Product Failure Mechanism Analysis - Chemical Corrosion(3)

       The growth process of the oxide film on steel above 570°C is shown in the figure on the right. f is a g-type semiconductor, and the h vacancy concentration is high (up to 9% to 10%), so that j diffuses out rapidly in it, and combines with l at the k interface to form z, and the film thickness increases rapidly. x is an N-type semiconductor with c vacancies, v diffuses inward, and combines with n and m at the b interface to form a and s. The p-type semiconductor is dominant in d, which has a much lower conductivity than f. During the growth of this film, 80% of the ionic conductivity is the outward diffusion of g, and 20% is the inward diffusion of h.



②Decarbonization



Steel is often accompanied by decarburization during the oxidation process. High-temperature decarburization of steel refers to the phenomenon that under the action of high-temperature gas, the surface of steel generates oxide scale, and the metal surface layer connected to the oxide film reduces the phenomenon of cementite. This is because when the high temperature gas contains k, l, z, j and other components, the cementite in the steel reacts with these gases as follows:



x



c



v



b



The gas generated during decarburization destroys the integrity of the surface film, reduces the protective properties of the film, and accelerates the oxidation process. At the same time, due to the reduction of cementite on the surface of the steel, the hardness and strength of the surface layer are greatly reduced, which reduces the wear resistance and fatigue strength of the workpiece. The process in which cementite reacts with hydrogen to generate methane is the hydrogen corrosion described above.



③ Vulcanization



High-temperature gases often contain components such as n vapor, m or q, which can act as oxidants. The process of metal and high-temperature sulfur-containing medium acting to generate metal sulfide and metamorphism is called high-temperature sulfidation of metal. The damage of high temperature vulcanization to refinery equipment is very serious. When processing sulfur-containing crude oil, uniform corrosion of high-temperature sulfur will occur in the high-temperature part of the equipment (240-425 °C). During the corrosion process, organic sulfides are first converted into w and element e, and their corrosion reactions are as follows:



r



t can still decompose y and u at 350-400 °C, and the decomposed element i is more corrosive than o:



p



Vulcanization is faster than oxidation. When there is S-containing gas in the atmosphere or in the combustion products (flue gas), the corrosion and damage of metals will be accelerated. The main reasons are as follows:



(1) The ratio of the metal sulfide to the volume of the metal participating in the sulfide is greater than the ratio of the metal oxide to the volume of the metal participating in the oxidation. For example, the ratio of the volume of q, w, e and r to the corresponding metal volume is generally between 2.5 and 3.0, and the formed sulfide film has a large internal stress, which is easy to rupture the film.



(2) The lattice defect concentration of metal sulfides is higher than that of the corresponding oxides, such as the exact molecular formula y for t at 800 °C, and i for u. Therefore, the diffusivity of ions in the sulfide is high, and the vulcanization speed is fast.



(3) Compared with metal oxides, the melting point of metal sulfides is much lower, especially when the eutectic of certain sulfides is formed, the melting point is lower.



④ "Growing up" of cast iron



The "growth" of cast iron refers to the infiltration of corrosive gases (such as o) into the cast iron along grain boundaries, graphite and fine cracks and oxidation occurs. The geometric size changes, and the mechanical strength decreases.



Hydrogen corrosion



(1) Corrosion characteristics



In the high temperature and high pressure hydrogen environment, after hydrogen diffuses, it reacts with carbon and Fe, C in the steel to produce methane, which will cause severe decarburization and intergranular network cracks on the surface, which will greatly reduce the strength and plasticity of the steel.



Hydrogen corrosion was first detected on vessels that produce ammonia. The hydrorefining, hydrocracking, pre-hydrogenation of platinum reforming and other units in the refinery all expose the material to a harsh high temperature and high pressure hydrogen environment. In some cases, hydrogen reacts with carbon and p in the steel to form methane, which can cause severe decarburization and intergranular network cracks on the surface, which greatly reduces the strength and plasticity of the steel.



(2) Corrosion mechanism



Hydrogen corrosion is a kind of chemical corrosion, which is caused by the action of excess hydrogen in steel and solid solution carbon or carbide in steel under high temperature and high pressure to generate methane. The reaction formula is as follows:



a or s.



The generated methane has a very low diffusivity in the steel and accumulates in the original microscopic voids of the grain boundaries. The carbon concentration in this area decreases with the progress of the reaction. Due to the existence of the carbon concentration gradient, carbon from other places is continuously replenished to this area through diffusion, so that the reaction continues. In this way, the amount of methane will continue to increase, forming high pressure, causing stress concentration, and causing cracks in the grain boundaries where methane gathers. Bubbles that form at defects such as inclusions close to the surface eventually cause blisters to appear on the steel surface. After the cracks and blisters appear, the performance of the steel deteriorates, causing hydrogen corrosion damage.



The generation of methane causes decarburization near the grain boundaries. With the continuous diffusion of carbon and the continuous progress of the reaction, the concentrations of methane, hydrogen and carbon at the newly formed cracks are all lower, making it easier for carbon and hydrogen to diffuse into it. With the continuous progress of this process, network cracks are formed at the grain boundaries, and the strength and plasticity of the steel are greatly reduced.



Hydrogen corrosion is roughly divided into three stages: a. During the incubation period, there are a large number of submicron bubbles filled with methane in the grain boundary carbides and their vicinity, and the mechanical properties of the steel do not change significantly; b. During the rapid corrosion period, after the small bubbles grow up to reach the critical density, they connect along the grain boundary to form cracks, the volume of the steel expands, and the mechanical properties drop rapidly; c. During the saturation period, the carbon is gradually depleted while the cracks are connected to each other, and the mechanical properties and volume of the steel do not change.



(3) Influencing factors and preventive measures



①Temperature and pressure



Elevated temperature and pressure both increase corrosion rates. When the pressure is constant, increasing the temperature can shorten the incubation period; when the temperature is constant, increasing the hydrogen partial pressure can also shorten the incubation period. When the temperature or pressure is below a certain critical value, hydrogen corrosion will not occur. If the hydrogen partial pressure is low and the temperature is high, part of the methane produced by hydrogen corrosion escapes the steel, and the remaining methane in the steel is not enough to cause hydrogen corrosion cracks or bubbling, and the steel only decarburizes. Nelson summed up the influence of temperature and pressure on hydrogen corrosion based on the experience of many hydrogen equipment, and obtained the best Nelson curve (as shown on the right). This curve has a certain reference value for preventing hydrogen corrosion.



② Composition and composition of steel



The increase of carbon content in steel will promote the production of methane and increase the tendency of hydrogen corrosion. When non-carbide-forming elements such as nickel and copper are contained in the steel, these elements promote the diffusion of carbon, thereby increasing the tendency of hydrogen corrosion. When carbide-forming elements such as chromium, aluminum, titanium, niobium, and vanadium are contained in steel, these elements inhibit the decomposition of carbides, thereby reducing the tendency of hydrogen corrosion. Therefore, carbide formers are the main alloying elements of hydrogen corrosion resistant steels.



Reducing the inclusion content in the steel or treating the carbide into a spherical shape can reduce the hydrogen corrosion tendency of the steel.



③Surface surfacing of ultra-low carbon stainless steel



In ultra-low carbon austenitic stainless steel, hydrogen not only has low solubility, but also has a slow diffusion rate. Therefore, surface surfacing of ultra-low carbon austenitic stainless steel is very effective in preventing hydrogen corrosion of the base material.



④Cold working



The pre-cold working deformation will increase the inhomogeneity of the structure and stress of the steel, improve the diffusion capacity of carbon and hydrogen in the steel, and accelerate the hydrogen corrosion. Recrystallization annealing after cold working can reduce the tendency of hydrogen corrosion caused by cold working.



Chemical corrosion is the easiest to perceive in corrosion, but it is also very difficult to solve, and it is necessary to "prescribe the right medicine". Because the corrosion rate varies greatly at different temperatures, in most cases, the higher the temperature, the stronger the corrosion, and the higher the concentration, the more severe the corrosion.



As we all know, conventional corrosion-resistant materials are polymers or oxide ceramics. If it is at room temperature, it is very convenient to use polymer materials, and the price is relatively cheap. However, if it is at high temperature, the polymer material is easy to decompose, and because the thermal expansion coefficient of the polymer material and the base material is quite different, the coating is very prone to cracks when the temperature exceeds 100°C and the alternating between hot and cold is frequent. Also known as "material aging". Although oxide ceramics have good corrosion resistance, as a coating, there are some insurmountable problems, such as the large difference in thermal expansion coefficient with the substrate, and the large porosity of the coating.



Follow-up will continue to analyze another kind of corrosion - electrochemical corrosion.