Issue 78

N. Mohan et alii, Frattura ed Integrità Strutturale, 78 (2026) 129-145; DOI: 10.3221/IGF-ESIS.78.08

parameters such as duration and temperatures is crucial. The details of the process optimization, layer composition and case depth and hardness values are reported in previous literature [6]. The important findings of the previous work are tabulated below (Tab. 2).

Fracture toughness (MPa √ m)

Elements

Case depth ( π m)

Layer composition

Hardness (Hv)

13Cr4Ni, 950°C 13Cr4-Ni, 1000°C 16Cr5Ni, 950°C 16Cr5Ni, 1000°C

40-55 75-90 35-50 55-70

FeB, Fe 2 B

1682 1735 1785 1804

3.3 2.8 3.9 2.6

FeB, Fe 2 B, Cr 2 B, Ni 2 B

FeB, Fe 2 B, Ni 2 B

FeB, Fe 2 B, CrB, Cr 2 B, NiB, Ni 2 B

Table 2: Case depth, composition, hardness and fracture toughness values.

Figure 2: Optical micrograph of 16Cr5Ni SMSS boronized at 950°C.

Potentiodynamic polarization tests The related corrosion potential (Ecorr) and corrosion current density (Icorr) values are provided in Tab. 3, and the potentiodynamic polarization curves of the bare and boronized samples are shown in Fig. 3. The boronized MSS samples show more active Ecorr values, moved by at least 200 mV and up to 310 mV, as compared to the bare specimens. For the boronized samples, there is also a minor rise in Icorr. The boronized samples experience constant corrosion, but the bare samples exhibit obvious passivation behavior. Repeated depassivation of metastable pits causes noticeable electrochemical noise in both anodic and cathodic areas for the boronized samples, with more noticeable fluctuations in 16Cr5Ni MSS boronized at 950 °C. All samples exhibit diffusion-controlled behavior in the cathodic region, suggesting that oxygen reduction in aerated NaCl solution controls the corrosion process and serves as the rate-limiting step. The anodic reaction, on the other hand, is very different: the bare samples show passivation at first, with Ecorr trending toward nobler potentials up to about 1 µA/cm², after which passive film disintegration causes a gradual increase in corrosion current. The boronized MSS samples, on the other hand, corrode constantly and exhibit no passivation. The boronized layer's surface inhomogeneities, which promote electrolyte penetration beneath the surface, are responsible for this behavior. Additionally, the alloy's capacity to form a stable passive film is diminished by partial sensitization and potential chromium depletion brought on by the production of CrB and Cr 2 B phases after boronizing. The literature has more information on chromium sensitivity and depletion [15].

Corrosion rate mm/year

Elements

Ecorr, mVSCE

icorr, µA/cm 2

0.007 0/078 0.095 0.011 0.050 0.091

13Cr4Ni MSS

Bare

456± 22 659 ± 12 664 ± 18 348 ± 24 646 ± 15 658 ± 19

0.6 ± 0.4 6.7 ± 0.7 8.1 ± 0.3 0.9 ± 0.3 4.3 ± 0.5 7.8 ± 0.2

Boronized at 950°C Boronized at 1000°C

16Cr5Ni MSS

Bare

Boronized at 950°C Boronized at 1000°C

Table 3: Corrosion parameters of boronized MSS

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