**2. Corrosion resistance of stainless steel and evaluation methods**

Due to the nature and change in the environment, many metallic materials are expected to possess a good corrosion resistance against corrosion attacks over time. However, the corrosion resistance ability of materials differs from each other, and corrosion does set in when the corrosion-resistant limit of a material is exceeded [21–23]. Hence, the major reason why many material scientists and corrosion experts always pay much attention on how to continuously protect the material surface from degradation and corrosion via surface treatment method, coatings, and other related techniques. For clarity's sake, the corrosion behaviour can be studied when the material is exposed to an aggressive corrosive environment alone (polarization) [9, 16] or under the action of both tensile stress and corrosion reaction (stress corrosion cracking—SCC) [18].

The conventional polarization tests are normally carried out using an electrochemical workstation consisting of the traditional three-electrode system; (1) reference electrode (RE), whose material can be made of saturated calomel electrode (SCE) or silver/silver chloride (Ag/AgCl), (2) counter electrode (CE), which can be Platinum (Pt), graphite, gold or carbon rod, and (3) working electrode (the testing material).

Generally, the corrosion resistance of metallic materials can be evaluated through electrochemical tests which can be done in the following ways (**Figure 1(a–f)**); (1) open circuit potential (OCP) study, (2) potentiodynamic polarization study, (3) potentiostatic polarization study including the current-time transient (CTT) study and double-log plot, (4) electrochemical impedance spectroscopy (EIS) analysis including the Nyquist plot, Bode impedance, and phase angle plots, and (5) Mott-Schottky analysis which is normally carried out to determine the semiconducting characteristics of the passive film.

To a large extent, the OCP test determines the stability of samples in the electrolyte before performing polarization and EIS tests. Here, it is believed that the higher the corrosion potential, the more stable the sample, and probably the better the corrosion resistance [5, 11], i.e., the sample "A" in **Figure 1(a)** possessed higher corrosion potential and is therefore expected to be more stable than sample "B". The potentiodynamic polarization shows the corrosion behaviour in terms of corrosion current density (*i*corr) and corrosion potential (*E*corr) which can be determined from the corrosion graph using the Tafel extrapolation method. It is generally believed that the lower the *i*corr and higher the *E*corr, the more the formation of the passive film, hence the better the corrosion resistance [7–9].

In addition, the anodic polarization process can be categorized into four regions, as illustrated in **Figure 1(b)**; (1) activation zone, where the *i*corr gradually increases with *E*corr, (2) activation-passivation transition zone, where the *i*corr decreases gradually and started forming passivation film, (3) passivation zone, which involves further decrease in *i*corr, signifying the formation of more passivation film, and (4) transpassive zone, signifying the degradation of the passive film with

*Corrosion Resistance, Evaluation Methods, and Surface Treatments of Stainless Steels DOI: http://dx.doi.org/10.5772/intechopen.101430*

**Figure 1.**

*Illustration of the corrosion properties of metallic materials; (a) open circuit potential (OCP), (b) potentiodynamic polarization, (c) Nyquist plot, (d) Bode phase angle plot, (e) Bode impedance plot, and (f) Mott-Schottky plot.*

a rapid increase in *i*corr*.* In EIS analysis, the samples with better charge-transfer resistance, higher impedance, and phase angle are believed to have a stabilized and more protecting passive film, hence possessing better corrosion resistance [10, 12]. For instance, sample "A" in **Figure 1(c–e)** is better than sample "B" in terms of corrosion resistance since it has a larger diameter of the semi-circle, higher phase angle, and impedance.

Corrosion fatigue is a common phenomenon that frequently occurs when materials are often exposed to simultaneous actions of corrosive environment and repeated stress, which leads to a markedly decrease in fatigue strength. In addition, unlike stress corrosion cracking which causes intergranular cracking and mostly

occurs in harmful environments, corrosion fatigue which causes transgranular cracking, can occur at any time and cannot be avoided in some cases [4, 5, 12].

Furthermore, Mott-Schottky analysis determines the electronic properties of the passive film by measuring the capacitance as a function of potential, which ultimately determines the semiconducting characteristics of the passive film. It is generally believed that a negative slope represents a p-type semiconductor while a positive slope signifies an n-type semiconductor. When a positive slope is obtained, it means there is no change in the semiconducting characteristics of the passive film, hence an enhancement in the stability of the passive film which can eventually increase the corrosion resistance. As illustrated in **Figure 1(f )**, the section denoted by "A" represents the flat band potential zone while the portion denoted by "B" represents the n-type semiconductivity zone.
