What is the corrosion of metal materials for industrial chemical pipeline?
The harm of metal corrosion is very common and serious. Corrosion will cause great direct or indirect losses, catastrophic accidents and personal safety. The running, emitting, dripping and leaking of production equipment and industrial chemical pipelines caused by corrosion will affect the production cycle and equipment life of production equipment, increase production cost, and also pollute the environment and endanger human health due to the leakage of toxic substances.
Classification according to the mechanism of corrosion
According to the mechanism of corrosion, it can be divided into chemical corrosion, electrochemical corrosion and physical corrosion.
Chemical corrosion refers to the damage caused by pure chemical interaction between metal surface and non electrolyte. The sulfur corrosion of metal in high temperature gas and high temperature oxidation of metal belong to chemical corrosion.
Electrochemical corrosion refers to the damage caused by electrochemical reaction between metal surface and ionic conducting medium. Electrochemical corrosion is the most common corrosion, such as the corrosion of metals in the atmosphere, sea water, soil and various electrolyte solutions.
Physical corrosion refers to the damage of metal caused by simple physical dissolution. Its characteristic is: when the metal with low melting point melts into the metal material, it will have the “splitting” effect on the metal material. Because the strength of the metal with low melting point is generally low, it will fracture preferentially in the state of stress, thus becoming the crack source of the metal material. It should be said that this kind of corrosion is rare in engineering.
Classification according to corrosion form
According to the classification of corrosion forms, it can be divided into three categories: overall corrosion, local corrosion and stress corrosion.
Overall corrosion, also known as uniform corrosion, is basically the same corrosion on a large area of the pipeline. Uniform corrosion is the least dangerous corrosion.
- ① The mechanical strength and service life of materials can be guaranteed by giving enough corrosion allowance in engineering.
- ② Uniform corrosion is usually evaluated by the corrosion depth of the corrosion medium to the metal material or the thinning amount of the wall thickness of the metal component (called corrosion rate) in unit time. According to sh3059 standard, materials with corrosion rate not more than 0.05mm/a are fully corrosion-resistant materials; materials with corrosion rate of 0.05-0.1mm/a are corrosion-resistant materials; materials with corrosion rate of 0.1-0.5mm/a are corrosion-resistant materials; materials with corrosion rate of more than 0.5mm/a are corrosion-resistant materials.
Local corrosion, also known as non-uniform corrosion, is far more harmful than uniform corrosion, because uniform corrosion is easy to be detected and protected, while local corrosion is difficult to predict and prevent, often without warning, the metal components are suddenly damaged, resulting in major fire or personal injury accidents. Local corrosion is very common. According to statistics, uniform corrosion accounts for 17.8% of the total corrosion, while local corrosion accounts for about 80%.
① Pitting corrosion, also known as pitting corrosion, refers to the deep corrosion concentrated on individual small points on the global surface. The diameter of the etched hole is equal to or less than the depth. The shape of the etched hole is shown in Figure 1.
Figure.1 Various profile shapes of pitting holes (selected from ASTM standard)
② Pitting is one of the most destructive and hidden corrosion forms of pipelines. Austenitic stainless steel pipe is most likely to produce pitting corrosion when transporting medium containing chloride or bromine ions. If the outer wall of stainless steel pipe is often wetted by sea water or natural water, pitting corrosion will also occur, because there are certain chloride ions in sea water or natural water.
③ The pitting process of stainless steel can be divided into two stages: the formation of pitting and the development of pitting.
The incomplete part of passivation film (outcrop dislocation, surface defect, etc.) is the source of pitting corrosion. In a certain period of time, it is in an active state, the potential becomes negative, a micro battery is formed between the passivation film and its adjacent surface, and it has a large cathode and small anode area ratio, which makes the metal in the source part dissolve rapidly, and the pitting begins to form.
The formed corrosion holes continue with the corrosion. The excess positive charge accumulated in the pore caused the migration of external Cl – to keep the electric neutral, and then the chloride concentration in the pore increased. Due to the hydrolysis of chloride, the solution in the pores is acidified and the dissolution of anode in the pores is further accelerated. As a result of this autocatalytic action, the etched pores develop to the depth continuously, as shown in Fig. 2.
Figure.2 Growth mechanism of pitting
④ Solution retention is easy to produce pitting; increasing flow rate will reduce pitting tendency, sensitization treatment and cold processing will increase pitting tendency of stainless steel; solid solution treatment can improve pitting resistance of stainless steel. The pitting resistance of titanium is higher than that of austenitic stainless steel.
⑤ Carbon steel pipes also suffer from pitting corrosion, usually in the steam system (especially in the low-pressure steam) and hot water system, which suffer from the corrosion of dissolved oxygen, and the temperature is the most serious between 80 ℃ and 250 ℃. Although the steam system is deaerated, it is difficult to ensure that the dissolved oxygen does not exceed the standard due to the lack of strict operation control, so the corrosion of carbon steel pipeline caused by dissolved oxygen often occurs.
When the material transported by the pipeline is electrolyte solution, crevice corrosion will occur at the crevices of the internal surface of the pipeline, such as flange gasket, single side welding incomplete, etc. Some blunt metals, such as stainless steel, aluminum and titanium, are prone to crevice corrosion.
The mechanism of crevice corrosion is generally considered as the principle of concentration difference corrosion battery, which is caused by the difference of oxygen concentration or metal ion concentration between the solution inside and around the crevice. Crevice corrosion occurs in many media, but the solution containing chloride is the most serious. Its mechanism is not only the effect of oxygen concentration cell, but also the autocatalytic effect like pitting corrosion, as shown in Figure 3.
Fig.3 Mechanism of crevice corrosion
Corrosion of welded joints
It usually occurs in stainless steel pipes and has three forms of corrosion.
① The corrosion of the weld is spongy, which is the selective corrosion of δ ferrite in austenitic stainless steel.
In order to improve the welding performance, austenitic stainless steel usually requires the weld to contain 3% – 10% ferrite structure, but in some strong corrosive media, selective corrosion of δ ferrite will occur, that is, the corrosion only occurs in δ ferrite phase (or further decomposed into σ phase), and the result is spongy.
② Heat affected zone corrosion. The reason for this kind of corrosion is that the temperature here is positive in the sensitized area during the welding process, and there is sufficient time for carbide precipitation, which results in intergranular corrosion.
Intergranular corrosion is a kind of corrosion form where the corrosion is limited to and near the grain boundary and the corrosion of the grain itself is relatively small. As a result, the grain will fall off or the mechanical strength of the material will be reduced.
The mechanism of intergranular corrosion is “chromium poor theory”. Stainless steel has high corrosion resistance due to its chromium content, which must be more than 12%, otherwise its corrosion resistance is similar to that of ordinary carbon steel. In the sensitization temperature range (450-850 ℃), the supersaturated solid solution carbon in austenite will be chromized to form Cr23C6 and precipitated along the grain boundary. As the diffusion rate of chromium in austenite is slower than that of carbon, the lead needed for the formation of Cr23C6 must be obtained from the vicinity of the grain boundary, resulting in chromium deficiency in the vicinity of the grain boundary. If the chromium content falls below 12% (the limit chromium content required for passivation), the chromium poor area is in an activated state. As an anode, it forms a galvanic cell for corrosion between it and the grains. The anode area of the chromium poor area is small, and the cathode area of the grains is large, which causes serious corrosion of the chromium poor area near the grain boundary.
③ The blade corrosion at the fusion line generally occurs in Nb and Ti stable stainless steel (347 and 321). Knife edge corrosion mostly occurs in oxidizing media. The schematic diagram of knife edge corrosion is shown in Figure 4.
Figure.4 Knife edge corrosion
Also known as erosion corrosion. When the corrosive fluid suddenly changes direction at the elbow, tee and other turning parts, it will cause mechanical erosion damage to the metal and the passivation film or corrosion product layer on the metal surface, and at the same time, it will cause severe electrochemical corrosion to the exposed metal fresh surface, resulting in more serious corrosion damage than other parts. The damage is that the metal is separated from the metal surface by its ions or corrosion products, rather than by solid metal powder as pure mechanical wear.
If there are bubbles or suspended solids in the fluid, it is most likely to wear and corrosion. The passive film of stainless steel has poor wear and corrosion resistance, while titanium is better. The steam system and h2s-h2o system have serious wear and corrosion on the elbow and tee of carbon steel pipeline.
For the hot corrosive gas pipeline with water vapor, condensation will occur on the inner wall at the stop or damage of the insulation layer due to the local temperature falling below the dew point, resulting in condensate corrosion, namely dew point corrosion.
Local atmospheric corrosion at coating damage
For the carbon steel pipe line of chemical plant, this kind of corrosion is sometimes very serious, because the atmosphere of chemical plant often contains acid gas, which is much more corrosive than the natural atmosphere.
The fracture of metal materials under the action of tensile stress and specific corrosion medium is called stress corrosion fracture. The time of stress corrosion cracking is long or short, some of them are cracked after several days, and some of them are cracked after several years, which shows that stress corrosion cracking usually has a long or short incubation period.
The stress corrosion cracks are in the form of withered branches, and generally develop in the direction perpendicular to the tensile stress. There are transgranular, intergranular (intergranular) and mixed types of cracks.
The source of stress, for pipes, welding, cold processing and installation of residual stress is the main.
Stress corrosion cracking is not caused by the interaction of any metal and medium. Among them, stress corrosion cracking occurs only in some specific corrosion environment. Table 1 lists the combination of pipe metal materials and corrosion environment that are easy to cause stress corrosion cracking.
The stress corrosion cracking of metals in alkaline solution is called alkali embrittlement. Carbon steel, low alloy steel, stainless steel and other metal materials can produce alkali embrittlement. The trend of alkali embrittlement of carbon steel (including low alloy steel) is shown in Figure 5.
Fig.5 Stress corrosion cracking zone of carbon steel in alkaline solution
It can be seen from Figure 5 that alkali embrittlement is likely to occur in carbon steel in all concentration ranges with sodium hydroxide concentration above 5%. The minimum temperature of alkali embrittlement is 50 ℃, and the concentration of alkali solution required is 40% – 50%. The high temperature zone near boiling point is the most likely to occur. The cracks are intergranular. The trend of alkali embrittlement of austenitic stainless steel is shown in Figure 6.
When the concentration of NaOH is more than 0.1%, the alkali embrittlement of 18-8 austenitic stainless steel can occur. At this time, the temperature of alkali embrittlement is about 115 ℃. The alkali embrittlement crack of ultra-low carbon stainless steel is transgranular type. When the carbon content is high, the alkali embrittlement crack is intergranular or mixed type. When 2% molybdenum is added to austenitic stainless steel, the alkali embrittlement limit will be narrowed and moved to the high concentration region of alkali. Nickel and nickel base alloys have high stress corrosion resistance, and their alkali embrittlement range becomes narrow, and they are located in high temperature concentrated alkali area.
Fig.6 Relationship between concentration of caustic soda and temperature for stress corrosion cracking
Note: the upper part of the curve is dangerous area
Chloride ion stress corrosion cracking of stainless steel
Chloride ion can not only cause pitting corrosion, but also stress corrosion cracking of stainless steel.
The critical chloride concentration of stress corrosion cracking decreases with the increase of temperature. At high temperature, the chloride concentration can cause cracking as long as it reaches 10-6. The critical temperature of chloride stress corrosion cracking is 70 ℃. The conditions of chloride concentration (repeated evaporation and wetting) are the most likely to break. Chloride ion stress corrosion cracking of stainless steel is very common in industry.
Chloride ion stress corrosion cracking of stainless steel occurs not only in the inner wall of the pipe, but also in the outer wall of the pipe, as shown in Figure 7.
Figure.7 Stress corrosion cracking of stainless steel pipeline
As the corrosion factor on the outer side of the pipe, it is considered as the problem of the thermal insulation material. The analysis results of the thermal insulation material show that it contains about 0.5% chloride ion. This value can be considered as the result of impurities contained in the insulation material, or brought in and concentrated by the damaged insulation layer and immersed rainwater.
Stress corrosion cracking of stainless steel with copolysulfuric acid
The stress corrosion cracking of stainless steel (H2SxO6, x = 3-5) is of great concern.
During normal operation, the pipeline is corroded by hydrogen sulfide, and the generated iron sulfide reacts with oxygen and water in the air to generate H2SxO6 during shutdown and maintenance. In Cr Ni austenitic stainless steel pipeline, stress corrosion cracks are produced in the parts with high residual stress (heat affected zone of weld, bend, etc.).
Sulfide corrosion cracking
① The stress corrosion cracking of metals in the medium containing hydrogen sulfide and water at the same time is called sulfide corrosion cracking. In the natural gas, oil collection, processing and refining, petrochemical and chemical fertilizer industries, pipeline and valve sulfur cracking accidents often occur. The time needed for sulfur cracking is a few days, a few months to several years, but there is no case of sulfur cracking in more than ten years.
② The cracks of sulfur cracking are relatively coarse, with few branches, mostly transgranular, intergranular or mixed. The concentration of hydrogen sulfide needed for sulfur cracking is very low, as long as it is slightly higher than 10-6, or even lower than 10-6.
The sensitivity of carbon steel and low alloy steel to sulfur cracking is the highest in the temperature range of 20-40 ℃, and most of the sulfur cracking of austenitic stainless steel occurs in high temperature environment. With the increase of temperature, the sulfur cracking sensitivity of austenitic stainless steel increases. In the medium containing hydrogen sulfide and water, if it contains acetic acid, or carbon dioxide and sodium chloride, or phosphine, or arsenic, selenium, antimony, tellurium compounds or chloride ions, it will promote the sulfur cracking of steel. For the sulfur cracking of austenitic stainless steel, chloride ion and oxygen play a promoting role. The sensitivity of 304L and 316L stainless steel to sulfur cracking is related as follows: H2S + H2O ＜ H2S + H2O + Cl – ＜ H2S + H2O + Cl – + O2 (the sensitivity of sulfur cracking is from weak to strong).
For carbon steel and low alloy steel, the microstructure of quenching + tempering is the best, and the microstructure of untempered martensite is the worst. The sulfur cracking resistance of steel decreases in the order of quenching + tempering structure → normalizing + tempering structure → normalizing structure → untempered martensite structure.
The higher the strength of steel is, the more prone to sulfur cracking. The higher the hardness of steel is, the more prone to sulfur cracking. In the case of sulfur cracking, the weld joint, especially the fusion line, is the most likely part to crack, because the hardness here is the highest. NACE has made a strict regulation on the hardness of carbon steel welds: ≤ 200hb. This is because the distribution of weld hardness is more complex than that of base metal, so the requirements of weld hardness are stricter than that of base metal. On the one hand, it is due to the effect of welding residual stress, on the other hand, it is the result of hardening structure of weld metal, fusion line and heat affected zone. In order to prevent sulfur cracking, it is necessary to carry out effective heat treatment after welding.
Hydrogen damage, also known as hydrogen damage, is the deterioration of metal properties caused by hydrogen permeation into the metal. Hydrogen damage can be divided into four types: hydrogen bubble, hydrogen embrittlement, decarburization and hydrogen corrosion.
① Hydrogen bubble and hydrogen induced step crack.
It mainly occurs in the medium containing wet hydrogen sulfide.
Dissociation of hydrogen sulfide in water:
Electrochemical corrosion of steel in aqueous hydrogen sulfide solution:
It can be seen from the above process that in this environment, steel will not only suffer from general corrosion due to anode reaction, but also because the adsorption of S2 – on the metal surface will hinder the hydrogen atom compound hydrogen molecule, thus promoting the hydrogen atom to penetrate into the metal. When hydrogen atoms permeate and diffuse into steel, they encounter cracks, delamination, voids, slag inclusions and other defects, so they gather together to form hydrogen molecules to cause volume expansion and generate great pressure (up to hundreds of MPA) inside the steel.
If these defects are near the steel surface, blisters are formed, as shown in Figure 8. If these defects are deep inside the steel, induced cracks will form. It is a parallel crack along the rolling direction, which is connected by short transverse cracks to form a “ladder”. Hydrogen induced step cracks cause steel embrittlement when they are light, and reduce effective wall thickness to pipe overload, leakage or even fracture when they are heavy.
Figure.8 Hydrogen bubbling
Hydrogen bubbling requires a critical concentration of hydrogen sulfide. It is reported that hydrogen bubbles will be produced when the partial pressure of hydrogen sulfide is 138pa. If there are phosphine, arsenic, tellurium compounds and CN – in the medium containing hydrogen sulfide at the same time, it is beneficial for hydrogen to permeate into steel. They are all hydrogen permeation accelerators.
Hydrogen bubbling and hydrogen induced step cracks usually occur on steel rolled pipes.
② Hydrogen embrittlement.
No matter how hydrogen enters the steel, it will cause steel embrittlement, that is, the elongation and reduction of area will decrease significantly, especially for high-strength steel. If the hydrogen in the steel is released (such as heating for hydrogen elimination treatment), the mechanical properties of the steel can still be restored. Hydrogen embrittlement is reversible.
The carbon steel pipeline corroded by h2s-h2o medium at room temperature can be hydrogen infiltrated, and it can also be hydrogen infiltrated in the high temperature and high pressure hydrogen environment; it can be hydrogen infiltrated in the pickling process without adding corrosion inhibitor or improper corrosion inhibitor, and it can also be hydrogen infiltrated in rainy days when welding or cathodic protection is excessive.
In the industrial hydrogen plant, high temperature hydrogen pipeline is easy to produce carbon damage. The cementite in steel reacts with hydrogen at high temperature to form methane:
The reaction results in the decrease of cementite in the surface layer, and the carbon diffuses from the adjacent unreacted metal layer to the reaction area gradually, so a certain thickness of metal layer becomes ferrite due to lack of carbon. Decarburization results in the reduction of surface strength and fatigue limit of steel.
④ Hydrogen corrosion.
When the steel is subjected to high temperature and high pressure hydrogen, its mechanical properties deteriorate, its strength and toughness decrease obviously, and it is irreversible. This phenomenon is called hydrogen corrosion.
The history of hydrogen corrosion can be explained in Figure 9.
Figure.9 The course of hydrogen corrosion
The process of hydrogen corrosion can be roughly divided into three stages: incubation period, no change in steel properties; rapid change in properties, rapid decarburization, rapid crack growth; the last stage, the carbon in solid solution has been exhausted.
The incubation period of hydrogen corrosion is important, which often determines the service life of steel.
There is an initial temperature for hydrogen corrosion under a certain hydrogen pressure, which is an index to measure the hydrogen resistance of steel. Below this temperature, the reaction rate of hydrogen corrosion is very slow, so that the incubation period exceeds the normal service life. The temperature of carbon steel is about 220 ℃.
There is also a starting point for hydrogen partial pressure (about 1.4mpa for carbon steel), that is, no matter how high the temperature is, when the pressure is lower than this partial pressure, only surface decarburization occurs without serious hydrogen corrosion.
The combination condition of temperature and pressure for corrosion of various hydrogen resistant steels is the famous Nelson curve (it is available in many standard specifications for selection of pipeline equipment, such as sh3059 general rules for selection of petrochemical pipeline design equipment).
Cold working deformation improves the diffusion ability of carbon and hydrogen and accelerates corrosion.
In a nitrogenous fertilizer plant, the high-pressure pipeline from the ammonia synthesis tower outlet to the waste heat boiler has a working temperature of about 320 ℃, a working pressure of 33 MPa, and a working medium of H2, N2 and NH3 mixture. Hydrogen resistant steel should be selected according to Nelson curve. One of them is a reducer short pipe, because of the misuse of ordinary carbon steel, it will rupture due to hydrogen corrosion soon after use, resulting in malignant accidents and heavy losses.
Source: China Chemical Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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