Types of Corrosion in Hydrogenation Units (marketing department of Shanghai HY Industry Co., Ltd)

The main types of corrosion in hydrocracking units are: hydrogen damage (including high-temperature hydrogen corrosion, hydrogen embrittlement, hydrogen-induced peeling), high-temperature H2+H2S corrosion, polythionate corrosion, temper embrittlement of Cr-Mo steel, high-temperature S corrosion, H2S+H2O corrosion and H2S+NH3+H2O corrosion at low temperature parts.

1. Hydrogen damage (FOR Corrosion in Hydrogenation Units)

【Definition】The destruction of the properties of metal materials due to the diffusion of hydrogen atoms into the metal body or the reaction with the metal is called hydrogen damage.
【Location】The main parts of hydrogen damage are: reaction system equipment and pipelines under high temperature and high pressure hydrogen environment.
【Classification】Hydrogen damage can be mainly divided into: hydrogen embrittlement, high temperature hydrogen corrosion and hydrogen induced peeling

(1) Hydrogen embrittlement

【Definition】When steel is used in the presence of hydrogen, hydrogen diffuses into the lattice in an atomic state, and accumulates in the grain boundary or around non-metallic slag inclusions in a molecular state.
【Characteristics】The physical process, which is reversible, is called a primary embrittlement phenomenon. The tensile strength or hardness of the material does not change significantly, but the notch strength or toughness of the material decreases under normal temperature conditions, and cracks sometimes occur. The ductility and toughness of the hydrogen embrittled material can be recovered after dehydrogenation treatment without cracks.

(2) High temperature hydrogen corrosion

【Definition】Hydrogen chemically reacts with inclusions (carbohydrate F3C or solid solution carbon C) or alloy additives (such as Si) in the alloy under high temperature (T>220℃) and high pressure to generate high-pressure gas. This leads to decarburization and grain boundary cracks in the steel. The tensile strength, ductility, and toughness of materials subjected to high-temperature hydrogen corrosion are significantly reduced.
[Features] High-temperature hydrogen corrosion is completely different from hydrogen embrittlement. It is a chemical reaction process and is irreversible, which is called permanent embrittlement. There are two main forms of high temperature hydrogen corrosion: one is surface decarburization; the other is internal decarburization.

Surface decarburization occurs when the steel comes into contact with hydrogen. Surface decarburization does not produce cracks, but the strength and hardness of the material decrease slightly, while the elongation increases. The main reactions that occur are: Fe3C+2H2→CH4+3Fe. This reaction generally starts from the surface of the steel and gradually advances to the inside. The generated methane gas is not easy to escape. They gather around the grain boundaries or impurities, and the local pressure formed can be as high as several thousand atmospheres. Therefore, not only the surface and The inner layer was decarburized and embrittled, and it also developed into severe bubbling cracking. The dissolved carbon in the steel will also react with the dissolved hydrogen in the steel: C+4H→CH4; Si+4H→SiH4.

High temperature hydrogen corrosion is characterized by an incubation period, which can be as short as a few hours or as long as several years depending on the material and environmental conditions. After the steel is corroded by high-temperature hydrogen, the dimensional change of the surface is small, which mainly reduces the yield strength and impact toughness of the material, thereby causing the material to crack. There is a combination of temperature and pressure for hydrogen corrosion of steel, and a latent value. If this limit value is exceeded, hydrogen corrosion will occur.
【Main factors affecting high temperature hydrogen corrosion】

■ Effects of temperature, pressure and exposure time. The higher the temperature or the higher the pressure, the earlier the onset of high temperature corrosion will be.
■Influence of alloying elements and impurity elements.
■The effect of heat treatment. The resistance to hydrogen corrosion of steel is also closely related to the microstructure of steel.
■The effect of stress. The higher the stress value, the stronger the corrosion.

(3) Hydrogen-induced stripping

【Definition】Hydrogen diffuses into the steel under high temperature and high pressure. When the temperature drops below 150°C during equipment maintenance or cooling, since the hydrogen cannot be released in time, part of the hydrogen is absorbed in the steel. Under certain conditions, it will The cracking phenomenon between the surfacing layer and the base metal occurs.

(4) Protective measures against hydrogen damage
■Strictly control the rate of cooling and depressurization, and cannot be too large. Cooling rate: 20～25℃/h, depressurizing rate: 1.0～1.5MPa/h. (It can well prevent hydrogen embrittlement and hydrogen-induced cracking).
■Control the content of impurity elements in the material, post-weld heat treatment.
■Hydrogen corrosion resistant materials are selected according to the latest version of the Nelson curve. Overheating and overpressure of the equipment are strictly prohibited.

2. High temperature H2+H2S corrosion (FOR Corrosion in Hydrogenation Units)

【Definition】High temperature H2+H2S corrosion refers to the corrosion and damage of metals in high temperature (204℃, also said 300~420℃), H2 and H2S environment.
【Corrosion part】The corrosion part of high temperature H2+H2S mainly occurs on the reaction system equipment after hydrogen mixing, such as: hydrogenation reactor, high pressure heat exchanger of reaction effluent, reaction furnace tube, heat high fraction and corresponding process pipeline. The corrosion form is the chemical corrosion of steel by H2S, which is manifested as uniform corrosion, hydrogen embrittlement and hydrogen corrosion.
【Corrosion factor】In the hydrogenation process, hydrogen is also a factor that causes equipment corrosion. It can not only directly corrode metals, but also play a role in promoting high temperature H2S. The influencing factors are:

■Concentration: When the concentration of H2S is below 1% (volume), the corrosion rate increases sharply with the increase of the concentration. When the concentration exceeds 1% (volume), the corrosion rate is basically unchanged.
■Temperature: When the temperature is 315-480℃, the corrosion rate increases correspondingly with the increase of temperature, and the corrosion rate increases by about 2 times for every 50℃ increase in temperature.
■Time: The corrosion rate decreases with the increase of time. Generally, the corrosion rate is the highest within 5000h of the start-up of the device. The corrosion rate is reduced by a factor of 2-10 in later times.
■Pressure: In the high temperature H2S+H2 corrosion, the pressure has no effect on the corrosion rate, but in the pure high temperature hydrogen, the pressure has a great influence on the corrosion.
[Protection measures] High temperature H2S+H2 causes uniform corrosion. The corrosion rate of the material should be estimated strictly according to the Couper curve, and the material should be reasonably designed and selected. Generally, when the temperature is below 250 °C, carbon steel can be selected; when the temperature exceeds 250 °C, use chromium-molybdenum steel (only H2 exists) or austenitic stainless steel (anti-H2+H2S corrosion).

3, even more sulfuric acid corrosion (FOR Corrosion in Hydrogenation Units)

[Corrosion part] Austenitic stainless steel equipment (such as reactor surfacing layer, furnace tube, austenitic stainless steel air cooling, heat exchanger, etc.).
[Protection measures] Use ultra-low carbon or stable austenitic stainless steel; try to eliminate or reduce residual stress caused by cold working and welding in manufacturing, and pay attention to processing so that no stress concentration or a structure with as small stress concentration as possible is formed; Nitrogen protection, keep the equipment temperature at about 150 ℃, neutralize and clean.

4. Temper brittleness of Cr-Mo steel (FOR Corrosion in Hydrogenation Units)

【Definition】When kept in the temperature range of 325~575℃ for a long time or slowly cooled from this temperature range, the fracture toughness of the material will deteriorate. This is due to the segregation of trace impurity elements and alloy elements in the steel to the prior austenite grain boundaries, which reduces the cohesion of the grain boundaries. Once the material undergoes temper embrittlement, its transition temperature shifts to the high temperature side.
[Corrosion part] The parts using Cr-Mo steel mainly occur on 2.25Cr-1Mo and 3Cr-1Mo material equipment (such as reactors, high-pressure heat exchangers, thermal high-grade, etc.).
【Main protective measures】

Strictly control the size of tempering brittleness coefficient (J-coefficient and (X) coefficient) (J=(Si+Mn)×(P+Sn)×104; (X)=(10P+5Sb+4Sn+As)× 10-2).
■The temper embrittlement degree vTr54 +3△vTr54 ≤ 0℃ of the material is controlled by the step cooling test.
■Adopt the hot-state start-up and shutdown scheme (heat up first and then boost, first depressurize and then cool down).

5. High temperature sulfur corrosion (FOR Corrosion in Hydrogenation Units)

【Definition】 High-temperature S corrosion in hydrocracking units is a phenomenon in which the sulfides (mainly H2S and elemental S) contained in the raw materials react with metals at temperatures above 240-260 °C to destroy the properties of metals.
【Corrosion part】 It is caused by uniform corrosion, which mainly occurs in the high temperature parts of the raw oil system and fractionation system before hydrogen mixing (the bottom of the tower, the elbows, tees, and large and small heads of the inlet and outlet pipelines of the reboiler). In the design, the corrosion rate of the material can be estimated according to the McConomy curve.
【Corrosion Mechanism】
H2S+Fe→FeS(<340℃); H2S→S+H2(<340℃); Fe+S→FeS.
From the above reaction equation, it can be seen that the corrosion should form a dense FeS protective film, which can prevent the corrosion reaction from proceeding, but when the medium flow rate is large, the corrosion product FeS is difficult to adhere to the metal surface, so that the protective effect of the application cannot be achieved. In the bare state, the resulting corrosion will continue to develop in depth.
【Anti-corrosion measures】
■In order to slow down the system corrosion of the debutanizer and the first fractionator, adopt the process of the depentanizer or increase the design load of the debutanizer.
■Upper circulating hydrogen desulfurization tower system.
■The corresponding material is Cr-Mo steel.

6. H2S+H2O corrosion at low temperature (FOR Corrosion in Hydrogenation Units)

【Definition】 H2S+H2O corrosion environment, also known as wet hydrogen sulfide corrosion, generally refers to the corrosion caused by hydrogen sulfide when liquid water and hydrogen sulfide coexist (or the water stream is below the dew point). The corrosion form of wet hydrogen sulfide is mainly manifested as uniform thinning of equipment and stress corrosion.
【Corrosion position】The corrosion of low temperature wet hydrogen sulfide mainly exists in the circulating hydrogen desulfurization tower, the rear path of high demarcation level, the main stripper, the debutanizer, the liquid hydrocarbon reflux tank, the cooler and other parts and the corresponding pipelines.

【Corrosion Mechanism】At low temperature, H2S corrosion only occurs in an environment where water and strong acid or oxygen exist at the same time. H2S is a weak acid and ionizes in water:
H2S=H++HS－
HS－=H＋＋S－
Therefore, the anodic reaction of electrochemical corrosion is:
Fe=Fe2++2e
Fe2++S-=FeS or Fe2++HS-=FeS+H++e
The cathodic reaction is: H++2e=2H=H2

【Factors Affecting Wet Hydrogen Sulfide Stress Corrosion Cracking】
■Hydrogen sulfide concentration. Wet hydrogen sulfide stress corrosion is considered when the H2S concentration is greater than 50PPm.
■The strength grade of steel. The higher the strength, the more prone to wet hydrogen sulfide stress corrosion cracking.
■The chemical composition of steel. S and P elements are prone to segregation in the process of steel formation and welding, resulting in the increase of martensite and bainite structures, and the increase of microstructure hardness of steel, which has an adverse effect on reducing wet hydrogen sulfide stress cracking.
■Hardness value. The higher the hardness value of the weld and heat affected zone, the more susceptible to hydrogen sulfide stress corrosion.

【Anti-corrosion measures】
■ Considering the economic and anti-corrosion aspects, it is better to choose the technological anti-corrosion measures. Such as injecting corrosion inhibitors into low temperature parts (such as the top of the main vapor tower, the top of the butane tower).
■After heat treatment, the hardness of weld seam and heat affected zone is HB<200.
■Reduce the content of S and P in the material. Such as using Q345R (R-HIC steel).

7. H2S+NH3+H2O corrosion (FOR Corrosion in Hydrogenation Units)

[Definition] H2S+NH3+H2O corrosion refers to the corrosion phenomenon caused by the combined action of H2S, NH3 and H2O. The corrosion is mainly manifested as under-scale corrosion and under-scale corrosion of NH4HS. Mainly occurs in high-pressure air coolers and downstream dehydration lines.
[Corrosion mechanism] In the H2S environment, H2S reacts with the metal to form a FeS protective film. However, if the flow rate of the process medium is too high, the protective film will be damaged due to scouring. The reaction formula is: Fe+H2S→FeS+H2. In a wet environment with high concentrations of H2S and NH3, that is, a high concentration of NH4HS, the reaction is carried out according to the following reaction formula, FeS and NH4HS form a complex, causing damage to the protective film and aggravating corrosion. The reaction formula is: FeS+6NH4HS→[Fe(NH3)6]2+. Through these reactions, severe wall thinning occurs at local high velocity and turbulent locations.

Causes of excessive local flow velocity: Bias and turbulence caused by contaminants and blockages; Bias caused by improper design. Corrosion may also occur at extremely low flow rates and at stagnant sites. The reason is that when the flow rate is small, deposits are generated at the low flow rate parts and the stagnant parts, and high concentrations of NH4HS are generated under these deposits, causing local corrosion. in addition. When iron sulfide is piled up, a galvanic cell is generated with iron, and iron sulfide becomes a cathode, which has the possibility of promoting corrosion.
According to the National Association of Anti-Corrosion Engineers (NACE) standards, the anti-corrosion measures are based on the Kp value of the corrosion coefficient of the air cooler pipe flow.
Kp= (H2S)mol%×(NH3)mol%
Wherein: H2S——the concentration of H2S in the logistics mol%;
NH3——the concentration of NH3 in the stream, mol%;
When Kp<0.07, carbon steel can be selected as the material.
When Kp=0.1%～0.5%, the material is carbon steel, and the suitable range of flow velocity is 4.6～6.09m/s.
When Kp>0.5%, when the flow velocity is lower than 1.5～3.05m/s or the flow velocity is higher than 7.62m/s, 3RE60 Monel or Incoloy800 high alloy material is selected. The reaction effluent air cooler requires a 316L protective sleeve.

【Anti-corrosion measures】
■Strictly control the Kp value.
■The pipe inlet is lined with 316L stainless steel protective sleeve.
■Inject water and water-soluble corrosion inhibitor before high-pressure air cooling to prevent the crystallization of amine salts.
■It adopts symmetrical balance structure to facilitate the even distribution of fluid.
■Avoid bias current in operation.

Material selection points
1. High-sulfur and low-acid value crude oil is mainly corroded by high-temperature sulfur at high temperature, and the equipment materials are mainly selected to contain an appropriate amount of Cr; Mo=2%～3%(wt) TP316 stainless steel is the most effective.
2. The maximum design corrosion allowance should not exceed 6mm, otherwise the material should be replaced.
3. Equipment operating in a high-temperature hydrogen environment is selected according to the operating hydrogen partial pressure and operating temperature with reference to the Nelson curve.
4. High temperature H2+H2S corrosion is selected according to the Couper curve.
5. High temperature sulfur corrosion is selected according to McConomy curve.
6. According to the corrosion rate of the preselected material, determine the main material according to the following principles:
(1) The corrosion rate of the selected material does not exceed 0.25mm/a;
(2) When chrome molybdenum steel is selected, its possible temper brittleness problem should be considered;
(3) When austenitic stainless steel is selected, stable austenitic stainless steel should be selected.

7. Material selection in wet acid corrosive environment (FOR Corrosion in Hydrogenation Units)

(1) For environments with wet hydrogen sulfide as the main corrosive medium, the selection of main materials should meet the following requirements:
①For the gas-phase medium environment with a small amount of condensed water, when the partial pressure of gas-phase hydrogen sulfide is less than 0.00035MPa, the main material should be carbon steel; when the partial pressure of gas-phase hydrogen sulfide is greater than or equal to 0.00035MPa, the main material should be carbon steel. And meet sulfide stress corrosion cracking (SSCC) requirements.
②For the medium environment of liquid phase or gas-liquid mixed phase, the selection of the main material should meet the following requirements: a) When the hydrogen sulfide content in the medium is less than 50μg/g, the main material can be carbon steel; b) When the sulfide in the medium is less than 50μg/g When the hydrogen content is 50～10000μg/g, if the pH value of the liquid phase is 5.5～7.5, the main material can be carbon steel; Anti-SSCC requirements, and steel welded steel pipes should use “hydrogen induced cracking (HIC) carbon steel”; c) When the hydrogen sulfide content in the medium is greater than 10000μg/g, no matter the liquid phase of the medium is neutral, acidic or alkaline The steel plate welded steel pipe should be made of “HIC carbon steel”, and the seamless steel pipe can be made of carbon steel, and meet the requirements of SSCC resistance; d) When the uniform corrosion rate of the selected material is greater than 0.25mm/a, it should be considered to increase materials, or take other measures.
③ When ammonia or amine exists in the medium at the same time, the influence of ammonia or amine on the uniform corrosion rate of the material and the influence of the stress corrosion cracking of amine should be considered.

(2) When ethanolamine is contained in the wet hydrogen sulfide corrosive environment and used as a desulfurizer, the selection of the main material should meet the following requirements:
① For the gas-phase medium environment with a small amount of condensed water, the main material should be carbon steel, and meet the requirements of SSCC resistance;
② For the liquid medium environment, the selection of the main material should be handled according to the following conditions: a) When the medium temperature is less than or equal to 110℃, the main material should be carbon steel, and meet the requirements of SSCC resistance and alkali stress corrosion cracking (ASCC) resistance; b) When the medium temperature is higher than 110℃, the main material should be ultra-low carbon austenitic stainless steel.

8. Material selection under high temperature sulfur and high temperature sulfide corrosion environment (FOR Corrosion in Hydrogenation Units)

(1) For pipelines with medium temperature greater than or equal to 240℃ and containing active sulfide corrosion medium, the influence of high temperature sulfide corrosion on material selection should be considered. In general, the total sulfur content in the medium and the operating temperature of the medium should be used as parameters to estimate the corrosion rate of the preselected material according to McConomy, and then the main material should be determined according to the following principles:
① According to the temperature distribution, properly divide the high-temperature oil pipeline of the whole device into several temperature sections, and select appropriate materials in each temperature section;
② Carbon steel, 1Cr5Mo should be preferred, and 1Cr9Mo material can be used if necessary;
③For large-diameter pipes, carbon steel + stainless steel composite plate rolled steel pipes should be used.

(2) When the flow velocity of the medium is greater than or equal to 30m/s, the materials that are resistant to erosion and corrosion should be considered.

9. Material selection in the presence of warm hydrogen and hydrogen sulfide in a corrosive environment (FOR Corrosion in Hydrogenation Units)

(1) For pipelines containing hydrogen and hydrogen sulfide whose mass temperature is greater than or equal to 200°C, the influence of high temperature hydrogen damage on material selection should be considered. In general, the medium temperature plus a certain margin and hydrogen partial pressure should be used as parameters. Pre-select materials according to Nelson curve.
(2) On the basis of the above, for the coexistence of hydrogen and hydrogen sulfide medium pipes with medium temperature greater than or equal to 200℃, the influence of high temperature hydrogen sulfide and hydrogen co-corrosion on the selection of materials should also be considered. The content of hydrogen sulfide and the temperature of the medium are used as parameters, and the corrosion rate of the preselected material is estimated by checking the Couper curve in combination with the light and heavy categories of hydrocarbon materials.

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