What are the corrosion of stainless steel pipe?

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.