Corrosion, Welding & Material Defects Flashcards

What is the primary cause of pitting corrosion in pressure vessels?

Answer: Localized breakdown of protective film

Pitting corrosion in pressure vessels is primarily caused by the localized breakdown of a protective passive film on the metal surface. This breakdown, often initiated by aggressive ions like chlorides, creates small anodic sites where corrosion accelerates rapidly, forming deep, localized pits. These pits can be particularly dangerous as they can lead to through-wall penetration without significant overall material loss.

Which welding defect is characterized by the failure of fusion between the weld metal and base metal?

Answer: Lack of fusion

Lack of fusion is a critical welding defect characterized by the failure of the molten weld metal to completely fuse with the base metal, or with previously deposited weld beads. This creates a discontinuity that acts as a stress riser, significantly reducing the strength and integrity of the welded joint. It is a serious defect that can lead to catastrophic failure in pressure vessels.

What is the typical cause of stress corrosion cracking (SCC)?

Answer: Tensile stress and corrosive environment

Stress Corrosion Cracking (SCC) is a dangerous form of material degradation caused by the synergistic action of a susceptible material, a specific corrosive environment, and tensile stress. The combination of these three factors leads to the initiation and propagation of cracks, often intergranularly, which can result in sudden and catastrophic failure of pressure vessels even at relatively low overall corrosion rates.

Which material defect can be detected by radiographic testing?

Answer: Slag inclusion

Radiographic Testing (RT) is highly effective at detecting internal volumetric defects, such as slag inclusions, due to its ability to visualize density differences within the material. Slag inclusions, which are non-metallic compounds trapped in the weld metal, appear as darker, irregular indications on a radiograph because they are less dense than the surrounding metal. This allows for their clear identification and assessment.

Which welding method is most prone to causing hot cracking?

Answer: Gas welding

Gas welding, particularly oxy-acetylene welding, is often considered more prone to hot cracking (solidification cracking) than other welding methods. This is due to the slower cooling rates and broader heat-affected zones associated with gas welding. These conditions can promote the segregation of impurities to grain boundaries, making the material susceptible to cracking as it solidifies under tensile stress.

What type of corrosion affects the grain boundaries of a material?

Answer: Intergranular corrosion

Intergranular corrosion is a specific type of corrosion that preferentially attacks the grain boundaries of a metal or alloy. These boundaries often have a different chemical composition or microstructure, making them more anodic and thus more susceptible to corrosive attack than the grains themselves. This can lead to a severe loss of strength and ductility, even with minimal overall material loss, making it particularly insidious.

Which defect is identified by a sharp groove at the weld toe?

Answer: Undercut

Undercut is a common welding defect characterized by a sharp groove melted into the base metal adjacent to the weld toe, without being filled by weld metal. This groove reduces the effective thickness of the base metal, creating a stress concentration point that can weaken the joint and lead to premature failure. It is typically caused by excessive welding current, high travel speed, or improper electrode angle.

What is the best inspection method to detect surface cracks in welds?

Answer: Liquid penetrant testing

Liquid penetrant testing (LPT) is highly effective for detecting surface-breaking discontinuities like cracks, laps, and porosity in welds. It works by applying a penetrant liquid that seeps into surface flaws, followed by a developer that draws the penetrant out, making the defects visible. Unlike ultrasonic or radiographic testing, LPT specifically targets and excels at identifying flaws open to the surface, making it ideal for surface crack detection.

Which condition can lead to hydrogen-induced cracking (HIC)?

Answer: Presence of hydrogen and stress

Hydrogen-induced cracking (HIC) occurs when atomic hydrogen diffuses into the metal, typically steel, and then combines to form molecular hydrogen at internal discontinuities, creating pressure. This process, combined with residual or applied tensile stress, can lead to the formation and propagation of cracks. Therefore, both the presence of hydrogen (often from welding or corrosive environments) and sufficient stress are critical for HIC to initiate and develop.