【Corrosion Technology Edition】Daily Question 20170820: What are the characteristics of the corrosion environment when high-temperature hydrogen and hydrogen sulfide coexist?
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When the temperature exceeds 240°C, the corrosion of equipment and pipelines is more severe than that caused by hydrogen sulfide alone on steel, and the corrosion rate generally increases as the temperature rises. In corrosion by high-temperature hydrogen and hydrogen sulfide, the corrosion rate depends only on the partial pressure of hydrogen sulfide, with little relation to the total operating pressure. At the same operating temperature, an increase in the hydrogen sulfide partial pressure leads to an increase in the corrosion rate
The corrosion of carbon steel equipment by hydrogen and wet hydrogen sulfide intensifies as the temperature rises; the corrosion rate is highest at 80 degrees Celsius, while it is lowest at 110–120 degrees Celsius.
In the presence of high-temperature hydrogen along with hydrogen sulfide, the corrosion of steel is more severe than when hydrogen sulfide is present alone, as hydrogen acts as a catalyst during the corrosion process, accelerating it.
When hydrogen sulfide is present in high-temperature hydrogen, the presence of hydrogen increases the corrosivity of hydrogen sulfide at temperatures of 200°C or higher. This constitutes a corrosive environment resulting from the coexistence of high-temperature hydrogen and hydrogen sulfide; generally, the presence of hydrogen sulfide leads to uniform corrosion that results in a reduction of thickness.
A medium containing both high-temperature hydrogen sulfide and hydrogen can cause more severe corrosion to equipment than either hydrogen alone or hydrogen sulfide alone.
Under the coexistence of high-temperature hydrogen and hydrogen sulfide, when the temperature exceeds 240°C, the corrosion of equipment and pipelines is more severe than that caused by hydrogen sulfide alone on steel; the corrosion rate generally increases as the temperature rises. Generally, low-chromium steels are no longer suitable for use in such environments. The factors affecting corrosion by high-temperature hydrogen and hydrogen sulfide include mass concentration, temperature, time, pressure, and alloy composition
High-temperature sulfur corrosion: When processing sulfur-containing crude oil, uniform corrosion caused by high-temperature sulfur occurs in the high-temperature areas of the equipment (240–425°C). In the process of corrosion under such harsh conditions, organic sulfides are first converted into hydrogen sulfide and elemental sulfur; these substances react directly with the surface of steel, causing corrosion. At high temperatures ranging from 375 to 425°C, the following reaction takes place: Fe + H2S → FeS + H2. Hydrogen sulfide can still decompose into S and H2 at temperatures between 350 and 400°C. The elemental sulfur (S) that is released causes more severe corrosion than H2S. Fe+S→FeS: The corrosion caused by sulfur at high temperatures starts at a high rate, and after a certain period of time, the corrosion rate stabilizes. This is due to the formation of a ferrous sulfide protective film.
Since the equipment is exposed to high-temperature and high-pressure hydrogen, hydrogen-induced damage is a significant problem. Corrosion is also severe when high-temperature and high-pressure hydrogen sulfide coexists with hydrogen. For this reason, to resist corrosion by high-temperature hydrogen sulfide, stainless steel (mostly austenitic stainless steel) coatings are also typically welded onto the inner surfaces of equipment such as reactors, and stainless steel materials are used to manufacture the internal components. This may again lead to defects such as hydrogen embrittlement in stainless steel, sulfide stress corrosion cracking in austenitic stainless steel, and hydrogen-induced delamination of the surfacing layer. Additionally, the temper embrittlement failure of Cr-Mo steel was also a problem that attracted worldwide attention
Due to the presence of hydrogen molecules in the environment, hydrogen atoms continuously penetrate beneath the scale layer formed by ferrous sulfide, causing the scale layer to become loose and porous. This allows hydrogen sulfide to diffuse further, accelerating corrosion. Therefore, the coexistence of hydrogen and hydrogen sulfide speeds up the rate of corrosion. Above 260 °C, the higher the temperature, the more pronounced the corrosion. At the same time, the presence of hydrogen sulfide hinders the recombination of hydrogen atoms into molecules, increasing the concentration of hydrogen atoms dissolved in the steel and thus promoting hydrogen corrosion.
When hydrogen sulfide is present in high-temperature hydrogen, the presence of hydrogen increases the corrosion rate of hydrogen sulfide at high temperatures when the temperature is 200°C or higher