A form of corrosion that can occur at the junction of dissimilar metals when they are joined together in a suitable electrolyte, such as a moist or aqueous environment, or soils containing moisture. 4.3.1.2 Affected Material All metals with the exception of most noble metals.
4.3.1.3 Critical Factors 关键因素 a) For galvanic corrosion, three conditions must be met: 1. Presence of an electrolyte, a fluid that can conduct a current. Moisture or a separate water phase is usually required for the solution to have enough conductivity. 2. Two different materials or alloys known as the anode and the cathode, in contact with an electrolyte. 3. An electrical connection must exist between the anode and the cathode.
b) The more noble material (cathode) is protected by sacrificial corrosion of the more active material (anode). The anode corrodes at a higher rate than it would if it were not connected to the cathode. c) A typical listing of the relative position of alloys in seawater is shown in Table 4-6. d) The farther the alloys are apart in the table, the higher the driving force for corrosion.
e) The relative exposed surface areas between anodic material and the cathodic material has a significant affect. 1. Corrosion rates of the anode can be high, if there is a small anode to cathode ratio. 2. Corrosion rates of the anode will be less affected if there is a large anode to cathode ratio. 3. If there is a galvanic couple, the more noble material may need to be coated. If the active material were coated, a large cathode to anode area can accelerate corrosion of the anode at any breaks in the coating. 4. The same alloy may act as both an anode and a cathode due to surface films, scale, and/or local environment (for example, old steel pipe connected to new steel pipe).
4.3.1.4 Affected Units or Equipment 影响单元/设备 a) Galvanic corrosion can occur in any unit where there is a conductive fluid and alloys are coupled. Heat exchangers are susceptible if the tube material is different from the tubesheet and/or baffles, particularly if salt water cooling is utilized. b) Buried pipelines, electrical transmission support towers and ship hulls are typical locations for galvanic corrosion.
Galvanic corrosion can occur in any unit where there is a conductive fluid and alloys are coupled. Heat exchangers are susceptible if the tube material is different from the tubesheet and/or baffles, particularly if salt water cooling is utilized.
4.3.1.5 Appearance or Morphology of Damage 破坏外观形 a) Damage occurs where two materials are joined at welded or bolted connections. b) The more active material can suffer generalized loss in thickness or may have the appearance of a crevice, groove or pitting corrosion, depending on the driving force, conductivity and relative anodic/cathodic areas ratio. c) Corrosion of the anode may be significantly higher immediately adjacent to the connection to the cathode, depending on solution conductivity (Figure 4-70 and Figure 4-71).
Figure 4-71 – Galvanic corrosion of a carbon steel nipple in a SS vessel in warm water service.
4.3.1.6 Prevention / Mitigation 预防/缓解 a) The best method for prevention/mitigation is through good design. b) Differing alloys should not be in intimate contact in conductive environments unless the anode/cathode surface area ratio is favorable. c) Coatings can be helpful, but the more noble material should be coated. d) For piping, specially designed electric insulating bolt sleeves and gaskets can eliminate the electrical connection. e) Galvanic corrosion is the principle used in galvanized steel, where the Zn corrodes preferentially to protect the underlying carbon steel. (If there is a break in the galvanized coating, a large anode to small cathode area prevents accelerated corrosion of the steel). This anode-to-cathode relationship reverses at water temperatures over about 150 F (66 C). °
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4.3.1.7 Inspection and Monitoring Visual inspection and UT thickness gauging are very effective methods for detecting galvanic corrosion. The damage may sometimes be hidden underneath a bolt or rivet head. 4.3.1.8 Related Mechanisms Soil corrosion (see 4.3.9).
This anode-to-cathode relationship reverses at water temperatures over about 150oF (66oC).