Corrosion is one of the three major failure modes of metals. Stainless steel is often used in more demanding environments to inhibit metal corrosion. However, engineers have discovered that even with stainless steel, components can still corrode under certain conditions. When pitting corrosion occurs in stainless steel, many engineers do nothing. The author believes that many engineers have misunderstandings in the selection of stainless steel materials. This misunderstanding is that stainless steel corrosion or even corrosion. There was a saying that said: The man has tears, but he does not flick, because he has not reached the point of his heart. This sentence can't be overemphasized for stainless steel. Stainless steel is not non-corroding, just because it doesn't encounter harsher corrosion environments. Here I will focus on the issue of local corrosion of stainless steel. I hope that some field projects will be relieved of some doubts in this area.
Brief Description of Local Corrosion of Stainless Steel
For chromium-nickel-containing stainless steel materials, there are two main forms of corrosion: one is uniform corrosion and the other is localized corrosion. Rust in the marine atmosphere is a typical example of general or uniform corrosion. Here the metal is evenly eroded over its entire surface. In this case, a loose layer is formed on the steel surface, and this layer of corrosion product is easily removed. Uniform corrosion is one of the easiest forms of corrosion because engineers can quantitatively determine the metal's corrosion rate and can accurately predict the life of the metal. Therefore, uniform corrosion is a form of corrosion that is minimally affected by rickets. Although it causes corrosion damage, it can be predicted and controlled.
However, the occurrence of localized corrosion often makes many engineers unprepared. This is because the damage caused by local corrosion is difficult to predict and the life of the equipment cannot be accurately calculated. One of the most annoying pittings, it is the most difficult type of local corrosion in metal. Because thousands of miles of the embankment, collapsed in the ant hole. This so-called pitting is an ant spot on a levee.
In the process of metal corrosion, two reactions occur at the same time on the electrode. One is the cathode reaction, and the non-metal is reduced at the cathode. The non-metal has electrons and the valence is reduced. The other is the anode reaction. When the anode reaction occurs, the metal loses electrons and the valence rises. The metal ions are detached from the metal surface. What I want to say is that the corrosion of metals depends on the reaction with the greatest resistance to corrosion. Therefore, this also provides a major guiding principle for solving the problem of metal corrosion.
Corrosion resistance design using the relationship between cathode and anode. If a large cathode face is connected to a small anode face, large current flows between the anode and the cathode. This situation must be avoided. On the other hand, when we reverse the situation by connecting a large anode surface with a small cathode surface, a small current flow will occur between the two metals. This situation is what we expect. We design the cathode of the weld metal in a container or tank as a cathode. The fastener device is designed so that the cathode fastener (small area) and the anode piece (large area) are connected together. An example of this concept is to rivet steel panels together with copper rivets and expose them to seawater with low flow rates. The copper fixture is a small cathode surface, while the steel plate is a large anode surface. This design is very convenient and produces good compatibility.
Pitting problem. Pitting can also be produced without gaps on the metal surface. The occurrence of pitting may come from two factors: the chloride ion in the environment and the heterogeneity of microstructures or components. The corrosion of stainless steel can be caused by the concentration of a special etchant such as chloride. If pitting occurs in stainless steel due to sensitization or other reasons, or when the chromium and nickel contents are not uniform or even fail to resist pitting corrosion, pitting corrosion may occur. Defects on the metal surface can also cause pitting. For example, a defect in a protective oxide layer of stainless steel or nickel alloy. Pitting can be prevented by using an alloy having a high corrosion resistance or by eliminating a chemical element that causes pitting. Another aspect of controlling metal pitting is the elimination of cathodic reactants in the environmental medium. Normally oxygen removal will have a better effect. As the bottom of the pit tends to be anodized, the surrounding area of the pit or gap tends to be cathodically so that the relationship of the battery current is formed. When the corrosion in the pit or crevice further expands, it becomes an autocatalytic reaction. Ferric ion interacts with chloride to form ferric chloride. The reaction is repeated and metal perforation occurs rapidly. Pitting or crevice corrosion is a very dangerous form of corrosion because it is highly localized and can quickly cause the metal to break through.
Brief Description of Local Corrosion of Stainless Steel
Subsurface corrosion problems. Just below the sediment or in the crevice, the oxygen content of the solution is low, and the oxygen content of the bulk solution in the outside of the crevice is very high. This establishes a battery with an anode under the sediment or in the crevice and outside. Is the cathode. Inside the gap containing the chloride medium, the pH drops and the chloride concentrates. This acidic chloride condition causes corrosion to accelerate and is automatically mediating. Then severe localized corrosion occurred. An example of this type of corrosion occurs when a stainless steel fastener is placed on a stainless steel plate and exposed to chloride-containing water. Crevice corrosion can occur when the bolt head or washer is used as the anode area. Preventing the formation of precipitates and scales or using materials with high alloy content will help reduce crevice corrosion.
Stripping corrosion. In this case, a loose, sheet-like corrosion layer is formed on the metal surface. Even a low velocity flow can easily remove loose layers of corrosives. As a result, new, unetched metal is exposed again, so that many additional sheet-like layers will be formed. Again, these platelets are easily removed and the process continues. The use of alloys that are not chemically reactive can avoid exfoliation corrosion.
Intergranular corrosion. Appearing in certain special alloys, intergranular corrosion may occur when they are heated to their sensitive temperature zone during welding or heat treatment. When certain stainless steel alloys are heated to 425-870°C, chromium carbides precipitate at the grain boundaries. This leads to the presence of chromium-depleted regions in the vicinity of the carbides and also affects the passivation of the grain boundary region. In special media, such as nitric acid or high-temperature water, corrosion may occur in the low-chromium zone. The grains appear on a sugary surface and are easily rubbed off when rubbed with a sampler. Intergranular corrosion of stainless steels and nickel alloys can be avoided by the use of low carbon alloys, the addition of carbide forming elements such as titanium or tantalum, or the use of stabilizing anneals.
Brief Description of Local Corrosion of Stainless Steel
Stress corrosion cracking. A typical example is an insulated steam line made of AISI 316 stainless steel (UNS S31600). Chlorides that may be present in the insulation material can be transferred to the metal surface when it is exposed to rain. This condition satisfies the stress corrosion crack generation conditions: a sensitive alloy—316 stainless steel; a special corrosive—chloride-containing water; and stress—cold-machined or welded pipes. If a cross-section metallographic examination is performed through the crack region, typical transgranular (spanning grain and grain boundaries) and branch cracks will be observed. This is the typical chloride stress corrosion cracking of austenitic stainless steels. Eliminating any of the above three conditions can prevent stress corrosion cracking.
Brief Description of Local Corrosion of Stainless Steel
Oxygen content affects corrosion. In general, the fresh and clean water flowing into the power plant is not corrosive. Steel works well in neutral water and its corrosion rate is directly related to the dissolved oxygen capacity. That is, the more oxygen content, the higher the corrosion rate. The corrosion of steel is also related to the pH value. When the pH is high, the corrosion rate of the steel is low. When the pH drops below 4, the steel rapidly erodes.
Temperature will also accelerate the corrosion of the steel. When the temperature is increased from 72°F to 104°F (22-41°C), it directly affects the corrosion rate of the steel. The flow rate has the opposite effect on the corrosion of the steel. When the seawater flow rate is higher than about 3 feet per second (0.9 m/s), the corrosion of the steel can be greatly accelerated. Mechanical removal of an unprotected corrosive material will result in a high corrosion rate because the removal of the corrosive material exposes a new metal with a high corrosion rate. At the same time, a high flow rate brings a large amount of oxygen to the exposed surface of the metal. Therefore, there is more oxygen to increase the corrosion rate.
If the austenitic stainless steel breaks due to stress corrosion cracking, the alternative material that should be considered is duplex stainless steel. Due to their different structure and composition, they have higher mechanical properties at room temperature up to 600°F (315°C) than 316 stainless steels. They also have higher stress corrosion cracking resistance. Dual-phase alloys can achieve higher resistance to pitting and crevice corrosion by increasing the chromium and molybdenum content.
Effect of chloride concentration on corrosion of stainless steel. When 304 or 304L stainless steel is used in fresh water, the chloride content should be less than 200 ppm. After the components are manufactured, residual iron must be removed. Because the residual iron will act like a gap, it will also react with the chloride to form ferric chloride to accelerate localized corrosion. 304 Pipes need to be periodically cleaned to remove crevices or deposits that can form gaps. Exposure of 304 or 304L-manufactured plant equipment to stagnant water (for example, a flow rate of less than 0.9 m/s) should be avoided because it will form deposits on the metal surface. Microbiological corrosion must also be controlled.
In order to successfully use Type 316L stainless steel in brackish water, the chloride content should be less than 1000 ppm unless the water is completely deoxygenated. Deoxygenated water will prevent pitting, cracking, and stress corrosion of 316L stainless steel. In the production process of the plant, the weld should be fully welded and smooth so as to obtain the best anti-corrosion effect. Electrodes with a high molybdenum content or that match the weld should be used. It is important that the surface of Type 316L stainless steel be cleaned like 304 to remove any residual iron. In general, the best way to remove residual iron is to use a HNO3-HF cleaning agent. In addition, any sediment should also be regularly removed. It is important to take care to avoid the situation of stagnant water. The flow rate of water should be a minimum of 0.9 m/s during the stoppage of the equipment to prevent the formation of deposits.
Metal corrosion is often a complex issue, and even some new forms of corrosion are not well understood by the public. It is recommended that field engineers learn more about corrosion and protection so that they can learn how to deal with corrosion of metal components.