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

instrument that applies a potential to a specimen which enables the modification of current flow. The commonly used electrochemical techniques are polarisation resistance technique, electrochemical impedance and Tafel extrapolation. The current can be measured by extrapolation procedure whereby a specimen is initially made to act as a cathode in the electrochemical cell containing the test solution [13]. Some of these techniques can be used to determine the lifetime of a metal by calculating the time required for initiation and propagation of corrosion to cause failure.

Studies have been conducted on commercial stainless steels such as 304SS and 316SS. Pitting or crevice corrosion resistance of these alloys in chloride environments can also be measured by immersion tests in metal-chloride solutions [14].

Bergstrom *et al.* [8] followed guidelines outlined on ASTM G48 method A and B [15] to test susceptibility of 201SS and 304SS to pitting corrosion. ASTM G48 method A is the practice for measuring pitting resistance and method B for crevice corrosion resistance. These two methods included immersion of coupons in 6 wt.% FeCl3.6H2O and measuring mass loss due to either pitting or crevice corrosion after 72 hours as shown in **Table 2**. The results showed that there was no significant difference in the mass loss of 201SS and 304SS. Therefore, this means that according to Bergstrom *et al.* [8], there was no difference in the corrosion behaviour of 201SS and 304SS in 6 wt.% FeCl3.6H2O even in the presence of an artificial crevice.

Similarly, Garcia-Alonso *et al*. [16] also performed corrosion tests of 304SS and 316SS rebars embedded in concrete with different chloride concentrations. Tests were also done for carbon steel and LNASSs for comparison. LNASSs had Ni composition of 0.2 wt. % and 1.5 wt. % with addition of Mn. The concrete was manufactured with additions of 2 wt. % and 4 wt. % chloride content of cement. Other rebars were also embedded in a portion of concrete without chlorides and immersed into 3.5 wt.% NaCl solution to evaluate the effect of diffusion of chloride ions through the non-chlorinated concrete in the corrosion behaviour of stainless steel rebars [16].

The icorr values of carbon steel, 304SS, 316SS and LNASSs were measured using electrochemical methods. In the absence of chlorides, icorr values for all test alloys were measured around 0.1 μA/cm2 . The icorr for carbon steel was observed to moderately increase after 30 days of immersion, which is attributed to diffusion of chlorine at the surface of the rebar. The icorr values for alloys that were embedded in concrete with 2 wt. % chlorides were measured to be 3–5 times higher for carbon steel compared to that of LNASSs and 316SS. In the slab with 4 wt. % chlorides, carbon steel was measured to have icorr value 10 times higher than of stainless steels. 304SS was measured to have an icorr value that is of at least one magnitude lower than other alloys in 4 wt.% chlorides concrete slab [16].

The response of these stainless steel and carbon steel alloys towards increased chlorides concentration can be attributed to local breakdown of the passive layer depicting the occurrence of pitting corrosion [1, 16].

Furthermore, Bautista *et al*. [14] also performed corrosion experiments for LNASS Type 204Cu stainless steel (204SS) in a solution simulating "pore solution" of the concrete. The composition of 204SS consisted of 1.89 wt. % Ni and 8.25 wt. % Mn.


**Table 2.**

*Results of ASTM-G48 A and B tests conducted at 22°C by Bergstrom et al. [8].*

Cyclic polarisation technique was used to test the susceptibility of 204SS rebars towards pitting corrosion against conventional 304SS and 316SS.

A number of mixtures of saturated calcium hydroxide concrete solutions were used with different NaCl additions. The pitting potential values were measured from the cyclic polarisation curves at the potentials where the current sharply increases when the working electrode is anodically polarised. No pitting was detected on the media without NaCl addition. However, pitting was detected for tests done with additions of NaCl. The greater the amount of NaCl added for each test, the lower the pitting potential obtained. The presence of chloride ions causes the passive layer to break down at potentials below the transpassive region and results in pitting corrosion. An example (Adapted from [14]) of pitting scans showing the effect of addition of NaCl in the corrosion behaviour of 204SS is shown in **Figure 3** [14].

The cyclic polarisation curve labelled 0% NaCl illustrates the typical behaviour of stainless steels in the absence of chloride ions. The curve shows passivity until it reaches a potential above 650 mV and evolution of oxygen is observed and referred to as transpassive region. The reverse scan for 0% NaCl curve shows current density values that are less than that of the forward scan. This means that in this medium, 204SS is not susceptible to pitting corrosion. Increasing NaCl concentration reduced the pitting potential value with a certain significant order and also increases the current density. This therefore means that the presence of chloride ions speeds up the initiation and propagation of pits. Moreover, it was noted that no repassivation or protection potential (Epro) was determined for tests with NaCl additions. Comparison of corrosion behaviour of 204SS with 316SS and 304SS in the solution with 0.5 wt.% NaCl is shown in **Figure 4** [14].

The pitting potential for 204SS was measured closer to that of 304SS and 316SS around 700 mV vs. SCE. Non-carbonated solution on its own is alkaline with the pH of 12.6 and without NaCl alloys did not experience pitting corrosion. When NaCl was increased to 5 wt. %, it was observed that 204SS had significantly lower pitting potential. The difference in Epit obtained at different NaCl concentrations is shown in **Figure 5** [14].

**Figure 3.** *Influence of chloride concentration on the pitting of 204SS (adopted from [14]).*

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

**Figure 4.** *Pitting of alloys in concrete with 0.5 wt.% NaCl (adopted from [14]).*

**Figure 5.**

*Variation of pitting potential of stainless steels with variation of NaCl [14].*

The general interpretation of pitting scans is: If the Epit value is more noble than Ecorr of a tested alloy, then pitting will not occur. Thus if the test solution constituents raises the pitting potential above what is estimated by the polarisation curve, that solution will protect the steel from pitting corrosion. Moreover, if the potential of the test solution is below the measured value of Epro, pitting corrosion will not take place. [Ca(OH)2] with 0.5% NaCl as an additive proved to be the safer one to use for concrete solution for all alloys that were tested because higher Epit values were obtained compared to solutions with 1 wt.% and 5 wt.% NaCl additive [14].

Bautista *et al.* [14] studies were in agreement with work done by Berke *et al.* [17], where carbon steel rebars were tested in solutions containing different chloride concentrations. It was observed that pit nucleation became more active with increasing chloride content [17]. It is therefore important to measure the chloride content of the test solution in order to determine if the test alloy will experience corrosion in a certain chloride environment or not.

In order to determine whether certain steel will corrode in a chloride environment, a critical chloride threshold level (CCTL) is calculated [18]. The CCTL is the measure of the chloride level that is enough to cause pitting. According to Bautista *et al.* [14], the CCTL for 204SS was measured to be 1 wt.% chloride and for 304SS was 5 wt.% [14].

Stainless steel producers have also ran in-solution tests to determine the CCTL of stainless steels and carbon steel in the concrete solution. The CCTL for carbon steel was measured to be less than 0.35 wt. % chlorides and 2.51 wt. % chlorides for 304SS. Garcia-Alonso *et al.* measured the CCTL of 304SS to be 2 wt.% chlorides in similar test conditions [16, 19].

Fajardo *et al.* [20, 21] also tested LNASS (4.32 wt.% Ni) against 304SS in a carbonated concrete solution with different chloride concentrations. The cyclic polarisation curves that were obtained showed that LNASS had almost similar pitting behaviour to that of 304SS, with 304SS having a slightly higher pitting potential at all chloride concentrations.

The results that were obtained by Fajardo *et al*. [20] were in agreement with the results that have been obtained by other researchers [14–17]. LNASS was expected to have pitting potentials significantly lower than those of 304SS, but further analysis of corroded samples showed that both LNASSs and 304SS had similar behaviour.

Thus, for the current work, we study the general corrosion behaviour of Hercules™ -a LNASS alloy using standard testing methods in comparison to 304SS. Cyclic polarisation technique and immersion tests are used. The objective was to evaluate whether or not the newly developed alloy has corrosion behaviour comparable to that of 304SS and therefore can be a candidate for applications such as reinforcement bars, fasteners and hot rolled stainless steel sheets.
