Corrosion Mechanisms and Monitoring Flashcards

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?

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

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.

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

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

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).

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?

Answer: Measurement error from inconsistent probe placement or surface preparation

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

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

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

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

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

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

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.

A pressure vessel operating in wet CO₂ service develops general thinning with a pH-dependent corrosion rate. What corrosion product is responsible for the partial protective film that forms?

Answer: Iron carbonate (FeCO₃) scale, also called siderite, formed at higher pH and temperature

In wet CO₂ corrosion (sweet corrosion), iron carbonate (FeCO₃/siderite) forms at higher pH and temperatures, providing partial protection that reduces corrosion rate when the film is stable.

What corrosion mechanism is described as 'liquid metal embrittlement' (LME) and what is a common example relevant to pressure vessel inspection?

Answer: Brittle fracture of a solid metal caused by contact with a liquid metal, such as zinc (galvanizing) on carbon steel at elevated temperatures

LME is brittle fracture of solid metal in contact with a liquid metal; zinc from galvanized coatings or fasteners contacting carbon or stainless steel at elevated temperatures is a common refinery example.