high temperatures in hostile conditions.
Such high-temperature corrosion products, in the form of compacted oxide layer glazes , prevent or reduce wear during high-temperature sliding contact of metallic or metallic and ceramic surfaces. Thermal oxidation is also commonly used as a route towards the obtainment of controlled oxide nanostructures, including nanowires and thin films. 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. Accelerated low-water corrosion ALWC is a particularly aggressive form of MIC that affects steel piles in seawater near the low water tide mark.
It is characterized by an orange sludge, which smells of hydrogen sulfide when treated with acid.
Corrosion rates can be very high and design corrosion allowances can soon be exceeded leading to premature failure of the steel pile. For unprotected piles, sacrificial anodes can be installed locally to the affected areas to inhibit the corrosion or a complete retrofitted sacrificial anode system can be installed.