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It is generally desirable to reduce the fluid velocity and promote laminar flow; increased pipe diameters are useful in this context. Rough surfaces are generally undesirable. Designs creating turbulence, flow restrictions, and obstructions are undesirable. Abrupt changes in flow direction should be avoided. Tank inlet pipes should be directed away from the tank walls, towards the center. Welded and flanged pipe sections should always be carefully aligned.
Impingement plates of baffles designed to bear the brunt of the damage should be easily replaceable. The thickness of vulnerable areas should be increased. Replaceable ferrules, with a tapered end, can be inserted into the inlet side of heat exchanger tubes to prevent damage to the actual tubes. Several environmental modifications can be implemented to minimize the risk of erosion corrosion. Abrasive particles in fluids can be removed by filtration or settling, while water traps can be used in steam and compressed air systems to decrease the risk of impingement by droplets.
De-aeration and corrosion inhibitors are additional measures that can be taken. Cathodic protection and the application of protective coating may also reduce the rate of attack. Exfoliation corrosion is a particular form of intergranular corrosion associated with high-strength aluminum alloys. Alloys that have been extruded or otherwise worked heavily, with a microstructure of elongated, flattened grains, are particularly prone to this damage.
Corrosion products building up along these grain boundaries exert pressure between the grains and the end result is a lifting or leafing effect. The damage often initiates at end grains encountered in machined edges, holes, or grooves and can subsequently progress through an entire section. The structural integrity of this part disappeared long ago. Fillform is a special form of crevice corrosion in which the aggressive chemistry build-up occurs under a protective film that has been breached.
Filiform corrosion normally starts at small, sometimes microscopic, defects in the coating. Lacquers and "quick-dry" paints are most susceptible to the problem. Their use should be avoided unless absence of an adverse effect has been proven by field experience.
Where a coating is required, it should exhibit low water vapor transmission characteristics and excellent adhesion. Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quality. Fretting corrosion refers to corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration.
Pits or grooves and oxide debris characterize this damage, typically found in machinery, bolted assemblies, and ball or roller bearings. Contact surfaces exposed to vibration during transportation are exposed to the risk of fretting corrosion.
Damage, as seen in this image Courtesy Mike Dahlager , can occur at the interface of two highly loaded surfaces, which are not designed to move against each other. The most common type of fretting is caused by vibration. The protective film on the metal surfaces is removed by the rubbing action and exposes fresh, active metal to the corrosive action of the atmosphere.
Galvanic corrosion also called ' dissimilar metal corrosion' or wrongly 'electrolysis' refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. It occurs when two or more dissimilar metals are brought into electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone.
Either or both metal in the couple may or may not corrode by itself themselves. When contact with a dissimilar metal is made, however, the self corrosion rates will change: corrosion of the anode will accelerate corrosion of the cathode will decelerate or even stop.
Galvanic coupling is the foundation of many corrosion monitoring techniques. The driving force for corrosion is a potential difference between the different materials. The bimetallic driving force was discovered in the late part of the eighteenth century by Luigi Galvani in a series of experiments with the exposed muscles and nerves of a frog that contracted when connected to a bimetallic conductor.
The principle was later put into a practical application by Alessandro Volta who built, in , the first electrical cell, or battery: a series of metal disks of two kinds, separated by cardboard disks soaked with acid or salt solutions. This is the basis of all modern wet-cell batteries and it was a tremendously important scientific discovery because it was the first method found for the generation of a sustained electrical current.
The principle was also engineered into the useful protection of metallic structures by Sir Humphry Davy and Michael Faraday in the early part of the nineteenth century. The sacrificial corrosion of one metal such as zinc, magnesium, or aluminum is a widespread method of cathodically protecting metallic structures. In a bimetallic couple, the less noble material will become the anode of this corrosion cell and tend to corrode at an accelerated rate, compared with the uncoupled condition.
The more noble material will act as the cathode in the corrosion cell. Galvanic corrosion can be one of the most common forms of corrosion as well as one of the most destructive. The relative nobility of a material can be predicted by measuring its corrosion potential.
The well-known galvanic series lists the relative nobility of certain materials in sea water. In this case, the galvanic current is concentrated onto a small anodic area. Rapid thickness loss of the dissolving anode tends to occur under these conditions. Galvanic corrosion problems should be solved by designing to avoid these problems in the first place.
Galvanic corrosion cells can be set up on the macroscopic level or on the microscopic level. On the microstructural level, different phases or other microstructural features can be subject to galvanic currents. This type of deterioration can be linked to corrosion and corrosion-control processes. It involves the ingress of hydrogen into a component, an event that can seriously reduce the ductility and load-bearing capacity as well as cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials.
Hydrogen embrittlement occurs in a number of forms but the common features are an applied tensile stress and hydrogen dissolved in the metal. Examples of hydrogen embrittlement are cracking of weldments or hardened steels when exposed to conditions that inject hydrogen into the component. Presently, this phenomenon is not completely understood and hydrogen embrittlement detection, in particular, seems to be one of the most difficult aspects of the problem.
Hydrogen embrittlement does not affect all metallic materials equally. The most vulnerable are high-strength steels, titanium alloys, and aluminum alloys. Sources of hydrogen causing embrittlement have been encountered in the making of steel, in processing parts, in welding, in storage or containment of hydrogen gas, and related to hydrogen as a contaminant in the environment that is often a by-product of general corrosion. It is the latter that concerns the nuclear industry.
Hydrogen may be produced by corrosion reactions such as rusting, cathodic protection , and electroplating. Hydrogen may also be added to reactor coolant to remove oxygen from reactor coolant systems. Hydrogen entry, the obvious pre-requisite of embrittlement, can be facilitated in a number of ways summarized below: Defence Standard , October Hydrogen diffuses along the grain boundaries and combines with the carbon, which is alloyed with the iron, to form methane gas.
The methane gas is not mobile and collects in small voids along the grain boundaries where it builds up enormous pressures that initiate cracks. Hydrogen embrittlement is a primary reason that the reactor coolant is maintained at a neutral or basic pH in plants without aluminum components.
If the metal is under a high tensile stress, brittle failure can occur. At normal room temperatures, the hydrogen atoms are absorbed into the metal lattice and diffused through the grains, tending to gather at inclusions or other lattice defects. If stress induces cracking under these conditions, the path is transgranular.
At high temperatures, the absorbed hydrogen tends to gather in the grain boundaries and stress-induced cracking is then intergranular. The cracking of martensitic and precipitation hardened steel alloys is believed to be a form of hydrogen stress corrosion cracking that results from the entry into the metal of a portion of the atomic hydrogen that is produced in the following corrosion reaction.
Hydrogen embrittlement is not a permanent condition. If cracking does not occur and the environmental conditions are changed so that no hydrogen is generated on the surface of the metal, the hydrogen can rediffuse from the steel, so that ductility is restored.
To address the problem of hydrogen embrittlement, emphasis is placed on controlling the amount of residual hydrogen in steel, controlling the amount of hydrogen pickup in processing, developing alloys with improved resistance to hydrogen embrittlement, developing low or no embrittlement plating or coating processes, and restricting the amount of in-situ in position hydrogen introduced during the service life of a part.
The microstructure of metals and alloys is made up of grains, separated by grain boundaries. Intergranular corrosion is localized attack along the grain boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected. This form of corrosion is usually associated with chemical segregation effects impurities have a tendency to be enriched at grain boundaries or specific phases precipitated on the grain boundaries.
Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity. The attack is usually related to the segregation of specific elements or the formation of a compound in the boundary. Corrosion then occurs by preferential attack on the grain-boundary phase, or in a zone adjacent to it that has lost an element necessary for adequate corrosion resistance - thus making the grain boundary zone anodic relative to the remainder of the surface. The attack usually progresses along a narrow path along the grain boundary and, in a severe case of grain-boundary corrosion, entire grains may be dislodged due to complete deterioration of their boundaries.
In any case, the mechanical properties of the structure will be seriously affected. A classic example is the sensitization of stainless steels or weld decay. Chromium-rich grain boundary precipitates lead to a local depletion of Cr immediately adjacent to these precipitates, leaving these areas vulnerable to corrosive attack in certain electrolytes.
Reheating a welded component during multi-pass welding is a common cause of this problem. In austenitic stainless steels, titanium, or niobium can react with carbon to form carbides in the heat affected zone HAZ causing a specific type of intergranular corrosion known as knife-line attack. These carbides build up next to the weld bead where they cannot diffuse due to rapid cooling of the weld metal. The problem of knife-line attack can be corrected by reheating the welded metal to allow diffusion to occur.
Exfoliation corrosion is a further form of intergranular corrosion associated with high strength aluminum alloys. The damage often initiates at end grains encountered in machined edges, holes or grooves and can subsequently progress through an entire section.
Recognized authority on all things materials vs. Corrosion that proceeds laterally from the sites of initiation along planes parallel to the surface, generally at grain boundaries, forming corrosion products that force metal way from the body of the material, giving it a layered appearance. It also indicates that it is synonymous to 'lamellar corrosion'.
The Encyclopedia Britannica indicates that this term suggests a composition or arrangement in the form of a thin, flat layer or scale. Nothing in the Glossary restricts the term to aluminum. Under Materials Selection, p. Don Sprowls discusses evaluation of exfoliation on pp. Under Temper Effects, p. In that version, no mention was made of exfoliation. Dale McIntyre updated this in as Volume 1 and 2.
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Start Now. Corrosion is a type of Electrochemical process. Corrosion is a natural process that converts a refined metal into a more chemically-stable form such as oxide, hydroxide, or sulfide. Electrochemical oxidation of metal in reaction with an oxidant such as oxygen or sulfates. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.
Electrochemical reaction, any process either caused or accompanied by the passage of an electric current and involving in most cases the transfer of electrons between two substances, one a solid and the other a liquid. Get Started for Free Download App. More Chemistry Questions Q1. Which of the following pair is correct? Isotopes - different mass number II.
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