Corrosion cracks in the tube sheet of stainless steel fixed tube sheet heat exchangers and anti-corrosion measures

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This post was last edited by Wang Wei2 on 2023-10-4 at 12:04: Corrosion Cracks in the Tube Sheets of Stainless Steel Fixed-plate Heat Exchangers and Anti-corrosion Measures. 1. Overview: After being in use for over a year, two stainless steel shell-and-tube heat exchangers owned by a certain company developed through-going corrosion cracks in their tube sheets. The cracks started at the expansion joint between the tube and the tube sheet, gradually extending toward the other tube opening. The junction between the tube and the tube sheet was covered with rust-like corrosion products; after removing these deposits, multiple corrosion cracks were visible around the tube opening, with the total length of these cracks exceeding 600 mm. 2. Structural type and operating conditions ⑴ Type: The heat exchanger is an 8-tube-side fixed-plate heat exchanger; the design pressure for the tube side is 0.71 Mpa, while that for the shell side is 0.3 Mpa ； ⑵Operating conditions: The design temperature is 170°C on the tube side and 100°C on the shell side. The fluid on the tube side is steam, while the fluid on the shell side is engine oil type 190. The tube sheet and head are made of 1Cr18Ni9Ti, while the heat exchange tubes are made of 0Cr18Ni9. ⑶Water quality: The steam used in the heat exchanger is used to circulate industrial water, which is then heated for use. The condensed water from the steam contains a high amount of Cl-. In the tests conducted on the water vapor, the chloride concentration (expressed as Cl-) was 25.5 mg/l, the total alkalinity (expressed as OH) was 23 mg/l, and the total hardness (expressed as Ca2+) was 18.2 mg/l. 3. Corrosion analysis: The fracture characteristics are consistent with those of intergranular corrosion (network-like) and stress corrosion (along grain boundaries). The main components of the corrosion products are Fe and Cr, with localized enrichment of Cl-. Generally, in water at 200°C, the presence of only 2PMCl- can cause stress corrosion in austenitic stainless steels, with pitting and crevice corrosion being the most common forms. The Cl- content in the steam condensate from this plant is as high as 25.5 mg/l, and this level of Cl- is sufficient to cause intergranular stress corrosion in stainless steel at temperatures between 200 and 300°C. The analysis results also show a significant local enrichment of Cl- within the crack surface. After microcracks form, stress is highly concentrated at the crack tips, causing rapid deformation and yielding in those tips and the adjacent areas. This leads to the formation of slip steps; the recurrence of these slip steps causes the tip surface to be torn open again. Each such tearing of the tip accelerates its further dissolution, and this process repeats itself, allowing the crack to propagate deeper over time. As corrosion progresses and corrosion products accumulate, closed-cell corrosion forms within the cracks; under the combined effect of stress and corrosion, this develops into cracks that extend deeper. Furthermore, the material of the stainless steel shell-and-tube heat exchanger did not meet the required standards, which reduced the substrate’s and grain boundary’s resistance to intergranular corrosion and intergranular stress corrosion. The Cl- ions in the medium, under the combined effect of grain boundary sensitization and residual stresses, caused intergranular stress corrosion; moreover, the accumulation of Cl- ions in the gaps between the tubes and the tube sheet, along with residual stresses and acidic autocatalysis, led to gap corrosion ； The intergranular non-metallic inclusions cause stress concentration and lead to larger grain sizes, which in turn exacerbates the occurrence and progression of corrosion, contributing to the aforementioned corrosion effects. 4. Rectification ⑴ Welding: Structurally, efforts should be made to avoid the formation of enclosed spaces; seal welding should be used between the tube sheet and tubes. Stress relief is necessary after welding, and the heat treatment temperature, duration, and rate of temperature change must meet the specified requirements in order to minimize the formation of chromium carbides, effectively control grain size and refine the grains, thereby reducing the likelihood of corrosion. ⑵Reduce carbon content: Use chromium-nickel austenitic stainless steels with a lower carbon content, such as 0Cr18Ni10Ti, 0Cr18Ni11Nb, or ultra-low carbon stainless steel like 00Cr19Ni10, in order to minimize the risks associated with grain boundary sensitization ； Strictly control the Cl- content to reduce the susceptibility of stainless steel to chloride stress corrosion cracking. After implementing the aforementioned anti-corrosion measures, the corrosion-induced damage caused by cracks in the stainless steel tube sheets was effectively addressed, ensuring the long-term safe operation of the equipment.