**1. Introduction**

Ni contributes about 60% of austenitic stainless steel manufacturing material price. This means that the price of austenitic stainless steel increases with an increase of Ni. Ni price fluctuation has led to major efforts to reduce its content in austenitic steels. Ni has been replaced with readily available, cheap elements such as Mn and N. Hercules™ is a low-Ni austenitic stainless steel alloy that was developed at Mintek-South Africa in the Advanced Materials Division. The typical content of Hercules™ comprises of 2 wt.% Ni, 9 wt.% Mn and 2.5 wt.% N [1–3].

When Hercules™ was tested for mechanical properties, it was found that it had higher tensile strength than 304SS in the hot rolled and annealed condition, hence it was termed Hercules™. This indicated that it can be used for structural applications where high strength is required. Possible applications for Hercules™ were targeted at reinforcement bars/rebars, fasteners and hot rolled channels for construction

purposes. Construction industry is likely to benefit from Hercules™ because it has higher strength and reduced cost than 304SS. Other industries will soon find benefit also because it could later be available in flat product such as sheet and coil [4].

A minimum yield strength required in structural applications is 400 MPa in hot rolled condition. Typical tensile properties and density for Hercules™ alloy in hot rolled condition are shown in **Table 1** [4, 5]. Fastener prototypes of Hercules™ have been manufactured and was divided into two types; the large head bolts (M16 and M24) and roof fasteners.

According to White *et al.* [6], 240,000 tonnes of fasteners are produced globally per year and South Africa contributes only about 35,000 tonnes per year. Production of corrosion resistant bolts and roof fasteners promises a viable business. It is required that new LNASSs be resistant to corrosion. The latter requirement is in response to specific environment such as swimming pool applications, which requires pitting and crevice corrosion resistance [6].

Furthermore, about 400,000 tonnes of stainless steel rebar is produced per year. South Africa has been using about 95% carbon steel rebars which are cheaper than 304SS. However, there was little success in marketing of carbon steel rebars because of poor corrosion resistance. Corrosion of rebar is detrimental in that it can cause concrete spalling which leads to infrastructure failure. An infrastructure repair is more costly than preventing failure. Hence there is a need to use stainless steel rebar in any concrete because they are more corrosion resistant than carbon steel rebar. The conventional LNASS 201 stainless steel (201SS) is hardly available in South Africa; therefore, Hercules™ could close the gap for applications that require 304SS properties at a lower price. The cost production of Hercules™ bar is around 25% less than 304SS using a similar production route [6].

The corrosion resistance was however compromised by addition of Mn and N in Hercules™. Thus, to counteract this, 0.5 wt. % Mo was added (Hercules™ B) [2]. The focus is on characterisation of the pitting behaviour of Hercules™ B (with 0.5 wt. % Mo) against Hercules™ A (without Mo addition) and 304SS.

Pitting corrosion is the local discontinuity of a passive film which results in small holes through the material. These holes are referred to as pits. Initiation of pits can be caused by mechanical imperfection such as surface damage or inclusions. The composition of stainless steel may cause formation of inclusions which become nucleation sites for pits at the inclusion-austenite matrix interface [7]. Bautista *et al.* [8] studied the morphology of pits that were formed on the surface of test coupon after polarisation in NaCl. Pits were found to nucleate preferentially at the point of strain and at geometrical irregularities that favoured formation of corrosion cells [8].

The effect of Mo content on pitting and crevice corrosion resistance of stainless steels in chloride environments has been studied by Kaneko *et al.* [9]. The pitting potential for austenitic stainless steel alloys with 2 wt. % and 5 wt. % Mo contents showed a dramatic increase compared to that of steels without Mo in chloride environment. It is understood that Mo is adsorbed at the dissolving interfaces of a corroding metal and hence inhibiting dissolution kinetics [3, 9].


**Table 1.**

*Typical mechanical properties of Hercules™ alloy [4].*

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

Kaneko *et al.* [9] findings are consistent with the work that was done by Pardo *et al.* [7]. He studied the effect of both Mn and Mo in the pitting corrosion resistance of 304SS and 316SS. Tests were performed by immersion in 6 wt. % FeCl3.H2O and cyclic polarisation technique in 3.5 wt. % NaCl. The scanning electron microscopy was used to examine the morphology of pits on the surface of corroded coupons. Images of corroded samples of 304SS that were electrochemically tested in 3.5 wt.% NaCl are shown in **Figure 1** [7].

Alloys with higher Mn content had larger pits compared to ones with lower Mn content. That is, higher Mn stainless steels experience high pitting because it was thought that since Mn has high affinity for sulphur, it reacts with Mn to form inclusions (MnS), which in turn are precursors for pit nucleation. When Mo was increased to 2.10 wt.%, pit density decreased and the size of pores evidently decreased as shown in **Figure 2** [7].

Pit initiation can also be influenced by surrounding conditions such as gaseous environment, temperature and the nature of the electrolyte. Stainless steels tend to form deep pits at specific areas when exposed to environments that contain solutions with chloride, bromide or hypochlorite [10, 11].

The occurrence of the electrochemical reactions is a result of a potential change created on a conductive metal when exposed to a conductive medium. Electrochemical potential is accompanied by electron movement which leads to electron availability at the metal surface. The electron movement or potential difference can affect the rate of corrosion reactions. Overall, the energy change provides the driving force and control for the spontaneous direction for a chemical reaction. The change in energy can be understood using thermodynamics to show how conditions of the corrosion cell can be adjusted to avoid corrosion. When a metal is immersed in a conductive solution, a charged surface of an alloy forms a complex interface. The interface is formed when the polar H2O molecules form an oriented solvent layer. The electric field formed around the solvent layer prevents easy charge transfer, thereby limiting electrochemical reactions at the surface of an alloy [10].

The positively charged ions such as Fe2+ at the anode are transferred to the conductive solution which acts as an electrolyte for the cell. The electrolyte consists of ions that create electrical connectivity with the metal. Oxygen and water act on the cathodic reaction and accept negatively charged ions to form hydroxyl ions (*OH* ) <sup>−</sup> .

Further anodic reactions occur simultaneously with cathodic reactions during corrosion. Typical anodic reactions are shown in Eqs. (1)–(3) and the cathodic reaction in Eq. (4). These type of corrosion reactions occur when the alternative air exposure and water is present, for example in the sea wave condition [12].

**Figure 1.** *Pitting corrosion of 304SS as a function of Mn (adopted from [7]).*

**Figure 2.** *Pitting corrosion of 304SS as a function of Mo (adopted from [7]).*

$$Fe \rightarrow Fe^{2+} + 2e^- \tag{1}$$

$$\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe}(\text{OH})\_2 \tag{2}$$

$$4Fe(OH)\_2 + 2H\_2O + O\_2 \to 4Fe(OH)\_3\tag{3}$$

$$4e^- + O\_2 + 2H\_2O \to 4OH^- \tag{4}$$

Stainless steels can corrode by pitting mechanism without a significant loss of weight on a whole structure being recognised. For example, chloride induced corrosion of stainless steel rebars in the concrete occur when there exists a difference of electric potential along the rebar. In the presence of chloride ions, the surface of the rebar is activated to act as the anode and the passivated region becomes the cathode. The reactions involved are shown in Eqs. (5) and (6) [12].

$$\text{Fe}^{2+} + 2\text{Cl}^- \rightarrow \text{FeCl}\_2 \tag{5}$$

$$\text{FeCl}\_2 + 2\text{H}\_2\text{O} \rightarrow \text{Fe(OH)}\_2 + 2\text{HCl} \tag{6}$$

The chloride ions migrate easily towards the interior of the pit and catalyse the hydrolysis reaction. An acidic environment is created in the pit solution as the reaction continues. During corrosion, more than one anodic reaction takes place because of different elements present in the alloy. The electrons produced by these anodic reactions are consumed by the cathodic reactions which includes hydrogen and metal reduction. Removing the cathodic sites therefore reduces the rate of corrosion. The potential required for corrosion to take place is denoted by potential corrosion Ecorr. This is the potential at which the total rate of all anodic reactions is equal to the total rate of all cathodic reactions. The corrosion current density at this point is denoted by icorr and it is used to measure corrosion rate of the metal as anionic species are released [13].

Electrochemical tests can be used to determine icorr and can be measured indirectly with the aid of a counter electrode and electronic equipment. This technique uses a potentiostat in conjunction with the reference electrode. Potentiostat is an
