Effect of solution temperature on corrosion resistance of 904L stainless steel in concentrated sulfuric acid solution
The effect of solution temperature on the electrochemical performance of 904L stainless steel in concentrated sulfuric acid solution was studied by potentiodynamic polarization curve and cyclic polarization curve. The results show that the solution temperature has little effect on the electrochemical behavior of the cathode, but has great difference on the anodic polarization curve. When the solution temperature is 1120 ℃ and the holding time is 0.5 h, the corrosion resistance of 904L stainless steel is the best.
904L stainless steel is a high chromium nickel molybdenum super austenitic stainless steel with good corrosion resistance. It has good corrosion resistance to uniform corrosion, pitting corrosion, intergranular corrosion, crevice corrosion, stress corrosion cracking and general corrosion, especially in HSO. At present, the hot rolled 904L stainless steel plate can only be put into use after solution treatment. The purpose is to make the carbide precipitated in the hot rolling process dissolve in the austenite at high temperature, and keep the austenite with solid solution C at normal temperature through rapid cooling, so as to reduce the ferrite content in the steel; by adjusting the solution temperature, the grain size of the steel is controlled to soften the steel structure and improve the plate material Quantity is of great significance. However, the solution temperature also has a great influence on the corrosion resistance of stainless steel [2-7].
In this paper, the effect of different solution temperature on the corrosion performance of 904L stainless steel in concentrated HSO solution was studied. The corrosion resistance mechanism of 904L stainless steel was discussed through the results. Through comparison and selection of appropriate solution temperature, the theoretical basis for improving the properties of 904L stainless steel is provided.
904L ultra-low carbon austenitic stainless steel plate produced by Sweden Outokumpu Avesta company was used as experimental material. The chemical composition (mass fraction,%) of the plate is C > 0.02, Cr 19 ~ 23, Ni 23 ~ 28, Mo 4.0 ~ 5.0, Mn > 0.20, Si > 1.0, s > 0.035, P > 0.045, Cu 1.0 ~ 2.0, Fe surplus. According to GB / T228-2002, the sample with a thickness of 4 mm was processed and treated in a box type resistance furnace. The solution temperature was 10801120 ℃ and 1160 ℃ respectively, and the solution holding time was 0.5 h. argon was introduced into the whole process as protective gas and water cooling was used. The metallographic samples of 904L stainless steel after solution treatment were corroded by aqua regia, and the microstructure was observed under DMI 5000m inverted metallographic microscope.
The size of the electrochemical sample is 1 cm × 1 cm × 0.3 cm. The copper wire is welded on the back side and encapsulated with epoxy resin. Before the experiment, the samples were polished by metallographic abrasive paper from No.0 to No.6, polished with polishing paste, and then cleaned with deionized water, acetone and anhydrous ethanol, and then dried for use.
Potentiodynamic polarization curve test was carried out in bio logic VSP electrochemical workstation. The classic three electrode system was used, 904L stainless steel was used as working electrode, Pt electrode was used as auxiliary electrode, and the experimental medium was 89% HSO analytical pure solution. Due to the strong corrosivity of corrosion medium, special Pt / PTO wire was used as reference electrode , and the volume of solution was 0.5 L. All potentials in this paper, unless otherwise specified, are relative to the reference electrode used. During the test, the working electrode was first poled at – 0.7 V for 5 min to remove the oxide film on the sample surface. Then, the working electrode was still immersed in concentrated HSO solution for 1 h, and then the action potential polarization experiment was conducted at a scanning rate of 2 MV / S (room temperature).
Results and discussion
Microstructure of 904L stainless steel at different solution temperatures
Fig. 1 shows the microstructure of 904L stainless steel after water quenching at different solution temperatures for 30 min. It can be seen that with the increase of solution temperature, the grain size gradually grows up and the grain size is gradually uniform. When the temperature is lower than 1120 ℃, the grain size increases slightly; when the temperature is higher than 1120 ℃, the grain size becomes coarser. This is due to the grain boundary migration caused by recovery and partial recrystallization or recrystallization of 904L stainless steel under the action of solution treatment. The grain boundary migration causes mutual annexation of grains and grain growth. Under a certain holding time, the higher the solution temperature is, the easier the defects in 904L stainless steel crystal will be rearranged or even partially eliminated, so the crystal is easier to recover and recrystallize. The faster the grain boundary migration speed, the larger the grain size.
Fig. 1 microstructure of 904L stainless steel at different solution temperatures
Self corrosion potential time curve
Fig. 2 shows the change curve of open circuit potential of 904L stainless steel with time at different solution temperatures. It can be seen that the corrosion potential of 904L stainless steel increases rapidly at 1080 ℃ and reaches the maximum value in a very short time. The potential during this period is caused by cathodic reduction . With the further extension of corrosion time, the corrosion potential gradually decreases and tends to be stable, which is related to the initial state of the sample surface, because after cathodic reduction treatment, there are Cr3 + oxides on the surface of the sample . These oxides are formed in the air before the sample invades the test medium, and these oxides gradually dissolve in the test medium on the decline surface of the corrosion potential, and finally the sample A stable corrosion state was established under the action of medium, and the corrosion potential kept stable at – 0.6 V; the corrosion potential at 1120 ℃ and 1160 ℃ increased slowly with the extension of time, and tended to a stable value. The time for the corrosion potential to stabilize at 1120 ℃ is faster than that at 1160 ℃, and the corrosion potential at both temperatures is stable at – 0.36 v.
Fig. 2 open circuit potential of 904L stainless steel in concentrated H2SO4 solution at different solution temperatures
Potentiodynamic polarization curve
Figure 3 shows the potentiodynamic polarization curve of 904L stainless steel in concentrated HSO solution. The electrochemical parameters measured by Tafel linear extrapolation method are shown in Table 1. In the weak polarization region, there is no obvious change in the cathodic polarization curve at three solution temperatures, and it increases monotonously without turning point. All of them are h-reduction process. The solution temperature has little effect on the electrochemical behavior of the cathode, but has great difference on the anodic polarization curve. Obvious passivation behavior occurred at the potential range of -0.02 to 0.8 V at 1080℃, indicating that a passive film was formed on the surface. The surface film layer was a layer of corrosion product film, which reduced the corrosion rate. However, with the further increase of polarization potential, the corresponding current density begins to increase and enter the over passivation region.
Fig. 3 potentiodynamic polarization curve of 904L stainless steel in concentrated H2SO4 solution
The main corrosion reactions of 904L stainless steel in concentrated sulfuric acid solution are as follows:
Cathodic reaction :
- 2 H + + 2 e → H 2 or O 2 + 4 H + + 4 E → 2 H 2O
The anodic reaction follows bockres mechanism [13,14]:
- (2) M e + H 2 O → M e ( H 2 O ) a d s
- (3) M e ( H 2 O ) a d s → M e ( O H – ) a d s + H +
- (4) M e ( O H – ) a d s → M e ( O H ) a d s + e
- (5) M e ( O H ) a d s → M e O H + + e
- (6) M e O H + + H + → M e 2 + + H 2 O
Formula (2) is the reaction process that occurs immediately when the metal surface contacts with aqueous solution. Formula (3) is that ho molecules adsorbed on the metal surface are ionized into (OH -) ads and H + in solution. This reaction goes on very quickly. Formula (4) is that Fe (or Cr) and (OH -) adsorbed on the metal surface combine to form an intermediate product adsorbed on the metal surface and can move in two dimensions. The reaction is also rapid. The combination of these three reactions is:
- (7) M e + H 2 O → M e ( O H ) a d s + H + + e
Formula (7) and (6) are reversible reactions, and compared with formula (5), they are fast reactions. Therefore, the two reactions can be regarded as equilibrium state approximately in the whole reaction process. Formula (5) is the reaction control step.
Fig. 4 shows the metallographic structures under the polarization curves of three solution temperatures. It can be seen that the scratches left during the preparation process can be seen on the surface of the samples. The metallographic surface at 1080 ℃ has uniform small pitting pits with large radius, while the metallographic surfaces at 1120 ℃ and 1160 ℃ have no pitting pits with larger radius. It shows that the corrosion at 1080 ℃ is more serious than that at 1120 ℃ and 1160 ℃.
Fig. 4 surface morphology of 904L stainless steel after polarization in concentrated sulfuric acid solution
Cyclic polarization curve
The cyclic polarization curves of 904L stainless steel in 89% concentrated HSO solution at different solution temperatures were measured. As shown in Fig. 5, the electrode current density in the positive scanning direction undergoes a process from negative to positive with the positive potential shift, which indicates that the components in the passivation film undergo oxidation reaction with the positive potential shift; when the potential is swept back, the current density shows a positive to negative process, in which the high valence oxides in the passivation film components will be reduced and converted into low valence oxides. The peak value of reduction current density appeared in the three kinds of solution temperature, that is, the minimum value of current density appeared, indicating that the structure of passive film at this potential was the most stable .
For the cyclic polarization curve at 1080 ℃, the current density of back scanning is higher than that of forward scanning in the range of 0.67 ~ 1.2 V, which indicates that the oxide film dissolves to a certain extent in the forward scanning process, so the self repair of the passive film occurs during the back scanning and the anode current is generated. In the range of – 0.5 ~ 0.67 V, the back scan current density is lower than the forward current density, which indicates that when the potential drops to a certain value, the material can still self passivate and form a better protective passivation film.
The cyclic polarization curves at 1120 ℃ and 1160 ℃ are similar to those at 1080 ℃, with hysteresis loops. It can be seen from Fig. 5 that the area of hysteresis loops increases with the increase of solution temperature. It shows that the degree of acidification and anionic aggregation in pitting pits of 904L stainless steel also increases with the increase of solution temperature . The area of hysteresis ring at 1120 ℃ and 1160 ℃ is almost the same, but it is much larger than that at 1080 ℃.
Protective potential (EP), also known as re passivation potential, is the potential value corresponding to the intersection point of the reverse polarization curve and the forward polarization curve. Under this potential, the growing of the corrosion hole will stop and be passivated again; after pitting, if the potential is above EP for a long time, the local corrosion will cause greater damage.
According to Fig. 5, EP at 1120 ℃ is the highest, which is -0.2026 V; at 1180 ℃, it is -0.2254 V; at 1080 ℃, it is -0.5369 v. The corrosion resistance of 904L stainless steel at different solution temperatures is 1120 ℃ > 1160 ℃ > 1080 ℃. This is consistent with the conclusion of potentiodynamic polarization curve.
Fig.5 Cyclic polarization curves of 904L stainless steel at different solution temperatures
- (1) With the increase of solution temperature, the grain size of 904L stainless steel gradually grows up and the grain size is gradually uniform. When the temperature is lower than 1120 ℃, the grain size increases slightly; when the temperature is higher than 1120 ℃, the grain size becomes coarser.
- (2) In the weak polarization region, the cathodic polarization curves of the three solution temperatures have no obvious change, and they rise monotonously without turning point, which are all hydrogen reduction reaction processes. The solution temperature has little effect on the electrochemical behavior of the cathode, but has great difference on the anodic polarization curve.
- (3) The corrosion resistance of 904L stainless steel at different solution temperatures is 1120 ℃ > 1160 ℃ > 1080 ℃. The corrosion resistance of 904L stainless steel is the best when the solution temperature is 1120 ℃ and the holding time is 0.5 H.
Authors: Zeng hongtao1,2, Xiang song1,2, he yonggang3, Liu songlin3
Source: China 904L Flanges Manufacturer – Yaang Pipe Industry (www.steeljrv.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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- Development and performance test of 904L super austenitic stainless steel
- Nickel Alloy: Nickel 200 (UNS N02200)/Nickel 201 (UNS N02201)
- Nickel Alloy: Hastelloy C-276 (UNS N10276)
- Nickel Alloy: Monel 400 (UNS N04400)
- Austenitic Stainless Steel: 904L (UNS N08904)
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