Talk about crevice corrosion of metals

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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, changes in the chemical and electrochemical conditions within the crevice solution (a decrease in pH and potential) can lead to severe crevice corrosion, or even perforation or cracking (in the presence of stress). 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 cells also exist in riveted joints; applying a layer of moisture-insensitive filler to such joints can yield good results. Sometimes, poor operation and management can also cause crevice corrosion. For example, a tank made of chromium-8 nickel-9 stainless steel used for storing concentrated chlorosulfonic acid shows very little corrosion in the concentrated acid. 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 filled with concentrated acid at all times, 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 serving as a sacrificial anode to provide cathodic protection for the stainless steel. Thickening the carbon steel portion can result in a reasonable service life. When stainless steel tube bundles are used and the baffles are made of carbon steel, corrosion of the stainless steel tubes is likely to occur. There is a gap between the baffle and the tube, resulting in carbon steel corrosion. Corrosion products adhere to the stainless steel pipes, thereby causing pitting corrosion on them. Therefore, when using stainless steel tube bundles, carbon steel plates should not be used for baffles. In general, to avoid crevice corrosion or prevent corrosive agents from entering cracks, it is necessary to improve installation, connection, 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.