Stress Corrosion Cracking

Unexpected or premature failure of chemical process equipment constitutes a serious hazard in terms of personnel, plant, and environmental safety. By weakening reliability, such failures also adversely affect productivity and profitability.

Modern experience in chemical plants has been that failures due to environmental cracking are among the most serious of such problems, making up about 20 to 30% of all corrosion failures. The purpose of this article is to discuss in simple terms some of the pertinent information.

General Description

Stress Corrosion Cracking (SCC) has been defined as failure by cracking under the combined action of corrosion and stress. The stress and corrosion components interact synergistically to produce cracks, which initiate on the surface exposed to the corrodent and propagate in response to the stress state.

For any given alloy-environment system, the engineering parameters of concern are as follows:

• Threshold stresses above which cracking occurs
• Metallurgical variables (heat treatment, structure, cold work) that render the alloy susceptible
• Environmental boundary conditions for cracking such as temperature, solution composition, pH, electrode potential, necessary impurities, etc.

It is important to realize that the conditions causing SCC may not occur during normal operation of equipment, but also during startup, shutdown, idle periods, or system upsets. Stresses and environmental conditions under these circumstances can be quite different than those encountered during normal operation.

Identification

Visual identification prior to failure is difficult due to the typical tightness of stress-corrosion cracks. A low-power hand lens will greatly aid determination. Detection of cracks can also be enhanced with ultrasonic, radiographic, or acoustic emission techniques.

Stress-corrosion cracks tend to branch along the metal surfaces. Typically, evidence of corrosion, such as accumulations of corrosion products, is not observed, although stains in the cracked region may be apparent. Stress-corrosion cracks tend to originate at physical discontinuities, such as pits, notches and corners. Areas that may possess high-residual stresses, such as welds or arc strikes, are also susceptible.

Cracking Locations

• REACTORS: All weldments; circumferential welds by which nozzles are attached; radius of dished head; external jackets especially half-pipe coils.
• HEAT EXCHANGERS: Weldments; nozzles; areas immediately adjacent to the tube-sheets; U-bends.
• COLUMNS: Weldments, especially circumferential nozzle welds; radii of dished heads; packing (e.g. Rasching rings); Expanded metal packing or mesh; Trays.
• PIPING: bends; welds

Preventive Measures

There are a number of different ways to control SCC. The method used depends on the application and may involve changing the mechanical, metallurgical and environmental conditions.

• Mechanical
  • Avoid stress concentrators
  • Relieve fabrication stresses
  • Introduce surface compressive stresses
  • Reduce operating stresses
• Metallurgical
  • Change alloy composition
  • Change alloy structure
  • Use metallic conversion coating
• Environmental
  • Modify environment
  • Apply anodic or cathodic protection
  • Add inhibitor
  • Use organic coating
  • Modify temperature

Laboratory and Plant SCC Testing

Material engineers use SCC tests for the following tasks:

• Identifying environments which cause cracking on certain materials
• Ranking materials for relative SCC resistance in certain environments
• Evaluating preventive measures

In plant equipment, stress corrosion cracks may take years to develop. Tests that precisely duplicate the stress level, metallurgical condition and environment anticipated in the plant might, therefore, require months to produce meaningful results. Since engineering decisions seldom allow the luxury of such extensive test times, almost all SCC tests are accelerated in one of several ways to truncate the time to failure. Laboratory and plant SCC testing are important sources of qualitative data for materials engineers.

Some Recent Failures

CASE 1: Fifty-pound steam pipe

Part: 20-inch standard wall C-steel pipe

Appearance: Intergranular stress corrosion cracks transverse and along the weld bead.

Cause: Cracking due to sodium hydroxide contamination in the steam line. Applied thermal tensile stresses and residual welding stresses provide sufficient tensile stresses, above the critical threshold stress, for SCC to occur.

Recommendations: Reduce residual stresses by stress relieving the welds. Reduce applied stresses by re-designing the pipe layout. Improve quality of steam by evaluating the effectiveness of the inhibitor program in utilities.

CASE 2. Third Stage Condenser

Part: Alloy 316L SS condenser tubes and alloy 304 SS shell.

Appearance: Branched cracking at U-bend of tubes and welds near nozzles on shell.

Cause: Chloride stress corrosion cracking. Contamination of cooling water with chlorides.

Recommendations: Material of construction of tubes should be changed from grade 316L stainless steel to a duplex alloy, and that of the shell should be changed from grade 304 stainless steel to grade 316L stainless steel.