H 2S and SSC – A form of corrosion that is routinely encountered in oil and gas production applications is sulfide stress cracking, or SSC. SSC occurs in sour gas and oil environments. In this context, the term ‘sour’ means that sulfur and hydrogen sulfide (H2 S) are present.
SSC is a function of the interaction between hydrogen molecules and a base metal. Hydrogen ions are a product of many corrosion processes. Refer to Figure 17. These ions pick up electrons from the base material and produce hydrogen atoms. Two hydrogen atoms may combine to form a hydrogen molecule. Most molecules will eventually collect, form hydrogen bubbles, and float harmlessly away; however, some percentage of the hydrogen molecules will diffuse into the base metal and embrittle the crystalline structure (a process that is referred to as hydrogen embrittlement). If a certain critical concentration of hydrogen is reached, and if a susceptible material is subjected to tensile stress, SSC will occur.
In many instances (particularly with low carbon and low alloy steels), the cracking will initiate and propagate along the grain boundaries (referred to as intergranular stress cracking). In other materials, the cracking will propagate through the grains (referred to as transgranular cracking).
The precipitating conditions for SSC are described below and they are shown in Figure 18.
• Concentration of H2 S – While the intensity of SSC increases as the concentration of H2S increases (refer to Figure 18), many users select corrosion resistant materials whenever any measurable amount of H2S is present.
• Fluid temperature – SSC is most severe at temperatures between 20 and 120 degrees F. Below this temperature range, the hydrogen diffusion rate is slow enough that the critical concentration is never reached. Above this temperature range, the diffusion rate is fast enough that the hydrogen passes through the material quickly and the critical concentration is never reached. The occurrence of stress corrosion cracking above 120 degrees F is still likely, but it will generally be of another form such as chloride stress cracking. As mentioned, many users select corrosion resistant materials whenever any measurable amount of H2 S is present, regardless of the temperature.
• Tensile stress – A susceptible component must be placed under tensile stress for SSC damage to occur; however, virtually all valve components are stressed. Tensile stress may result from process pressure that acts on valve components, from misalignment of piping, from thermal expansion, and from the residual stress of cold work, welding, or heat treatments.
Figure 19 is an illustration of a valve plug guide that has been severely damaged by sulfide stress cracking. The valve body from which the component was removed was so severely damaged that it would not hold line pressure; i.e., the process fluid seeped through the body wall.
The susceptibility of a material to SSC is related to its hardness level. Hardness is a physical property that relates the resistance of a material to penetration or indentation. In metals, hardness is usually measured in the laboratory by loading an indenter into a material and measuring either the depth or the surface area of the indentation. Several test procedures and scales of hardness have been established. A popular scale is the Rockwell C scale, which is abbreviated as HRC (Hardness Rockwell C).
The range for the Rockwell C scale is from HRC 20 to HRC 60. Generally speaking, most trim materials for general service applications have a minimum hardness in the range of 25 to 35 HRC. For example, untreated 316 stainless steel bar stock has a hardness of approximately HRC 20, although this material may be hardened through various treatments. Harder trims that are designed for erosive applications commonly have a hardness of 38 to 45 HRC. For example, 17-4 stainless steel that is treated to the H1075 condition has a hardness of 35-40, and Alloy 6 hard facing has a hardness of approximately 43. Trim for extremely erosive applications may require material hardness of up to 50 to 60 HRC. 440 C stainless steel in the fully hardened condition has a hardness of 55-60 HRC.
When control valve trim components are heat treated to progressively higher hardness levels, the time to SSC induced failure decreases dramatically. Figure 20 illustrates the relative time to failure (in hours) of bolting materials with varying hardness levels. Because of the relationship of hardness levels and SSC, the hardness of valve construction materials must be less than allowable hardness levels that have been determined by test and evaluation.