Chemical Degradation of High-Density Polyethylene
Specimen: High-density Polyethylene Sulfuric Acid Storage Tank (mining industry)
Material: High-density Polyethylene
Background: The plastic sulfuric acid storage tank had been in service for approximately 15 years. An inspection of the tank was performed to determine if it was still suitable for further service.
Service Life: Approximately 15 years.
Findings: Examination of the interior of the tank revealed the interior surface was riddled with surface cracking – caused by chemical attack of the sulfuric acid on the HDPE. A cross-section through the tank wall revealed the presence of micro-cracking. Based upon the condition of the tank, it was deemed unfit for further service and it was recommended that the tanks be replaced immediately.
Failure Due to Poor Design
Specimen: Fractured steel gear shaft from a slag car (mining industry)
Material: AISI/SAE 1038 medium carbon steel
Background: The service history of the shafting was unknown. However, it was known that the gear shaft was subject to abnormal bending and torsional loads due to gear misalignment.
Service Life: Unknown.
Findings: Examination of the fracture surface revealed the cracking had initiated at the keyway in the shaft. Closer examination of the keyway revealed the presence of cracking at the 90° machined corner in the keyway. A sharp corner can act as a stress concentration site in the keyway causing local stresses to be as much as 10 times higher than the average nominal stress. The failure of this shaft could have been avoided if the design had called for:
- A properly machined keyway with a fillet radius to reduce the stress concentration in the vicinity of the keyway, and
- The use of a tougher steel (i.e. able to absorb energy and deform plastically before fracturing), such as an AISI/SAE 4340 (which has an appreciable nickel content to increase the shaft toughness coupled with a higher tensile strength).
Metallurgical Replication of a High Pressure – High Temperature Steam Line
Environment: Steam header, exposure to elevated temperatures of 750°F and pressures of 400 psi.Material: ASTM A-53 Grade B carbon steel pipeService Life: 60 + yearsFindings: Metallurgical Replication of the high temperature–high pressure steam header which had been exposed to 60+ years of service indicated that the header had come to the end of its safe service life. The microstructure indicated a primarily a soft and weak ferrite matrix with ~5% fraction pearlite. Damage from 60+ years of elevated temperature service is shown by the presence of iron carbide particles which have precipitated along grain boundaries from decomposed pearlite lamellae within the grains. Recommendations were made to have the steam header, and adjacent piping replaced.
Specimen: Digester Agitator Blade (Mining Industry)
Material: Alloy 31
Background: The blade is used to mix up a solution of mined slurry and sulfuric acid in a 100,000 liter digester tank.
Findings: The blades had fractured and cracked due to sensitization of the heat affected zone of the weld. The sensitization of the heat-affected zone of the Alloy 31 blades resulted in severe, localized metal loss along the heat-affected zone of the welds (Figure A and B). Sensitization is defined as the precipitation of carbides and intermetallic phases, usually at grain boundaries, on exposure to high temperatures, leaving the near-grain boundary region depleted of corrosion resistant elements and therefore susceptible to preferential attack by a corroding medium. In the situation of the corroded agitator blades, the high temperature of the welding would have caused the precipitation of carbides (most likely chromium carbides). The precipitation of the carbides caused the heat affected zones of the blades to become susceptible to corrosion in the corrosive environment of the digester. The localized metal loss along the heat-affected zone of the weld(s) weakened the load carrying cross-section of the blades resulting in the observed fracture and cracking. It is important to note that this grade of Alloy 31 did not contain any carbide stabilizing elements, such as niobium or titanium. The lack of carbide stabilizers makes the blades perfect candidates for sensitization and preferential corrosion of the heat affected zone. Therefore, the Alloy 31 blades should not have been welded.