*3.2. Pitting Corrosion*

For the evaluation of steels sensitivity in pitting corrosion, available standards allow characterization of the formation, the shapes and the density of pits per unit area [52]. For the establishment of Figures 6–9, it was necessary to determine the density of pitting corrosion (counting the number of pits per unit area).

**Figure 7.** Pitting test results of the transverse and longitudinal surfaces for various grades of steels (0.1 M FeCl3 test medium at 37 ◦C for 15 days).

**Figure 8.** Pitting test results of the transverse and longitudinal surfaces for various grades of steels (0.5 M NaCl test medium at 37 ◦C for 26 days).

**Figure 9.** Pitting test results of the transverse and longitudinal surfaces for various grades of steels (artificial sweat test medium at 37 ◦C for 30 days).

In all test environments, the transverse surfaces showed a higher pitting density compared to the longitudinal ones.

Using a Kontron KS 300 Version 1.2 image analysis program, the cross-sectional area of alloys #1, #2, #3, #4, #5, #7, #8 and #10, tested in 0.5 M FeCl3 at 50 ◦C for 2 h was analyzed statistically in relation to the area size of the pits. The following classes were established accordingly: <20, 20–50, 50–150, 150–500, 500–1000 and >1000 μm2. The densities (number of pits/cm2) according to the above mentioned classes are presented in Figure 10.

The image analysis revealed a density of pits which can be significant (more than 10,000 for sample #2, Figure 10). Under these conditions, it was necessary to define criteria which enable easier identification of pitting corrosion. Examination of the surfaces revealed numerous cavities which were not necessarily pitting.

**Figure 10.** Number of pits counted by class, according to the size of the pit surface area.

At this point, it is essential to clarify the definition of pitting corrosion. According to the ASTM, a pitting corrosion is electrochemically active if an anodic dissolution of the alloy occurs within the cavity. Thus, by definition, a pitting corrosion releases cations in the electrolyte. It is therefore sufficient to carry out the corrosion test in an electrolyte containing traces of an analytical reagent which forms, with one of the cations released by the active corrosion pits, an insoluble colored compound which will deposit near the pit. The common element for all the alloys in question is iron, the iron dissolution mechanism being based on ferrous ions (Fe2+). Tests carried out on several reagents showed that potassium ferricyanide (K3[Fe (CN6]) is suitable to form an insoluble colored complex. Corrosion pits, revealed as blue discs encircling the pits (Figure 11), were used to determine the number per unit area.

**Figure 11.** Corrosion pits revealed as blue discs (cross section sample #2).

In conclusion, the FeCl3 solution, according to the ASTM G48-11 [31] standard, was aggressive with respect to the pitting corrosion behavior of the alloys studied; within two hours, most steel grades show readily identifiable macroscopic corrosion pits. On the other hand, in case of NaCl 0.5 M or artificial sweat ISO 3160-2 [30], the evaluation of the pits density was problematic given the difficulty of identifying the effectively electrochemically active pits. The use of potassium ferricyanide, which precipitates in the form of Turnbull blue in the presence of Fe2<sup>+</sup> ions, greatly facilitated the evaluation of the density of pits after immersion in this type of electrolyte.

According to Blackwood [2], pitting corrosion was a common problem with the early 304 stainless steels. In the case of 316L stainless steels, the addition of 2–3 wt% Mo has greatly reduced the number of failures due to pitting corrosion [2].

After initiation, pits either keep growing or repassivation may occur. According to Virtanen, an alloy with a high pitting corrosion resistance should ideally combine low susceptibility to pit initiation, low pit propagation rate, and fast repassivation [53]. According to Melchers, in case of metals with electron-conducting passive films such as stainless steels, the number of pits usually correlates inversely with their average depth, since the cathodic current consumed by the large passive surface area fosters anodic dissolution inside the pits [54]. The pits may grow at different rates, depending on the number of active pits [54]. According to Abbasi Aghuy et al., changes in electrochemical behavior of metal due to grain refinement as a consequence of changing grain boundary densities may occur [55].
