2205 Corrosion in seawater fisheries

There is welding processing, the welding position is shown in the picture above

. . . . No answer, I will analyze it myself. Dear masters, please think about it, right?: The common types of corrosion are stress corrosion, intergranular corrosion, point and crevice corrosion, etc. Surface corrosion is inconsistent with this question. 1. 2205 dual-phase steel contains ferrite and is relatively stable. Generally, stress corrosion occurs rarely. However, it is relatively close to the welding position, so it is not sure whether it is stress corrosion. 2. The carbon content is also relatively low, and intergranular corrosion rarely occurs normally. 3. Pitting corrosion. The chloride ion content in seawater is about 19,000 ppm. Under normal ambient temperature, it should not meet the pitting corrosion requirements. 4. Crevice corrosion. When the equipment is salvaged, it invades the water part and is completely covered by aquatic plants such as algae. Refer to the crevice corrosion process: In the narrow gap between metal or non-metal surfaces, the movement of the solution is blocked. After the oxygen in the solution in the gap is depleted, chloride ions migrate from the outside of the gap to the gap, and due to the hydrolysis of the metal chloride and the autocatalytic acidification process, the passivation film ruptures, resulting in localized corrosion similar to autocatalytic pitting corrosion. Above, I am not sure whether it is stress, crevice corrosion, or the heat-affected zone of welding processing. Can anyone provide further analysis? I would be very grateful!

I didn't see any obvious corrosion, I just saw a lot of sludge, and I don't know how to tell whether it's pitting corrosion or stress corrosion.

Clean the surface and at least see the corroded part and its surroundings. Where is the corroded part? It is estimated that it should be around the welding seam. Due to welding, there is chromium deficiency around the welding seam. It’s best to take some partial and full photos after cleaning it up.

This post was last edited by tie008 on 2018-1-1 18:24 I can’t see the basis for judgment from the photos~~~~If it is expressed according to corrosion classification, I think: 1. A Cl- environment of 19000ppm is already a relatively strong pitting corrosion environment. ； 2. Corrosion in the oxygen-rich zone of the waterline has entered the category of electrochemical corrosion ； (The so-called "concentration battery" appears near the waterline. The oxygen-rich area serves as the cathode, while the steel specimen near the lower side of the waterline serves as the anode and suffers corrosion. Usually called waterline corrosion) 3. It is far-fetched to explain the adhesion of aquatic organisms in terms of crevice corrosion environment~~~It is more reasonable to analyze it from the perspective of chemical corrosion ； 4. The condition of the weld is unclear and difficult to judge (from experience, it often appears as an anode). Finally, if you want to analyze this problem in depth, refer to the opinions on the 6th floor to make preparations.

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It is important to analyze the problem, but more important is how to solve the corrosion problem. Unexpected problems will inevitably occur during the forging, processing and use of steel, and corrosion is inevitable. It is difficult to achieve long-term anti-corrosion solely by relying on the material itself, and it goes against the laws of nature. Through effective anti-corrosion treatment, such as inorganic anti-corrosion coating, it not only increases the insurance factor during the use of materials, but also effectively reduces the material selection requirements of the original profiles. The coating of inorganic anti-corrosion coatings (ZS-711 series, originally used for the coating of amphibious tanks) can prevent seawater corrosion (chloride ion corrosion) and can also effectively prevent the attachment and reproduction of microorganisms (inorganic products, no food source). Through coating, the original stainless steel material can be changed into ordinary carbon steel, reducing equipment costs. The choice of any material is definitely not something that can be done once and for all when faced with corrosion. One hero has three gangs, and the composite application of multiple materials is the best way to truly solve the defects of complementary corrosion material selection.

The maximum temperature resistance is 400℃ and the minimum temperature is minus 30℃. However, if Ph≤4, other products need to be selected for protection.