NACE TM0177 SSC

ST conducts Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments.

What is Sulfide Stress Cracking (SSC)?

Sulfide Stress Cracking is the cracking of a metal under the combined action of tensile stress and corrosion in the presence of water and hydrogen sulfide (a form of hydrogen stress cracking). Sulfide stress cracking is a form of hydrogen embrittlement that occurs in high-strength steels and in localized hard zones in weldment of susceptible materials when the environment contains wet H2S.

Stress-corrosion cracking—a cracking process requiring the simultaneous action of a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast fracture. It also excludes intercrystalline or transcrystalline corrosion which can disintegrate an alloy without either applied or residual stress.

NACE TM 0177 Method A—NACE Standard Tensile Test , Method B—NACE Standard Bent-Beam Test / NACE TM0316 & Method C—NACE Standard C-Ring Test are conducted at our lab.

This standard covers the testing of metals subjected to tensile stresses for resistance to cracking failure in low-pH aqueous environments containing H2S. Carbon and low-alloy steels are commonly tested for EC resistance at room temperature where SSC susceptibility is typically high. For other types of alloys, the correlation of EC susceptibility with temperature is more complicated.

ST conducts Sulfide Stress Cracking Tests at High Temperatures / Pressures in special Autoclaves as per NACE TM-0177 Method A, Method B- ASTM G39 (FPB) / NACE TM0316 and Method C.

The dominant cracking mechanisms for most classes of materials in the presence of H2S vary with temperature. Ferritic steels and ferritic and martensitic stainless steels crack primarily by a hydrogen (i.e., cathodic) mechanism and have maximum susceptibility near room temperature. For austenitic stainless steels, as temperature increases, cracking susceptibility increases due to the major contribution from anodic processes. Duplex stainless steels exhibit mixed behavior, with maximum susceptibility to cracking in a mid-range of temperatures. To facilitate testing in simulated service conditions or to predict worst-case conditions, and to facilitate testing with H2S partial pressure exceeding 100 kPa (absolute) (14.5 psia), the following modified techniques are available.

Testing at elevated temperatures and pressures involves additional safety considerations compared to room temperature and atmospheric pressure testing. Because H2S may be consumed during the test, gas replenishment and continuous gas bubbling techniques are described. The H2S loss rate and its effect on the corrosiveness of the test environment are functions of several factors, including the corrosion rate of the test material and the partial pressure of H2S in the test environment. Partial Pressure, H2S presence in required amount shall be demonstrated, by measuring its concentration in the test solution.

Test Method A—Standard Tensile. Room Temperature and Elevated Temperature / Pressure

Method A, the NACE standard tensile test, provides for evaluating metals for EC resistance under uniaxial tensile loading. It offers a simple unnotched test specimen with a well-defined stress state. EC susceptibility with Method A is usually determined by time-to-failure. Tensile test specimens loaded to a particular stress level give a failure/no-failure test result. When multiple test specimens are tested at varying stress levels, an apparent threshold stress for EC can be obtained.

where: σ = maximum tensile stress, E = modulus of elasticity, t = thickness of specimen, y = maximum deflection (between outer supports), H = distance between outer supports, and A = distance between inner and outer supports. The dimensions are often chosen so that A = H⁄4.

Testing at Room Temperature and Elevated Temperature / Pressure

Test Method B—Standard Bent-Beam. ASTM G39 / NACE TM0316

Method B, the NACE Standard Bent-Beam Test, provides for testing carbon and low-alloy steels subjected to tensile stress to evaluate resistance to cracking failure in low-pH aqueous environments containing H2S. It evaluates EC susceptibility of these materials in the presence of a stress concentration. The compact size of the bent-beam test specimen facilitates testing small, localized areas and thin materials. Bent-beam test specimens loaded to a particular deflection give a failure/no-failure test result. When testing multiple test specimens at varying deflections, a statistically based critical stress factor (Sc) for a 50% probability of failure can be obtained. NaCl is not added to the test solution for this test method. Laboratory test data for carbon and low-alloy steels have been found to correlate with field data.

ASTM G39-The bent-beam test is best suited for flat product forms, such as sheet, strip, and plate. For plate material the bent-beam specimen is more difficult to use because more rugged specimen holders must be built to accommodate the specimens. A double-beam modification of a four-point loaded specimen to utilize heavier materials is shown in fig C to the left.

Significance and Use:

The bent-beam specimen is designed for determining the stress-corrosion behavior of alloy sheets and plates in a variety of environments. The bent-beam specimens are designed for testing at stress levels below the elastic limit of thealloy. For testing in the plastic range, U-bend specimens should be employed (see Practice G30). Although it is possible to stress bent-beam specimens into the plastic range, the stress level cannot be calculated for plastically-stressed three- and four-point loaded specimens as well as the double-beam specimens. Therefore, the use of bent-beam specimens in the plastic range is not recommended for general use.

Testing at Room Temperature & Elevated Temperature / Pressure

Test Method C—Standard C-Ring Test.

Method C, the NACE Standard C-Ring Test, provides for evaluating the EC resistance of metals under conditions of circumferential loading. It is particularly suitable for making transverse tests of tubing and bar. EC susceptibility with the C-ring test specimen is usually determined by time-to-cracking during the test. C-ring test specimens, when deflected to a particular outer fiber stress level, give a failure/no-failure result. When testing multiple C-ring test specimens at varying stress levels, an apparent threshold stress for EC can be obtained.