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Corrosion. Haseeb Ullah Khan Jatoi Department of Chemical Engineering University of Engineering and Technology Lahore. Wear Types. Corrosion. Corrosion is the disintegration of a material into its constituent atoms due to chemical reactions with its surroundings . Types of Corrosions
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Corrosion Haseeb Ullah Khan Jatoi Department of Chemical Engineering University of Engineering and Technology Lahore
Corrosion Corrosion is the disintegration of a material into its constituent atoms due to chemical reactions with its surroundings. Types of Corrosions MetallicCorrosion is classified into eight types.
Uniform Corrosion Uniform corrosion is the attack of a metal at essentially the same at all exposed areas of its surface. Uniform attack is a form of electrochemical corrosion that occurs with equivalent intensity over the entire exposed surface and often leaves behind a scale or deposit In a microscopic sense, the oxidation and reduction reactions occur randomly over the surface. Uniform corrosion occurs when there are local anodic and cathodic sites on the surface of the metal. In uniform corrosion, the metal loss occurs at essentially the same rate over the entire metal surface.
Smooth surfaces are usually roughened during uniform corrosion. It can be controlled by cathodic protection or by using paints or coatings.
2. Galvanic corrosion Galvanic corrosion is a localized corrosion mechanism by which metals can be preferentially corroded. Galvanic corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. The more reactive metal will experience corrosion and the less reactive metal, will be protected. When galvanic couple is formed, one of the metal in the couple becomes anode and corrodes faster than it would alone, while other become cathode and corrodes slower than it would alone. Galvanic corrosion can occur in a single metal placed in two different surroundings or electrolytes.
For example, steel screws corrode when in contact with brass in a marine environment, or if copper and steel tubing are joined in a domestic water heater, the steel will corrode in the vicinity of the junction. For galvanic corrosion to occur three conditions must be present. Electrochemically dissimilar metals must be present. These metals must be in electrical contact. The metals must be exposed to an electrolyte.
Preventions of Galvanic Corrosion. 1. If coupling of dissimilar metals is necessary, choose two that are close together in the galvanic series. 2. Electrically insulate dissimilar metals from each other. 3. Cathodic protection.
3. Pitting corrosion It is a form of extremely localized corrosion that leads to the creation of small holes or pits in the metal. Driving power for pitting corrosion is lack of oxygen around a small area. Holes ordinarily penetrate from the top of a horizontal surface downward in a nearly vertical direction. It often goes undetected and with very little material loss until failure occurs, so it is more dangerous. In this type, oxidation occurs within the pit itself, causing reduction at the surface.
It is supposed that gravity causes the pits to grow downward. A pit may be initiated by a localized surface defect such as a scratch or a slight variation in composition. Corrosion products often cover the pits with minimal overall metal loss, can lead to the failure of an entire engineering systems.
Preventions Control the environment to low chloride content(Pitting occurs in stainless steels in neutral or acid solutions containing halides, primarily chlorides (CI– ), for example seawater) and low temperature if possible. Fully understand the environment. Use alloys sufficiently high in chromium, molybdenum and/or nitrogen to ensure resistance. Prepare surfaces to best possible finish. Remove all contaminants, especially free-iron.
4. Crevice Corrosion It is type of corrosion occurring in spaces to which the access of working fluid is limited. These spaces are generally called crevices. It is a localized form of corrosion usually associated with a stagnation solution on the micro- environmental level. Such stagnant micro-environments tend to occur in crevices (shielded area) such as those under gaskets, washers, insulations materials and surface deposits.
Preventions By using welded instead of bolted joints. Using non absorbing gaskets such as Teflon. Removing accumulated deposits frequently. Designing vessels to avoid stagnant areas and ensure complete drainage.
5. Stress Corrosion Cracking Stress corrosion cracking is the intergranular cracking of a material due to the combined action of tensile stress and a corrosive environment. In fact, some materials that are virtually inert in a particular corrosive medium become susceptible to this form of corrosion when a stress is applied. Small cracks form and then propagate in a direction perpendicular to the stress with the result that failure may eventually occur cracks may form at relatively low stress levels, significantly below the tensile strength. Most alloys are susceptible to stress corrosion in specific environments, especially at moderate stress levels.
For example, most stainless steels stress corrode in solutions containing chloride ions, whereas brasses are especially vulnerable (weak) when exposed to ammonia. The stress that produces stress corrosion cracking need not be externally applied; it may be as a results from rapid temperature changes and uneven contraction, or for two-phase alloys in which each phase has a different coefficient of expansion.
Elimination of stress corrosion is done, By lowering the magnitude of the stress. This may be accomplished by reducing the external load or increasing the cross-sectional area perpendicular to the applied stress. Furthermore, an appropriate heat treatment may be used to remove any residual thermal stresses.
6. Erosion Corrosion Erosion–corrosion arises from the combined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion. Virtually all metal alloys, to one degree or another, are susceptible to erosion–corrosion. It is especially harmful to alloys that passivate by forming a protective surface film; the abrasive action may erode away the film, leaving exposed a bare metal surface. Relatively soft metals such as copper and lead are also sensitive to this form of attack.
It can be identified by surface grooves and waves having contours that are characteristic of the flow of the fluid. The nature of the fluid can have influence on the corrosion behavior. Increasing fluid velocity normally enhances the rate of corrosion. Also, a solution is more erosive when bubbles and suspended particulate solids are present. Erosion–corrosion is commonly found in piping, especially at bends, elbows, and abrupt changes in pipe diameter-positions where the fluid changes direction or flow suddenly becomes turbulent. Propellers, turbine blades, valves, and pumps are also susceptible to this form of corrosion.
Preventions Change the design to eliminate fluid turbulence and impingement effects. Furthermore, removal of particulates and bubbles from the solution will lessen its ability to erode. Elbow
7. Intergranular Corrosion Intergranular corrosion occurs preferentially along grain boundaries for some alloys and in specific environments. The net result is that a macroscopic specimen disintegrates along its grain boundaries. This type of corrosion occurs in some stainless steels. When heated to temperatures between 500 and 800C for sufficiently long time periods, these alloys become sensitized to intergranular attack. It is believed that this heat treatment permits the formation of small precipitate particles of chromium carbide (Cr23C6)by reaction between the chromium and carbon in the stainless steel.
Chromium carbide particles that have precipitated along grain boundaries in stainless steel, and the attendant zones of chromium depletion.
Both the chromium and the carbon must diffuse to the grain boundaries to form the precipitates, which leaves a chromium-depleted zone adjacent to the grain boundary. Consequently, this grain boundary region is now highly susceptible to corrosion. Intergranular corrosion is an especially severe problem in the welding of stainless steels, when it is often termed weld decay. Intergranular corrosion of stainless steels near welded areas; caused by chromium carbide precipitation along grain boundaries of alloy subject to prolonged heating in the temperature range 400-850°C.
Preventions Subjecting the sensitized material to a high-temperature heat treatment in which all the chromium carbide particles are re dissolved. Lowering the carbon content below 0.03 wt% C so that carbide formation is minimal. Alloying the stainless steel with another metal such as titanium, which has a greater tendency to form carbides than does chromium so that the Cr remains in solid solution.
8. Selective Leaching Selective leaching (dealloying, demetalification, parting and selective corrosion) is found in solid solution alloys and occurs when one element or constituent is preferentially removed (leached) as a consequence of corrosion processes. The less noble metal is removed from the alloy by microscopic-scale galvanic corrosion mechanism The most common example is the dezincification of brass, in which zinc is selectively leached from a copper–zinc brass alloy. Similar process in other alloy system such as loss of nickel, tin and chromium from copper alloy.
Preventions Use alloys not susceptible to grain boundary depletion, Use a suitable heat treatment, Altering the environment (e.g. lowering oxygen content), Use cathodic protection.
9. Fretting Corrosion It refers to corrosion damage at the asperities (roughness) of the contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration.
Hydrogen Embrittlement • Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and fracture following exposure to hydrogen. Hydrogen embrittlement is often the result of unintentional introduction of hydrogen into susceptible metals during forming or finishing operations and increases cracking in the material.
