API 510 Corrosion, Welding & Material Defects

Question 1

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

Accepted Answer

Localized breakdown of protective film

Answer

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.

Question 2

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

Accepted Answer

Lack of fusion

Answer

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.

Question 3

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

Accepted Answer

Tensile stress and corrosive environment

Answer

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.

Question 4

Which material defect can be detected by radiographic testing?

Accepted Answer

Slag inclusion

Answer

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.

Question 5

Which welding method is most prone to causing hot cracking?

Accepted Answer

Gas welding

Answer

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.

(API 510) Certified Pressure Vessel Inspector Practice Test

(API 510) Certified Pressure Vessel Inspector Practice Test

API 510 Corrosion, Welding & Material Defects