API 510 Corrosion Mechanisms and Monitoring 5

Question 1

Which inspection technique is specifically recommended for detecting stress corrosion cracking (SCC) that may be tightly closed and not visible by conventional ultrasonic shear wave methods?

Accepted Answer

Time-of-flight diffraction (TOFD) or phased array UT

Answer

TOFD and phased array UT are more effective than conventional shear wave UT for detecting tight SCC because they use diffraction signals from crack tips rather than reflection from crack faces.

Question 2

Caustic stress corrosion cracking (caustic SCC) in pressure vessels most commonly occurs at what location and under what condition?

Accepted Answer

Weld and HAZ regions under tensile stress with concentrated caustic above 120°F (50°C)

Answer

Caustic SCC preferentially attacks weld and HAZ regions where residual tensile stresses are highest, and requires concentrated caustic (typically >5%) above approximately 120°F (50°C).

Question 3

When reviewing a corrosion monitoring program, an inspector finds that ultrasonic thickness measurements at permanent monitoring points show readings jumping erratically between successive inspections. What is the most likely cause?

Accepted Answer

Measurement error from inconsistent probe placement or surface preparation

Answer

Erratic UT readings at permanent monitoring points most commonly result from inconsistent probe placement, coupling variation, or surface condition differences between inspection rounds.

Question 4

In the context of API 510, what is 'injection point corrosion' and why does it require special inspection attention?

Accepted Answer

Accelerated, localized corrosion downstream of chemical or water injection nozzles due to mixing turbulence and concentration gradients

Answer

Injection point corrosion is accelerated, localized attack downstream of injection locations where turbulence, incomplete mixing, and concentration gradients create highly aggressive conditions.

Question 5

Which alloying element is most effective at improving the resistance of carbon steel to high-temperature sulfidic (H₂S) corrosion above 500°F (260°C)?

Accepted Answer

Chromium additions, with 5-9% Cr providing significantly better resistance

Answer

Chromium is the primary alloying element for sulfidic corrosion resistance; the Modified McConomy curves show that 5-9% Cr steels have dramatically lower corrosion rates than carbon steel.

(API 510) Certified Pressure Vessel Inspector Practice Test

(API 510) Certified Pressure Vessel Inspector Practice Test

API 510 Corrosion Mechanisms and Monitoring 5