**4.2 Severe pitting in ferric chloride**

The mass loss of test alloys was measured and the corrosion rate due to pitting was calculated. No significant difference in mass loss of all test alloys was observed, with corrosion rates measured to be 11.8 ± 0.2 mm/y for Hercules™ A, 12.6 ± 0 mm/y for Hercules™ B and 14.1 ± 0.1 mm/y for 304SS.

This behaviour is similar to results that were obtained by Bergstrom *et al*. [8] when 201SS and 304SS were tested in 6 wt.% FeCl3 for 72 hours. Both alloys showed a corrosion rate of 0.0228 g/cm2 and similar pit depth of 0.0762 mm [8].

The 6 wt. % FeCl3.6H2O solution is generally used as a test for localised corrosion for accelerated tests. Therefore, test alloys were immersed in the solution for 72 hours as recommended in the ASTM G84 standard. However, this test solution is used to simulate a very rough composition environment within a localised corrosion site in a stainless steel. It can be very aggressive for low-alloyed steels such as 304SS and 201SS. This has also been confirmed by tests that were conducted by Ujiro *et al*. [28]. Alloys with higher Mo and Cr contents (above 26 wt.% Cr and 4 wt. % Mo) showed more corrosion resistance than the ones with the compositions approximately similar to that of 304SS [28].

Furthermore, 6 wt. % FeCl3.6H2O serves as a chemical potentiostat by forming the Fe3+/Fe2+ redox couple which has an approximate potential of 450 mV with high chloride concentration. The solution is highly acidic with a pH of 1.44, which is enough to create a large current without a need to polarise the specimen as with the electrochemical tests [27]. From the electrochemical tests, it has been established that chloride concentration from neutral 1 wt. % NaCl was enough to cause a Epit that is less than 450 mV. The 6 wt. % FeCl3.6H2O solution has higher chloride concentration than NaCl, hence Hercules™ alloys and 304SS corroded aggressively. The high potential of the test solution almost guarantees that the pitting potential of each alloy was exceeded. The acidic nature of FeCl3.6H2O also inhibits repassivation and lowers passive film strength by cathodic reactions that occur on the surface of the sample via Fe3+/Fe2+ ions [27].

The difference in the individual pit morphology was observed. Hercules™ A had irregular shaped pits and with high depth, whilst Hercules™ B had wide and round shallow pits. This means that pitting propagated quicker in Hercules™ A than in Hercules™ B. **Figure 13** shows the one of the deepest representative pit that was observed in Hercules™ A after 72 hours of immersion in FeCl3.6H2O, along with the pits measurements. Although pit density of all test alloys was almost similar, Hercules™ A showed severe pitting because of larger pit opening.

**Figure 14** shows that the size of pit opening observed in Hercules™ B is smaller than that of Hercules™ A. **Figure 15** shows the extent of pitting that was observed for 304SS. Overall, the pit evaluation proves that FeCl3.H2O is an aggressive solution for testing LNASSs and 304SS. Even the addition of 0.5 wt. % Mo for Hercules™ B is not enough for it to be used for applications in aggressive environments.

**Figure 13.** *Micrographs (scale bar = 1 mm) and representative pit of Hercules™ A in FeCl3.H2O.*


**Figure 14.** *Micrographs (scale bar = 1 mm) and representative pit of Hercules™ B in FeCl3.H2O.*


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

**Figure 15.** *Micrographs (scale bar = 1 mm) and representative pit of 304SS in FeCl3.H2O.*


### **4.3 Passivity behaviour in sulphuric acid**

A stainless steel can be considered resistant to uniform corrosion in a particular environment if the corrosion rate does not exceed 0.1 mm/y [19] . In the current work all test alloys demonstrated resistance in 5 wt. % H2SO4 as shown by polarisation curves in **Figure 16**.

All alloys displayed the ability to passivate spontaneously in 5 wt. % H2SO4. Lower Ecorr for Hercules™ A can be attributed to the kinetics of passivity for stainless steels, whereby if the cathodic reaction becomes more dominant at lower potentials and thus remaining in the active region will result in favourable conditions for anodic reactions to overtake the redox reactions at lower potentials. Polarisation scans of some stainless steel will show these cathodic reactions by the presence of anodic current peak, which is an indication of non-uniform passivity due to less OCP test times. Thus, it can be observed that with all test alloys, if given enough time to form a passive layer during OCP tests, the anodic peak current can be avoided and alloy are fully passivated with no disruption of the protective film when exposed to H2SO4. Therefore, H2SO4 is considered a safe environment for LNASSs and so is 304SS.

Furthermore, the absence of a hysteresis loop is an indication that tested alloys did not undergo any type of localised corrosion even though an artificial crevice was introduced in each sample. The presence of an artificial crevice creates passivation current (ipass) (Current density at the passive region) that is higher than icorr. However, the conditions were not sufficient to activate sample surface for formation of pits or cause crevice corrosion since the test solution did not contain chlorides [31].

**Figure 16.** *Cyclic polarisation scans of austenitic SSs in H2SO4.*

The corrosion rates calculated from polarisation curves for Hercules™ B and 304SS were comparable at 0.001 ± 0.008 mm/y and 0.002 ± 0.004 mm/y, respectively. The corrosion rate of Hercules™ A was measured to be 0.016 ± 0.029 mm/y, which is a magnitude higher than 304SS and Hercules™ B.

However, the corrosion rate of test alloys in 5 wt. % H2SO4 was calculated to be higher in the immersion tests. The corrosion rate of Hercules™ A was calculated from the mass loss incurred to be 1.863 ± 0.028 mm/y. Hercules™ B and 304SS had the corrosion rate less than 0.100 ± 0.020 mm/y, with 304SS having the lowest. It is often assumed that corrosion rate of stainless steels is linear with the function of time during immersion tests, but this is not always true for some stainless steels immersed for a longer time [32].

Hercules™ A was observed to react aggressively for the first 24 hours at a presumably higher corrosion rate but the vigorous reaction decreased as days progressed. Hercules™ B did not react aggressively in the beginning and throughout the entire exposure time [32].

Based on observations made in the current project, the solution in which Hercules™ A was immersed had a dark bluish precipitates after a few hours of immersion and the reaction was more aggressive than Hercules™ B. The solution with Hercules™ B and 304SS did not show any change of colour and the reaction was less aggressive. Therefore, it can be established that passive films formed by stainless steels may be broken down in a prolonged exposure period and hence higher corrosion rates are obtained in immersion tests than polarisation tests. Although mass loss is negligible after a certain period it can still add up to the final mass loss measurement. Electrochemical tests took less than 4 hours to obtain a complete scan. Therefore, the corrosion rate measured tend to be less than that obtained from immersion tests [27].
