Effect of Alloying Elements on the Susceptibility to Sulfide Stress Cracking of Line Pipe Steels

• Published in: Corrosion (Houston, Tex.) Tex.), 2004-03, Vol.60 (3), p.262-274
• Main Authors: Koh, S.U., Yang, B.Y., Kim, K.Y.
• Format: Article
• Language: English
• Subjects: Alloying additive; Alloying effects; Alloying elements; Analytical methods; Applied sciences; Chemical precipitation; Copper; Corrosion; Corrosion environments; Corrosion mechanisms; Crack initiation; Crack propagation; Cracking (corrosion); Cracks; Current density; Diffusion rate; Electron microscopy; Elongation; Evaluation; Exact sciences and technology; Fluctuations; Flux; Fracture mechanics; Hardness; Hydrogen; Hydrogen permeation; Light microscopy; Linear polarization; Mechanical properties; Metals. Metallurgy; Microscopy; Microstructure; Nickel; Nucleation; Optimization; Oxidation; Penetration; Pipes; Precipitates; Scanning electron microscopy; Spectroscopy; Steel; Structural steels; Sulfide; Sulfides; Sulphides; Susceptibility; Water hardness
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3287730

ABSTRACTThe effects of alloying elements (Cu-Ni, Mo, and Ti) in line pipe steels on the susceptibility to sulfide stress cracking (SSC) was studied with respect to hydrogen permeation, crack nucleation, and crack propagation. The SSC susceptibility was evaluated using a constant elongation rate test (CERT) method in a NACE Standard TM0177 solution. The corrosion properties were evaluated with potentiodynamic, potentiostatic, and linear polarization methods in the same solution used in CERT. The hydrogen diffusion rate through the steel matrix was determined electrochemically by measuring the hydrogen oxidation current density using Devanathan-Stachurski cells. Crack nucleation and propagation behavior were investigated using optical microscopy (OM), scanning electron microscopy (SEM), and energydispersive spectroscopy (EDS). The cracks nucleated predominantly at nonmetallic inclusions and propagated through the steel matrix in a quasi-cleavage manner regardless of the test materials' compositions. The number of inclusions, which acted as crack nucleation sites, was not influenced by the addition of the alloying elements. Addition of Cu-Ni (Steel A) and Mo (Steel B) increased SSC susceptibility by increasing the hardness value of the steel matrix due to the formation of banitic components in the microstructure, and by increasing hydrogen permeation flux through the steel matrix. Addition of Mo (Steel B) increased hydrogen permeation flux by both inducing pitting in sulfide film, which formed on the steel surface, and by decreasing theSulfide stress cracking (SSC) is a kind of hydrogeninduced degradation, which occurs in an environment containing hydrogen sulfide (H2S). Hydrogen sulfide retards the recombination of hydrogen atoms to the molecular hydrogen gas.1 This poisonous effect accelerates diffusion of atomic hydrogen into the steel leading to catastrophic failure of line pipes. Phenomenologically it has been established that, on the basis of the morphological features of failed steels, two forms of steel degradation by hydrogen atoms occur depending on the hardness of the steel matrix.2 Hard steels with a Rockwell hardness above HRC 22 (~248 Hv) are susceptible to SSC, which proceeds perpendicular to the steel surface. On the contrary, soft steels are resistant to this kind of cracking because nucleated cracks can be surrounded by crack-resistant soft matrix. However, soft steels suf-