PSI - Issue 58

Alan Vaško et al. / Procedia Structural Integrity 58 (2024) 48–53 A. Vaško et al. / Structural Integrity Procedia 00 (2019) 000–000

52

5

higher corrosion resistance) than nickel-manganese nodular cast iron. Both non-austenitic nodular cast irons have a significantly higher average corrosion rate (and therefore lower corrosion resistance) compared to austenitic nodular cast irons, with a difference of approximately 3 to 4 times. Silicon-copper nodular cast iron has the highest corrosion rate (and thus the lowest corrosion resistance).

Fig. 2. Dependence of the average corrosion rate on the exposure time.

Fig. 3. Dependence of the current density i on the potential E.

The results of the electrochemical potentiodynamic polarisation test of chosen austenitic and non-austenitic nodular cast irons are shown in Fig. 3. The corrosion potential E corr , corrosion current density i corr and average corrosion rate v corr (recalculated for 1 year) were determined from the potentiodynamic polarisation curves using Tafel analysis (Table 5). Nickel-chromium nodular cast iron has a higher corrosion potential (and therefore higher thermodynamic stability) and a slightly higher corrosion current density and corrosion rate than nickel-manganese nodular cast iron. Both austenitic nodular cast irons have significantly higher corrosion potential and considerably lower corrosion current density and corrosion rate compared to non-austenitic nodular cast irons. The shift of the corrosion potential in the positive direction indicates a higher thermodynamic stability of austenitic nodular cast irons; a lower value of the corrosion current density means a lower corrosion rate of austenitic nodular cast irons (thus a higher corrosion resistance). Non-austenitic nodular cast irons have a 2.5 to 3 times higher corrosion rate compared to austenitic nodular cast irons. Silicon-copper nodular cast iron has the highest corrosion rate (and thus the lowest corrosion resistance).

Table 5. Electrochemical parameters determined by Tafel analysis. NCI E corr (V) i corr (μA cm -2 ) v corr (mm year -1 ) NiMn -0.476 2.660 0.031 NiCr -0.396 3.117 0.037 SiMo -0.645 7.207 0.084 SiCu -0.668 8.117 0.094

The outcomes of both corrosion tests prove that the corrosion resistance of austenitic nodular cast irons in salt solution is considerably higher than the resistance of chosen non-austenitic nodular cast irons. These non-austenitic cast irons corrode intensively in salt water and should not be used for any applications in sea water or other salty environments. The corrosion resistance of silicon-molybdenum nodular cast iron is described in Stawarz (2022). 4. Conclusions Different chemical compositions and microstructures of austenitic nodular cast irons cause different mechanical and corrosion properties compared to non-austenitic nodular cast irons.