Corrosion patterns of stainless steel in industrial water systems
1. Corrosion of cooling water to the stainless steel heat exchanger
Chromium-nickel steel, especially 18Cr-8Ni austenitic stainless steel, is most widely used in the chemical industry due to its high stability in many chemical media and its ability to resist high-temperature gas corrosion. Although stainless steel has a very low overall corrosion rate in various industrial waters, under actual industrial production conditions, accidents of corrosion damage to stainless steel equipment, especially various industrial water coolers, are very frequent.
In industrial water, the pitting corrosion and stress corrosion cracking of stainless steel is caused by chloride ions in the water, so people often hope to find out the critical chloride ion concentration that causes stress corrosion cracking, but due to the factors that cause localized corrosion of stainless steel in the actual operating device It is difficult to determine, so it is often found that the life of two equipment with roughly the same conditions is very different; the same stainless steel has stress corrosion cracking in cooling water with a low concentration of chloride ions (only 10~20mg/L). However, it is safe to use for a long time in seawater with a high concentration of chloride ions. Although the mechanism of stress corrosion cracking has not been fully understood so far, and the boundary conditions for completely avoiding or eliminating stress corrosion cracking cannot be proposed, the statistical analysis of the operation of a large number of industrial equipment and many in-depth laboratory studies have enabled people to understand To the main factors affecting stainless steel pitting corrosion and stress corrosion cracking, put forward some statistical laws, these works are very beneficial to prolong the operating life of stainless steel equipment.
The corrosion resistance of stainless steel often depends on the oxide film existing on the metal surface, and the oxide film can only be formed when there is oxygen, oxidant, or anodic polarization. Once the necessary oxidation conditions are lost, the oxide film will be destroyed, such as in crevices and under deposits. Such damage conditions may occur. In water with fully dissolved oxygen, as long as the water flow rate is not lower than 1.5m/s, no deposits will appear on the surface of the stainless steel, and the integrity of the oxide film can be maintained. Under the actual operating conditions of industrial water, the water flow rate is often lower than 1.5m/s, various solid particles will deposit, microorganisms in the water will cause dirt, and sometimes there will be gaps in the equipment structure, especially in the water. Aggressive chloride ions, result in pitting corrosion and even stress corrosion cracking. Of course, there must be tensile stress in order to produce stress corrosion cracking. In addition, pitting corrosion and stress corrosion cracking are related to chloride concentration, pH value, oxygen concentration, temperature, stress level, cation, and other factors, but the degree of dependence is different.
2. Pitting corrosion and crevice corrosion of stainless steel
Stainless steels immersed in chloride-containing aqueous solutions are prone to pitting and crevice corrosion. When the stainless steel is above the critical pitting potential in the corrosive medium, the active area will be polarized by the anode current due to the existence of a passivation-activation battery, which will intensify the development of pitting corrosion. Once pitting is induced and developed, the chloride ions in the pits will autocatalyze concentration (enrichment). Numerous studies have revealed that the pH value and chloride ion concentration of the solution where pitting and crevice corrosion occur are completely different from the bulk solution. The pH value at the place of occurrence is significantly lowered, and the concentration of chloride ions is highly concentrated. There are literatures that use artificial corrosion pits to study the corrosion of 304L, 316L, and 18Cr15Ni5Mo (at 70°C, 0.5mol/L, NaCl solution), the measurement results of the composition and pH value of the solution in the hole are shown in the table below.
The concentration of original chloride ions in raw water or circulating water does not have decisive significance on whether pitting corrosion occurs in stainless steel. In the system where pitting occurs, the test results of chloride ions in the pits prove that the concentration of chloride ions is amazing. For example, the cooling water of a factory contains only 50mg/L of Cl-, but the concentration of Cl- in the corrosion pit is as high as 104mg/L. Usually, the Cl-concentration in the corrosion pit is several thousand mg/L, and the CI-concentration in the artificial crevice can reach 105mg/L. This fact also exists in production practice. 1Cr18Nil0Ti steel has pitting corrosion in tap water containing 196mg/L of chloride ions and in river water with low chloride ion concentration, and pitting corrosion occurs under sediments. Once steps are taken to remove the deposit or rust, the problem is avoided. As another example, in a water system with low chloride ion content, austenitic stainless steel was also damaged, and a high concentration of chloride deposition was found at the damaged site. These facts show that despite the large difference in the Cl-concentration of the system, as long as there is sediment, it will eventually produce the same bad results. Although the increase of CI- concentration will negatively shift the pitting corrosion potential of stainless steel, the effect is not obvious under the conditions of normal temperature and low Cl- concentration (<10< span=””>3mg/L).
3. Stress corrosion cracking of stainless steel
In industrial water, the stress corrosion cracking of austenitic stainless steel is induced by pitting corrosion. The influencing parameters of the two are the same, but the critical values required by each are different. As for the effect of stress when stress corrosion cracking occurs, the 18-8 austenitic stainless steel was studied in 0.05mol/L NaCI with the specimen in the occluded area under tension, and it was found that when the specimen was anodically polarized (oV, SCE), The crack is the active area, the tensile stress promotes the rupture of the passivation film, and makes it difficult to repair the film. As a result, the potential here becomes more negative, the pH value drops faster, and the corrosion is more serious.
4. Intergranular corrosion of stainless steel
In most cases, intergranular corrosion of austenitic stainless steels is caused by chromium-depleted regions adjacent to grain boundaries. Stainless steel must have a certain amount of chromium. If the chromium content decreases, its corrosion resistance will deteriorate. When the carbon content is ≥0.02%, in the temperature range of 510~788°C, chromium carbide Cr23C8 or carbon will precipitate at the grain boundary. In this way, chromium will be separated from the solid solution in the form of chromium carbide, which will reduce the chromium content near the grain boundary and form a chromium-depleted area. The chromium-depleted area adjacent to the grain boundary corrodes due to poor corrosion resistance. 18-8 stainless steel (type 304) generally contains 0.06% to 0.08% carbon, and there is enough carbon and chromium to form chromium carbide precipitation to form a chromium-depleted area between the grains, as shown in the figure below. Weld corrosion is a special intergranular corrosion of stainless steel. The weld corrosion zone is usually on a strip (heat-affected zone) slightly away from the weld on the base plate, and this part of the stainless steel has been heated in the sensitizing temperature range during the welding process.