In stress corrosion, the majority of the surface usually remains intact; however, fine cracks appear in the microstructure, making the corrosion hard to detect.
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The cracks typically have a brittle appearance and form and spread in a direction perpendicular to the location of the stress. Selecting proper materials for a given environment including temperature and management of external loads can mitigate the potential for catastrophic failure due to SCC. Galvanic corrosion is the degradation of one metal near a joint or juncture that occurs when two electrochemically dissimilar metals are in electrical contact in an electrolytic environment; for example, when copper is in contact with steel in a saltwater environment.
However, even when these three conditions are satisfied, there are many other factors that affect the potential for, and the amount of, corrosion, such as temperature and surface finish of the metals. Large engineered systems employing many types of metal in their construction, including various fastener types and materials, are susceptible to galvanic corrosion if care is not exercised during the design phase.
Choosing metals that are as close together as practicable on the galvanic series helps reduce the risk of galvanic corrosion. In aqueous environments, metals may be exposed to not only uniform corrosion, but also to various types of local corrosion including pitting, crevice, intergranular, stress, and galvanic corrosion.
In areas where corrosion is a concern, stainless steel products offer value and protection against these threats. Nickel helps to stabilize the microstructure, increasing SCC resistance. Manganese, in moderate quantities and in association with nickel, will perform many functions attributable to nickel and helps prevent pitting.
The addition of molybdenum the additional element in Type SS that increases its performance with respect to Type SS , helps increase resistance to pitting and crevice corrosion. Reduced levels of carbon, such as those found in L and L will help prevent intergranular corrosion. Choosing stainless steel can help greatly reduce the risk of corrosion and yield long-term savings by avoiding the costs associated with reinstallation of inferior products. For corro- sion to be adequately addressed, designers need to know about the mechanisms of corrosion failure and to know when they need assistance from a corrosion specialist in selecting materials or operating parameters.
Purchasers have the responsibility for ensuring that their chosen system func- tions safely and efficiently, and they bear the financial burden of its maintenance. Roads, bridges, planes, pipeline systems, and the electrical grid are examples of systems where a comprehensive understanding of corrosion can lead directly to lower maintenance costs, longer service lifetimes, and less risk of failure. End users may have little direct connection to the designer but must be con- fident that the designer has taken corrosion into account.
While the monetary cost of corrosion can be estimated, the cost of risks to public safety cannot be so easily measured without performing a complex risk assessment. Public safety and the environment are the main reason end users and the public should be concerned about the state of corrosion engineering education and the implementation of that education by the engineering design workforce. From the evidence the committee examined, corrosion will clearly continue to have a major impact on key industries and infrastructure systems being planned.
How industries function and how systems are built will be strongly influenced by their response to the environment in which they must operate. As discussed above, designers of devices and structures, those who purchase them or maintain them, and, of course, their end users should at the very least be aware of the impact of corrosion.
The sections that follow describe the crucial role of corrosion in the infrastructure systems that are the lifeblood of the U. The next decade may see the gradual electrification of the automobile with its concomitant dependency on fuel cells, batteries, and the electricity grid.
A new set of corrosion problems accompanying this transforma- tion is likely to limit the service lives of the batteries and fuel cells. The production of hydrogen in quantities large enough to make an impact on the transportation infrastructure will probably require thermochemical or electrochemical processes using very high temperatures and highly corrosive solutions.
The storage tanks for hydrogen will have to resist degradation such as hydrogen embrittlement. The present pipeline infrastructure for deliv- ering natural gas, crude oil, and refined gasoline products is also operating at its limits. For example, there have recently been significant failures associated with corrosion at Prudhoe Bay, Alaska. In addition, modifications to gasoline formulations that use renewable resources ethanol have now made these formulations more sensitive to water uptake and increased the potential for corrosion during storage and transport.
Accessed March I m p o rta n c e of C o r r o s i o n E n g i n e e r i n g E d u c at i o n 17 Engineered Devices and Systems Many new engineering structures employ lightweight materials to save money and energy. Lightweight magnesium is considerably more reactive than steel or aluminum. Graphite composites can be made into very thin, stiff, light- weight structures but will require greater environmental resistance to maintain structural integrity. The push for extended component lifetimes and less design effort means that environmental degradation will become a greater concern.
Energy Infrastructure The nation and the world will be challenged to rebuild the energy infrastruc- ture in a way that avoids greenhouse gas emission and maximizes efficiency of electricity production. Wind turbines, solar cells, biofuels, nuclear energy, and clean coal are all set for significant development and increased implementation. Corrosion is likely to be a key issue in solar cell lifetime and wind turbine perfor- mance and will become more important in large central power plants. Strategies for scrubbing emissions and capturing carbon will likely be limited by corrosion.
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Similarly, high efficiencies of central power plants are achieved through the use of very high temperature working fluids, which means much more expenditure for corrosion protection than we see today. As the country considers commissioning more nuclear power plants, the storage and eventual disposal of nuclear waste are largely an issue of containment vessel corrosion rates and possible failure modes.
Virtually all energy sources will see an increasing cost of corrosion and new forms of corrosion. Therefore, developments in corrosion technology are key to improved efficiency in energy production. Health Care Health care is increasingly dependent on biomedical devices that monitor and control bodily functions and deliver drugs. The drive to minimize size, maximize capability, and extend device lifetime places demands on the materials of construc- tion and on their tolerance for degradation before function is affected.
The use of new materials over longer and longer times will require knowing more about the interaction between these materials and the human body environment. Their exposure to drugs will call for such devices to have particularly high resistance to chemical interactions. New uses of such devices and implants are limited by the need for them to resist corrosion for extended lifetimes.
As more medical devices are implanted to serve an ever-aging population, unexpected uses and failures can. However, designers will not be able to solve these problems unless they have extensive training in corrosion science. The insertion of smart devices and the more widespread the deployment of sensors in all types of systems and structures puts electronics into ever-harsher use environments.
Although a cell phone is now a commodity product, the owner still expects that the instrument will reliably switch micro-amp currents through its various switches for a very long time. Corrosion on these contacts can destroy instrument quality and reliability. Sensors are ever more important in daily life, from monitoring biological activity in the body to controlling our cars and providing information on wind, precipitation, chemical contamination, and so on.
The increase in sensor utility is driven by advances that shrink the devices to the micrometer level or less, resulting in significantly larger surface areas for the same active volume. The result is that surface and interface cor- rosion processes will become much more important than they are today and will pose an increasing threat to device and system reliability. Figure shows corrosion of a circuit board that had been in a data logger used in a moist environment.
National Defense Defense readiness is highly sensitive to corrosion, and future defense systems will still present new challenges as new materials are inserted into defense platforms. Ground vehicles designed for cold war battlefields are being used in desert environ-. Right: Rebar corrosion, bridge on Highway in Ontario, Canada.
Courtesy of Tim Mullin. The future soldier will probably be clad in multifunctional uniforms that possess communica- tion capability, power sources, and armor, all of which will require considerable innovation in durability. Exceedingly important from the standpoint of national defense is the impact that corrosion damage has on overall defense readiness. At any given time, 20 to 50 percent of the U. Air Force tanker fleet is in repair; many U. Army trucks and HMMWVs are in repair or are being used at less than full capacities owing to general wear and corrosion.
Little is being done to train and prepare present and future professionals in handling this problem properly, and DOD struggles to train its workers to deal well with corrosion.
Public Infrastructure The national infrastructure and its maintenance is an important issue see Figure The sustainability of the modern infrastructure depends on the proper design and maintenance of its major com- ponents, and this in turn demands a cadre of engineers and scientists capable of choosing and using materials so as to minimize environmental degradation. Historical Interest Even our historical artifacts are constantly undergoing degradation and must be maintained to preserve them for present and future generations. Structures and objects from the past require special handling and restoration and protec- tion methods.
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Bronze statuary of historical interest has become a special problem recently as acid rain has begun to attack bronze alloys that were formerly inert to environmental attack Figure The Statue of Liberty was found to have suf- fered considerable corrosion when it was restored in the early s. In Our Homes Within our homes, as new building materials are introduced, different cor- rosion hazards present themselves.
Composites that do not degrade like natural materials or metals will be used increasingly in building construction. Connecting the copper piping of a home to steel mains is difficult, as is the incorporation of magnesium anodes for protection of water heaters. See Figure for an example of a corroded water heater. In Summary In general, we are pushing the limits of operability with all of the materials we use in the modern world. Baboian, E. Ballante, and E. Mastbaum, Humanity is expanding into harsher environments on Earth, requiring systems, objects, and structures that can sup- port human activity at great ocean depths, deep underground, and in desert and arctic environments.
All of these demands will require a workforce conscious of environmental attack on all types of systems and able to anticipate and design for sustainability under extreme conditions.
One of the biggest driving factors is the trend to extend the useful lifetimes of items beyond their original design lifetimes. It is rare to stop using a bridge, for example.
Commercial and military aircraft in daily use in the United States are operating well beyond their expected lifetime Figure With a bridge, about 10 percent of the construction cost of the structure controls the lifetime cost. The components of lifetime expense are the costs associated with replacement, readi- ness, safety, and reliability. But all too often such practices are not employed because of a shortfall in investments or a lack of knowledge on the part of designers.
While better and more cost-effective corrosion management procedures could significantly extend the service life of existing systems and reduce maintenance costs and replacement requirements, the value of preventive strategies is often not recognized and they are not even applied. There is no identifiable advocate for corrosion control as there is for, say, the steel or aluminum industries.
Environmental Effects on Engineered Materials
While there are interested par- ties, such as the corrosion mitigation industry and professional societies like NACE International, corrosion is not a product per se and there is no national advocate for corrosion programs. As it reached its flight altitude, the cockpit crew heard a The skilled crew landed the plane safely with only one death.
The aircraft had been designed to sustain major structural failure and survive. Also contributing to the accident were the failure of the Federal Aviation Administration FAA to require inspection of all the lap joints proposed by Boeing after the discovery of early production difficulties in the cold bond lap joint, which resulted in poor bond durability, corrosion, and premature fatigue cracking. Volume 22 Issue Dec , pp. Volume 21 Issue Dec , pp. Volume 20 Issue 6 Dec , pp. Volume 19 Issue Dec , pp. Volume 18 Issue 6 Dec , pp. Volume 17 Issue Dec , pp. Volume 16 Issue 4 Dec , pp.
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