Hydrogen induced cracking (HIC) testing of low alloy steel in sour environment: Impact of time of exposure on the extent of damage

Abstract

The selection of materials for application in the oil and gas industry in environments containing hydrogen sulphide (H 2 S) is an important matter for the safety of equipments. In the presence of H 2 S, hydrogen atoms originating from the cathodic corrosion reaction can diffuse into the steel, resulting in cracking mechanisms such as hydrogen-induced cracking (HIC) or sulphide stress cracking (SSC). For the SSC cracking mode, a diagram defining different regions of environmental severity was established in the 1990's. The requirements for SSC testing of steels in a given environment are well defined and accepted by the industry. On the other hand, fit-for-purpose (FFP) testing for HIC remains a subject of debate. The most common practice consists in 96 hours exposure under 1 bar H 2 S. But this test environment is extremely severe, and is often considered to be too conservative for the selection of steels for use in mildly sour environments. However, a universally applicable FFP test method for HIC has not yet been established. In addition to pH and H 2 S partial pressure, test duration is another major parameter for HIC testing. Therefore, this experimental study was launched to investigate the impact of time of exposure on HIC. Experiments consisted of HIC exposure tests based on the NACE TM0284-2003 standard method. Experiments were conducted in different nodes of the pH-pH 2 S diagram respectively between pH 6.5 and 3.5, and between 10 mbar and 1 bar of H 2 S. For each condition, tests with different times of exposure were realised, from only a few hours and up to three months. Characterisation of HIC consisted in ultrasonic testing. Permeation experiments were also performed in selected pH-pH 2 S conditions. Permeation transients were analysed for calculation of diffusion coefficients. Modelling of hydrogen profiles in HIC specimens was then realised, and compared with the results of HIC immersion tests. For each test condition, it was possible to draw the evolution with time of the extent of HIC. Depending on the environment, the minimum duration necessary to initiate HIC was extremely variable, from a few hours in the most severe conditions to several weeks in low severity conditions. The extent of cracking at equilibrium also seemed to vary with pH and H 2 S. A good correlation was found between HIC and permeation results, confirming that HIC is strongly linked with the increase of hydrogen concentration in the steel. These results confirm that low severity conditions require longer exposures to reach equilibrium levels of internal hydrogen. This must be taken into account for FFP HIC testing in mildly sour environment. With that aim, permeation experiments could represent an interesting technique to assess the time to reach a certain level of hydrogen in the material.

Effects of environmental conditions on hydrogen permeation of X52 pipeline steel exposed to high H2S-containing solutions

“…Sosa [6], Cabrera-Sierra [7], Kittel [8], Hernández-Espejel [9] and Zhou [10] reported that the protective property of the sulphide film depended on the factors such as the H 2 S partial pressure (PH 2 S), temperature and pH. The sulphide layer performed better protective property in the acid solution than that in the neutral solution under 1 bar H 2 S at ambient temperature [8,11]. However, Tang [12] and Kim [13] reported that the corrosion rate and the hydrogen permeation rate of carbon steel increased with decreasing pH values of the solution under 1 bar H 2 S because the solubility of sulphide increased with decreasing pH value.…”

“…The corrosion scale acted as a diffusion barrier for the species involved in the corrosion process can slow down the corrosion process and prevent the underlying steel from further dissolution [5]. Sosa [6], Cabrera-Sierra [7], Kittel [8], Hernández-Espejel [9] and Zhou [10] reported that the protective property of the sulphide film depended on the factors such as the H 2 S partial pressure (PH 2 S), temperature and pH. The sulphide layer performed better protective property in the acid solution than that in the neutral solution under 1 bar H 2 S at ambient temperature [8,11].…”

Corrosion Behavior of API X100 Steel Material in a Hydrogen Sulfide Environment

“…It is believed that hydrogen atoms produced due to surface corrosion of the steel diffuse into it through microstructural defects that exist in the material [38]. The accumulation of a critical amount of hydrogen in the metal defeats results in HIC initiation and propagation, as recently reported by Kittel et al [39].…”

Corrosion Behavior of API 5L X52 Steel under High Concentrations of H<sub>2</sub>S/CO<sub>2</sub> Gases

“…These fractures can be the beginning of more serious damage for the integrity of API 5L X52 steel, when exposed to environments with high hydrogen sulfide contents. Under these conditions, the development of some characteristic corrosion processes such as hydrogen induced cracking (HIC), sulfide stress cracking (SSC) and stress-oriented-hydrogen-induced cracking (SOHIC) [32] [33] [34] has been observed. In this particular case and according to what has been established by some researchers, this corrosion type would seem to be the most common failure for the steel, being the most dangerous mechanism for a metal fracture, as it can occur instantaneously without previous warning [21].…”