Aiming at the problem of corrosion failure of laser cladding layer of oil cylinder in coal mine environment, the mine environment such as underground water, coal powder soaking liquid water, coal quality, etc. was analyzed, the microstructure of the cladding layer was studied with a metallographic microscope, and the corrosion state and chemical elements of the cladding layer were studied using SEM images, EDS energy spectrum, XPS and other methods. The corrosion mechanism of the cladding layer in coal mine environment was discussed. The results show that the cladding layer and the substrate are well metallurgically bonded, and no defects are found inside; the structure is distributed in dendritic crystals and elliptical and granular shapes, and the closer to the substrate, the finer the dendrites; the depth of the corrosion pit is 50 to 150 μm, which can reach 1/4 of the thickness of the cladding layer, and the size and depth vary; the chemical composition of the cladding layer is uniform, the element content meets the design requirements, and there is no obvious element segregation; EDS energy spectrum, XPS and other test results show that the corrosion starts from the chromium-depleted area between the dendrites of the cladding layer, and elements such as S and Cl participate in the corrosion process; pitting and contact corrosion between the coal powder containing corrosive media such as S and Cl and the cladding layer cause the cladding layer to fail. The study provides a reference for the surface treatment process design of the oil cylinder in the coal mine environment.
Hydraulic support is the most numerous and invested coal mine comprehensive mining equipment in the underground working face, and the hydraulic cylinder is the most frequently moving component on the hydraulic support. Its stability and durability determine the mining progress of the coal mine. Laser cladding, as a green, metallurgical, high-hardness, and corrosion-resistant surface treatment technology, has been applied to the hydraulic support cylinder industry of coal mine machinery for more than 10 years. It has replaced about 40% of the electroplating process every year. Laser cladding technology has gradually been recognized by the coal machinery industry. The underground environment of my country’s coal mines is complex and diverse. The content of harmful ions such as Cl- and SO2-4 in water reaches 100 to 3,000 mg/L, and the air contains harmful gases such as CO, CO2, SO2, and H2S. These harmful gases and substances are dissolved in humid air and underground water to generate various corrosive electrolyte solutions, which gather together with coal dust and adhere to the outer surface of the hydraulic support cylinder, which has a great impact on the service life of the laser cladding layer. The laser cladding technology used in the coal mine machinery industry mainly uses martensitic iron-based stainless steel powder. In recent years, the problem of corrosion of the cladding layer has also occurred from time to time.
The underground environment of a coal mine in my country is harsh and belongs to a typical highly corrosive coal mine environment. The martensitic cladding layer has rust problems during underground use. The hydraulic cylinder cladding layer of the mine was selected as the research object. The field data was collected. Combined with the actual corrosion of the cylinder, the corrosion morphology and corrosion products of the middle cylinder were studied from the perspective of the mine environment factors. The uniformity, metallographic structure and hardness, chemical element distribution, corrosion products, etc. of the cladding layer were analyzed to explore the corrosion mechanism of the martensitic cladding layer in a highly corrosive coal mine environment.
1 Experiment
1. 1 Experimental materials
The secondary cylinder of the cylinder (hereinafter referred to as the middle cylinder) uses φ290 mm, 27SiMn alloy steel as the raw material. The chemical composition of the middle cylinder matrix is shown in Table 1. The outer cylindrical surface treatment adopts the laser cladding process, and the net thickness of the finished cylinder cladding layer is 0.5 ~ 0.7 mm. The cladding layer has corrosion problems during its use in coal mines, as shown in Figures 1 and 2. A large number of rust spots appear on the outer circle. The whole circle of processing lines in the figure is a turning mark for the convenience of sample preparation.
The corrosion cylinder and wire cutting sampling locations are shown in Figures 3 and 4. The rust at the corrosion location in the figure was tested for corrosion products, and the samples were cut, sampled, and tested to analyze the failure mechanism of the cladding layer.
1. 2 Chemical composition and cladding parameters of the cladding layer
Laser cladding uses iron-based stainless steel powder with a particle size of 50 to 180 μm. The main chemical composition is shown in Table 2, and the laser cladding parameters are shown in Table 3.
1. 3 Coal ash water analysis
The coal sample was crushed and sieved, and particles with a particle size of less than 0.3 mm were taken and soaked in distilled water. 100 g of coal + 400 mL of distilled water were soaked for 24 h, and the leaching liquid was taken for water quality analysis. The number of samples is 2, and the volume of each sample is 0.3 L.
1. 4 Coal powder analysis
The coal samples were crushed to more than 100 mesh, dried at 110℃ for 7 hours, and then subjected to elemental analysis and industrial analysis, and the conductivity of the coal powder was tested. The industrial analysis of the coal samples was carried out according to the “Method for Industrial Analysis of Coal” (GB/T 212-2008), the total sulfur analysis was carried out using the “Method for Determination of Total Sulfur in Coal” (GB/T 214-2007), the carbon and hydrogen analysis was carried out using the “Method for Determination of Carbon and Hydrogen in Coal” (GB/T 476-2008), and the nitrogen analysis was carried out using the “Method for Determination of Nitrogen in Coal” (GB/T 19227-2008). The conductivity of coal samples was tested by the four-terminal method ST2722-SD with ST2643 high resistance meter. The number of samples was 2, each sample was 0.25 g, the test temperature was 28 ℃, and the humidity was 45%.
1.5 Morphology, phase structure and hardness test
In order to observe the corrosion of the cross section of the cladding layer, the cut corroded surface and uncorroded surface samples were inlaid, polished and polished, and then their cross sections were observed. The laser cladding layer was corroded by high ferric chloride hydrochloric acid aqueous solution, and the macroscopic morphology of the surface and cross section of the cladding layer was observed by a stereo microscope, and its metallographic structure was observed at different magnifications; the cladding layer structure, cladding layer cross section, surface corrosion pits and element distribution were analyzed by scanning electron microscopy. The corrosion products were scraped from the surface of the severely corroded cladding layer, and the corrosion products were analyzed by EDS energy spectrum, XPS, etc.
2 Results and discussion
2.1 Corrosion environment analysis
The test results of water used for flushing the oil cylinder surface are shown in Table 4. It can be seen from Table 4 that the concentrations of Cl- and SO2-4 in the water are relatively high, among which the Cl- content reaches 1460 mg/L. Cl- is highly active and can penetrate the passive film of stainless steel to corrode the cladding layer.
It can be seen from Table 5 that in the water soaked with 100 g coal + 400 mL distilled water, the Cl- content is 10.68 mg/L and the SO2-4 content is 93.02~97.14 mg/L. When the water used to flush the bracket in Table 4 is used to soak the coal powder, the amount of Cl- and SO2-4 will be superimposed, resulting in an increase in the concentration of Cl- and SO2-4. Under actual working conditions, a large amount of coal dust will be deposited on the surface of the cylinder when the bracket is flushed, and water will splash onto the coal powder. Due to the alternating effect of dry and wet, various ions in the water will be concentrated and enriched in the coal powder, resulting in an increase in ion concentration. In a humid environment, its corrosiveness is enhanced.
The results of industrial analysis and elemental analysis of coal samples are shown in Table 6.
It can be seen from Table 6 that the coal samples tested this time are within the range of lignite. According to the “Fixed Carbon Classification of Coal” (MT/T 561-2008), they belong to low fixed carbon coal. Inorganic sulfur is more common in my country’s coal, and most of the inorganic sulfur in coal exists in the form of pyrite sulfur (FeS2), and a small amount is marcasite sulfur (FeS2).
The test data of dry coal powder conductivity are shown in Table 7. It can be seen from Table 7 that the resistivity of dry coal powder can reach tens of GΩ·cm under a relatively low pressure, which can be considered as insulating.
Wet coal powder soaked in mine water was used for sample preparation and tested with a low resistance meter. Two samples were also used, each with a mass of 0.5 g. The test temperature was 28 °C and the humidity was 45%. The resistivity test data of wet coal powder are shown in Table 8. It can be seen from Table 8 that under a relatively small pressure, the resistivity of wet coal powder is between 10 and 20 kΩ·cm, which is significantly lower than the GΩ·cm of dry coal powder. The decrease in resistivity indicates that wet coal ash has better conductivity and adheres to the surface of the cladding layer, making it more corrosive to the cladding layer.
2.2 Macromorphology analysis of cladding layer
The morphology of the corrosion part of the cylinder is shown in Figure 5. Figure 5 (a) shows the side of the cladding layer in contact with coal ash. The surface is severely corroded with flakes of black-brown rust; Figure 5 (b) is a photo of the corroded part after grinding. After grinding the corrosion products on the surface, the metallic luster is exposed, but there are still many corrosion pits on the surface of the cladding layer. Figure 5(c) is the morphology of the cladding layer in Figure 5(a) under a stereo microscope. It can be seen that the surface of the cladding layer is covered with a layer of black-brown rust products, and the corrosion pits are distributed in shallow flakes. Some corrosion pits are relatively large and gather together to form large pieces; some corrosion pits are just beginning to form and are only the size of a pinhole. The bottom of the corrosion pit shows a gray luster under light. This luster is different from the luster of the metal itself, but is the “diffuse reflection” type of oxide surface luster.
The metallographic structure of the cross section of the cladding layer is shown in Figure 6. It can be seen from Figure 6(a) that the cladding layer forms a good metallurgical bond with the substrate, with dendritic crystals and elliptical and granular shapes, and carbides are distributed on the martensite in the form of elongated blocks and strips. Among them, the cladding layer forms a white bright layer near the fusion line, which is a plane crystal, and then forms a thick dendrite growing vertically upward near the substrate. The thick dendrite near the surface layer is transformed into slightly fine dendrites, and the structure is relatively uniform; the corrosion pit is a circular pit, and no cracks are found at the bottom of the pit. Due to the uneven local corrosion, there are undulations at the bottom of the corrosion pit.
From Figure 6 (b) and 6 (c), it can be seen that there are many corrosion pits in the cladding layer. Most of the corrosion pits are shallow, and some are deep. The bottom of the corrosion pit is relatively smooth, and no sharp cracks are found in the cladding layer at the bottom of the pit; the corrosion does not penetrate the entire cladding layer, and the corrosion depth is 50 to 150 μm. Cracks and dendrite structures can be observed in the corrosion pits, indicating that they have not been completely corroded and transformed into oxides. No defects are found in other parts of the cladding layer.
2. 3 SEM and energy spectrum analysis of cladding layer
2. 3. 1 Energy spectrum analysis of the cross section of the cladding layer
The energy spectrum test of the cross section of the cladding layer is shown in Figure 7. As can be seen from Figure 7, no obvious defects are found in the macroscopic view, and its structure is basically consistent with the metallographic structure observed above. The metal composition of the cladding layer is relatively uniform, and no obvious element segregation is observed. The element spectrum test is shown in Figure 8. As can be seen from Figure 8, the Cr content on the dendrite trunk is high, and the content between the dendrites is slightly lower, and there is a certain degree of difference. This difference is caused by the solute redistribution during the solidification process and is also related to the formation of Cr carbides. Figure 8 (a) At the position of the dendrite trunk, the Cr content is about 20.69%; Figure 8 (b) At the position between the dendrites, the Cr content is about 16.49%, which is because a chromium-poor area is formed between the dendrites during the melting and condensation of the powder.
2. 3. 2 SEM analysis of the cross section of the cladding layer
The SEM of the cross section of the cladding layer is shown in Figure 9. As can be seen from Figure 9, the corrosion pit is circular or elliptical, with a depth of 50 to 150 μm, which is similar to the metallographic observation results in Figure 6.
The corrosion pit on the left is deeper, but the degree of corrosion is shallow. There are two large cracks in the corrosion pit, and fine cracks are formed in other places in the corrosion pit. There are more cracks at the bottom of the pit. After corrosion, oxides are produced, the volume expands, and the corrosion products peel off. Although the corrosion pit on the right is shallow, many fine cracks are also formed in the corrosion pit. It is obvious that there is a difference in the corrosion of dendrite branches and interdendritic. The corrosion rate of interdendritic is faster, and the corrosion rate of dendrite branches is slower, thus forming many holes. The element spectrum results in Figure 8 show that the Cr content between dendrites is low, and the Cr content of dendrite branches is high. The difference in corrosion rate between the two is negatively correlated with the Cr content.
The SEM of the cladding surface is shown in Figure 10. As shown in Figure 10, the surface of the cladding layer is relatively smooth, and the corrosion pits are shallow flakes. After the surface oxides are removed, the bottom of the pit is exposed. There are obvious irregular cracks at the bottom of the pit. Due to the corrosion, the bottom of the pit presents a similar organizational morphology to metallographic corrosion. There are also fine pitting corrosion in other places, which is relatively dense. After magnification, it is found that there are cross-shaped holes on the surface of the sample. Analysis shows that these holes may be micro-shrinkage formed before the corrosion expands, and they become corrosion channels first. As shown in Figures 9 and 10, the corrosion of the cladding layer starts to expand from the chromium-poor area between the dendrites, and the volume change of the cladding layer leads to defects such as cracks and pores. In the chromium-poor area, the chromium content is low, and harmful substances such as Cl and S in the coal ash or water in the mine are more likely to destroy the passivation film in the chromium-poor area, forming corrosion gaps, and then forming corrosion points or cracks.
2. 4 Corrosion product analysis
2. 4. 1 EDS spectrum characterization of corrosion sites on the surface of cladding layer
The surface of cladding layer was tested by spectrum test, and the results are shown in Figures 11 and 12. As can be seen from Figures 11 and 12, the Fe and Cr elements in the corrosion sites still maintain a high content, which are the two elements with the largest proportion in stainless steel. The Ni, Nb, Si and other elements contained are consistent with the elements in stainless steel powder; but the C and O contents increase a lot, indicating that the cladding layer has undergone significant oxidation; in addition, the S, K, Ca and other elements increase significantly, indicating that these elements are involved in the corrosion process, especially the S element, the content of which reaches 1.27% to 3.58%. S generally exists in the form of anions in water, and the corrosion effect on the cladding layer is more obvious.
2. 4. 2 EDS spectrum analysis of corrosion products
The spectrum analysis results of the corrosion products scraped from the cladding layer are shown in Figure 13.
As shown in Figure 13, the ratio of Fe and Cr in the two corrosion products is quite different, which may be related to the test method of energy spectrum, which mainly tests the distribution of surface elements. If the corrosion is severe, the composition of the corrosion product will be more uniform; if the corrosion is light, the oxide on the surface of the corrosion product will play a shielding role. The Fe, Cr, Si and other elements in the corrosion product are the chemical composition of the stainless steel itself, and the Na, Al, S, Cl, Ca and other elements are the chemical composition of the underground water quality or coal ash in the coal mine. The test results show that S and Cl elements exist. In aqueous solution, S and Cl generally appear as anions, which have a greater corrosive effect on metals. Among them, Cl- is adsorbed on the metal surface, which can reduce the corrosion potential of the metal, causing the metal to lose electrons and dissolve and pit.
2. 4. 3 XPS analysis of corrosion products
The XPS test results of corrosion products are shown in Figure 14. After wide scanning, it can be seen from Figure 14 that Fe, O, Cr, Si, N, and S elements all have a certain content, and their corresponding peak areas are shown in Tables 9 and 10.
From the calculation results and spectra, the ratio of Fe and Cr is normal, and S has a spectrum peak, indicating that it is involved in the corrosion process. SO2 gas in the mine is very easy to dissolve in the water film on the metal surface, and can react to generate H2 SO3 or H2 SO4, which is acidic. H2 SO3 is a strong depolarizer and has an accelerating effect on corrosion.
3 Analysis and discussion of the failure mechanism of the cladding layer
From the above analysis results, it can be seen that the cladding layer is well metallurgically bonded to the substrate, and no obvious defects are found. The thickness of the cladding layer is 0.6-0.7 mm. The dendrites of the cladding layer are obvious. Some dendrites are relatively long. No obvious element segregation is observed. There is a difference in Cr elements between the dendrite branches and dendrites. This is an inherent phenomenon in the solidification process. The Cr content of the cladding layer meets the design requirements.
From the previous corrosion morphology analysis, it can be seen that the entire corrosion process is caused by the adhesion of coal powder to the surface of the cladding layer. Coal powder can cause contact corrosion and pitting corrosion.
First, the corrosion of the cladding layer at the bottom of the wet coal powder is similar to the corrosion of metal in the soil. At the bottom of the coal powder, due to insufficient O2 supply and low potential, it becomes an anode, while at the edge without coal powder coverage, O2 is relatively sufficient and the potential is high to form a cathode, thus forming an oxygen concentration difference corrosion cell. The passivity of the metal surface is easily destroyed, which accelerates the corrosion of the metal at the bottom of the coal powder, as shown in Figure 15. From the previous analysis of the corrosion products, we know that there is S element in it. The gas in the coal mine contains SO2, which has the greatest impact on metal corrosion. The SO2 content in the gas is relatively low, but the solubility of SO2 in aqueous solution is hundreds of times higher than that of O2, resulting in extremely high SO2 concentration in the liquid film on the metal surface. SO2 accelerates the metal corrosion rate by strengthening the anodic depassivation and cathode depolarization.
Pitting is caused by the competitive adsorption of anions such as Cl- and oxygen. The metal oxide film has the function of metabolism and self-repair, that is, the passivation film is in a dynamic equilibrium state of continuous dissolution and repair. If the film adsorbs active anions such as Cl-, because Cl- can preferentially and selectively adsorb on the passivation film, it squeezes out oxygen atoms and combines with cations in the passivation film to form soluble chlorides. The dynamic equilibrium of the passivation film is destroyed, dissolution is dominant, and small pits are generated at specific points of the newly exposed base metal. The pore size is mostly 20 to 30 μm. These small pits are called pitting nuclei. The Cl- and SO2-4 in the mine water are mixed with the S-containing coal powder, which can easily destroy the passivation film at the bottom of the coal powder and the contact part of the cladding layer, causing pitting on the surface of the cladding layer, as shown in Figure 16. Once the pitting hole is formed, the metal in the hole is in a local activation state
(low potential), which is the anode; the large surface outside the pitting hole is still in a passivation state (high potential), which is the cathode. Therefore, the inside and outside of the pit constitute an activation-passivation battery composed of a small anode and a large cathode, which accelerates the development of the pit.
Coal contact is the primary factor leading to the failure of the cladding layer. When the contact corrosion destroys the passivation film on the surface of the stainless steel, the corrosive media such as Cl and S in the underground environment are superimposed, causing the volume of the corroded part of the cladding layer to change and expand. The generated pores, cracks and other macro defects provide channels for the corrosive media, accelerate the development of corrosion, and further produce oxides inside the corrosion pit. When the metal becomes oxide, the volume expands and breaks, which eventually leads to the failure of the cladding layer.
4 Brief analysis of countermeasures to improve the life of cladding layer
During underground coal mining, there is a risk of coal gangue collapsing onto the surface of the cylinder coating. The hydraulic support cylinder coating generally uses martensitic stainless steel, and the hardness of the cladding layer is HＲC50~55. However, from the underground use and the above analysis, it can be seen that the corrosion resistance of martensitic stainless steel in highly corrosive coal mines is general and cannot meet the harsh working conditions underground. In the working face of the mine where the failed cylinder is located in this paper, the safety valve and fastening bolts used in the cylinder are made of 304 or 316 stainless steel, and no rust occurs. The stainless steel with austenitic structure has better corrosion resistance, but the hardness is HＲC20~30, which is not suitable for the outer cylindrical coating of the underground cylinder in coal mines. Considering that the outer cylindrical coating of the coal machine cylinder should have the characteristics of high corrosion resistance and high hardness, a duplex stainless steel cladding sample containing austenite and martensite structures was prepared, and a hanging block test was carried out on the working face of the cylinder corrosion in this paper (Figure 17). It can be seen that after 1 year of hanging block test, the bidirectional stainless steel sample did not have rust and other problems. Its corrosion resistance and hardness are more suitable for use in coal mines, and it is a development direction for coal machine cylinder coating.
5 Conclusion
The rust of the cladding layer of the middle cylinder is mainly concentrated on the side in contact with coal ash. The corrosion pits are shallow and of varying sizes. The depth of the corrosion pits is 50 to 150 μm. Cracks can be observed in the corrosion pits. There are many small cracks in the corrosion pits, which are caused by the difference in corrosion between dendrite branches and dendrites. The corrosion starts from the chromium-poor area between dendrites of the cladding layer, and elements such as S and Cl participate in the corrosion process. The contact corrosion and pitting between coal powder containing corrosive media such as S and Cl and the cladding layer are the primary factors leading to the failure of the cladding layer. When the passivation film on the surface of the stainless steel is damaged, the harmful medium will accelerate the corrosion of the cladding layer, causing the cladding layer to expand, rust, break, and fail. Martensitic stainless steel is not suitable for use in highly corrosive coal mines. Duplex stainless steel containing austenite and martensite is a better development direction.
