PSI - Issue 14

A. Poonguzhali et al. / Procedia Structural Integrity 14 (2019) 705–711 Poonguzhali et al. / Structural Integrity Procedia 00 (2018) 000–000

707

chloride concentration and increased deformation from 0 to 20% CW. Chloride ions results in the breakdown of the passive film and enhance active dissolution of the exposed surface resulting in initiation of the pit. Fig. 2a exhibited a range of passivity between -0.2 V (SCE) to 0.4 V (SCE) approximately and the passive currents in the range of 3 to 5 μA/cm 2 was observed for CW specimens in acidified 1M NaCl environment and fig.2b exhibited a range of passivity between -0.1 to -0.3 V (SCE) and passive current in the range of 2 to 13 μA/cm 2 was observed for CW specimens in acidified 5M NaCl + 0.15 M Na 2 SO 4 environment. The pitting potential decreased with the degree of deformation due to the microstructural modifications caused by deformation that are deleterious to the corrosion resistance. As deformation provides enhanced dislocation pile-ups, results in unstable passive film causing increased pitting corrosion attack in the CW specimen. The deformation bands with a high density of dislocations are highly stressed areas which act as preferential sites for pitting (Kamachi Mudali et al., 2002). The current density ( I c ) has increased from 7 to 21 µA/cm 2 in lower chloride concentration and 21 to 46 µA/cm 2 in higher chloride concentration with increasing cold work level indicative of the reduced protection of the passive film. The corrosion current density increases with increasing the concentration of NaCl, due to the electrical conductivity of Cl - ions. As the concentration increases, the number of ions in the solution increases, this leads to an increase in the conductivity of the solution and thereby resulting in the breakdown of the passive film (Lameche et al., 2006).

a

b

Fig. 1. Potentiodynamic anodic polarization curved of cold worked type 316L SS with 0.11 wt.% nitrogen (a) acidified 1M NaCl and (b) acidified 5M NaCl + 0.15 M Na 2 SO 4 .

The corrosion fatigue results obtained for SS316LN steel in chloride medium for as-received and different cold worked are presented in fig.2a in the form of S-N curve with two different regimes which are dependent on cyclic frequency, environment, mean stress and cold work as, corrosion fatigue failure occurs due to the synergistic effect of cyclic frequency and mean stress and is a rate-controlled process. However, the environmental effect disappears at higher frequencies. For the base material, in regime I (σ mean  375 MPa), the number of cycles to failure decreases with increase in σ mean , due to higher crack-tip strain rate and rupture of the passive film and thereby resulting increase in material dissolution (Poonguzhali et al., 2015). Also, at higher stress levels, the barriers to dislocation motion are overcome and thereby resulting in faster crack initiation. In view, of the environmental effect, crack tip opening increases resulting in the interaction of the steel with the corrosive environment due to increased supply of chloride ions in the crack tip and promoting crack propagation easier. However, 20% CW SS shows better fatigue life as compared to other steels as a cold worked material due to the high strain energy and high defect density results in blunting of the crack tip and hence more time would be required for the crack to resharpen and propagate. Fatigue strength of 316LN SS increases with increasing yield strength (YS) and ultimate tensile strengths (UTS). It was observed that YS and UTS increase with an increase in CW. In regime II (σ mean ≤ 375 MPa), the as-received material showed the marginal difference with a number of cycles to failure. Similarly, the cold worked material 5% and 20 % in regime I (σ mean  375 MPa) showed a similar trend as observed for the base material. However, at lower stress regimes the cold worked material 5% and 20 % shows the merging trend. It is known that different mechanisms are operating at different fatigue regimes based on the applied stress levels. However, in the present case the linear part of the S-N curve has been considered for fatigue life predictions analysis. Also, the crack initiation mechanism is predominantly from the surface can be seen in the fractographic image. The linear regime is fitted