PSI - Issue 68

Tsanka Dikova et al. / Procedia Structural Integrity 68 (2025) 99–105 Tsanka Dikova & Natalina Panova / Structural Integrity Procedia 00 (2025) 000–000

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boundary between the laser melted layer and the base metal. 4. Discussion

It was found in our previous research that the morphology and chemical composition of the microstructure of as delivered AISI 321 steel were relatively inhomogeneous (Dikova et al. (2024). The microstructure was mainly two phase, consisting of relatively large austenite grains and striped δ-ferrite (Fig. 3). Small amount of spherical carbides was observed along the grain boundaries. The microstructure of the laser melted layers on the steel surface was fine grained, more homogeneous and free of inclusions. It had dendrite morphology with two-phase structure – austenite in dendrites and δ-ferrite in the inter-dendritic areas (Fig. 4c). These results are consistent with the research of Cui et al. (2011), Stavrev et al. (2011) and Chan et al. (2018). During the electrochemical corrosion test of the wrought AISI 321 steel, a micro-galvanic couple is formed on the surface due to the two-phase microstructure (Zhao et al. (2021). As a result, the phase with lower corrosion resistance - δ-ferrite is destroyed. The corrosion failure depends on the shape, sizes and distribution of the δ-ferrite grains. This leads to formation of pits with irregular shape and sizes varying within the large range (10 µm - 400 µm) (Fig. 1a,b; Fig. 2a,b). As δ-ferrite is in the shape of stripes, the corrosion failure penetrates in large depth. Similar results are obtained by Revilla et al (2020) which found much deeper and wider pits on the surface of the wrought AISI 316 L steel. The inter-granular corrosion along the austenite grain boundaries (Fig. 3a) can be associated with the presence of carbides from one hand and lower stability of the boundary region due to the disordered atom arrangement and high dislocation density from the other hand (Zhao et al. (2021). In the present study, the specific more homogenous fine-grained microstructure of laser melted surface of AISI 321 steel defines the corrosion failure characterizing with shallow and nearly round shaped pits (Fig. 1c-h and Fig. 2c-h) regardless of the corrosion medium used. As it can be seen in Fig. 4b,d, during the electrochemical corrosion test the destruction of the δ-ferrite mainly occurs in the laser melted layers, analogically to the wrought samples. Our findings confirm the results of Revilla et al (2020) and Zhao et al. (2021). According to the first author, the selective corrosion restrains the initiation and expansion of pitting corrosion in the regions with dendritic structure. May be this is the main reason for propagation of the pitting corrosion in horizontal direction in laser cladded Cr-Ni steels observed by Zhang et al. (2016). From the other hand, the fine-grained microstructure of laser melted layers of austenite stainless steel is characterized by higher densities of dislocations, grain and sub-grain boundaries resulted in a thicker passive oxide film (Man et al. (2019), Kong et al. (2020), Revilla et al. (2020). Tarasov et al. (2019) is proposed that a Cr oxide passive film is formed preferentially on the δ-ferrite grains because the δ-ferrite lattice is enriched by chromium. All these processes prevent the sample’s surface of further corrosion attack and retard the development of corrosion pits in depth. Inter-granular corrosion in laser melted layers of AISI 321 steel is not observed due to more homogeneous microstructure and lack of inter-metallic or non-metallic inclusions which confirms the results of Laleh et al. (2019) and Man et al. (2019). Therefore, the corrosion failure of untreated and laser melted AISI 321 steel in Ringer’s solution and AS with different acidity is identical and runs by selective destruction of the corrosion non-resistant δ-ferrite phase in the form of pits. However, the morphology and size of the corrosion pits are different, which is determined by the δ-ferrite morphology in the microstructure. 5. Conclusions The failure of wrought and laser melted surfaces of austenite AISI 321 stainless steel under electrochemical corrosion in artificial saliva and Ringer’s solution were investigated in the present study. It was found that regardless of the type and acidity of the environment, two types of corrosion processes took place on the samples’ surface - pitting and crevice. 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. The pits on the surface of the laser melted layers had rounded shape, sizes of 10-400 µm, less depth and rough walls. The corrosion destruction in depth of the surface layer of wrought steel was selective, mainly of δ-ferrite, in the form of pits of irregular shape and great depth (50-300 µm). Inter-granular corrosion was present at the austenite grain boundaries near the surface. The