Common corrosion environments in the petrochemical production process

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1. Metal materials and environmental combinations that can lead to stress corrosion cracking: (1) For carbon steel and low-alloy steel, the media include alkaline solutions, nitrate solutions, anhydrous liquid ammonia, wet hydrogen sulfide, acetic acid, etc； (2) For austenitic stainless steels, the media include chloride ions, chlorides + steam, hydrogen sulfide, alkaline solutions, etc ； (3) For molybdenum-containing austenitic stainless steels, with media such as alkaline solutions, chloride aqueous solutions, and sulfuric acid+copper sulfate aqueous solutions, etc ； (4) For brass, the media include ammonia gas and solutions, ferric chloride, wet sulfur dioxide, etc ； (5) For titanium, the media include methanol or ethanol containing hydrochloric acid, molten sodium chloride, etc ； (6) For aluminum, the media include wet hydrogen sulfide, hydrogen-sulfur-containing gases, seawater, etc ； The engineering measures to prevent stress corrosion cracking in materials include the following: firstly, reducing the stress levels, avoiding or minimizing local stress concentrations, and eliminating residual stresses from processing and welding ； The second is to control sensitive environments, such as by adding corrosion inhibitors, raising the pH value of the medium, and employing electrochemical protection measures ； Thirdly, the right materials should be selected to avoid situations in which environments that can lead to stress corrosion cracking occur. 2. Intergranular corrosion of austenitic stainless steel: During welding of austenitic stainless steel, the areas 2–3 mm on either side of the weld can be heated to 400–910°C; this is what is known as the intergranular corrosion sensitization zone. In this region, chromium and carbon at the grain boundaries combine to form Cr23C6, which precipitates out of the solid solution. Chromium has poor mobility and does not easily diffuse from within the grains to the grain boundaries; as a result, chromium-deficient regions form at these boundaries. Steel must contain more than 11% chromium to exhibit good corrosion resistance; the chromium content in the chromium-deficient regions can drop to around 11%. In certain corrosive solutions, a \"chromium carbide (cathode) – chromium-deficient region (cathode) cell\" is formed, leading to corrosion in the chromium-deficient regions at the grain boundaries. The main engineering measures to prevent intergranular corrosion in austenitic stainless steels are: (1) performing solution treatment ； (2) Reduce the carbon content in stainless steel, bringing it below 0.03% ； (3) Austenitic stainless steel containing stabilizing elements (mainly titanium and niobium) is used. 3. Environments that lead to wet hydrogen sulfide stress corrosion 1) An environment of wet hydrogen sulfide stress corrosion is present when hydrogen sulfide is present in the medium and one of the following conditions is met: (1) The partial pressure of hydrogen sulfide in the medium is greater than or equal to 0.000345 MPa ； (2) The medium contains liquid water or the operating temperature is below the dew point ； (3) The pH of the medium is less than 6, but it can be greater than 7 when cyanides are present in the medium. 2) When the medium creates an environment conducive to wet hydrogen sulfide stress corrosion cracking, the selected materials shall meet the following requirements ； (1) The yield strength specified in the material standards shall be less than or equal to 355 MPa ； (2) The actual measured tensile strength of the material shall be less than or equal to 650 MPa ； (3) The material should be in the normalized, normalized and tempered, annealed, or quenched and tempered condition ； (4) Carbon equivalent limit: It should be less than or equal to 0.4 for carbon steel and carbon-manganese steel, and less than or equal to 0.45 for low-alloy steel ； (5) Hardness limit: Whether for the material itself or the welds and heat-affected zones, the hardness must be less than or equal to HB200 ； (6) Stress-relief heat treatment or other equivalent heat treatment should be carried out after welding. 4. Environments that lead to stress corrosion cracking in liquid ammonia 1) When the medium is liquid ammonia and one of the following conditions is met, an environment conducive to stress corrosion cracking in liquid ammonia is established: (1) The medium is liquid ammonia with a water content of 0.2% (wt) or less, and it may be contaminated by air (oxygen or carbon dioxide) ； (2) The operating temperature of the medium is above -5°C. 2) When the medium creates an environment conducive to stress corrosion cracking in liquid ammonia, the materials to be used must meet the following requirements: (1) The yield strength specified in the material standards shall be less than or equal to 355 MPa ； (2) The actual measured tensile strength of the material shall be less than or equal to 650 MPa ； (3) The material should be in the normalized, normalized and tempered, annealed, or quenched and tempered condition ； (4) Carbon equivalent limit: It should be less than or equal to 0.4 for carbon steel and low-alloy steel, and less than or equal to 0.45 for low-alloy steel ； (5) Hardness limit: The hardness, whether of the material itself or of the weld and heat-affected zone, shall be less than or equal to HB185 ； (6) Stress-relief heat treatment or other equivalent heat treatment should be carried out after welding. 5. Conditions for stress corrosion in sodium hydroxide solution pipelines: Under certain conditions, sodium hydroxide solution pipelines can cause stress corrosion cracking (alkaline embrittlement) in carbon steel materials. Factors that influence stress corrosion cracking in carbon steel include the concentration of the alkaline solution, operating temperature, and residual stresses present in the material. Under normal circumstances, stress-relief heat treatment should be applied to the weld when the concentration of NaOH (wt) % and the operating temperature exceed the specified values. See Table 5 for details. Table 5: Concentration of NaOH (wt) % 5 10 15 20 30 40 50 60 Operating temperature, °C: 85 76 70 65 54 48 43 40 When the concentration of NaOH (wt) % and the temperature exceed the specified values, it is necessary to consider using nickel-based alloys. See Table 6 for details. Table 6 Concentration of NaOH (wt)%: 10, 20, 30, 40, 50. Operating temperature in °C: 105, 110, 97, 82, 77. 6. Hydrogen embrittlement and hydrogen corrosion: (1) Hydrogen embrittlement: At high temperatures and pressures, hydrogen molecules partially decompose into atomic hydrogen; or in a wet corrosive environment, hydrogen gas undergoes electrochemical reactions to produce atomic hydrogen. These atomic hydrogen atoms penetrate into the steel, reducing the bonding force between steel grains, which results in a decrease in the steel’s elongation rate, reduction in cross-sectional shrinkage, and a drop in strength. This phenomenon is known as hydrogen embrittlement. (2) Hydrogen corrosion: When steel is in prolonged contact with high-temperature, high-pressure hydrogen gas, hydrogen atoms or molecules undergo a chemical reaction with the carbides (cementite) present in the steel to produce methane (Fe3C2+2H2→3Fe+CH4). When this chemical reaction occurs on the surface of the steel, it is referred to as surface decarburization ； When it occurs inside the steel, it is called internal decarburization. Internal decarburization and external decarburization are collectively referred to as hydrogen corrosion. In the case of internal decarburization of steel, the generated methane gas cannot diffuse out of the steel; instead, it accumulates between the grains, creating localized high pressure. This leads to stress concentration, which in turn causes microcracks or blistering in the steel. As a result, the strength and toughness of the steel are reduced—that is, the steel becomes brittle. Hydrogen embrittlement is a one-time embrittlement and is reversible ； Hydrogen corrosion causes permanent embrittlement and is irreversible. Measures to prevent hydrogen embrittlement in engineering include: avoiding use in temperature-sensitive areas, selecting materials with lower strength, and reducing the stress levels of metal components.