PSI - Issue 68

102 Tsanka Dikova et al. / Procedia Structural Integrity 68 (2025) 99–105 Tsanka Dikova & Natalina Panova / Structural Integrity Procedia 00 (2025) 000–000 corrosion destruction was carried out on preliminary prepared cross sections using OM Leica M 80 with digital camera Leica IC 90 E and Zeiss Evo 10 SEM (Jena, Germany). 3. Results obtained Investigation of the surface morphology after electrochemical tests have shown that regardless of the type and acidity of the environment, two types of corrosion processes took place on the samples’ surface - pitting and crevice (Fig. 1 and Fig. 2). Corrosion pits of different shape and sizes were observed on the surface of all samples, as their number was higher in the laser melted layers. In both corrosion media, the pits on the untreated steel surface were characterized with irregular shape, smooth walls and sizes between 10-300 µm (Fig. 1a,b and Fig. 2a,b). The pits on the surface of laser melted layers had rounded shape, less depth and rough walls (Fig. 1c-h and Fig. 2c-h). After corrosion test in Ringer’s solution, the sizes of pits on laser melted layers varied from about 10 µm for single pits up to about 400 µm for the coagulated ones. The pits’ dimensions in AS were in the range 150-300 μm regardless of different acidity.

Fig. 1. SEM images of corrosion damage on the surface of AISI 321 steel in Ringer solution. (a), (b) Untreated surface of sample 0; and laser

The chemical composition was investigated in different points on the surface of laser melted layers (point 1 in Fig. 1e; point 1 in Fig. 2e) and in the pits (point 2 in Fig. 1f; points 2 and 3 in Fig. 2f). The results, presented in Table 2, showed that comparing to the surface, the oxygen content on the pits walls was increased while that of the chromium and nickel was decreased.

Table 2. Chemical composition of laser-melted surface of sample 4 after galvanic corrosion test. Fluids Ringer solution*

Artificial saliva, pH 5.6**

Point

1

2

1

2

3

Element quantity

Wt.% 54.08 17.21

At.% 32.78 11.20

Wt.% 55.14 14.70

At.% 34.18

At.% 69.44 19.60

At.% 64.21 21.07

At.% 62.20 16.74

Fe Cr Ni

9.79 5.67

6.41

3.70

9.61

9.93 1.04

4.48 9.84

6.11

O 14.95 *The measurement points are shown in Fig. 1e and Fig. 1f. **The measurement points are shown in Fig. 2e and Fig. 2f. 15.00 31.73 12.36 26.75