Stress Corrosion Cracking

Description

SCC is the brittle cracking of a metal due to the result of combined effects from localized corrosion and tensile stress. there are many examples in which specific metals and environments in combination cause such problems. a few examples include:

• brass - SCC in solutions with ammonia
• steel - SCC in caustic (high ph), amine solutions
• stainless steels and aluminum alloys - SCC in solutions containing chlorides.
• ti-alloys - SCC in nitric acid or methanol.

Stress Corrosion Cracking of Stainless Steel -

The example shown indicates many intersecting, branched cracks with a transgranular propagation mode. These are typical of stress corrosion cracking (SCC) in austenitic stainless steel. In this case, however, the alloy was reported to be resistant to SCC in the NaCl brine service environment. The location of cracking was limited to a region covered by an elastomeric sleeve. Under the sleeve, evidence of severe general and pitting corrosion were found and evidence of sulfur-containing corrosion products. Analysis of the elastomer indicate that it was not the correct grade and chemical degradation had occurred in service to produce organic acids and sulfur compounds. This local environment resulted in enhanced localized susceptibility of the material to pitting corrosion and SCC.

Prevention or Remedial Action

• lower either applied or residual tensile stresses.
• modification of the environment to eliminate specific scc agent(s).
• change alloy or increase alloy content (i.e. stainless steels and nickel base alloys).
• cathodic protection to change corrosion potential out of scc range.
• add chemical inhibitor.

Standard Test Methods

• ASTM G-30 - practice for making and using U-bend ssc test specimens.
• ASTM G-38 - practice for making and using C-ring scc test specimens.
• ASTM G-39 - practice for preparation and use of bent-beam scc test specimens.
• ASTM G-44 - practice for evaluation of scc resistance of metals and alloys in 3.5% NaCl solution.
• ASTM G-49 - practice for preparation and use of direct tension scc test specimens.
• ASTM G-58 - practice for preparation of scc test specimens for weldments.
• aluminum alloys: ASTM G-44 (seawater - alternate immersion), ASTM G-47(high
• stainless steels and nickel base alloys: ASTM G-35 (polythionic acid),
• ASTM G-36 stainless steels (boiling MgCl₂ solution)
• ASTM G-37: copper-zinc alloys (ammonia solution).
• ASTM D-807 steels (caustic).
• ASTM F-945 titanium (aircraft engine cleaning materials).
• ASTM G129: Slow Strain Rate Testing of Materials for Environmentally Assisted Cracking
• ASTM G142 - Tensile tests method in hydrogen environments
• NACE TM0274 - dynamic corrosion testing of metals in high temperature water.

Evaluation of SCC and other forms of Environmentally Assisted Cracking (EAC)

The evaluation of SCC and EAC (e.g. , Stress Corrosion Cracking, Hydrogen Embrittlement, Liquid Metal Embrittlement) requires understanding of various materials, mechanical and environmental factors that come together to produce resistance or susceptibility to cracking. In many cases, SCC involves the combination of tensile stress and local anodic attack which dictate the period of incubation prior to the initiation of SCC. Therefore, to conduct tests for SCC, either mechanical or electrochemical means are often utilized to promote localized corrosion so that the inherent susceptibility of the material can be determined. Such techniques include the use of slow strain rate, cyclic slow strain rate, fracture mechanics and electrochemical potential control.

In some cases, where constant load tests are used, environmental cracks can initiate but not propagate through the entire cross-section of the specimen. Therefore, the specimen may not fail, but significant cracking may take place.