API 653 Corrosion Assessment 3 — Questions and Answers
Question 1: In API 653, what is the maximum allowable corrosion rate used to project the retirement thickness when two inspection data points are available?
- The average of all historical rates
- The long-term corrosion rate or short-term corrosion rate, whichever is greater (Correct answer)
- The most recent measured corrosion rate only
- The minimum corrosion rate observed
Correct answer: The long-term corrosion rate or short-term corrosion rate, whichever is greater
API 653 requires using the greater of the long-term or short-term corrosion rate to conservatively project remaining life.
Question 2: Which type of soil condition is generally considered most corrosive to unprotected tank bottoms?
- Sandy, well-drained soil with high resistivity
- Clay-rich, wet soil with low electrical resistivity (Correct answer)
- Dry, alkaline limestone-rich soil
- Coarse gravel with excellent drainage
Correct answer: Clay-rich, wet soil with low electrical resistivity
Clay-rich, wet soils with low electrical resistivity provide an excellent electrolyte path, promoting corrosion of unprotected steel tank bottoms.
Question 3: What is the API 653 definition of a 'critical zone' on a tank shell for corrosion assessment purposes?
- The entire shell from bottom to top
- The area within 12 inches above the tank bottom (Correct answer)
- The top one-third of the shell
- The weld heat-affected zones only
Correct answer: The area within 12 inches above the tank bottom
API 653 defines the critical zone as the shell plate area within approximately 12 inches above the tank bottom, where corrosion is typically most severe.
Question 4: When assessing pitting corrosion in a tank shell, API 653 allows evaluation using which standard?
- ASME B31.3
- API 579-1/ASME FFS-1 (Correct answer)
- NACE SP0169
- API 650 Appendix A
Correct answer: API 579-1/ASME FFS-1
API 579-1/ASME FFS-1 provides fitness-for-service assessment methods for pitting corrosion that are referenced by API 653.
Question 5: Which NDE method is specifically recommended by API 653 for locating weld seam corrosion on tank floors?
- Dye penetrant testing (PT)
- Radiographic testing (RT)
- Vacuum box testing (Correct answer)
- Magnetic particle testing (MT)
Correct answer: Vacuum box testing
Vacuum box testing is used to check weld seams on tank floors for leaks and is commonly used during internal inspection to identify corrosion-related failures at welds.
Question 6: What is the significance of the 'retirement thickness' concept in API 653 corrosion assessment?
- The thickness at which the tank is scheduled for painting
- The minimum acceptable thickness below which the tank must be repaired or taken out of service (Correct answer)
- The original design thickness from the fabrication drawing
- The thickness that triggers a change in inspection interval
Correct answer: The minimum acceptable thickness below which the tank must be repaired or taken out of service
Retirement thickness is the minimum shell or floor thickness below which the tank can no longer safely operate and must be repaired or retired.
Question 7: What causes concentration cell corrosion, and where is it most commonly found in storage tanks?
- High-velocity fluid impingement on the shell
- Differences in oxygen or ion concentration in the electrolyte at different locations on the metal surface (Correct answer)
- High-temperature oxidation at the roof
- Hydrogen embrittlement near welds
Correct answer: Differences in oxygen or ion concentration in the electrolyte at different locations on the metal surface
Concentration cell corrosion is driven by differences in electrolyte concentration (typically oxygen) and commonly occurs under deposits, coatings, or scale on tank floors.
API 653 Corrosion Assessment 3
In API 653, what is the maximum allowable corrosion rate used to project the retirement thickness when two inspection data points are available?