Examples of mistakes in material selection

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Examples of mistakes in material selection: According to investigations and estimates by corrosion scientists, a large portion of corrosion-related losses is caused by a lack of understanding regarding corrosion. By widely applying the existing knowledge of corrosion science and engineering, one can recover as much as 1/4 to 1/5 of all corrosion losses. Since every industry, factory, enterprise, and mode of transportation faces issues related to the use and selection of materials, and the vast majority of engineering and management professionals lack knowledge of corrosion science or have only an incomplete understanding of it, they often make mistakes when encountering certain corrosion problems due to insufficient comprehension, resulting in significant losses. The following summarizes ten common mistakes made by decision-makers (though certainly not limited to these ten). If all professionals in the technology and management fields could become familiar with the correct ways to handle such issues, it is believed that losses of hundreds of millions, even billions, could be reduced each year! ①No research is conducted on materials in advance; unless one is certain, it is necessary to consult corrosion data and related information before selecting a material, as corrosion phenomena are generally complex, and new types of corrosion keep emerging. Thinking that material selection is a simple task is a mistaken notion. The severity of the losses caused by this illusion can be seen from the following example. In the U.S. Apollo moon landing program, titanium alloy tanks were used to store the fuel N2O4. Once, pressure tests were conducted on 20 titanium tanks; due to the danger associated with N2O4, methanol was used instead as the test fluid. Due to some similar physical properties between methanol and N2O4, stress corrosion cracking occurred in the titanium tank during pressure testing, resulting in losses of around $1.5 million. It was a fact discovered ten years before this pressure testing that titanium alloys suffer from stress corrosion cracking in methanol; had the responsible personnel taken the time to consult relevant literature in advance, such substantial waste could have been avoided.   In China, similar situations are also common, such as using 18/8 stainless steel for evaporators for magnesium chloride and calcium chloride ； Use aluminum containers to hold alkali, cement, and lime water ； Treating oxygen-containing acids and the like with copper and copper alloys has caused significant unnecessary losses. ②It is believed that stainless steel is a universal corrosion-resistant material. Besides stainless steel, some people also consider titanium, zirconium, or plastics to be universal materials as well. Some factories use \"white steel\" (the common term used in factories for stainless steel) to address any corrosion problems, which often results in waste. It’s not a universal corrosive material! There are many types of stainless steel, and plastic varieties are even more diverse. Each material has unique physical, chemical, and biochemical properties. For example, 18/8 chromium-nickel stainless steel is much more resistant to the effects of the atmosphere than carbon steel (which is where its name comes from), but it is just as susceptible to reducing acids such as hydrochloric acid and dilute sulfuric acid as carbon steel. Its resistance to stress corrosion cracking caused by chloride ions is far inferior to that of carbon steel. They are superstitious about the \"universal material,\" and their mistake lies in not doing any research – they haven’t read any books on material corrosion at all. ③There is a lack of attention to the corrosion caused by air, water, distilled water, etc. Some people think that only strong acids have a high degree of corrosivity, but this is not necessarily the case. The factors affecting corrosion are complex and vary depending on the material. It is a mistake to think that the corrosion caused by air, water, and even distilled water must be mild. 18/8 austenitic stainless steel can suffer from stress corrosion cracking in water containing just a few ppm of Cl-, while carbon steel can also experience stress corrosion cracking in an atmosphere with trace amounts of H2S. High-strength steels developed in recent years (martensitic steels, precipitation-hardening steels, maraging steels, with a yield strength of around 1034 MPa) are very useful materials, but it was soon discovered after their application that they can suffer from stress corrosion cracking in both atmospheric conditions and pure water. All of the above situations have led to serious accidents, such as equipment damage, bridge collapses, and plane crashes.   Some domestic plants that were established by importing equipment failed to address water corrosion by not adopting any treatment technologies, nor did they find effective solutions on their own, resulting in water corrosion becoming a serious problem. In short, technological work should not be taken for granted; it needs to be approached scientifically. ④The design does not take into account the relationship between the structure and corrosion. Some people check relevant data before selecting materials; sometimes, materials deemed suitable for use according to manuals suffer from severe corrosion. This is generally not due to an error in the manuals, but rather because corrosion factors are complex and not fully taken into consideration. The following will discuss, in sequence, some issues that should have been considered but were not, thereby leading to corrosion. Among them, not taking into account the influence of the design structure may be one of the important reasons.   The presence of numerous gaps is the main design flaw. While it is difficult to completely avoid it, gaps should be minimized as much as possible, and efforts must be made to avoid them in critical areas. The gap will create an oxygen concentration cell, which subsequently leads to acidification within the gap and causes rapid corrosion. When the exterior dries out, liquid remains in the seam, continuing the corrosion. If repeated dry-wet cycles occur, the ions inside the seam (such as Cl-) will become increasingly concentrated, leading to dangerous stress corrosion cracking.   The following figures show several examples of avoiding gaps. The gap can also be filled, or the surface of the gap can be protected with a coating containing anti-corrosion additives. In addition to gaps, there are also other structural defects that should be avoided, such as dead corners where drainage is difficult, hot or cold spots with excessively high or low local temperatures, and structures that cause the corrosion solution to condense.  ⑤Failing to pay attention to galvanic corrosion: A workshop or a piece of equipment may involve the use of several different materials. When selecting materials, one should not consider just a single device or component in isolation; rather, it is necessary to carefully take into account whether there are other components made of different materials that are in contact with it or even located at some distance away. Contact between different materials, due to their differing potentials, forms a battery, leading to galvanic corrosion. Generally, the greater the potential difference between two metals, the more severe the corrosion of the anode metal with the lower potential. The order of standard metal potentials can be found in corrosion or electrochemistry books, and there may be some differences in actual environments. For example, the order of electrode potentials measured in seawater differs from the standard potentials. If iron and zinc come into contact, the accelerated corrosion of zinc is not significant ； When copper and zinc come into contact, zinc corrodes severely. In addition, the polarization behavior of the electrode also has a significant impact; for example, if polarization is high, corrosion is minimal even when the open-circuit potential difference is large ； Conversely, if the polarization is low, even a small potential difference can promote anodic corrosion.   Sometimes the cathode can also be damaged, as neutral metals such as lead and tin are corroded by the OH- ions produced at the cathode ； The oxide film on stainless steel loses its protective properties due to cathodic reduction ； Since hydrogen is generated at the cathode and hydrogen atoms penetrate into the metal, hydrogen embrittlement may occur.   In galvanic corrosion, attention must be paid to the area factor, that is, the ratio of the cathode to anode area. If the cathode is large and the anode is small, the cathodic current becomes concentrated, resulting in a high current density that facilitates perforation. If the areas of the cathode and anode are similar, or if the anode is larger, corrosion is distributed evenly over the anode surface, posing little risk.   Violating the aforementioned principles often results in costly losses. There is an example that can illustrate such problems well. A factory in the country uses hundreds of steel tanks, which are coated with phenolic paint for protection. The bottom coating is prone to wear and damage, which can lead to product contamination. To address this issue, the bottom of the steel tank is lined with stainless steel, while the top and tank walls are made of steel; the steel walls are welded to the stainless steel bottom. To prevent corrosion, the steel wall is painted up to just below the weld. Just a few months after driving it, the walls of the tank developed many small holes. This is because the coating contains many micro-pores, and the steel at the bottom of these pores forms a cell with the bottom of the stainless steel tank, resulting in very rapid corrosion at a rate of about 25 mm per year. Without such “improvements,” the groove walls can generally last 10 to 20 years. This is an example of turning good intentions into bad outcomes. The remedy is to \"protect\" the stainless steel as well with paint, thereby reducing the cathode area as well.   Another simple method to prevent galvanic corrosion is to insert a non-metallic insulator between different metals whenever possible; for example, when connecting stainless steel pipes and aluminum pipes, a section of plastic pipe can be used as insulation in between. ⑥Without considering the impact of manufacturing processes on corrosion, an important effect of these processes is that they induce localized stresses in the equipment, which can lead to dangerous stress corrosion cracking in suitable environments. There are many sources of stress, the important ones including: welding, cutting, shearing, punching, bending, riveting, heat treatment, improper assembly, and uneven tightening of bolts. During the operation of the equipment, periodic heating, thermal expansion, vibration, rotation, and loads can also cause local stress concentration.   An effective method to prevent stress corrosion cracking is to eliminate stress through heat treatment. However, for unstable stainless steels, the stress-relief annealing temperature of 730–870°C falls within their sensitization range; thus, while stress is removed, intergranular corrosion is induced, which is why stabilized stainless steels containing titanium or niobium need to be used.   Welding not only generates residual stress but also raises the temperature near the weld to a sensitization level (around 500–800°C), causing intergranular corrosion in austenitic stainless steels. Good results can be achieved by using stabilized steel or ultra-low carbon steel. Another approach is to subject the sensitized steel to high-temperature solution treatment, that is, heating it to around 1100°C and then quenching it in water. At 1100°C, the chromium carbide formed during the sensitization temperature dissolves, resulting in a more homogeneous alloy. Most austenitic 18/8 stainless steels available on the market are in this state. However, solution quenching is difficult for large-scale equipment. The quenching process also needs to be carefully controlled; if cooling is slow, the entire structure is prone to intergranular corrosion.   If it is understood that manufacturing processes can have a profound impact on corrosion and if this is addressed properly, many severe cases of corrosion can be avoided. ⑦Without taking into account the effect of the environment on strength, designs are all carried out based on the standard strength values of materials specified in the manuals; as a result, many devices experience accidental failures at stresses well below their nominal yield strength. This is because none of the materials are ideal; they contain microdefects and microcracks within them. Under the influence of corrosive environments, it can also promote stress corrosion cracking.   Over the past two decades, thanks to the use of fracture mechanics in the study of stress corrosion cracking, an important achievement has been the ability to determine design strength values that are suitable for practical use. The definition of K is: K = σC, where the crack length in an infinitely large plate is 1/2. For non-limiting plates, the correction factor αoK=ασK has units of kgf/mm2 or kgf·mm-3/2, ksi.   When the tensile stress increases to σc (the critical value), K reaches Kc; Kc is known as the fracture toughness of the material. At this point, the crack rapidly becomes unstable and propagates, leading to brittle fracture. When the crack is in a plane-strain state (thick plates), Kc at this time is called the plane-strain fracture toughness, denoted as K1c.