**4. Corrosion performance**

### **4.1 Localised corrosion mechanisms in sodium chloride**

Polarisation scans are shown in **Figure 8**. All test alloys performed poorly in 3.56 wt. % NaCl, with just a slightly higher Epit around 72 ± 19.4 mV. SCE for 304SS. However similar corrosion behaviour was observed for all test alloys. The Epit values for Hercules™ A and Hercules™ B were measured to be almost similar at more electronegative potentials. Epro was measured at potentials below the pitting potential Epit for all test alloys, which is an indication of severe pitting corrosion as observed by higher anodic currents at the Epit, that is, there was a sudden increase of current at lower potentials. The Tafel extrapolation showed that Ecorr values were almost similar for all test alloys and presumptuously this could be expected because of similar corrosion constants *K*.

In 1 wt. % NaCl the Epit was measured at 219 ± 17.5 mV. SCE for Hercules™ A which is more positive than the one obtained from 3.56 wt. % NaCl test, as shown in **Figure 9**. The Epit for Hercules™ B was measured at 579 ± 0.8 mV. SCE. There was a significant difference in Epit values for each alloy due to reduction of NaCl concentration. All test alloys scans showed incapability to repassivate as the Epro was measured below Ecorr.

The behaviour of 304SS and Hercules™ in 1 wt. % NaCl can be explained by considering the effects of Cr since LNASSs have lower Cr content than 304SS. Ujiro *et al*. [28] has studied the effect of Cr additions in the corrosion behaviour of austenitic SSs in NaCl solution. It was noted that increasing Cr reduced the rate of increase of current density above Epit, that is, the size of hysteresis loop was reduced. In the alloy with less Cr content, the hysteresis loop will show higher anodic currents than one with higher Cr contents as observed with 304SS and LNASSs comparisons. This demonstrates the inhibitory effect of Cr on initiation of localised corrosion.

**Figure 8.** *Polarisation scans of austenitic SSs in 3.56 wt. % NaCl.*

**Figure 9.** *Polarisation scans of austenitic SSs in 1 wt. % NaCl solutions.*

Typically, in stainless steels a Cr-rich passive oxide film will form when exposed to an oxidising environment. It has been previously reported that the film consists of layers of different compositions formed due to dissimilar conditions at the metaloxide interface and oxide-environment interface of the passive film. Depending on the composition of an alloy, it has been observed that the metal adjacent to the passive film is enriched with Ni, while the passive film itself consists of Cr-rich oxide inner layer and an Fe-rich hydroxide outer layer. The Ni-rich layer is formed because *The Evaluation of the Comparative Corrosion Behaviour of Conventional and Low-Nickel… DOI: http://dx.doi.org/10.5772/intechopen.102381*

of diffusion of Fe and Cr out the bulk metal into the oxide layer. **Figure 10** shows that Cr and Mo are more significant in the formation of oxide layer because of their high affinity for oxygen. On the other hand, Ni does not participate in the formation or stabilisation of a passive layer [29].

Moreover, Ujiro *et al.* [28] studied the effect of Mo in the corrosion behaviour of austenitic SSs containing 26 wt. % Cr and varying Mo contents from 0 to 4 wt. %. It was observed that addition of Mo decreased the anodic current density; that is, the current measured before Epit or before the onset of pitting. Extended passive region was evident in Hercules™ B compared to Hercules™ A and 304SS. However, both Hercules™ alloys show similar hysteresis loop behaviour. The hysteresis loop closes at potentials lower than Ecorr.

To further substantiate polarisation results micrographs of corroded samples surface were evaluated. The micrographs of corroded samples surface are shown in **Figure 11**. It is evident from the optical micrographs that all samples experienced crevice corrosion underneath the crevice washer and as a result Epro was measured at potentials below Epit.

In a crevice metal-solution system, there lies a crevice critical solution, of which a minor shift in potential gradient changes the corrosion behaviour of an alloy from passive to active. The longer it takes for an alloy to reach that crevice critical solution defines the resistance of an alloy to corrosion [30]. Amongst other factors, crevice critical solution is mainly affected by alloy composition. The cationic metal species react with water to generate acidity in the crevice region. In stainless steels, the typical chemical reaction to take place under a crevice is: ( ) <sup>2</sup> <sup>2</sup> <sup>2</sup> *Fe H O Fe OH H* 2 2 <sup>+</sup> +→ + <sup>+</sup> .

The reaction involves other alloying elements such as Cr, Mn and Mo. Mo was added in Hercules™ B with an expectation that it will inhibit the localised corrosion reactions by lowering rate of generation of acidic hydrogen and consequently lower the corrosion rate. However, the reaction rate is controlled by two different kinetic phenomena. The first is charge transfer or activation control. In this case, the reaction rate is controlled by the size of the driving force, which is either hydrogen evolution or water reduction reaction. As the driving force increases, so does the reaction rate [30].

Other mechanism controlling the rate of reaction is mass transfer through the electrolyte to the electrode surface, that is, oxygen reduction reaction. Since the reaction rate is controlled by diffusion, it cannot increase indefinitely as the driving force increases. Instead, the current reaches a maximum current density which

**Figure 10.** *Contents of optimal passive layer formed in NaCl (adopted from [29]).*

**Figure 11.**

*Micrographs of corroded coupons (scale bar 1 mm) showing crevice corrosion in 3.56% NaCl solution for: Hercules™ a (left), Hercules™ B (middle) and 304SS (right).*

is itself a function of the concentration of the species of interest in the solution as well as its diffusivity. Once the rate for a particular reaction has reached its limiting value, further increases in driving force will not result in any additional increase of the reaction rate. For this current work, addition of Mo was expected to reduce rate of propagation of corrosion by slowing down diffusion at the crevice. However, the concentration of added Mo (0.5%) was not sufficient to lower the corrosion rate in Hercules™ B and thus, similar corrosion behaviour was observed for all test alloys [29, 30]. Perhaps, higher amount of Mo should be added, but bearing in mind costrelated issues. Micrographs are in agreement with the polarisation scan measurements, which showed similar corrosion behaviour.

The difference was observed with micrographs obtained from 1 wt. % NaCl test, as shown in **Figure 12**, which showed pitting corrosion within the crevice area. Ujiro *et al.* [28] explained that crevice corrosion can occur either by depassivation or pitting. Depassivation type occurs by corrosion of surface underneath the crevice, due to pH drop and extensive destruction of the passive film under the crevice washer (observed with 3.56 wt. % NaCl). Pitting type occurs by pitting inside the crevice area as a result of chloride concentration increase in the inner solution.

Ujiro *et al.* [9] also investigated the relationship between the type of corrosion and the Ecorr. It was observed that ferritic alloys which corroded by depassivation had lower Ecorr than the ones that corroded by pitting. Crevice corrosion by depassivation is related to Ecorr because it involves an intensive metal dissolution at lower potentials and not just a single pit. Similar to the current work, Ecorr values obtained from 1 wt. % NaCl were measured to be higher than those obtained from 3.56 wt. %

### **Figure 12.**

*Micrographs of corroded coupons (scale bar 1 mm) showing pitting corrosion in 1 wt. % NaCl solution for: Hercules™ A (left), Hercules™ B (middle) and 304SS (right).*

*The Evaluation of the Comparative Corrosion Behaviour of Conventional and Low-Nickel… DOI: http://dx.doi.org/10.5772/intechopen.102381*

NaCl. This means that 1 wt. % NaCl did not contain a critical chloride concentration required to reach crevice critical solution for the complete dissolution of a metal underneath the crevice. Hence, instead of depassivation, pitting was observed. The solution in the pit is more aggressive once pitting has started as shown by an increase of current density. In some alloys, once pitting initiates, propagation is faster and it becomes difficult for a formed pit to repassivate as observed in the CP scans. However, the onset of pitting was delayed for Hercules™ B in 1% wt. % NaCl, as indicated by an extended passive region than in other test alloys, owing to the inhibiting effect of Mo.
