Corrosion Protection for Air Cooled Heat Exchangers (Part Two)
3. Erosion Corrosion
Erosion corrosion, encompassing erosion and wear corrosion, results from the high-speed relative motion between a metal surface and a fluid medium. The eroded metal surface appears bright, typically taking the form of grooves, valleys, or teardrops, devoid of corrosive deposits. Unlike corrosion under other stresses, factors influencing erosion corrosion are multifaceted. In addition to the chemical composition, microstructure, mechanical properties, surface roughness, and corrosion resistance of materials, considerations include temperature, pH, dissolved oxygen content, granularity, and hardness of the medium. Furthermore, the shape, structure, flow velocity, and regime of the flowing part play a role.
In air cooled heat exchangers, erosion corrosion is most likely to occur at the joint between the base pipe and the tube plate, as these are typically single-layer or double-layer welded. The fillet weld's height ranges between 1.5 and 2 mm, and prolonged sluicing can thin the weld to the point of leakage. Therefore, a well-thought-out design of the heat exchanger structure can effectively mitigate the rate of erosion corrosion. Common strategies include altering the fluid flow direction through a return pipe or introducing a resistance belt in the base pipe to act as a buffer, reducing media flow rate and altering flow dynamics.
4. Stress Corrosion
Stress corrosion, resulting from the joint action of stress and a corrosive environment, represents a detrimental loss of brittleness. Materials under stress conditions may experience mechanical fracture or fatigue fracture, and when combined with a corrosive environment, corrosion damage can lead to cracking. This type of corrosion is particularly common in air cooled heat exchangers, often manifesting as sulfide stress corrosion cracking (SSCC). SSCC refers to the brittle fracture phenomenon caused by tensile stress in the tube bundle bearing materials within a sulfide medium. Conditions for stress corrosion cracking involve the metal's susceptibility to such cracking, the structure being in contact with a selective corrosive medium, and the presence of tensile stress exceeding a certain threshold.
5. Intergranular Corrosion
Intergranular corrosion is a locally selective corrosion occurring along the grain boundaries of metallic materials in specific corrosive media. The grain boundary, representing the junction between different grains, serves as a site for corrosion due to structural defects. Conditions conducive to intergranular corrosion involve impurities in the metal or alloy, second-phase precipitation along the grain boundary, and differences in chemical composition between the grain boundary and the grain itself. In suitable media, a corroded battery forms, with the grain boundary acting as an anode and the crystal grain as a cathode. Specific corrosive mediums can trigger severe intergranular corrosion, as seen in certain alloy-dielectric systems.
Five basic intergranular corrosion test methods and evaluation approaches are outlined in standards, including oxalic acid electrolytic etching, 65% nitric acid boiling, boiling sulfuric acid-ferric sulfate, boiling sulfuric acid-copper sulfate, and nitric acid-hydrogen fluoride.
