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SSC/SSCC Test

SOHIC开裂检测 Test Item

Sulfide Stress Cracking, SSC

Test Purpose

Examine the Damage Resistance of Metals in Acidic Aqueous Solution Containing H2S

Test Object

Petroleum Pipeline, Flange, Valve, Automatic Instrument, Boiler, Liquid Gas Storage Tank and other Pressure Vessels, metal products

 

 

 Project Introduction

 Te

     Sulfide stress corrosion cracking (SSCC) is that metal material subjected to tensile stress in sulfide medium occur brittle fracture due to the coupling effect of medium and stress, and the same as the sulfide stress cracking (SSC). The test is performed according to NACE TM0177-2005. SSC test mainly adopts constant load stress corrosion test (Method A) and three point bending test(Method B).

    In acidic environment, sulfide stress corrosion cracking (SSCC) is a kind of corrosion that has the largest destructiveness and dangerousness, and gains universal attention by experts both at home and abroad. SSCC is the destruction under the dual function of applied stress and corrosive environment, which has three necessary conditions: sensitive material, acidic environment and tensile stress. SSCC has some differences with common stress corrosion. In the common stress corrosion, the role of environment is the anodic dissolution, but SSCC is cathodic hydrogen charging. The SSC mechanism has not yet been fully revealed. But at the moment, most scholars tend to that the cracking is caused by hydrogen, and sulfur ions or hydrogen sulfide have a certain role in promoting the diffusion of hydrogen into the materials.

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 Project Introduction

        At present, the research on the mechanism of SSCC is hydrogen embrittlement theory, which is thought that the cathode reaction of corrosion to produce hydrogen, hydrogen atoms diffuse into the metal, spread to the crack tip, and make this area to be fragile, finally, metal happens brittle fracture under the action of tensile stress. Hydrogen plays a major role in stress corrosion, but the views on how to cause the brittle fracture by hydrogen are different: some scholars believe that hydrogen reduces the crack front atomic key binding energy; some scholars believe that the effect of adsorbed hydrogen decrease surface energy; also some scholars argue that existence of hydrogen causes high inner pressure, promotes the dislocation activities and so on.

      In recent years, with the deepening of stress corrosion cracking under the action of stress, some corrosion cracking phenomenon can be analyzed better by combining anodic dissolution with embrittlement of hydrogen diffusion.It is generally believed that the role of hydrogen in stress corrosion should be based on specific circumstances: in some corrosion system, hydrogen embrittlement is given priority, and anodic dissolution is given priority in other corrosion system. Metal rupture caused by sulfide in high strength steel is thought to be caused by hydrogen brittleness, hydrogen is an important factor of stress corrosion fracture, but in the low strength steel, hydrogen is not the main factor of stress corrosion cracking.

      At present the mechanism about the process of H2S promoting hydrogen permeability has many hypothesis, but the real test date is not much. Some scholars believe that the existence of H2S can lower Fe-H bond energy, and hydrogen is easy to transmit from the metal surface to the depths; some scholars believe that, in H2S solution, the hydrogen in the H2S is easier to get into metal by dissociation than hydrogen atom adsorbed on the metal surface; some scholars think that H2S molecules adsorbed on the metal surface are easy to decompose to new hydrogen atoms, H2S+2e→2H吸附+S2, that results higher concentration of hydrogen on the surface of the metal; some scholars think that H2S acts as catalyst; some scholars believe that dissolved & unionized H2S molecular promote hydrogen embrittlement, because that H2S molecular adsorbed on the surface of steel play a role of bridge ligand for proton discharge, increasing the discharge reaction and making hydrogen into the steel. The above studies provide the basis of the H2S accelerating penetration process of hydrogen, illustrating the H2S stress corrosion cracking is controlled by the diffusion process, and the lattice diffusion is the main controlling factors of the brittle fracture. High strength steel is prone to rupture in acidic H2S environment, because that H2S promotes hydrogen atoms diffusing to crack front in metal and that makes hydrogen embrittlement happen faster. From microscopic perspective, hydrogen embrittlement caused by the internal corrosion goes through the four stages: adsorption of hydrogen atom → dissolution (adsorption) → lattice diffusion → forming hydride → crack or bubble.,

 

 

 Test method and standard

Test Method

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Sample Requirement

Remark

Method A—Tensile Test

NACE TM0177-2005,

GB/T 4157-2006,

ASTM G39-99,

ISO 7539-2011,

Simulated test of working condition

Additional blueprint

Temperature: 25±3℃;

Loaded Stress: 80% yield strength;

Test Medium:

Solution A: NaCl 5%, acetic acid 0.5%, solution saturated by H2S;

Solution B: NaCl 5%, acetic acid 0.23%, sodium acetate 0.4%, solution saturated by H2S;

 

Method B—Bent-Beam Test

American standard (Three-Point Bend):

67。3mm*4。57mm*1。52mm。

Chinese standard (Two-Point Bend): 110-255mm long by 15-25mm wide。 0。8-1。8mm thickness for specimens need be loaded big strength (200MPa)

Method C—C-Ring Test

American standard: Outer diameter ≥15.9mm;Chinese standard : Outer diameter ≥15mm. Specific sizes refer to the drawings

Method D—DCB Test

American and Chinese standards: 110mm*30mm*10mm

Four-Point Bend

110-250mm long by 15-50mm wide, thickness is supplied by client.

Galvanically Induced Hydrogen Stress Cracking (GHSC)

NACE TM0177-2005

Refer to the conditions of H2S test

GHSC test shall be carried out according to the above requirements of the SSC test, and meet the following additional requirements, options and instructions.

a) CRA specimens form a thermocouple with unalloy steel (carbon steel) immersed fully in test solution. The ratio of the area of unalloy steel to the submersed area of CRA specimen shall be 0.5~1 according to NACE TM 0177-1996. Stressing jig shall be insulated with specimens and coupled carbon steel. For assessment of specific application, CRA can coupled with low alloy material which will be coupled with CRA in use and is in use.

b) Test environment is solution A of NACE TM0177-1996, partial pressure of H2S is 0。1MPa, and the temperature is 24℃±3℃。 For assessment of specific application, the SSC test environment described in Clause 2。5 of Appendix E can be used。

 

 Test equipment:Stress Ring

       Our lab is strength, and there are enough stress rings to perform 120 groups of SSC tests and 200 groups of HIC tests at the same time.

硫化氢应力腐蚀应力环

   Sample requirements

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