Now I'd like to talk about the corrosion of stainless steel. Stainless steel is quite different from other carbon steel, and its main properties are corrosion and mechanical properties. So the corrosion of stainless steel is very important. And as you can see in your daily life, there are many kinds of corrosion, and here in this figures you can see some forms of corrosion. The first one here is usually called intergranular corrosion. That means corrosion occurs along these grain boundaries and it causes harmful effects on steel. And this one here, you can see the corrosion of the overall surface, so this is called general corrosion or uniform corrosion. On the left side, the austenite phase and the ferrite phase are coexistent. In this case, if one phase of the corrosion property is superior to the other phase, then the inferior phase corrodes firstly; this is called galvanic corrosion. And here, in the right side diagram, you can see many kinds of pits which can be seen to be very small pits. And this is called pitting corrosion. In stainless steel, there are so many forms of stainless steel you can meet. However, the most important properties of corrosion are written in red color, and I will talk only about these corrosion properties very shortly. The first one is intergranular corrosion, and the second one is pitting corrosion which is very similar to crevice corrosion. The third one is stress corrosion cracking, and the final one is high temperature oxidation because stainless steel is usually applied at higher temperatures as well. So firstly, I'm going to talk about intergranular corrosion. Here in this figure, you can see the corrosion along grain boundaries, and this intergranular corrosion is associated with the formation of the carbide which means M23C6 at grain boundaries. As I talked in the former topic, these M23C6, usually Cr23C6 carbides are formed at high temperatures around 600 degrees Celsius. So that's why we have to avoid the formation of these carbides when we process stainless steel during heat treatment. And by the formation of these Cr23C6 carbides, the grain boundary region is depleted of chromium in the vicinity of those carbides because the chromium is gathered into chromium carbide. So the grain boundary region loses its corrosion resistance because of the depleted chromium content. That's why it leads to intergranular corrosion. This is so-called sensitization phenomena, and in order to be sensitized in stainless steel, two conditions should be met. The first one is that M23C6 carbide should be formed. And the second one is that the chromium depletion around the carbides must be such that the local chromium content is less than 12 percent. So even though the chromium is depleted by the formation of chromium carbides, if that depleted zone of chromium content is higher than 12 percent, you won’t have the intergranular corrosion. That means we can avoid intergranular corrosion. The second one is the pitting corrosion like I said. Pitting corrosion occurs at very localized locations. As you can see in this figure, the size of the pit is very small. So pitting appears very minor at the surface. Sometimes we cannot see it with the naked eye. However, it has a larger cross-sectional area deeper in the metal. That's why pitting is very important phenomena, and an important property in stainless steel. And this pitting corrosion occurs in sealing areas such as seat packets and gaskets. In this case we sometimes call it crevice corrosion. Pitting occurs because of chlorides, and these chloride ions usually break down the passive film area, as shown in this diagram. So these chloride ions are very harmful elements which cause pitting corrosion. And molybdenum dramatically increases the resistance to pitting. So in order to avoid pitting, we need to add molybdenum. However, since molybdenum is a very expensive alloying element, we have to control the amount of molybdenum optimally. One more important feature of pitting corrosion is that it's very difficult to predict because it’s very small in size and it’s not easy to see on the surface. That's why it's very difficult to predict. So nowadays, we use pitting resistance equivalent number, which is called PREN, and usually it is expressed like this one: the percent chromium and some weighting factor of molybdenum and some weighting factor of nitrogen. That means molybdenum, nitrogen, and chromium are the most important alloying elements which can determine the pitting corrosion ability or resistance to pitting corrosion. And the pitting corrosion resistance is expressed by this PREN number. This is obtained by analyzing many experimental results of specific corrosion tests, and people made these empirical equations. The first one is the ordinary one. And the second one is when we add the molybdenum and nitrogen simultaneously, sometimes we need to consider the synergistic effect of molybdenum and nitrogen. So this PREN number is a little modified from this simple one, and this PREN number contains some synergistic effect of molybdenum and nitrogen. And if we add some manganese, it causes a slightly harmful effect. Here as you can see in this table, usually the weighting factor of molybdenum is 3.3 or sometimes three. And the nitrogen factor is from 16 to 36 sometimes. And molybdenum and nitrogen are simultaneously added. In that case we have to add some more factors to this synergistic factor. And if we add manganese, we need to subtract some numbers. So anyway, if you see this PREN number, chromium, molybdenum, and nitrogen are the most important alloying elements which can give the ability of pitting resistance. And the third form of corrosion of stainless steel is the stress corrosion cracking. As the name indicates – you can see that stress and corrosion – if those two items are combined, then this stress corrosion cracking occurs. And this is a very dangerous corrosion phenomenon because usually failure occurs very drastically without any prediction. So we need to know about stress corrosion cracking. In this diagram you can see that there is a sensitive material and a corrosive environment, and also we need some tensile stresses. So if all these conditions are met, then stress corrosion cracking occurs like this one. And the forms of stress corrosion cracking are shown here: the transgranular cracking or intergranular cracking. And these cracking occurs very drastically and very abruptly. That's why we need to avoid stress corrosion cracking. By stress corrosion cracking, usually the ductile metals and alloys show brittle failure by the combined action of tensile stress and some specific corrosion environment. Here, I just want to show some important features in stress corrosion cracking. Usually ferritic stainless steel is immune to stress corrosion cracking. And austenitic stainless steel is prone to low stress or residual stress corrosion cracking. So stress corrosion cracking is very important in austenitic stainless steel. The reason why austenitic stainless steel is very prone to this kind of stress corrosion cracking is shown in this diagram. As you can see here, the x-axis is the percent of nickel and this y-axis is time to failure which is expressed in hours. If you see this diagram, at around 8 percent of nickel content usually the stress corrosion cracking occurs within an hour or a very short time. However, the problem is that austenitic stainless steel usually contains 18 percent or around 18 percent chromium, and 8 to 12 percent nickel. So this is the range of the nickel content in austenitic stainless steel. That’s why they are very prone to this kind of stress corrosion cracking. And this diagram is sometimes called the Copson-curve. So stress corrosion cracking resistance of stainless steel is obtained by lowering or just adding very high nickel. By decreasing the nickel content, we can obtain ferritic stainless steel. So this ferritic stainless steel is immune to stress corrosion cracking. And also by increasing the nickel content, we can make nickel-based alloys, and by making nickel-based alloys, we can avoid stress corrosion cracking. Finally, I want to talk about high temperature oxidation which we can see in stainless steel because stainless steels are widely applied from low temperature to high temperature, and sometimes even up to 900 degrees Celsius as shown here. In the automotive exhaust system, you can see that exhaust manifolds which are exposed to very high temperature[s] up to 900 degrees Celsius. So in this case, the high temperature oxidation property is very important. And from this exhaust manifold to muffler or tailpipe, the temperature decreases, and depending on these temperatures we use different kinds of stainless steel. For example, for the exhaust manifold, catalytic converter, flexible pipe, and muffler and tailpipe, the temperature changes from 900 degrees Celsius up to around 100 degrees Celsius. Depending on these conditions – temperature condition and outside atmosphere – we can select different kinds of stainless steel. So far I talked about the corrosion of the stainless steel briefly.
