Local Corrosion Resistance of Three Common Stainless Steels
Austenitic stainless steels have good room temperature and low temperature toughness, weldability, corrosion resistance and heat resistance, and have been widely used. Among them, 1Cr18Ni9Ti, 304, 316L three kinds of austenitic stainless steel, in the field of synthetic fiber, textile, petroleum, chemical, atomic energy, aerospace and other fields for the manufacture of various containers and corrosion-resistant parts [1-3]. Although 1Cr18Ni9Ti, 304, 316L stainless steel has a low overall corrosion rate, it has poor local corrosion resistance. These localized corrosions mainly include intergranular corrosion, pitting and stress corrosion. Intergranular corrosion failure is not easy to detect. In severe cases, it loses its metal characteristics and taps and breaks. Severe pitting can penetrate thin tubes, plates and other parts and cause leakage. Pitting is also a source of stress corrosion cracking and crevice corrosion; stress corrosion is fast, damage is severe, and brittle fractures often occur without any obvious macroscopic deformation and without any omen, endangering personal safety or causing economic losses [ 4]. Therefore, it is important to have a deep understanding of local corrosion resistance. The above three stainless steels have similar physical properties such as density, thermal conductivity, and crystal structure, but their trace elements are different in content. The composition of the material affects its performance, and the composition of the stainless steel affects the quality of its surface passivation film. However, regarding the influence of different trace element content on local corrosion resistance, lack of systematic research. The effect of trace element content on the local corrosion resistance of 1Cr18Ni9Ti, 304, 316L austenitic stainless steel was studied.
|Results and discussion|
The thickness of three cold rolled steel plates is 3 mm, and the chemical composition is listed in Table 1.
Table 1 Chemical composition of three kinds of austenitic stainless steel (%, mass fraction, Fe rest)
The samples of intergranular corrosion, stress corrosion and pitting corrosion were cut from three kinds of stainless steel plates along the rolling direction. The solution treatment was carried out. The heat treatment system was water cooling after holding at 1050 C for 20 minutes.
In the study of intergranular corrosion properties, the samples after solid solution were sensitized and heat treated. The system was air-cooled after holding at 650 C for 2 hours. According to GB/T 4334-2008 Standard “Corrosion of Metals and Alloys – Intergranular Corrosion Test Method of Stainless Steel”, the intergranular corrosion tendency of 1Cr18Ni9Ti, 304 and 316L austenitic stainless steels were studied by “Sulfuric acid-Copper Sulfate Corrosion Test Method of Stainless Steel”. Details of the experiment: 100 g analytical pure copper sulfate was dissolved in 700 ml distilled water, then 100 ml superior pure sulfuric acid was added, and then diluted to 1000 ml with distilled water to form sulfuric Acid-Copper sulfate solution. The solution was packed into a conical flask with a reflux condenser. Copper chips with 99.8% purity were laid on the flask and a sample with a size of 80 mm *20 mm *3 mm was placed. The flask was placed in the heating device and cooled with cooling water. The experimental solution was heated and kept in a slight boiling state. After 16 hours, the flask was washed and dried. The flask was bent 180 degrees and then observed.
The pitting corrosion behavior of three solid solution stainless steels in 3.5% NaCl solution was measured by potentiodynamic method. The reference electrode in the electrochemical workstation is a saturated calomel electrode (SCE), the auxiliary electrode is a platinum sheet, and the working electrode is a sample to be tested. The area of the square surface with a cross section of 6 mm x 6 mm is 0.36 cm2. All specimens were sanded with the same process. Potential scanning range: 1Cr18Ni9Ti is -0.4 V~1.5 V, 304 is -0.6 V~0.8 V, 316L is -0.4 V~1.5 V. The actual termination potential varies with pitting breakdown potential. The test temperature is room temperature and the scanning rate is 0.002 V/s.
The three stainless steels have different compositions, so the stress corrosion susceptibility can be evaluated by using HB7235-1995 Slow Strain Rate Stress Corrosion Test Method, which can measure the different stress corrosion susceptibility caused by different compositions. In the experiment, the mechanical properties of the samples under slow strain rate (10-6 s-1) in corrosive medium (42% boiling magnesium chloride solution) and inert medium (silicone oil protection) were tested, and the stress corrosion sensitivity coefficient was calculated.
2.1 Intergranular corrosion behavior of three stainless steels
Fig. 1 shows the photographs of three stainless steels boiled continuously for 16 hours in sulfuric Acid-Copper sulfate solution and bent 180 degrees. The results show that the outer surface and side of the bending parts of 1Cr18Ni9Ti and 316L specimens are relatively smooth, and no cracks are observed at low magnification, while the outer surface of 304 stainless steel bending parts is obviously rough, and the cracks are obvious on the side. These results indicate that intergranular corrosion may occur in 304 stainless steel after solution and sensitization. Because the cracks occur at the edges and corners of the bending part, according to GB/T 4334-2008 requirements, further examination of the metallographic structure is needed to determine whether intergranular corrosion has occurred.
Fig.1 Photographs of bended stainless steels at sensitization state after intergranular corrosion test (a)-(c) front view, (d)-(f) side view
Fig. 2 shows the metallographic photographs of the bending specimens (Fig. 1) of the three materials after intergranular corrosion experiments. Fig. 2a-c, Fig. 2d-f and Fig. 2g-i show the typical structures of the bending, flattening and grain boundary of three stainless steels respectively. The black arrows in figs. 2B and 2E indicate the cracking position of 304 stainless steel along grain boundary. Figure 2 shows that for 304 stainless steel, obvious cracking along grain boundaries can be observed in both bending and flattening parts, indicating that intergranular corrosion does occur. For another 304 samples with the same heat treatment regime (solution and sensitization) but without corrosion test, only the same degree of bending was carried out, and no cracks were found. This shows that the crack of 304 stainless steel in Fig. 2 is not caused by bending, but by intergranular corrosion. Figure 2g-i shows the characteristics of grain boundaries of three samples after intergranular corrosion test. As indicated by the black arrow in the figure, there are discontinuous small dots and continuous carbides on the grain boundaries of solid solution and sensitized 304 stainless steel. However, no fine carbide precipitation was observed at the grain boundaries of 1Cr18Ni9Ti and 316L, which was consistent with the cracking observed at low magnification (Fig. 1), indicating that no intergranular corrosion occurred. These results indicate that the occurrence of intergranular corrosion is closely related to the precipitation of carbides near grain boundaries.
Fig.2 Mettallographic photos of typical microstructures of three kinds of steels after intergranular corrosion test: (a)-(c) bending area, (d)-(f) flat area, (g)-(i) grain boundary area
At room temperature, the solubility of C element in austenite is very small, about 0.02%-0.03%. At sensitization temperature, such as 650 (?) for a certain time, C diffuses continuously to the austenite grain boundary, and forms Cr23C6 near the grain boundary with Cr23C6. This results in the formation of Cr-poor regions near grain boundaries. When the local content of Cr is less than 11.7%, the corrosion resistance of  decreases significantly, which shows that it is susceptible to intergranular corrosion. Ti and C have stronger bonding ability than Cr. Preferential bonding with C to synthesize stable carbides can avoid the formation of Cr-poor zone in austenite and reduce the generation of intergranular corrosion . When C content is less than 0.03%, the intergranular corrosion resistance of stainless steel is significantly improved . According to the data in Table 1, compared with 1Cr18Ni9Ti and 316L, 304 stainless steel has the highest C content (about 0.051%), while 316L stainless steel has the highest C content (0.019%) and 1Cr18Ni9Ti has the lowest C content (0.023%). Therefore, the intergranular corrosion resistance of 316L is better than 304, and the addition of stabilizing element Ti in 1Cr18Ni9Ti hinders the formation of Cr23C6, greatly reduces the Cr-poor zone, and makes 1Cr18Ni9Ti more resistant to intergranular corrosion than 304 stainless steel.
The results show that the low C content and the addition of stabilizer Ti can improve the intergranular corrosion susceptibility of austenitic stainless steel.
2.2 Pitting corrosion resistance of three stainless steels
The potentiodynamic polarization curves of 304, 316L and 1Cr18Ni9Ti stainless steel in 3.5% NaCl solution are shown in Table 2.
Fig.3 Potential dynamic polarization curves of 1Cr18Ni9Ti, 304 and 316L stainless steels in 3.5% NaCl solution at room temperature
Table2 Characteristics of polarization curves for three kinds of steels in 3.5% NaCl solution, room temperature
At room temperature, the self-corrosion potential (Ecorr) of the three materials in NaCl solution with 3.5% concentration (mass fraction) was -0.164 V, -0.215 V, -0.282 V, respectively. The order from high to low was 316L, 1Cr18Ni9Ti, 304 stainless steel, and the self-corrosion current density (Icorr) was 5.38 *10-8 A/cm2, 7.44 *10-8 A/cm2, 1.69 *10-7/cm2, respectively. The order from low to high was 316L, 1Cr18Ni9Ti and 304 stainless steel. Solid solution 316L has the highest self-corrosion potential and the smallest self-corrosion current density. The results show that 316L stainless steel passive film has the best protective performance and corrosion resistance under natural conditions, followed by 1Cr18Ni9Ti and 304 stainless steel.
The anodic polarization curves of the three materials in Fig. 3 show that the passivation zone is not vertical upward and inclined to some extent. The results show that the passivation process of the three materials is not significant, and the passivation film has the characteristics of transition to activation. The dissolution rate of the passivation film is slightly faster than the growth rate of the passivation film. The current density on the anodic polarization curve reflects the dissolution rate of passive film. Compared with current density at the same potential, 316L passivation film dissolves slowest, that is, the stability of passivation film is the best, followed by 1Cr18Ni9Ti, and 304 stainless steel is the fastest. The passivation intervals of 304 stainless steel, 316L stainless steel and 1Cr18Ni9Ti stainless steel decreased in turn, which indicated that the passivation ability of solid solution 1Cr18Ni9Ti stainless steel was weaker than that of solid solution 316L and solid solution 304 stainless steel. Comparing the characteristics of passivation interval and dimension passivation current density, it can be seen that although 304 passivation film dissolves quickly, its regeneration ability is strong, so its passivation interval is relatively large; while 316L passivation film is stable and dissolves slowly, its regeneration ability is slightly lower than 304; 1Cr18Ni9Ti is the worst.
The order of pitting breakdown potential (Eb) of three stainless steels is 304 (0.413 V)>316L (0.268 V)>1Cr18Ni9Ti (0.092 V). The potential indicates the potential of forming stable pitting pits and reflects the pitting resistance of materials. It can be concluded that the pitting sensitivity of solid solution 304, 316L and 1Cr18Ni9Ti in 3.5% NaCl solution at room temperature is the lowest, followed by 316L and 1Cr18Ni9Ti.
The etching pits on the working electrode surface after polarization are shown in Fig. 4. Fig. 4a-b, 4c-d and 4e-f show the surface morphologies of 1Cr18Ni9Ti, 304 and 316L stainless steel after polarization in 3.5% NaCl solution at low and high power, respectively. Pitting pits of 304 stainless steel are small and few along the direction of wear marks (fig. 4c, d); 316L pits distribute more evenly than 304 and some larger pits appear (fig. 4e, f); 1Cr18Ni9Ti pits are the densest and larger than 304 and 316L pits (fig. 4a, b). The order of pit density is 304 < 316L < 1Cr18Ni9Ti, which is consistent with the conclusion of polarization curve. 304 stainless steel in 3.5% NaCl solution has the strongest pit corrosion resistance, followed by 316L and 1Cr18Ni9Ti stainless steel.
Fig.4 Micrographs of the stainless steels after pitting corrosion in 3.5wt.% NaCl solution (a) and (b) 1Cr18Ni9Ti; (c) and (d) 304; (e) and (f) 316L
The chemical composition of the material is one of the important factors affecting the dissolution of passive film, its repair ability and pitting breakdown potential. A large number of studies have shown that the most effective elements to improve pitting corrosion resistance of stainless steel are Cr and Cr, mainly to improve the repair ability of passive film of steel [4,6-8]. Mo and N are beneficial to improve pitting corrosion resistance, while Ti is harmful . MoO42-formed during corrosion dissolves in solution. When the membrane breaks in Cl-environment and exposes the active metal surface, Mo adsorbs on the active metal surface in the form of MoO42-so as to inhibit the dissolution of the metal surface and prevent the further destruction of the film. Therefore, the stability of the passive film can be improved [6-7] [10-12]. According to G.P. Halada et al., N forms nitrides at the metal/passivation film interface to prevent the dissolution of alloy elements ; M.B. Ives et al. believe that NH3 or NH4+ can combine with free Cl-ions to form compounds, which can prevent the oxidation of elements in stainless steel and improve the resistance to local corrosion . High C content is not conducive to intergranular corrosion resistance, but as interstitial atoms, dislocation movement is limited and pitting corrosion resistance is favorable . Among the three stainless steel materials tested, 304 has the highest content of Cr, N and C, which makes it have the highest pitting potential and the best pitting resistance. 316L stainless steel has the smallest Cr content, but adds elements Mo and N, and its Ni content is the highest. The addition of Mo makes 316L have the lowest passivation current density and the highest self-corrosion potential, i.e. the highest stability of passivation film. Ni2 is not the main component of passivation film. Ni2 improves the re-passivation ability by inhibiting the depletion of Cr in the transition layer between the outermost layer of passivation film and the base metal, thereby improving the corrosion resistance of the material . The pitting corrosion resistance of 316L is better than that of 1Cr18Ni9Ti, but slightly weaker than that of 304. The experimental results also show that the pitting corrosion resistance is improved significantly by containing Cr and N. As for 1Cr18Ni9Ti stainless steel, Ti is easy to form chemical compounds with C and N. Precipitated phase leads to inhomogeneous passivation film, which is easy to become the adsorption and preferential dissolution site of active ions, and promotes the occurrence of pitting corrosion. Especially 1Cr18Ni9Ti stainless steel does not add N and other elements which have strong pitting resistance, so its pitting resistance is the worst.
2.3 STRESS CORROSION RESISTANCE BEHAVIOR OF THREE KINDS OF STAINLESS STEEL
Stress corrosion sensitivity coefficient (ISSRT) reflects the resistance of materials to stress corrosion. The smaller the ISSRT, the better the resistance to stress corrosion. The calculation method is as follows:
R m and R’m represent the tensile strength of materials in inert and corrosive environments respectively, A and A’represent the elongation of materials after fracture in inert and corrosive environments respectively. The measured parameters are shown in Table 3.
Table 3 Results of stress corrosion experiment by slow strain rate test technique
The average stress corrosion sensitivity coefficients of 304, 316L and 1Cr18Ni9Ti are 0.35, 0.266 and 0.366, respectively. The results show that the order of stress corrosion susceptibility is 1Cr18Ni9Ti > 304 > 316L, 1Cr18Ni9Ti is the most susceptible to stress corrosion, and 316L is the least susceptible to stress corrosion. Previous studies have shown that Ni plays an important role in improving the stress corrosion resistance of austenitic stainless steel [8-9]. The fracture source of stress corrosion is usually pitting corrosion. Adding N, Mo can improve pitting resistance . Therefore, N and Mo elements are beneficial to stress corrosion resistance. Comparing the compositions of the three materials (Table 1), it can be seen that 316L has the strongest stress corrosion resistance because it contains the most Ni elements and adds the elements Mo and N. The content of Ni in 1Cr18Ni9Ti is lower than 316L and higher than 304, but the addition of element Ti is disadvantageous to the stress corrosion resistance of 1Cr18Ni9Ti stainless steel in chloride solution . Therefore, the stress corrosion sensitivity of 1Cr18Ni9Ti stainless steel is the highest. The results show that 316L is the best choice of materials under stress corrosion environment.
- (1) Ti addition and low C content are beneficial to the intergranular corrosion resistance of austenitic stainless steel. Solid solution and sensitized 316L and 1Cr18Ni9Ti stainless steels have no tendency of intergranular corrosion. They can be used in the sensitized temperature range, while 304 stainless steels form carbides of Cr near the grain boundary at the sensitized temperature, forming poor Cr region, which is prone to intergranular corrosion failure.
- (2) The effect of Cr and N on pitting corrosion resistance is remarkable. The addition of Ti results in non-uniform passivation film and pitting corrosion. The pitting corrosion resistance of 1Cr18Ni9Ti in NaCl solution with 3.5% concentration is the worst due to the combined action of trace elements.
- (3) 316L with the highest Ni content has the best stress corrosion resistance in boiling magnesium chloride with 42% concentration, and 1Cr18Ni9Ti has the worst stress corrosion resistance.
The authors have declared that no competing interests exist.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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