2.4.1. Chemical Tests

The standardized tests [36–38] to assess the sensitivity of stainless steel to intergranular corrosion, according to ASTM A262-15 [36], have been summarized in Table 4.

**Table 4.** ASTM A262-15 [36] Standard practice for detecting susceptibility to intergranular attack in austenitic stainless steels.


Evaluation of intergranular corrosion according to standard ASTM A262-15-the Strauss test [36]. The test medium was a solution of copper sulphate-50% sulfuric acid in the presence of metallic copper, brought to the boiling point (125 ◦C) for 120 h. This test investigated the intergranular corrosion behavior of steel in the potential range between 110–350 mV. The test setup was in accordance with ASTM A262-15 [36]. The samples to be tested, tubes of #3, 1.4435/316L steel, were placed in specific glass cradle.

### 2.4.2. Electrochemical Tests

Electrochemical Potentiodynamic Reactivation (EPR) measurements, single or double loop, are methods of examining and assessing the corrosion sensitivity of austenitic steels [37–47].

The ASTM G108-94 (2015) [48] was practiced with the single loop method. Using the single loop test, a sample polished to a 1 μm finish, was polarized for two minutes at 200 mV vs. SCE in a solution of 0.5 M H2SO4 + 0.01 M KSCN. Subsequently, the potential was decreased, at a rate of 6 V/h, to the corrosion potential, Ecorr. This decrease resulted in reactivation of the specimen, involving breakdown of the passive film covering chromium depleted areas of material. The area under the large loop generated in the curve of potential vs. current (Figure 3) was proportional to the electric charge Q, that depends on surface area and grain size. In non-sensitized material, the passive film was intact and the loop size was small.

**Figure 3.** Procedures of single loop EPR test method according to ASTM G108–94 (2015) [48].

The samples tested were tubes of 304L AISI, used for manufacturing medical endoscopes (Table 5). The tubes #Test 1 and #Test 2 were suspected to be sensitized in intergranular corrosion. For the evaluation test, reference samples #Brut 1 and #Brut 2 and three samples from the supplier stock, subjected to heat treatments at 500, 620 and 750 ◦C, respectively, were used (Table 5).


**Table 5.** Test samples #1, 14306/304L tubes.

The sample (Table 5), transversely cut, was incorporated into a resin and "mirror" polished. The resin was machined to be adapted to the working electrode. The mounting of the electrodes, the electrochemical cell and the EPR measurement conditions were those of ASTM G108-94 (2015) [48]. The ASTM G108-94 method allows a quantitative evaluation of the intergranular corrosion sensitization of steels AISI 304 and AISI 304L. The purpose of the test was to evaluate the intergranular corrosion of the transverse surface by an electrochemical scanning method from +200 mV to −400 mV vs. SCE. In the mathematical calculation for evaluating the sensitivity to intergranular corrosion, another parameter, the corresponding value of the grain index, has to be considered, according to ASTM E112-13 [49].
