API 571 : GALVANIC CORROSION
(API 570 EXAM STUDY GUIDE)
Galvanic Corrosion is the first damage mechanism under the heading "Uniform or Localized Loss of Thickness". There are 7 damage mechanisms to study under this heading for API 570 exam.
4.3.1 Galvanic Corrosion
Galvanic Corrosion occurs when a corrosion cell is formed by the joining of dissimilar metals.
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| CORROSION CELL Image is taken from NACE CIP Study Material |
Description Of Damage
Occurs at the junction of dissimilar metals
A Suitable electrolyte is needed for galvanic corrosion to occur
Electrolyte: A moist environment, An aqueous environment, soils containing moisture
Affected Material
All metals
Exception: Most noble metals
Critical Factors
Actually, four conditions must meet for the occurrence of galvanic corrosion.
1. Anode
2. Cathode
3. Electrolyte
4. Metallic Pathway (or external conductor)
In the context of API 571, the presence of anode and cathode is considered as a single condition. So, we can say that three conditions are needed for the galvanic corrosion to occur.
1. Anode and Cathode
Two different materials or alloys.
2. Electrolyte
A fluid that can conduct a current.
Moisture or a separate water phase is usually required for the solution to have enough conductivity
3. Metallic Pathway (or external conductor)
The contact can be through welding, bolting, riveting or any other similar means.
Active material: Anode
Noble Material: Cathode
The noble material is protected by sacrificial corrosion of the more active material.
The anode corrodes at a higher rate than it would if it were not connected to the cathode.
The farther the alloys are apart in the table 4-6, higher will be the corrosion.
The exposed surface area of anode and cathode has a significant effect on galvanic corrosion.
The noble material may need to be coated.
The same alloy can act as both cathode and anode.
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| THE SAME ALLOY CAN ACT AS CATHODE AND ANODE Image is taken from NACE CIP Study Material |
Affected Units of Equipment
Heat exchange's where the tube material, tube sheet and /or baffle are dissimilar material.
Click on the below link to see, how cathodic protection is done inside a heat exchanger.
CATHODIC PROTECTION OF HEAT EXCHANGER
Buried pipelines
Ship hulls
Electrical transmission support towers.
Appearance or Morphology of Damage
The more active material can suffer generalized loss in thickness.
The appearance of a crevice, groove or pitting corrosion
Corrosion of the anode may be significantly higher immediately adjacent to the connection to the cathode.
Prevention/Mitigation
The Best method is through good design
The most noble material should be coated
Specially designed electric insulating bolt sleeves and gaskets can eliminate the electrical connection.
Galvanized steel: Zn corrodes preferentially to protect the underlying carbon steel. The cathode to anode relationship reverses at water temperatures over about 66 degree Celsius.
Inspection and Monitoring
Visual Inspection
UT thickness gauging
The damage may sometimes be hidden underneath a bolt or rivet head
Related Mechanisms
Soil Corrosion
LECTURE VIDEO
QUESTIONS AND ANSWERS
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