Module 5.1 Corrosion Prevention and Control PDF

M E 4 1 3 : 5 M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Chapter Corrosion Prevention and Control Inte...

M E 4 1 3 : 5 M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Chapter Corrosion Prevention and Control Intended Learning Outcomes After studying this chapter, you should be able to do the following: 1. Differentiate the different types of corrosion. 2. Identify the cause of corrosion. 3. Know how to prevent and control the corrosion in a material. CORROSION Corrosion is defined as the destructive and unintentional attack on a metal; it is electrochemical and ordinarily begins at the sur face. The problem of metallic corrosion is significant; in economic terms, it has been estimated that approximately 5% of an industrialized nation’s income is spent on corrosion prevention and the maintenance or replacement of products lost or contaminated as a result of corrosion reactions. The consequences of corrosion are all too common. Familiar examples include the rusting of automotive body panels and radiator and exhaust components. Corrosion is a dangerous and extremely costly problem. Because of it, buildings and bridges can collapse, oil pipelines break, chemical plants leak, and bathrooms flood. Corroded electrical contacts can cause fires and other problems, corroded medical implants may lead to blood poisoning, and air pollution has caused corrosion damage to works of art around the world. Corrosion threatens the safe disposal of radioactive waste that must be stored in containers for tens of thousands of years. FORMS OF CORROSION It is convenient to classify corrosion according to the manner in which it is manifest. Metallic corrosion is sometimes classified into eight forms: uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosion-corrosion, and stress corrosion. The causes and means of prevention of each of these forms are discussed briefly. UNIFORM ATTACK 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. Familiar examples include general rusting of steel and iron and the tarnishing of silverware. This is probably the most common form of corrosion. It is also the least objectionable because it can be predicted and designed for with relative ease. Module No. 5 – Corrosion Prevention and Control 1 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Uniform Attack Corrosion Source: https://www.nitty-gritty.it/en/morphology-of-corrosion/ How to prevent uniform corrosion? Uniform corrosion or general corrosion can be prevented through a number of methods: Use thicker materials for corrosion allowance Use paints or metallic coatings such as plating, galvanizing or anodizing Use Corrosion inhibitors or modifying the environment Cathodic protection (Sacrificial Anode or Impressed Current -ICCP) and Anodic Protection GALVANIC CORROSION Galvanic corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. The less noble or more reactive metal in the particular environment experiences corrosion; the more inert metal, the cathode, is protected from corrosion. As examples, steel screws corrode when in contact with brass in a marine environment, and if copper and steel tubing are joined in a domestic water heater, the steel corrodes in the vicinity of the junction. Sample of Galvanic Corrosion Source: https://www.nace.org/resources/general-resources/corrosion-basics/group-1/galvanic- corrosion Galvanic Series This represents the relative reactivities of a number of metals and commercial alloys in seawater. The alloys near the top are cathodic and unreactive, whereas those at the bottom are most anodic; no voltages are provided. Comparison of the standard emf and the galvanic series reveals a high degree of correspondence between the relative positions of the pure base Module No. 5 – Corrosion Prevention and Control 2 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G metals. Most metals and alloys are subject to oxidation or corrosion to one degree or another in a wide variety of environments—that is, they are more stable in an ionic state than as metals. Galvanic Series How to prevent galvanic corrosion? Galvanic corrosion can be prevented through a number of methods: Select metals/alloys as close together as possible in the galvanic series. Avoid unfavorable area effect of a small anode and large cathode. Insulate dissimilar metals wherever practical Apply coatings with caution. Paint the cathode (or both) and keep the coatings in good repair on the anode. Avoid threaded joints for materials far apart in the galvanic series. CREVICE CORROSION Electrochemical corrosion may also occur as a consequence of concentration differences of ions or dissolved gases in the electrolyte solution and between two regions of the same metal piece. For such a concentration cell, corrosion occurs in the locale that has the lower concentration. A good example of this type of corrosion occurs in crevices and recesses or under Module No. 5 – Corrosion Prevention and Control 3 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G deposits of dirt or corrosion products where the solution becomes stagnant and there is localized depletion of dissolved oxygen. Corrosion preferentially occurring at these positions is called crevice corrosion. The crevice must be wide enough for the solution to penetrate yet narrow enough for stagnancy; usually the width is several thousandths of an inch. Sample of Crevice Corrosion Source: Chemical Engineering World The major factors influencing crevice corrosion are: crevice type: metal-to-metal, metal-to-non-metal crevice geometry: gap size, depth, surface roughness material: alloy composition (e.g. Cr, Mo), structure environment: pH, temperature, halide ions, oxygen How to prevent crevice corrosion? Crevice corrosion can be designed out of the system Use welded butt joints instead of riveted or bolted joints in new equipment Eliminate crevices in existing lap joints by continuous welding or soldering Avoid creating stagnant conditions and ensure complete drainage in vessels Use solid, non-absorbent gaskets such as Teflon. Use higher alloys (ASTM G48) for increased resistance to crevice corrosion PITTING CORROSION Pitting is another form of much localized corrosion attack in which small pits or holes form. They ordinarily penetrate from the top of a horizontal surface downward in a nearly vertical direction. It is an extremely insidious type of corrosion, often going undetected and with very little material loss until failure occurs. The mechanism for pitting is probably the same as for crevice corrosion, in that oxidation occurs within the pit itself, with complementary reduction at the surface. It is supposed that gravity causes the pits to grow downward, the solution at the pit tip becoming more concentrated and denser as pit growth progresses. Module No. 5 – Corrosion Prevention and Control 4 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Causes: lack of homogeneity of the metal surface (high roughness or presence of superficial scales) localized loss of liabilities ferrous or non-metallic surface contamination (e.g. inclusion of sulphides) mechanical disruption or antioxidant coating chemistry. How to prevent pitting corrosion? Pitting corrosion can be prevented through: Proper selection of materials with known resistance to the service environment Control pH, chloride concentration and temperature Cathodic protection and/or Anodic Protection Use higher alloys (ASTM G48) for increased resistance to pitting corrosion INTERGRANULAR CORROSION As the name suggests, 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 is especially prevalent in some stainless steels. When heated to temperatures between 500C and 800C (950F and 1450F) 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. These particles form along the grain boundaries, as illustrated in Figure 7.12. Figure 7.13 shows this type of intergranular corrosion. Module No. 5 – Corrosion Prevention and Control 5 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Figure 7.12 The pitting of a 304 stainless steel plate by an acid–chloride solution. (Photograph courtesy of Mars G. Fontana. From M. G. Fontana, Corrosion Engineering, 3rd edition. Copyright © 1986 by McGraw-Hill Book Company. Reproduced with permission.) Figure 7.13 Schematic illustration of chromium carbide particles that have precipitated along grain boundaries in stainless steel, and the attendant zones of chromium depletion. Prevention: Stainless steels may be protected from intergranular corrosion by the following measures: (1) subjecting the sensitized material to a high-temperature heat treatment in which all the chromium carbide particles are re dissolved, (2) lowering the carbon content below 0.03 wt% C so that carbide formation is minimal, and (3) alloying the stainless steel with another metal such as niobium or titanium, hich has a greater tendency to form carbides than does chromium so that the Cr remains in solid solution. SELECTIVE LEACHING Selective leaching is found in solid solution alloys and occurs when one element or constituent is preferentially removed as a consequence of corrosion processes. The most common example is the dezincification of brass, in which zinc is selectively leached from a copper–zinc brass alloy. The mechanical properties of the alloy are significantly impaired because only a porous mass of copper remains in the region that has been dezincified. In addition, the material changes from yellow to a red or copper color. Selective leaching may also occur with other alloy systems in which aluminum, iron, cobalt, chromium, and other elements are vulnerable to preferential removal. Sample of Selective Leaching Module No. 5 – Corrosion Prevention and Control 6 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G How to prevent dealloying? Dealloying, selective leaching and graphitic corrosion can be prevented through the following methods: Select metals/alloys that are more resistant to dealloying. For example, inhibited brass is more resistant to dezincification than alpha brass, ductile iron is more resistant to graphitic corrosion than gray cast iron. Control the environment to minimize the selective leaching Use sacrificial anode cathodic protection or impressed current cathodic protection 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. 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 Prevention: One of the best ways to reduce erosion corrosion is to change the design to eliminate fluid turbulence and impingement effects. Other materials may also be used that inherently resist erosion. Furthermore, removal of particulates and bubbles from the solution lessens its ability to erode. STRESS CORROSION Stress corrosion, sometimes termed stress corrosion cracking, results from the combined action of an applied tensile stress and a corrosive environment; both influences are necessary. 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. Failure behavior is characteristic of that for a brittle material, even though the metal alloy is intrinsically ductile. Furthermore, 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 when exposed to ammonia. The stress that produces stress corrosion cracking need not be externally applied; it may be a residual one that results from rapid temperature changes and uneven contraction or occur for two-phase alloys in which each phase has a different coefficient of expansion. Also, gaseous and solid corrosion products that are entrapped internally can give rise to internal stresses. Prevention: Probably the best measure to take to reduce or completely eliminate stress corrosion is to lower 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 anneal out any residual thermal stresses. Module No. 5 – Corrosion Prevention and Control 7 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G HYDROGEN EMBRITTLEMENT Various metal alloys, specifically some steels, experience a significant reduction in ductility and tensile strength when atomic hydrogen (H) penetrates into the material. Fig. 7.15 Fig. 7.16 Figure 7.15 Impingement failure of an elbow that was part of a steam condensate line. (Photograph courtesy of Mars G. Fontana. From M. G. Fontana, Corrosion Engineering, 3rd edition. Copyright © 1986 by McGrawHill Book Company. Reproduced with permission.) Figure 7.16 A bar of steel bent into a horseshoe shape using a nutand-bolt assembly. While immersed in seawater, stress corrosion cracks formed along the bend at those regions where the tensile stresses are the greatest. (Photograph courtesy of F. L. LaQue. From F. L. LaQue, Marine Corrosion, Causes and Prevention. Copyright © 1975 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.) The mechanism starts with lone hydrogen atoms diffusing through the metal. At high temperatures, the elevated solubility of hydrogen allows hydrogen to diffuse into the metal (or the hydrogen can diffuse in at a low temperature, assisted by a concentration gradient). When these hydrogen atoms re-combine in minuscule voids of the metal matrix to form hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength up to the point where it cracks open (hydrogen induced cracking, or HIC). High-strength and low-alloy steels, nickel and titanium alloys are most susceptible. Tensile stresses, susceptible material, and the presence of hydrogen are necessary to cause hydrogen embrittlement. Residual stresses or externally applied loads resulting in stresses significantly below yield stresses can cause cracking. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component. How to prevent hydrogen embrittlement? Hydrogen embrittlement can be prevented through: Control of stress level (residual or load) and hardness. Avoid the hydrogen source. Baking to remove hydrogen. Module No. 5 – Corrosion Prevention and Control 8 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Video: https://www.youtube.com/watch?v=YF_FhNN5D1w&t=154s https://www.youtube.com/watch?v=M_a5hG9sInY&t=4s https://www.youtube.com/watch?v=UpS1chG2Bas&t=1s https://www.youtube.com/watch?v=K87KvHPwiIU https://www.youtube.com/watch?v=4HCsBMI7nSg&t=269s https://www.youtube.com/watch?v=EYjdCKUMhPM https://www.youtube.com/watch?v=5Sd6TEenwEE https://www.youtube.com/watch?v=Aa8WOKGjm4s Module No. 5 – Corrosion Prevention and Control 9 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G References: 1. Materials Science and Engineering: An Introduction, 9th Edition, William D. Callister, Jr. Department of Metallurgical Engineering The University of Utah with special contributions by David G. Rethwisch The University of Iowa. 2. TWI Lt (n.d). What is Galvanic Corrosion and How can it be Prevented. Available at: https://www.twi-global.com/technical-knowledge/faqs/faq-what-is-galvanic-corrosion-and-how- can-it-be-avoided 3. Steelfab, 2017. A Guide To Crevice Corrosion & How To Treat It. Available at: https://steelfabservices.com.au/a-guide-to-crevice-corrosion-how-to-treat-it/ 4. Webcorr Corrosion Consulting Services,n.d. Different Types of Corrosion. Available at: https://www.corrosionclinic.com/types_of_corrosion/uniform_corrosion.htm Module No. 5 – Corrosion Prevention and Control 10