**3. Corrosion tests**

**Table 3** represent the summary of corrosion tests that were conducted.

### **3.1 Cyclic polarisation**

Cyclic polarisation technique was used to investigate the susceptibility of Hercules™ and 304SS alloys to pitting. This technique is generally used to measure the pitting tendencies of alloys in a given metal-solution system. The experiment starts by applying the potential scan beginning at Ecorr and continuing in the anodic direction until there is a large increase in current (e.g. a sustained anodic current density ≥ 10 μA/cm−2). The resulting graph is a plot of applied potential vs. the logarithm of current density. When the scan reaches the programmed current density limit value, it reverses and begins scanning in the negative direction. An example of a typical cyclic polarisation plot is shown in **Figure 7** [22, 23].

The Epit is the potential at which stable pits initiate and propagate as applied potential increases. Epro is the potential below which no initiation of pits will occur. Pits that form above Epit will eventually repassivate below Epro hence the potential is also referred to as the repassivation potential. Both Epit and Epro are used to explain the kinetics of pitting and repassivation [23].

The size of the hysteresis loop can give a rough indication of the extent of propagation of initiated pits. The longer time it takes for pits to repassivate, the bigger the hysteresis loop. This imply that formed pits are severe and stable. In some cases, pits show no tendency to repassivate by the hysteresis loop closing at a potential less than Ecorr [23].

In our work cyclic polarisation technique has been used to evaluate the corrosion behaviour of Hercules™ alloys and conventional 304SS. The procedure outlined in the ASTM G61 standard [24] was used to conduct pitting corrosion tests. The 12 mm diameter disc shaped samples were prepared from each alloy. Three test solutions were used, 3.56 wt. % NaCl, reduced concentration to 1 wt. % NaCl and 5 wt. % H2SO4.


**Table 3.** *The summary of corrosion tests.*

**Figure 7.** *A typical cyclic polarisation plot [22].*

Test samples were ground to 600 grit SiC paper finish. The corrosion test conditions were set as shown in **Table 4**. An ACM potentiostat was used to apply the potential on the working electrode and to measure the current flow between the counter electrode and the working electrode. A graphite rod was used as a counter electrode and a saturated calomel electrode (SCE) was used as a reference electrode. The scan was set to increase the potential stepwise starting from the corrosion potential to 1200 mV. Duplicate scans were performed.

The corrosion potential (Ecorr), pitting potential (Epit) and protection potential (Epro) were measured from the cyclic polarisation curves and they were analysed using the Origin program. The corroded coupons were further taken for analysis under stereomicroscope at 50X magnification. Information about the extent of passivation region, stability of passive state and the ability of tested alloys to spontaneously passivate in a given environmental system was obtained. The corrosion rate (*CR*) was calculated using Eq. (7) [25]. The critical current density (icorr*)* was obtained by extrapolating the Tafel region of the cathodic and the anodic regions of the polarisation curve.

$$CR = 0.011 \times i\_{com} \times 1000 \left( mm / \,\text{y} \right) \tag{7}$$


**Table 4.** *Cyclic polarisation test parameters.* *The Evaluation of the Comparative Corrosion Behaviour of Conventional and Low-Nickel… DOI: http://dx.doi.org/10.5772/intechopen.102381*

### **3.2 Immersion tests**

Immersion tests were performed in 5 wt. % H2SO4 for 10 days. Tests were done following the guidelines outlined in ASTM G31 [26]. 25 by 50 mm2 coupons of Hercules™ A, Hercules™ B and 304SS were ground to 600 grit SiC paper finish. Test coupons were left for a minimum of 24 hours to allow them to passivate in air, in order to spontaneously form the passive layer possibly disrupted by sample preparation [27]. Test coupons were then immersed in a test solution at room temperature. The amount of solution in the beaker was calculated to a ratio: 0.20 ml/mm2 [26].

ASTM G48-Method A was used for immersion tests in FeCl3.6H2O. The temperature was maintained at 26°C ± 2°C. The 25 by 50 mm2 coupons were also prepared. Test coupons were immersed in 6 wt. % FeCl3.6H2O for 72 hours. Then coupons were removed and rinsed with water and ethanol. Corroded coupons were then reweighed and examined for pitting [15]. Pit depth and density were measured by visual examination of images taken at 10X magnification using the Light microscope.

Mass loss due to corrosion was measured and corrosion rate calculated using Eq. (8) [26].

$$\text{Corrosion rate} = \frac{\text{K} \times \text{W}}{\text{A} \times \text{T} \times \text{D}} \tag{8}$$

Where:

K = <sup>4</sup> 8.76 10 × for corrosion rate in millimetres per year (mm/y), W = mass loss in grams, A = area in cm2 , T = time of exposure in hours, D = density in g/cm3 .
