What are the corrosion of metal materials used in pressure pipeline?
The harm of corrosion is very common and serious. Corrosion will cause great direct or indirect losses, cause catastrophic accidents and endanger personal safety. The running, emitting, dripping and leaking of production equipment and pipelines caused by corrosion will affect the production cycle and equipment life of production units, increase production costs, and pollute the environment and endanger human health due to the leakage of toxic substances.
According to the mechanism of corrosion
According to the mechanism of corrosion, it can be divided into chemical corrosion, electrochemical corrosion and physical corrosion.
The chemical damage to the surface of the metal is not caused by the chemical action of the electrolyte. Sulfur corrosion of metals in high temperature gas and high temperature oxidation of metals 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 and common corrosion, such as the corrosion of metals in the atmosphere, sea water, soil and various electrolyte solutions.
Physical corrosion refers to the destruction of metals caused by simple physical dissolution. Its characteristic is: when the low melting point metal dissolves into the metal material, it will produce the “splitting” effect on the metal material. Because the strength of the metal with low melting point is generally low, it will fracture preferentially under the stress state, and become the crack source of metal materials. It should be said that this kind of corrosion is rare in engineering.
Classification according to corrosion morphology
According to the types of corrosion, it can be divided into three categories: total corrosion, local corrosion and stress corrosion.
Overall corrosion, also known as uniform corrosion, is basically the same degree of corrosion in a large area of pipeline. Uniform corrosion is the least dangerous corrosion.
- ① In engineering, the mechanical strength and service life of materials can be guaranteed by giving enough corrosion allowance.
- ② Uniform corrosion is usually evaluated by the corrosion depth of metal materials or the wall thickness reduction of metal components (called corrosion rate) in unit time. According to sh3059 standard, materials with corrosion rate less than 0.05mm/a are fully corrosion-resistant materials; materials with corrosion rates of 0.05-0.1mm/a are corrosion-resistant materials; materials with corrosion rates of 0.1-0.5mm/a are corrosion-resistant materials; materials with corrosion rates more than 0.5mm/a are non-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. It often causes sudden damage to metal components without warning, thus causing major fire or personal injury and death accidents. 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, is called pitting corrosion. The diameter of the etch hole is equal to or less than the depth.
② Pitting corrosion is one of the most destructive and hidden corrosion forms of pipelines. Pitting corrosion of austenitic stainless steel pipeline is most likely to occur when conveying medium containing chloride ion or bromine ion. 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 seawater 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.
As a pitting source, the incomplete parts of the passive film (such as exposed dislocations, surface defects, etc.) are active in a certain period of time, and the potential becomes negative. A micro cell is formed between the passivation film and its adjacent surface, which has a large cathode and small anode area ratio, which makes the metal in the pitting source rapidly dissolve and the etch hole begins to form.
As the corrosion continues, the etched holes are formed. The excess positive charge accumulated in the pores caused the migration of Cl – from the outside to keep neutral, and then the chloride concentration in the pores increased. Due to the hydrolysis of chloride, the solution in the hole is acidified, which further accelerates the dissolution of anode in the hole. As a result of this autocatalytic action, the etch hole is continuously developed to the depth, as shown in Fig. 1.
Fig. 1 growth mechanism of pitting hole
④ The results show that the solution retention is easy to produce pitting corrosion; increasing the flow rate will reduce the pitting corrosion tendency, sensitization treatment and cold working will increase the pitting corrosion tendency of stainless steel; the solid solution treatment can improve the pitting corrosion resistance of stainless steel. The pitting corrosion resistance of titanium is higher than that of austenitic stainless steel.
⑤ Pitting corrosion also occurs in carbon steel pipes, usually in steam systems (especially low-pressure steam) and hot water systems, where the corrosion is most serious in the temperature range of 80-250 ℃. Although the steam system is deoxidized, it is difficult to ensure that the dissolved oxygen content does not exceed the standard due to the loose operation control. Therefore, the pitting corrosion of carbon steel pipeline caused by dissolved oxygen often occurs.
When the material transported by pipeline is electrolyte solution, crevice corrosion will occur in the cracks on the inner surface of the pipeline, such as flange gasket, incomplete penetration of one-sided welding, etc. Some passive metals, such as stainless steel, aluminum and titanium, are prone to crevice corrosion.
The mechanism of crevice corrosion is generally considered to be the principle of concentration difference corrosion cell, 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 it is most serious in chloride containing solution. Its mechanism is not only the effect of oxygen concentration cell, but also the autocatalytic effect like pitting corrosion.
Corrosion of welded joints
It usually occurs in stainless steel pipes and there are three forms of corrosion.
① This is the selective corrosion of δ ferrite in austenitic stainless steel.
In order to improve the welding performance, austenitic stainless steel usually requires 3% – 10% ferrite structure in the weld. However, in some strong corrosive media, selective corrosion of δ ferrite occurs, that is, corrosion only occurs in δ ferrite phase (or further decomposes into σ phase), and the result is sponge like.
② Heat affected zone corrosion. The reason for this corrosion is that the temperature here is favorable to precipitate carbides in the sensitized zone for sufficient time during welding, thus resulting in intergranular corrosion.
Intergranular corrosion is a kind of corrosion form in which the corrosion is limited to the grain boundary and near the grain boundary, but the corrosion of the grain itself is relatively small.
The mechanism of intergranular corrosion is “poor chromium theory”. Stainless steel has high corrosion resistance because of its chromium content. The chromium content must be more than 12%, otherwise its corrosion resistance is similar to that of ordinary carbon steel. In the range of sensitization temperature (450 ～ 850 ℃), the supersaturated solid solution carbon in austenite will react with chromium to synthesize CR 23 C 6, which precipitates 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 Cr 23 C 6 must be obtained from the vicinity of grain boundary, which results in chromium deficiency in the region near the grain boundary. If the chromium content is less than 12% (the limit chromium content required for passivation), the poor chromium area is in an active state. As an anode, it forms a corrosion galvanic cell between the grain and it. The anode area of the poor chromium area is small, and the area of the grain cathode is large, which results in serious corrosion of the poor chromium area near the grain boundary.
③ The knife edge corrosion at the fusion line generally occurs in stainless steel (347 and 321) stabilized with Nb and Ti. Most of knife edge corrosion occurs in oxidizing medium.
Wear and corrosion
Also known as erosion corrosion. When the corrosive fluid suddenly changes direction in the elbow, tee and other turning parts, it will cause mechanical erosion damage to the metal and the passive film or corrosion product layer on the metal surface, and at the same time, it will cause severe corrosion damage than other parts of the metal surface.
This kind of damage is that the metal is separated from the metal surface by its ions or corrosion products, rather than by solid metal powder like pure mechanical wear.
If there are bubbles or suspended solids in the fluid, it is easy to wear and corrosion. The corrosion resistance of passive film of stainless steel is poor, while that of titanium is better. The steam system and h2s-h2o system have serious wear and corrosion on carbon steel pipe elbows and tee joints.
For the hot corrosive gas pipeline containing water vapor, condensation will occur on the inner wall of the insulation layer where the insulation layer is suspended or damaged due to the local temperature falling below the dew point, resulting in condensate corrosion, i.e. dew point corrosion.
Local atmospheric corrosion at coating damage
For the carbon steel pipe line of chemical plant, this kind of corrosion can be very serious sometimes, because the atmosphere of chemical plant often contains acid gas, which is much more corrosive than natural atmosphere.
The fracture failure of metal materials under the joint action of tensile stress and specific corrosive medium is called stress corrosion cracking. The time of stress corrosion cracking may be long or short. Some of them will crack after a few days, and others will crack after several years. This shows that stress corrosion cracking usually has a long or short incubation period.
The stress corrosion cracking is in the form of withered tree branches and develops along the direction perpendicular to the tensile stress. The micro morphology of the crack is transgranular, intergranular (intergranular) and mixed.
For the pipeline, the residual stress during welding, cold working and installation is the main source of stress.
Stress corrosion cracking is not caused by the joint action of any metal and medium. The stress corrosion cracking of metal materials occurs only in some specific corrosion environments.
The stress corrosion cracking of metals in alkaline solution is called alkali embrittlement. Alkali embrittlement can occur in carbon steel, low alloy steel and stainless steel.
The lowest temperature of alkali embrittlement is 50 ℃ and the concentration of alkali solution is 40% ～ 50%. The high temperature zone near the boiling point is the most easy to occur.
The cracks are intergranular. When the concentration of sodium hydroxide is more than 0.1%, alkali embrittlement of 18-8 type austenitic stainless steel can occur. When the concentration of NaOH is 40%, the temperature of alkali embrittlement is about 115 ℃.
The alkali embrittlement crack of ultra-low carbon stainless steel is transgranular, and when the carbon content is high, the alkali embrittlement crack is intergranular or mixed. When 2% molybdenum is added into austenitic stainless steel, the limit of alkali embrittlement is reduced and moves to the high alkali concentration area. Nickel and nickel base alloys have high stress corrosion resistance, and their alkali embrittlement range becomes narrow and located in high temperature concentrated alkali zone.
Chloride stress corrosion cracking of stainless steel
Chloride ion can not only cause pitting corrosion of stainless steel, but also cause stress corrosion cracking of stainless steel.
The critical concentration of chloride ion for stress corrosion cracking decreases with the increase of temperature. At high temperature, as long as the chloride concentration reaches 10-6, it can cause fracture. The critical temperature of chloride stress corrosion cracking is 70 ℃.
The conditions with chloride concentration (repeated evaporation and wetting) are the most prone to rupture. 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 pipeline, but also in the outer wall of the pipeline.
As the corrosion factor on the outside of the pipe, it is considered to be the problem of thermal insulation materials. According to the analysis results of thermal insulation materials, it is found that there are about 0.5% chloride ions. This value can be considered as the impurity contained in the insulation material, or the result of concentration brought in by the damaged insulation layer and immersed in rain water.
Stress corrosion cracking of stainless steel with polysulfate
The stress corrosion cracking (SCC) of stainless steel (H2SxO6, x = 3-5) has attracted much attention. When the pipeline is in normal operation, it is corroded by hydrogen sulfide. When the pipeline is shut down for maintenance, it reacts with oxygen and water in the air to form H2SxO6. Stress corrosion cracking (SCC) occurs in the parts of Cr Ni austenitic stainless steel pipe with high residual stress (weld heat affected zone, elbow, etc.).
Sulfide corrosion cracking
① The stress corrosion cracking of metals in the medium containing hydrogen sulfide and water is called sulfide corrosion cracking. In natural gas, oil collection, processing and refining, petrochemical and chemical fertilizer industries, sulfur cracking accidents often occur in pipelines and valves. The time required for sulfur cracking varies from a few days in a short period to several months to several years in a long time. However, no sulfur cracking has occurred in more than ten years.
② The cracks of sulfur cracking are coarser and have fewer branches. Most of them are transgranular, and there are intergranular or mixed types. The concentration of hydrogen sulfide required for sulfur cracking is very low, which can occur only slightly more than 10-6 or even less than 10-6.
Carbon steel and low alloy steel are the most sensitive to sulfur cracking in the temperature range of 20 ~ 40 ℃. Sulfur cracking of austenitic stainless steel mostly 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 acetic acid, carbon dioxide and sodium chloride, or phosphine, or arsenic, selenium, antimony, tellurium compounds or chloride ions are contained at the same time, the sulfur cracking of steel will be promoted.
The sensitivity of 304L and 316L stainless steels to sulfur cracking is 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 and tempering is the best, and the microstructure of non tempered martensite is the worst. The sulfur cracking resistance of steel decreases in the order of quenching + tempering → normalizing + tempering → normalizing → tempering martensite.
The higher the strength of steel, the more likely sulfur cracking occurs. The higher the hardness of the steel, the easier sulfur cracking occurs. In the accident of sulfur cracking, the welding seam, especially the fusion line, is the most prone part to crack, because the hardness here is the highest.
NACE strictly stipulates the hardness of carbon steel weld: ≤ 200hb. This is because the distribution of weld hardness is more complex than that of base metal, so the requirement of weld hardness is 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 in 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 caused by hydrogen penetration into metal. Hydrogen damage can be divided into four types: hydrogen bubbling, hydrogen embrittlement, decarburization and hydrogen corrosion.
① Hydrogen bubbling and hydrogen induced step cracks.
Hydrogen sulfide dissociates in water:
It mainly occurs in the medium containing wet hydrogen sulfide. In this environment, not only the general corrosion of steel will occur due to the anodic reaction, but also the adsorption of S2 – on the metal surface will hinder the hydrogen atom from combining with the hydrogen molecule, thus promoting the hydrogen atom to penetrate into the metal.
When hydrogen atoms permeate and diffuse into the steel, the defects such as cracks, delamination, voids and slag inclusion are encountered, and then they gather together to form hydrogen molecules, causing volume expansion, and generating a great pressure (up to hundreds of Mpa) in the steel.
If these defects are near the surface of the steel, they will form bubbles; if they are deep inside the steel, they will form induced cracks. They are parallel cracks along the rolling direction and connected by short transverse cracks to form a “ladder”.
Hydrogen induced step cracks cause steel embrittlement in light cases, and reduce effective wall thickness to overload, leakage and even fracture of pipelines in severe cases.
Hydrogen bubbling requires a critical concentration of hydrogen sulfide. It has been reported that hydrogen bubbling will occur when hydrogen sulfide partial pressure is 138pa. If there are phosphine, arsenic, tellurium and CN – in the wet hydrogen sulfide medium, it is favorable for hydrogen to penetrate into the steel. They are all accelerating agents for hydrogen permeation.
Hydrogen bubbling and hydrogen induced step cracks usually occur on the pipe made of steel plate.
② Hydrogen embrittlement.
No matter what way hydrogen enters into steel, it will cause steel embrittlement, that is, elongation and reduction of area decrease significantly, especially for high strength steel. If the hydrogen in the steel is released (such as heating for dehydrogenation), the mechanical properties of the steel can still be recovered. Hydrogen embrittlement is reversible.
Hydrogen permeation can be carried out in h2s-h2o corrosive carbon steel pipeline at room temperature, and also in high temperature and high pressure hydrogen environment; hydrogen permeation can be carried out in acid pickling process without adding corrosion inhibitor or improper inhibitor, and it can also be penetrated when welding in rainy days or excessive cathodic protection.
In the industrial hydrogen production plant, high temperature hydrogen pipeline is easy to produce carbon damage. The cementite in steel reacts with hydrogen to form methane at high temperature
As a result of the reaction, the cementite in the surface layer decreases, and the carbon gradually diffuses from the adjacent unreacted metal layer to the reaction zone, and the metal layer with a certain thickness becomes ferrite due to lack of carbon. The results of decarburization reduce the surface strength and fatigue limit of steel.
④ Hydrogen corrosion.
The mechanical properties of steel are deteriorated, and the strength and toughness are obviously reduced after being subjected to high temperature and high pressure hydrogen. This phenomenon is called hydrogen corrosion.
The process of hydrogen corrosion can be roughly divided into three stages: in the incubation period, the properties of steel do not change; in the stage of rapid change in properties, decarburization and crack propagation rapidly; in the final stage, carbon in the 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 of 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 ℃.
The hydrogen partial pressure also has a starting point (about 1.4mpa for carbon steel), that is, no matter how high the temperature is, below this partial pressure, only surface decarburization will occur without serious hydrogen corrosion.
The combined conditions of temperature and pressure for corrosion of various hydrogen resistant steels are the famous Nelson curve (this curve is available in many standard specifications for the selection of pipeline equipment, such as sh3059 “general rules for selection of equipment for petrochemical pipeline design”).
Cold working deformation improves the diffusion ability of carbon and hydrogen and accelerates the corrosion.
In a nitrogenous fertilizer plant, the high pressure pipeline from ammonia synthesis tower outlet to waste heat boiler has a working temperature of 320 ℃ and a working pressure of 33Mpa. The working medium is mixture of H2, N2 and NH3. Hydrogen resistant steel should be selected according to Nelson curve. One of the short diameter reducing pipes was damaged by hydrogen corrosion due to misuse of common carbon steel, which caused serious accident and heavy loss.
Source: China Pressure 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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