*Dependence of Corrosion Resistance of Austenitic Chromium-Nickel Steels on the Magnetic… DOI: http://dx.doi.org/10.5772/intechopen.102388*

corrosion behavior, two groups of the most common steels were selected: AISI 304, AISI 321 and their analogues 08Cr18Ni10, 08Cr18Ni10Ti in **Table 1**.

The samples were made of rolled sheet metal with a thickness of 1 mm of industrial melts of these steels. Taking into account that the uneven distribution of δ-ferrite over the width of the cold-rolled sheet [5] and for further averaging of the obtained results 7 … 10 flat samples with dimensions of <sup>7</sup> <sup>3</sup> 1 mm<sup>3</sup> were cut from different places of the sheet of each smelting. Specific magnetic susceptibility χ was determined by a magnetometric unit (Faraday balance). The content of δferrite and the specific magnetic susceptibility χ<sup>0</sup> of austenite (according to [7]) of these samples were determined. The average content of δ-ferrite *P*<sup>δ</sup> and χ<sup>0</sup> for each melt was calculated. The obtained results (*P*<sup>δ</sup> and χ0) [8] and corrosion rate *K* (attracted from the source [9]) in chloride-containing solution are given in **Table 2**.

Analysis of experimental hyperbolic dependences of the corrosion rate *K* [9] on the specific magnetic susceptibility of χ<sup>0</sup> austenite steels AISI 304, 08Cr18Ni10 and AISI321, 08Cr18Ni10Ti shows (**Figure 1**) [8]: that the larger the value χ0, the higher the corrosion resistance (lower corrosion rate *K*). Since χ<sup>0</sup> determines the atomicmagnetic structure of austenite, it can be assumed that the corrosion behavior of austenitic chromium-nickel steels depends on the atomic-magnetic (paramagnetic) state of austenite, pre-formed to interact with the chloride-containing medium.


### **Table 1.**

*Chemical composition of the investigated steels, % wt.*


### **Table 2.**

*The average values of* P*δ, χ0,* K *of each investigated steelmaking.*

### **Figure 1.**

*Relationship between the corrosion rate* K *and the specific amagnetic susceptibility χ<sup>0</sup> of austenite of the studied steels. a—AISI 304 (melt 1–5) and 08Cr18Ni10 (melt 6); b—AISI 321(melt 1–5) and 08Cr18Ni10Ti (melt 6).*

In [4], an electron-probe analysis of the distribution of chemical elements for two local regions (without corrosion and with corrosion) of steel 12Х18Н10Т (heat exchange tube) was performed. According to the results of microanalysis in the vicinity of corrosive ulcers, the chemical composition of the main elements was as follows (% wt.): 72.4 Fe; 15.2 Cr; 10.8 Ni; 1.1 Ti, which practically corresponds to the composition of steel 12Х18Н10Т, except for the tendency to decrease the concentration of chromium. However, as the authors [4] showed, the chemical composition of steel has changed radically in the ulcer itself, where locally, along with the increase in carbon concentration, the concentration of chromium has increased significantly. Excess carbon content causes the formation of chromium carbides and depletion of the surrounding austenite with chromium with a concentration below 12%, which no longer ensures the corrosion resistance of steel.

Other authors [10] showed that the level of concentration fluctuations of chemical elements from the boundary into the grain grain of austenitized steel *Dependence of Corrosion Resistance of Austenitic Chromium-Nickel Steels on the Magnetic… DOI: http://dx.doi.org/10.5772/intechopen.102388*

12Х18Н10Т decreases: for nickel—�1.5 times its average content in steel, chromium—20% of the average content, for sulfur and phosphorus—dozens of times. Therefore, it should be expected that in industrial austenitic chromiumnickel steels the corrosion is local and depends on the atomic-magnetic structure of austenite, the content of which in the studied steels �99.5% (δ-ferrite not more than 0.5%). Many works are devoted to the ambiguous (sometimes opposite) influence of the α-phase (δ-ferrite, α<sup>0</sup> -martensite) on the corrosion resistance of austenitic chromium-nickel steels [11]. At the content of 0.06...0.08% of δ-ferrite in the austenitic paramagnetic matrix, the magnetic moments of δ-ferrite and austenite are equal. Therefore, to more accurately determine the amount of δ-ferrite in the sample by magnetometric method, the magnetization of the austenitic matrix was taken into account.

In **Figure 2** shows the relationship between the corrosion rate *K* and the content of *P*<sup>δ</sup> δ-ferrite in steels AISI 304 and AISI 321. As you can see, the amount of δ-ferrite can determine the corrosion rate. However, a significant effect of this amount of *P*<sup>δ</sup> δ-ferrite (0.005...0.12% in steels AISI 304, 08X18H10 and 0.01...0.30% in steels AISI 321, 08X18H10T) is unlikely due to insignificant (compared with austenite) the contact surface of δ-ferrite with an aggressive environment.

To further elucidate the role of low δ-ferrite content on the corrosion process, an experimental dependence of *P*<sup>δ</sup> on χ<sup>0</sup> was constructed (**Figures 3** and **4**).

The low amount of *P*<sup>δ</sup> δ-ferrite correlates with the specific paramagnetic susceptibility χ<sup>0</sup> of austenite (the amount of which is �99.7%), and hence with the atomic-magnetic (paramagnetic) state of austenite (χ0). In the studied steels, the amount of δ-ferrite is in thermodynamic equilibrium with the paramagnetic state of austenite, the smallest violation of which causes a change in the amount of δ-ferrite in the austenitic matrix. Hence there is an indirect dependence of the corrosion rate *K* on the content of *P*<sup>δ</sup> δ-ferrite, which, in turn, corresponds to the atomic-magnetic state of austenite (parameter χ0). Therefore, it is assumed that the low content of δ-ferrite indirectly (not directly) affects corrosion, i.e. is a measure (indicator) of the corrosion rate *K*.

It is experimentally established that the corrosion resistance of austenitic chromium-nickel steels depends on the atomic-magnetic state of almost 100% austenite—paramagnet, which is characterized by a specific magnetic susceptibility

### **Figure 2.**

*The relationship between the corrosion rate* K *(involved with [9]) and the content of* P*<sup>δ</sup> δ-ferrite in the studied steels. а—AISI 304 (melt 1–5) and 08Cr18Ni10 (melt 6); b—AISI 321 (melt 1–5) and 08Cr18Ni10Ti (melt 6).*

**Figure 3.**

*Tendency of change of low amount of* P*<sup>δ</sup> δ-ferrite from atomic-magnetic state of austenitic matrix (parameter χ0) in steels AISI 304 (melt 1–5) and 08Cr18Ni10 (melt 6).*

### **Figure 4.**

*Tendency of change of low amount of* P*<sup>δ</sup> δ-ferrite from atomic-magnetic state of austenitic matrix (parameter χ0) in steels AISI 321 (melt 1–5) and 08Cr18Ni10Ti (melt 6).*

χ0: the greater χ0, the higher the corrosion resistance (lower corrosion rate *K*), and vice versa. It is assumed that the low content of *P*<sup>δ</sup> δ-ferrite (0.005...0.5%) is an indicator of corrosion intensity, because *P*<sup>δ</sup> depends on χ<sup>0</sup> (with increasing χ<sup>0</sup> austenite increases *P*δ), and *K* depends on χ<sup>0</sup> (with increasing χ<sup>0</sup> decreases *K*). It should be noted that as the amount of *P*<sup>δ</sup> δ-ferrite increases, the magnetic susceptibility χ of steel (rather than χ<sup>0</sup> austenite), which contains both austenite and δferrite, increases. It is shown that with increasing *P*<sup>δ</sup> (at low δ-ferrite content) the corrosion rate *K* decreases (and vice versa), i.e. low δ-ferrite content can also be a measure (indicator) of the corrosion process in austenitic chromium-nickel steels.
