What are the corrosion of stainless steel pipe?

What are the corrosion of stainless steel pipe?

The harm of metal corrosion is very common, but also very serious. Corrosion can cause significant direct or indirect losses, cause catastrophic accidents, and endanger human safety. The running, leakage, dripping and leakage of production equipment and pipelines caused by corrosion will affect the production cycle and equipment life of production devices, increase production costs, and pollute the environment because of the leakage of toxic substances, endangering human health.

According to the mechanism of corrosion

According to the mechanism of corrosion, it can be divided into three categories: chemical corrosion, electrochemical corrosion and physical corrosion.

1. Chemical corrosion

Chemical corrosion refers to the destruction of metal surface by direct and pure chemical interaction with non-electrolyte. Sulfur corrosion of metal in high temperature gas and oxidation of metal at high temperature belong to chemical corrosion.

2. Electrochemical corrosion

Electrochemical corrosion refers to the destruction of metal surface caused by electrochemical reaction with ionic conductive 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.

3. Physical corrosion

Physical corrosion is the destruction of a metal by simple physical dissolution. Its characteristic is: WHEN the metal of low melting point is dissolved into the metal material, it will produce "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, thus becoming the crack source of the metal material. It should be said that such corrosion is rare in engineering.

Classification according to corrosion morphology

According to the corrosion form, it can be divided into three categories: comprehensive corrosion, local corrosion and stress corrosion.

1. Comprehensive corrosion

Overall corrosion, also known as uniform corrosion, is basically the same degree of corrosion in a large area of the pipeline. Uniform corrosion is the least dangerous kind of corrosion.

①In engineering, sufficient corrosion allowance is often given to ensure the mechanical strength and service life of materials.

② Uniform corrosion commonly used unit time corrosion medium corrosion depth of metal materials or metal component wall thickness thinning (called corrosion rate) to assess. SH3059 standard: corrosion rate does not exceed 0.05mm/ A material is fully corrosion resistant material; 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 still corrosion-resistant materials; Materials whose corrosion rate exceeds 0.5mm/a are considered non-corrosion resistant.

2, local corrosion

Local corrosion, also known as non-uniform corrosion, is far more harmful than uniform corrosion, because uniform corrosion is easy to be detected, easy to prevent, while local corrosion is difficult to predict and prevent, often without warning, metal components suddenly destroyed, 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

①  The larger depth of corrosion concentrated on individual small spots on the global surface is called pitting corrosion, also known as pitting corrosion. The diameter of the etching hole is equal to or less than the depth.

② Pitting corrosion is one of the most destructive hidden corrosion forms of pipelines. The austenitic stainless steel pipeline is most likely to produce pitting corrosion when conveying medium containing chloride ions or bromine ions. If the outer wall of stainless steel pipe is often wet by seawater or natural water, it will also produce pitting corrosion, because seawater or natural water contains certain chloride ions.

③The pitting corrosion process of stainless steel can be divided into two stages: the formation of corrosion hole and the development of corrosion hole.

The incomplete part of the passivation film (outcrops dislocation, surface defects, etc.), as the pitting source, is active in a certain period of time, the potential becomes negative, and the micro-cell is formed between it and the adjacent surface, and the area ratio of large cathode to small anode makes the metal in the pitting source rapidly dissolve, and the corrosion hole begins to form.

The corrosion holes that have been formed continue with the corrosion. An excess of positive charge accumulates in the pore, which leads to the migration of external Cl- to maintain electrical neutrality, followed by the increase of chloride concentration in the pore. Chloride hydrolysis acidifies the solution in the pore and further accelerates the dissolution of the anode in the pore. As a result of this autocatalysis, the corrosion hole develops deeper and deeper.

④ Solution retention is easy to produce pitting corrosion; Increasing the flow rate will reduce the pitting tendency, sensitization and cold work will increase the pitting tendency of stainless steel. Solution treatment can improve the pitting corrosion resistance of stainless steel. Titanium has higher pitting resistance than austenitic stainless steel.

⑤ Seamless carbon steel pipe  also occurs pitting, usually in the steam system (especially low pressure steam) and hot water system, suffered from the corrosion of dissolved oxygen, the temperature between 80 and 250℃ is the most serious. Although the steam system is deoxygenated, but because the operation control is not strict, it is difficult to ensure that the dissolved oxygen does not exceed the standard, so dissolved oxygen caused by carbon steel pipe pitting often occurs.

(2) Crevice corrosion

When the material transported by the pipeline is electrolyte solution, crevice corrosion will occur at the crevice of the inner surface of the pipeline, such as the flange gasket and the welding without penetration on one side. Some blunt metals such as stainless steel, aluminum, titanium, etc., are prone to crevice corrosion.

The mechanism of crevice corrosion is generally considered to be the principle of concentration difference corrosion of cells, that is, due to the difference in oxygen concentration or metal ion concentration between the solution in and around the crevice. Crevice corrosion occurs in many media, but it is most serious in solutions containing chloride, and its mechanism is not only the action of oxygen concentration cells, but also autocatalysis such as pitting corrosion.

(3) Corrosion of welded joints

Usually occurs in stainless steel pipes, there are three forms of corrosion.

① The welded meat is corroded into spongy shape, which is the selective corrosion of δferrite in austenitic stainless steel.

In order to improve the welding performance, the weld of austenitic stainless steel is usually required to contain 3% ~ 10% ferrite structure. However, in some highly corrosive media, δferrite selective corrosion occurs, that is, corrosion only occurs in δferrite phase (or further decomposed into σ phase), and the result is spongy.

②heat affected zone corrosion. The reason for this corrosion is that during the welding process, the temperature here is just in the sensitization zone, and there is sufficient time to precipitate carbide, resulting in intergranular corrosion.

Intergranular corrosion is a form of corrosion confined to the grain boundaries and near the grain boundaries, but the corrosion of the grain itself is relatively small. As a result, grain shedding or mechanical strength of the material will be caused.

The mechanism of intergranular corrosion is "chrome-poor theory". Stainless steel has high corrosion resistance due to chromium content, the chromium content must be more than 12%, otherwise its corrosion resistance and ordinary carbon steel is similar. In the sensitization temperature range of stainless steel (450 ~ 850℃), the carbon in the austenite susaturated solid solution and chromium will form Cr23C6, precipitate along the grain boundary. Because chromium diffuses more slowly than carbon in austenite, the lead needed to produce Cr23C6 must be obtained near the grain boundary, resulting in chromium deficiency in the area near the grain boundary. If the chromium content falls below 12% (the limit chromium content required for passivation), the chrome-depleted zone is in the activated state. As an anode, it forms a corrosion galvanic cell between it and the grain. The anode area of the chrome-depleted zone is small, and the cathode area of the grain is large, resulting in serious corrosion of the chrome-depleted zone near the grain boundary.

③ Knife edge corrosion at the fusion line generally occurs in stainless steel stabilized with Nb and Ti (347 and 321). Knife-edge corrosion mostly occurs in oxidizing media.

(4) Wear and corrosion

Also known as erosion corrosion. When the corrosive fluid suddenly changes direction in the bending parts such as elbow and tees, it will produce mechanical erosion and destruction on the metal and the passivation film or corrosion product layer on the metal surface, and fierce electrochemical corrosion will occur on the constantly exposed fresh surface of the metal, resulting in more serious corrosion damage than other parts. This damage occurs when the metal is detached from the metal surface by its ions or corrosion products, rather than by a solid metal powder as in pure mechanical wear.

If there are bubbles or solid suspended solids in the fluid, it is most prone to wear and corrosion. The corrosion resistance of stainless steel passivation film is poor, titanium is better. The wear and corrosion of carbon steel pipe elbow and tee are serious in steam system and H2S-H2O system.

(5) condensate corrosion

For the hot corrosive gas pipeline containing water vapor, in the insulation layer at the end or the inner wall of the damage, because the local temperature falls below the dew point, condensation phenomenon will occur, resulting in condensate corrosion, namely dew point corrosion.

(6) Local atmospheric rust at the coating damage

This corrosion can sometimes be severe for carbon steel pipes in chemical plants, because the atmosphere in chemical areas often contains acidic gases that are much more corrosive than the natural atmosphere.

3. Stress corrosion

The fracture of metal material under the joint action of tensile stress and specific corrosion medium is called stress corrosion rupture. The time of stress corrosion cracking is long or short, some cracking after a few days, and some cracking after several years, which indicates that stress corrosion cracking usually has a long or short incubation period.Stress corrosion cracks are dendritic and generally develop in the direction perpendicular to the tensile stress. The microstructure of cracks includes transgranular type, intergranular type (intergranular type) and mixed type of both.The source of stress, for pipes, welding, cold working and installation of residual stress is the main.Not all metals and media together cause stress corrosion cracking. Stress corrosion cracking of metal materials occurs only in some specific corrosion environments.

(1) Alkali brittleness

Stress corrosion cracking of metals in lye is called alkali brittleness. Carbon steel, low alloy steel, stainless steel and other metal materials can be alkali brittle.

The carbon steel with sodium hydroxide concentration above 5% is almost likely to produce alkali brittleness in the whole concentration range. The minimum temperature of alkali brittleness is 50℃, and the concentration of alkali liquid is 40%-50%, which is most likely to occur in the high temperature area near the boiling point. The crack is intercrystalline.

Type 18-8 austenitic stainless steel can be alkali brittle when the concentration of sodium hydroxide is above 0.1%. The concentration of sodium hydroxide 40% is the most dangerous, when the temperature of alkali brittle is about 115℃. The alkali brittle crack of ultra-low carbon stainless steel is transgranular. When the carbon content is high, the alkali brittle crack is intergranular or mixed. When 2% molybdenum is added to the austenitic stainless steel, the alkali-brittle limit is reduced and the alkali-high concentration area is moved. Nickel and nickel-based alloys have high stress corrosion resistance, and their alkali brittleness range becomes narrow, and they are located in the high temperature concentrated alkali zone.

(2) chloride ion stress corrosion cracking of stainless steel

Chloride ions can not only cause corrosion of stainless steel, but also cause stress corrosion cracking of stainless steel.

The critical chloride concentration for stress corrosion rupture decreases with the increase of temperature. At high temperature, as long as the chloride concentration reaches 10-6, it can cause rupture. The critical temperature for chloride stress corrosion rupture is 70℃. Conditions with chloride concentration (repeated evaporation, wetting) are most prone to rupture. Chloride stress corrosion cracking of stainless steel is quite common in industry.

Stainless steel chloride stress corrosion cracking occurs not only in the inner wall of the pipeline, but also in the outer wall of the pipeline.

As a corrosion factor on the outside of the tube, it was considered to be a problem of the insulation material, and the results of the analysis of the insulation material were tested to contain about 0.5% chloride ions. 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, immersed rainwater.

(3) stainless steel with polysulfuric acid stress corrosion cracking

Stress corrosion cracking of stainless steel with polysulfuric acid (H2SxO6, x= 3-5) is the most typical hydrodesulfurization unit.

In normal operation, the pipeline is corroded by hydrogen sulfide, and the generated iron sulfide reacts with oxygen and water in the air to form H2SxO6 during maintenance. Stress corrosion cracks occur in the parts of CR-Ni austenitic stainless steel pipes with high residual stress (weld heat affected zone, bend, etc.).

(4) Sulfide corrosion rupture

① The stress corrosion fracture of metal in the medium containing hydrogen sulfide and water is sulfide corrosion fracture, referred to as sulfur crack. In natural gas, petroleum collection, processing and refining, petrochemical and chemical fertilizer and other industrial sectors often occur pipeline, valve sulfur cracking accident. The time required for the occurrence of sulfur fracture can be as short as a few days, as long as a few months to several years, but there is no case of sulfur fracture occurring over a decade.

② The cracks of sulfur cracking are thicker, with fewer branches, and most of them are transgranular, and some are intergranular or mixed. The concentration of hydrogen sulfide required for sulfur cracking to occur is very low, only slightly above 10-6, and even at concentrations less than 10-6.

Carbon steel and low-alloy steel are the most sensitive to sulfur cracking in the temperature range of 20 ~ 40℃, while austenitic stainless steel's sulfur cracking mostly occurs at high temperature. The sensitivity of sulfur cracking of austenitic stainless steels increases with increasing temperature. In the medium containing hydrogen sulfide and water, if also 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 ions and oxygen play an promoting role. The sensitivity of 304L and 316L stainless steel to sulfur cracking has the following relationship: H2S+H2O < H2S+H2O+Cl- < H2S+H2O+Cl-+O2 (sulfur cracking sensitivity from weak to strong).

For carbon steel and low alloy steel, the microstructure of quenched and tempered is the best and the microstructure of untempered martensite is the worst. The sulfur cracking resistance of steel decreases in the order of quenching + tempered structure → normalizing + tempered structure → normalizing structure → untempered martensite structure.

The higher the strength of steel, the more prone to sulfur cracking. The higher the hardness of steel, the more easily sulfur cracking occurs. In the accident of sulfur cracking, the weld, especially the fusion line, is the most prone to rupture, because the hardness here is the highest. NACE has strict rules on the hardness of welding seams for carbon steel: ≤200HB. This is because the distribution of weld hardness is more complex than the base metal, so the provisions on weld hardness are stricter than the base metal. On the one hand, it is due to the residual stress of welding, on the other hand, it is the result of the hardening of the weld metal, the fusion line and the heat affected zone. In order to prevent sulfur cracking, effective heat treatment is necessary after welding.

(5) Hydrogen damage

Hydrogen penetrates into the metal interior and causes metal properties deterioration called hydrogen damage, also known as hydrogen destruction. Hydrogen damage can be classified into four different types: hydrogen bubbling, hydrogen embrittlement, decarbonization and hydrogen corrosion.

① Hydrogen bubbling and hydrogen-induced step crack.

It mainly occurs in the medium containing wet hydrogen sulfide. Hydrogen sulfide dissociates in water, and electrochemical corrosion of steel occurs in hydrogen sulfide aqueous solution. In this environment, steel will not only undergo general corrosion due to the anodic reaction, but also promote the penetration of hydrogen atoms into the metal due to the hindering effect of S2- adsorption on the metal surface on hydrogen atom composite hydrogen molecules. When hydrogen atoms permeate and diffuse into steel, they meet defects such as cracks, layers, voids, and slag inclusions. They gather and combine into hydrogen molecules, resulting in volume expansion and extreme pressure (up to hundreds of mpa) inside the steel.

If these defects are near the steel surface, bubbles form. If these defects are deep inside the steel, induced cracks form. It is produced along the rolling direction of parallel cracks, are connected by short transverse cracks to form a "ladder". Hydrogen-induced step cracks cause embrittlement of steel, while hydrogen-induced step cracks reduce effective wall thickness to overload, leakage or even fracture of pipeline.

Hydrogen bubbling requires a critical concentration of hydrogen sulfide. Information is introduced, hydrogen sulfide partial pressure at 138Pa will produce hydrogen bubbling. If phosphine, arsenic, tellurium compounds and CN- exist in the wet hydrogen sulfide medium at the same time, it is conducive to hydrogen infiltration into the steel, they are all hydrogen infiltration accelerators.

Hydrogen bubbling and hydrogen-induced step cracks generally occur in steel coil pipe.

② hydrogen embrittlement.

No matter what way hydrogen enters the steel, will cause steel embrittlement, that is, elongation, section shrinkage significantly decreased, high strength steel is especially serious. If the hydrogen in the steel is released (such as heating for hydrogen removal), the mechanical properties of the steel can still be restored. Hydrogen embrittlement is reversible.

H2s-h2o medium corrosion carbon steel pipeline at room temperature can seep hydrogen, at high temperature and high pressure in hydrogen environment can also seep hydrogen; Hydrogen can be seeped in the pickling process without corrosion inhibitor or improper corrosion inhibitor. Hydrogen can also be seeped when welding in rainy days or when cathodic protection is excessive.

③ the decarburization.

In industrial hydrogen production plant, high temperature hydrogen pipeline is easy to produce carbon damage. The cementite in steel reacts with hydrogen at high temperature to produce methane. The reaction results in the reduction of cementite in the surface layer, and the carbon gradually diffuses into the reaction zone from the adjacent unreacted metal layer, so a certain thickness of the metal layer becomes ferrite due to the lack of carbon. As a result of decarbonization, the surface strength and fatigue limit of steel are reduced.

④ Hydrogen corrosion.

Steel subjected to high temperature and high pressure hydrogen action, its mechanical properties deteriorate, strength, toughness is significantly reduced, and is irreversible, this phenomenon is called hydrogen corrosion.

The process of hydrogen corrosion can be roughly divided into three stages: during the incubation period, the properties of steel do not change; In the stage of rapid performance change, rapid decarbonization and rapid crack propagation; In the final stage, the solid solution is depleted of carbon.

The incubation period of hydrogen corrosion is important and 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 so slow that the incubation period exceeds the normal service life. For carbon steel, the temperature is about 220℃.

Hydrogen partial pressure also has a starting point (carbon steel about 1.4MPa), that is, no matter how high the temperature, below this partial pressure, only surface decarbonization without serious hydrogen corrosion.

The combination of temperature and pressure for corrosion of various hydrogen resistant steels is the famous Nelson curve (this curve is available in many standard specifications for pipeline equipment selection, such as SH3059 "General Principles for Selection of Petrochemical Pipeline Design Equipment").

The cold deformation improves the diffusion ability of carbon and hydrogen and accelerates corrosion.

In a nitrogen fertilizer plant, the high-pressure pipeline from the outlet of ammonia synthesis tower to waste heat boiler has a working temperature of about 320℃, a working pressure of 33MPa, and a working medium of H2, N2 and NH3 mixture. Hydrogen resistant steel should be selected according to Nelson curve. One of the short reducers, due to the wrong use of ordinary carbon steel, soon after use due to hydrogen corrosion and rupture, resulting in a vicious accident, very heavy losses.

Read more ： What are the reasons for the corrosion of seamless steel pipes?