Chapter 4
4.6 Corrosion-Resistant Materials
Tadashi Shinohara
Materials Reliability Center, National Institute for Materials Science
1. Corrosion resistance of materials
Corrosion progresses due to the same electrochemical reactions as electric cells. Its extent depends on the oxidizing nature of the environment, and the electrode potential is its index. The acidity
(pH) is one more representative item serving as an index of the environment, and a phase diagram (potential-pH diagram) that takes these two indices as the two axes is employed as the means for determining the corrosion resistance of metal materials. Figure 4.6.1(a)1) is a potential-pH diagram of iron (Fe). It is determined that the given metal corrodes if it is in the stable region of ionic species, and this metal does not corrode if it is in the stable region of the metal itself or a substance thereof (an oxide or hydroxide). The oxides or hydroxides of many metals are stable and have corrosion resistance in a specified pH range. However, only Fe has a corrosive region over the full pH range. Such skillful use of Fe can be acknowledged as the place for a researcher in the materials field to show his skill. Even in the case of Fe, though, if the oxidizing nature of the environment is high, the potential enters the stable range of Fe2O3, and a thin (nanometer order) and delicate film is produced, and this exhibits corrosion resistance. It is well known that the iron nails placed in concentrated nitric acid exhibit corrosion resistance. Such a state where the metal surface is covered with a thin film is called passivity. Titanium (Ti), nickel (Ni), chromium (Cr), etc. also display corrosion resistance as passive metals. On the other hand, when Cr is added to Fe, the passive region of Cr (Figure 4.6.1 (b) 1)) overlaps on the corrosive region of Fe, and ends up becoming passive and having corrosion resistance as long as it does not become considerably low pH, and such a Fe-Cr alloy is stainless steel. The critical value at which it can no longer be rendered passive is called,
pHd (Figure 4.6.22)), and is the index showing the corrosion resistance of stainless steel, and it is not possible to use this stainless steel in an environment with a pH lower than the critical value. When a chloride ion (Cl-) is present even in an environment where stainless steel has corrosion resistance, localized corrosion like pitting or crevice corrosion – the corrosion that occurs under the crevice structure – occurs, and these serve as the isitiation site of stress corrosion cracking (SCC).1, 2) The value of pHd also serves as an index of resistance to localized corrosion.
2. Research trends
2.1 Stainless steel
Austenitic stainless steel
Since austenitic stainless steel is superior in corrosion resistance and ease of processing, and retains its toughness even at extremely low temperatures, it can be used for a wide range of purposes. However, its most important drawback is that it causes SCC to occur with the corrosion part as the isitiation site in a chloride environment. For example, the upper critical temperature Tc that does not cause SCC is Tc = 50 °C in the case of type 304 steel (18 Cr-8 Ni), which is widely used as austenitic general purpose stainless steel, and attempts are being made to raise the Tc to 100 °C and above with a composition that is close to this kind of steel.3)
On the other hand, one more of the drawbacks of austenitic stainless steel is that it contains Ni, which is a problem due to metal allergies. Even more than the corrosion resistance, Ni is added in order to make the steel into an austenite single phase. High nitrogen austenitic stainless steel, which is set as an austenite phase by adding nitrogen (N) instead of Ni, is being
Fig. 4.6.1 E-pH Diagrams of Fe and Cr |
Fig. 4.6.2 Effects of alloy elements on pHd (reference 2) |
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Materials Outlook for Energy and Environment
examined.4)
Ferritic stainless steel
Ferritic stainless steel has the characteristics that it does not include Ni, which is a problem due to metal allergies, and tends not to cause stress corrosion cracking, and so on. However, intergranular corrosion tends to occur easily, and pitting corrosion resistance also declines, due to the carbon (C) and N contained as impurities in the steel. In the past, it was not possible to reduce both of these impurity elements sufficiently, and it has been thought that the corrosion resistance of ferritic stainless steel is inferior to that of austenitic stainless steel. From around 1970, the refining technology for stainless steel improved by leaps and bounds, and it became easy to mass produce steel materials with remarkably low amount of C + N (known as “high purity steel”), and the development of high purity ferritic stainless steel with excellent corrosion resistance was actively pursued.5) The corrosion resistance of ferritic stainless steel is chiefly determined by the amount of Cr and the amount of molybdenum (Mo), and it is thought that the addition of Ni is effective in improving the mechanical properties, and particularly the toughness. However, Ni addition of 2% and above causes the SCC resistance to decline, so a composition with a good balance of Cr, Ni and Mo is being examined.
2.2 Ni base Alloy
Ni exhibits good corrosion resistance towards alkali and nonoxidizing acid, and not only has excellent corrosion resistance such as its passivity in a neutral natural environment, but also is superior in its ease of processing and mechanical properties. In addition, alloys that are superior in their chloride stress corrosion cracking resistance are also being developed by making alloys with Cr and Mo.2)
Since these alloys are superior in corrosion resistance, they are used in harsher environments like chemical plants. Owing to this, information about what kind of environment (temperature, Cl- concentration and electrode potential, etc.) it is where these can be used without causing corrosion, in other words the usable range,1), 2) is also important.
2.3 Titanium
Ti has superior corrosion resistance owing to the passive film that is formed on the surface. Although it is known that it causes crevice corrosion or SCC in the presence of Cl-, alloys with excellent corrosion resistance even in a chloride environment such as Ti-0.15 Pd are being developed. However, since Ti forms a hydroxide due to hydrogen absorption, and this itself is brittle, it exhibits embrittlement as a material (hydrogen embrittlement), so elucidation of the invasive behavior of hydrogen or hydrideforming behavior is awaited.2)
3. Challenges in materials science
The addition of Ni and Mo is effective in improving corrosion resistance. However, the large increase in the price of both metals has become a problem, and metal allergies are an additional problem in the case of Ni. It has become necessary to examine methods for reducing even a little bit the amount of these elements that is added, or replacing them with some other elements.
On the other hand, if this is viewed from the standpoint of the user of the materials, it is said that type 304 steel causes SCC at temperatures of 50 °C and above, but in reality there are also cases where SCC does not occur in type 304 steel in a Cl- environment of 50 °C and above, and it is necessary not only to elucidate the mechanism of these forms of corrosion but also to clarify “when, where and how” it occurs and continues to grow.
References
1)Japan Society of Corrosion Engineering, ed., Primer in Materials Environmental Studies, Maruzen (1993).
2)Japan Society of Corrosion Engineering, ed., Corrosion and Corrosion Prevention Handbook (CD-ROM edition, second edition) (2005).
3)T. Mizoguchi, Symposium Materials – Effects of Alloy Elements on Corrosion Resistance of Stainless Steel and Development Trends in Research Conserving Steel, Iron and Steel Institute of Japan (2008) 33.
4)D. Kuroda, T. Hanawa, R. Yamamoto, A. Yokoyama and N. Oda: Materia, 44 (2005) 691.
5)Y. Hosoi: Zairyo-to-Kankyo, 50 (2007) 436.
Chapter 4
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Chapter 4. Chemical Energy Materials