Corrosion Resistance, Evaluation Methods, and Surface Treatments of Stainless Steels

Stainless steels are widely recognized and find applications in many engineering industries and companies due to their excellent properties including high resistance to corrosion as a result of their minimum 10.5% chromium content, exceptional strength and durability, temperature resistance, high recyclability, and easy form-ability. In the present book chapter, the basic concepts of stainless steel including its applications, classifications, and corrosion properties will first be discussed. Thereafter, their corrosion behaviour will then be explained. The various methods by which the corrosion resistance behaviour can be significantly improved including surface treatments such as coatings/electrodepositions, alloying, mechanical treatment, and others will be discussed in detail.


Introduction
Stainless steels (iron-based alloys) are widely recognized due to their high machinability, hardness, mechanical strength, good heat resistance and excellent corrosion resistance [1][2][3].Compared to other steels, the superior corrosion resistance exhibited by many stainless steels can be attributed to the chromium content (about 10.5 wt.%) which initiated the formation of a stable layer of chromium oxide on the steel surface [2,3] thereby preventing chemical reactions with the bulk material hence reducing corrosion attack to the minimum.Chromium is one of the major elements that play a vital role in the corrosion resistance of stainless steels.However, when exposed to water for a long period, their corrosion resistances reduce at elevated temperatures, hence the need to come up with more robust techniques for protecting the sample surface.
Even though they exhibit good corrosion resistance, efforts have continually been made to further improve the corrosion resistance behaviour of stainless steels.Heat treatment such as annealing due to the oxidation of steel surface [4][5][6], surface mechanical treatment [7][8][9][10][11][12], coatings/electrodepositions [13][14][15][16], alloying, machining/molding [17] and many more are presently in use to further improve the corrosion resistance behaviour of stainless steels.Other protection methods include epoxy coating, cathodic protection, and thicker concrete cover.For instance, surface modifications, as well as heat treatments of the modified sample surface by low temperature annealing, were used to enhance the corrosion resistance of 301, 17-4PH, 304 steels, 316, and mild steels [4,5,9,12].To sum it up, stability, compaction, chemical composition, thickness, and many more are the main factors influencing the corrosion resistance of stainless steels [18][19][20].
In the present study, the general overview of stainless steels including their properties and application is presented.The corrosion resistance of stainless steel and evaluation methods are evaluated.The surface treatment methods aimed at enhancing the overall corrosion and mechanical properties are then presented.

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][22][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][8][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 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

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.
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.

Surface treatments: enhancing the overall corrosion and mechanical properties
The overall corrosion and mechanical properties of stainless steels can be enhanced by subjecting them to different types of surface treatments including anodization, electrodeposition, dip-coating, micro-arc oxidation (MAO), and surface mechanical treatments.Anodization is a surface modification technique involving the passage of constant current and voltage through the cathode and anode resulting in the deposition of the natural oxide layers on the material surface [24] with an improved thickness and properties thereby turning the metal surface into an excellent finish which is anodic oxide in nature, corrosion-resistant, durable, and wear/abrasion-resistant.
As a form of surface treatments (anticorrosion processing methods), electrodeposition is a conventional phenomenon, and the combination of reduction and oxidation reactions whereby dissolved metals and alloys cations are cathodically reduced by the passage of electric current in electrolytes leading to the formation of a thin layer coating on electrodes [25].By this, thin layers of functional materials including alloys and metals can be electrolytically deposited on the surface of Mg alloys (acting as the cathode in the electrolytic cell) to improve the corrosion properties and the overall mechanical behaviour.
The dip-coating process can be highlighted in five ways depending on the immersion time and speed as well as the withdrawal speed, (a) dipping the substrate into the desired solution, (b) removal of the dipped substrate from the solution, (c) deposition of the film on the substrate after removal, (d) removal of excess liquid from the material surface, and (e) dispersal of the solvent from the liquid film.Meanwhile, micro-arc oxidation is the electrochemical oxidation process through which hard, dense, and protective ceramic oxide coatings are formed on metal surfaces for corrosion protection under the influence of various processing conditions and parameters [26][27][28].

Conclusion
Despite their good corrosion resistance, stainless steels still experience failure, especially when exposed to an aggressive environment for a prolonged period of time.This can be corrected by adopting the right surface treatment methods including alloying, heat treatment, coating/electrodeposition, and surface mechanical treatment.The corrosion resistance of stainless steels often depends on the stability, compaction, chemical composition, thickness of the passive film generated on the material surface.