3.4.2. Electrochemical Tests: EPR Method ASTM G108–94 (2015)

Electrochemical Tests were carried out according to the EPR Method ASTM G108–94 (2015) [48]. The grain index was determined according to the ASTM E112-13 method [49] (Figures 18 and 19).

(**a**) (**b**)

**Figure 18.** (**a**)#Test 1 and (**b**) #Test 2. Grain index (ASTM E112-13) G = 11.

**Figure 19.** #Brut 1. Grain index (ASTM E112-13) G = 11.

Grain index values are given in Table 9.

**Table 9.** Grain index according to ASTM E112-13 [49].


According to the EPR method ASTM G108-94 (2015) [48], after the cyclic polarization scans, the evaluation parameter is the normalized charge (Pa), measured in coulombs/cm2, calculated with the formula:

$$P\_{\mathbf{a}} = \mathbf{Q} / \mathbf{\mathcal{X}} \tag{1}$$

where Q = measured on current integration measuring instrument (coulombs), normalized for both specimen size and grain size X <sup>=</sup> As[5.1 <sup>×</sup> <sup>10</sup>−3*e*0.35G], where As <sup>=</sup> specimen area (cm2), G <sup>=</sup> grain index at 100× according to ASTM E112-13 [49].

In the derivation of the equation, it was assumed that the Q value was the result of the attack on the specimen surface that was distributed uniformly over the entire grain boundary region of a constant width of 2 <sup>×</sup> (5 <sup>×</sup> <sup>10</sup><sup>−</sup>5) cm. This may not represent the actual physical processes.

The potentiokinetic electrochemical reactivation results are presented in Table 10.

**Table 10.** Potentiokinetic electrochemical reactivation results.


Eoc = Initial open circuit potential, Ir = maximum anodic current density.

Figure 20 shows the potentio kinetic reactivation curves in linear axes.

 

 

**Figure 20.** Potentio kinetic reactivation curves recorded for #Brut 1, #Test 1, #500\_1, #620\_2, and #750\_2.

The peak valuesfor Ir, given in Table 10, were specific to the intergranular corrosion degradation of the tubes. The higher the intensity, the greater the degradation. Thus, according to Figure 20, it was noted that the highest sensitization of the tubes was generated by the heat treatment at 620 ◦C. The minimum sensitization corresponded to the 500 ◦C heat treatment. The overall results (Table 10) for the normalized charge (Pa) calculated (Figure 21) for all the samples indicated that the heat treatment over 500 ◦C for 304 steels is not indicated, the risks of inducing an intergranular corrosion process being obvious. Consequently, the 500 ◦C heat treatment should be used in the manufacturing process.

**Figure 21.** Normalized charge (Pa) measured by EPR.

Type 304 steel was more sensitive to intergranular corrosion compared to other steels. Consequently, in the manufacturing process, great importance must be given to this type of corrosion morphology. The temperature of 620 ◦C was critical for generating the process and therefore used in the ASTM tests (A262-15 and G108-94) for evaluating intergranular corrosion. The goal is to near the behavior of the tube in raw state (#Brut 1).

The susceptibility to intergranular corrosion of stainless steels is not always due to heat treatment with precipitation of chromium carbides. Under certain conditions, the precipitation of intermetallic compounds of (Fe, Cr)Mo2 or (Cr, Ni, Fe)3P2 type can occur. According to Stonawská et al., the structural sensitization of 316 L steel is due to the precipitation of secondary phases along the grain boundaries [59]. The studies of Liu et al. regarding 316 L steels [60] and Fujii et al. [61] regarding 304 steel, also supports this statement. According to Liu et al. [62], the chromium-depleted zones near grain boundaries represent the corrosion nucleation sites for austenitic steels. According to Eliaz, since the carbon content in 316 stainless steel was lowered in the 316L and 316LVM grades, sensitization of this steel is less problematic as it used to be [7].
