Key points of material selection for stress corrosion of pressure vessels
Stress corrosion cracking
Stress corrosion cracking (SCC) of a metal material occurs only in a specific corrosion environment. The following table shows the combination of pressure vessel steel and corrosion environment that are easy to cause stress corrosion cracking. It can be seen that almost every material is likely to have stress corrosion cracking.
Table.1 Combination of Pressure Vessel Steel/Corrosion Environment Easily Causing Stress Corrosion Cracking
|Metallic materials||Corrosive medium|
|Carbon steel and low alloy steel||NaOH solution; KOH solution; Nitrate solution; HCN solution; HzS aqueous solution; CO+CO2+H2O system; Gas liquid or coke oven gas liquid (CH4+H2+CO2+HCN+H2S+NH3+H2O); Liquid ammonia; K2CO3; NH4Cl; CaCl2; MgCl2; HsPO,; H2SO4; Red fuming nitric acid; Acetic acid; Hydrochloric acid; Chromic acid; Ethylamine; Industrial and marine atmosphere; NH4SCN aqueous solution; Liquid zinc; Liquid cadmium; Liquid lithium|
|Austenitic stainless steel||NaOH solution; KOH solution; NaOH+sulfide aqueous solution; Containing CI aqueous solution; F-; Br-; I-; HzS aqueous solution; High temperature water and aqueous solution; Water vapor (260C); Seawater; Copolysulfuric acid (H2S, O); Dichloroethane; HzSO4; H2SO4+chloride; HzSO4+CuSO4; NaCIO3; NazSO3; NaH2PO4; NazPO4; NO5; NaNO3; MgCl2; BaCl2; CaCl2; ZnClz; LiCl; NH4Cl; NaCl; Seawater; Concentrate boiler water; Ocean atmosphere; Liquid zinc|
|Ferritic chromium stainless steel||NaOH solution; H2S aqueous solution; Seawater; High temperature water; Nitric acid; Sulphuric acid; NaCl solution; NHs aqueous solution; High temperature and high pressure water; Industrial atmosphere; Ocean atmosphere; High temperature alkali; H2SO4; HNO3; Cl-; F-; Br-; I-.|
According to the above stress corrosion cracking characteristics, the common prevention methods are:
- ① Try to eliminate tensile stress or apply compressive stress. After machining or welding, it is better to conduct stress relief annealing or shot peening to cause surface compressive stress.
- There are three sources of tensile stress: one is the pressure load or non pressure load stress of equipment (such as thermal stress); The second is the residual stress generated in the process of processing, manufacturing and installation; The third is the stress caused by the expansion of corrosion products, that is, the “wedging effect” caused by the larger volume of corrosion products than that of the corroded metal.
- ② Stress corrosion cracking can be prevented by changing the corrosivity of the medium to make it completely non corrosive (including making it stable and passive), or by making it completely corrosive. The former uses corrosion inhibitor; For the latter, if the medium composition of the parts that can be replaced frequently is changed, it will cause overall corrosion.
- ③ Metal materials resistant to stress corrosion cracking shall be selected.
- ④ Cathodic protection is adopted.
The stress corrosion cracking of metal in sodium hydroxide solution is called alkali embrittlement. Alkali embrittlement can occur in carbon steel, low alloy steel, stainless steel and other metal materials. Almost all carbon steels may produce alkali embrittlement when the concentration of sodium hydroxide is above 5%; When the temperature and sodium hydroxide concentration of carbon steel and low alloy steel exceed the limit value corresponding to the curve, stress relief heat treatment shall be carried out after welding. The corrosion allowance of steel plates for carbon steel and low alloy steel pressure vessel shells shall not be less than 3mm to enhance the resistance to uniform corrosion and reduce the shell stress level. Carbon steel and low alloy steel welded vessels containing potassium hydroxide solution shall also refer to the above requirements.
The relationship shown in the figure below has been published as early as the early 1950s and has become the most cited data in corrosion works. However, alkali embrittlement damage accidents of carbon steel equipment continue to occur today, and the environmental temperature conditions are obviously within the fracture zone (stress relief). Some of the reasons are related to the heating mode, that is, hot water heating is the main method, but steam heating is the auxiliary method in winter, causing overheating (more than 50 ℃). The following aspects can be considered as solutions: change the heating mode, reconsider the material selection and heat treatment scheme; Set temperature detection points to prevent overheating, etc.
Figure.1 Temperature and concentration limits of carbon steel used in sodium hydroxide
For austenitic stainless steel, when the sodium hydroxide concentration is above 0.1%, 18-8 austenitic stainless steel can suffer from alkali embrittlement. The most dangerous is when the sodium hydroxide concentration is 40%. At this time, the temperature of alkali embrittlement is about 115 ℃. When 2% molybdenum is added to austenitic stainless steel, the limit of alkali embrittlement can be narrowed and moved to the area with high alkali concentration. Nickel and nickel base alloys have high stress corrosion resistance, and their alkali embrittlement range becomes narrow, and they are located in the high temperature concentrated alkali zone.
Chloride ion can not only cause pitting corrosion of austenitic stainless steel, but also cause stress corrosion cracking. The critical chloride concentration for 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 for chloride stress corrosion cracking is 60 ℃. The conditions with chloride ion concentration (repeated evaporation and wetting) are most likely to crack. Chloride stress corrosion cracking of austenitic stainless steel is quite common in pressure vessels.
Oxygen ion stress corrosion cracking of austenitic stainless steel occurs not only on the inner wall of equipment, but also on the outer wall of equipment and pipeline. The air always contains chloride, and the chloride content is higher in marine atmosphere and industrial area atmosphere. The repeated condensation and evaporation of moisture in the atmosphere on austenitic stainless steel bolts will cause chloride concentration on the bolt surface. The alternation of dry and wet is very beneficial to chloride concentration. Therefore, stainless steel equipment in the alternation of dry and wet environment is prone to stress corrosion cracking. Chlorine ion may also come from impurities contained in insulation materials, or the result of damage of insulation layer, rainwater entering and concentration, such as asbestos insulation materials. In order to avoid stress corrosion of stainless steel caused by chlorine concentrated after the insulation material of austenitic stainless steel is wet, the chlorine content of the insulation material can be specified when necessary.
Stress corrosion cracking of stainless steel in polythionic acid
In the petroleum refining industry, the stress corrosion cracking of stainless steel with polysulfuric acid (H2SxO6, x=3~5) is very noticeable, taking the hydrodesulfurization unit as a typical example. During normal operation, the equipment is corroded by hydrogen sulfide, and the generated iron sulfide reacts with oxygen and water in the air to generate H2SxO6 during shutdown for maintenance. Stress corrosion cracking occurs in parts of Cr Ni austenitic stainless steel equipment and pipes with large residual stress (weld heat affected zone, elbow, etc.).
Sulfide corrosion cracking (SSCC sulfur cracking)
The stress corrosion cracking of metal in the medium containing hydrogen sulfide and water is called sulfide corrosion cracking, which is called sulfur cracking for short. The time required for sulfur cracking is as short as a few days, and as long as a few months to a few years. However, there is no case of cracking in more than ten years. The concentration of hydrogen sulfide required for sulfur cracking is very low, slightly more than 10-6, or even less than 10-6. NACE takes 355Pa hydrogen sulfide partial pressure as the criterion to determine whether a specific gas component (oil field) is in the sulfur fracture latency zone. The time required for sulfur cracking is generally shortened with the increase of hydrogen sulfide concentration. The critical stress decreases with the increase of hydrogen sulfide concentration.
Carbon steel and low alloy steel are most sensitive to sulfur cracking in the temperature range of 20~40 ℃, but 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 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.
For the sulfur cracking of austenitic stainless steel, chloride ions and oxygen play a role in promoting the sulfur cracking. The sensitivity of 304L and 316L stainless steels to sulfur cracking is related as follows: H2S+H2O<H2S+H2O+CI -<H2S+H2O+Cl -+O2 (the sensitivity to cracking is from weak to strong).
For carbon steel and low alloy steel, the quenched and tempered metallographic structure has the best sulfur crack resistance, and the untempered martensitic structure is the worst. The sulfur cracking resistance of steel decreases in the order of quenching+tempering structure, normalizing+tempering structure, normalizing structure and untempered martensite structure.
The higher the strength of steel is, the easier sulfur cracking will occur. The higher the hardness of the steel, the easier the sulfur cracking occurs. NACE standard stipulates that HRC of steel containing flowing oil and gas field is less than 22. It is now widely used internationally as the standard for carbon steel and low alloy steel in wet hydrogen sulfide environment. There is no theoretical basis for this, just a summary of on-site experience. The exceptions are very few.
The welding seam, especially the fusion line, is the position most prone to sulfur cracking, because the hardness here is the highest. NACE strictly stipulates the hardness of carbon steel welds: HB ≤ 200. Fracture often occurs at the weld seam, on the one hand, due to the role of welding residual stress, on the other hand, due to the hardening structure of weld metal, fusion line and heat affected zone. In order to prevent sulfur cracking, it is necessary to conduct effective heat treatment after welding (appropriately increase the holding time of 625 ℃± 25 ℃).
Measures for controlling sulfur cracking: 18-8 austenitic stainless steel and carbon steel and low alloy steel with standard tensile strength greater than 540MPa should not be used as pressure components in sulfur cracking environment; Reduce the content of S and P in steel plate; Increase non-metallic inclusion and grain size inspection to avoid inclusion exceeding the standard and ensure fine grain requirements; When materials not recommended in the standard are selected, or the manufacturing process of materials is changed, or materials without use experience are selected, SSCC resistance performance evaluation test shall be conducted for materials; The corrosion allowance of steel plate for shell shall not be less than 3mm; Copper and various copper alloys shall not be used; Dissimilar metal welding joints between ferritic steel or duplex stainless steel and austenitic copper are not allowed.
The control measures for carbon steel and low alloy steel in other stress corrosion cracking environments are basically the same as the above sulfur cracking control measures. Because “stress concentration” and “medium concentration” are two unavoidable and very important factors in the process of pressure vessel design and use, and are closely related to structural design, the critical stress and critical concentration for stress corrosion cracking given in the data are only of reference value and relative significance, and are acceptable engineering control indicators or practical measures suitable for use. This is also the complexity of engineering projects.
Other common stress corrosion cracking systems
① Stress corrosion cracking of carbon steel and low alloy in liquid ammonia (ammonia embrittlement)
Pure liquid ammonia will not cause stress corrosion cracking. When air (O0, N2, CO2) is mixed with liquid ammonia, such as agricultural liquid ammonia in fertilizer industry, stress corrosion cracking will occur in both liquid and gas phases. If the water content in liquid ammonia exceeds 0.2%, the cracking can be inhibited. It is necessary to carry out heat treatment to eliminate residual stress on the weld.
② Stress Corrosion Cracking of Carbon Steel in CO-CO2-H2O Environment
Such damage accidents often occur in the decarburization system of synthetic ammonia, hydrogen production, gas system, organic synthesis, petroleum gas and other industries.
③ Stress corrosion cracking of austenitic stainless steel in high temperature water
In power industry and nuclear industry, such damage accidents often occur. Dissolved O2 is the promoting factor, and the crack is intergranular, but if Cl – is contained, the crack will be transgranular.
④ Carbon steel is sensitive to stress corrosion cracking in nitrate solution, gas liquid and coke oven gas.
For example, cracks of carbon steel in coke oven gas (mainly CH4 and H2, containing CO2, H2S, NH3, HCN, H2O) can be found after one year of use at 35 ℃.
Source: China Pressure Vessels Manufacturer – www.secmachinery.com
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