Let’s talk about metal intergranular corrosion

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This post was last edited by Wang Wei2 on 2025-3-7 09:42. Gaps are the main design flaw that causes corrosion, but they are also inevitable in design. Although the corrosion rate of metals in the external solution is very low, the chemical and electrochemical conditions in the gap change (with a decrease in pH and potential), which can lead to severe crevice corrosion, and even perforation or rupture (when stress is present). The characteristics of crevice corrosion must be taken into account when selecting materials. The following examples illustrate the occurrence of crevice corrosion and the protective measures that can be taken. Welds are often the parts of equipment most prone to corrosion. The welding method is very important; using double-sided welding and continuous welding is better than lap welding and spot welding, as the latter create gaps. Oxygen concentration difference cells also exist in the rivet joints; applying a moisture-proof filler to these joints can yield good results. Sometimes, poor operation and management can also cause crevice corrosion. For example, a chromium-8 nickel-9 stainless steel tank used to store concentrated chlorosulfonic acid suffers from corrosion and degradation in such strong acids. After the groove was emptied, it was washed with water; the concentrated acid stored in the groove turned into dilute acid, resulting in severe corrosion inside the groove. Obviously, this corrosion can be avoided by keeping the tank always filled with concentrated acid, or by thoroughly cleaning and neutralizing it. Another example is the crevice corrosion between the tubes in the tube sheet (stainless steel) of a stainless steel heat exchanger; cooling water containing Cl- accumulated in these crevices, causing stress corrosion cracking of the stainless steel tubes, which resulted in damage within 1 to 9 months. Later, a carbon steel plate was used as a lining for the stainless steel plate, with the carbon steel acting as a sacrificial anode to provide cathodic protection for the stainless steel. Thickening the carbon steel section can achieve a reasonable service life. When a stainless steel tube bundle is used and the baffle is made of carbon steel, it can easily cause corrosion to the stainless steel tubes. There is a gap between the baffle and the tube, resulting in carbon steel corrosion. Corrosion products adhere to the stainless steel pipes, causing pitting corrosion on them. Therefore, when using stainless steel tube bundles, baffle plates should not be made of carbon steel. In summary, to avoid crevice corrosion or prevent corrosive agents from entering cracks, it is necessary to improve installation, joining, and welding methods to ensure that the joints are airtight; the cracks can be filled with solder or plastic, or the surface of the cracks can be protected with coatings containing corrosion inhibitors. When using gaskets, it is best to use water-resistant materials, and remove wet gaskets promptly after the vehicle has stopped.