You may wonder what it means whenever you see a NACE HIC symbol on a steel product or product line. The answer is very simple. It’s an acronym that stands for “NACE Hydrogen Induced Cracking.” It’s a standard test to determine the number of sulfide inclusions in a material. This is important because it determines how resistant the material is to stress-cracking. This test is used in manufacturing pipelines, refineries, petrochemical plants, and other industries prone to high-temperature corrosion.
Results of tests
Generally, results of NACE HIC tests can be obtained by exposing unstressed test specimens to a standard test solution. The sample is then removed after 96 hours. The pH of the solution is adjusted with HCl or NaOH. The solution can then be run at a reduced partial pressure. The dissolved oxygen content is measured by oximetry.
Typical durations of test samples are 4 days or 30 days. However, longer test periods can be considered representative. In some cases, tests for mildly sour or transition regions may require long exposure times.
Various tests can be used to determine the amount of hydrogen permeability on the metal surface. These tests are designed to determine the degree of environmental severity. These tests can also create environmental severity maps.
A recent study describes the results of a series of NACE HIC tests. The experiments involved five different commercially available sweet service line pipe steels. Each of the steels was tested using the NACE TM0284-2003 test method.
Susceptibility to sulfide stress cracking (SSC)
Several factors contribute to Sulfide Stress Cracking (SSC). The testing temperature is an important factor. The National Association of Corrosion Engineers (NACE) has published several standard test methods. One of them is the method TM0177, which is used to determine the SSC resistance of metals. It uses a double cantilever beam test to assess the effects of tensile stresses and H2S corrosion.
In the case of SSC, the local concentration of hydrogen at the stress concentration area is the dominant contributor. However, the local concentration of hydrogen at the test specimen cannot be measured with existing methods. The test specimen is exposed to a saturated H2S gas and a buffered brine solution. Moreover, the pH of the solution is kept at constant values of 3.5 and 4.5.
The effect of the testing temperature on SSC susceptibility has been discussed in the literature. The mechanism behind this relationship is not fully understood.
Effects of sulphide inclusions on NACE HIC resistance
Various microstructures of pipeline steels have different hydrogen permeation rates. This is a factor affecting HIC resistance. The present study aims to analyze the effect of sulfide inclusions on NACE HIC resistance.
The hydrogen atoms from the steel surface are enriched at the lattice defects caused by inclusions. The higher the input of hydrogen, the faster the growth rate of the HIC crack.
The size of the inclusions, the volume fraction of the inclusions, and their spatial distribution are important determinants of HIC susceptibility. These factors directly impact the fracture toughness and initiation of the HIC cracks. The interconnection of individual cracks can drastically reduce the growth rate.
A series of eight test samples were evaluated to determine the effect of sulfide inclusions on NACE HIC resistance. The specimens were characterized by their tensile strength, yield strength, and sulfide stress corrosion cracking resistance. Each of these plates was immersed in a solution containing 5.0 wt% NaCl and 0.5 wt% CH3COOH in deionized water. The test was conducted according to the international NACE standard TM 0177-A.
Acceptance criteria
Identifying acceptance criteria for NACE HIC is a key step in evaluating a pipe for a specific project. Several tests are conducted to determine if the material can withstand hydrogen-induced cracking. This testing method is used for both welded and low-alloy steels. Depending on the test, the results of each section are reported, or the overall average of all the specimens is determined. These tests are often recommended for welded pipelines.
The European Pipeline Research Group is currently in the process of developing acceptance criteria for sweet service steels. They conducted full-scale tests and laboratory tests for two materials. These tests were compared with the HIC tests and showed that both materials exhibited a degree of propensity for HIC. However, the full-scale tests revealed that there was no apparent cracking. This meant that the pipes passed the test. The manufacturer’s statements backed the test results.
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