Introductory Chapter: Protection of Stainless Steels in Corrosive Media

1. Introduction

Corrosion has now become the part and parcel of metals and alloys in reactive environments. It is an interdisciplinary action that takes place in various disciplines. It includes chemistry, physics, mechanical engineering, electrical engineering, civil engineering, metallurgy, materials science, new energy devices, electronics, marine engineering, dentistry, bio-implants, aviation, military, and biology. Several mechanisms of action are involved depending on the harsh media and metal surface. Since corrosion is omnipresent wherever metals are present, it causes significant loss of materials and hampers the economy globally. Almost all the industries have to spend a lot of money for repairing, replacement, and supervision of corrosion worldwide. It has to be mitigated in order to run the industries smoothly and to reduce the shutdown and failures. There are numerous methods to mitigate corrosion, but still only 20–30% can be mitigated. So, there is always a motivation and urge to develop new materials or to use prevalent corrosion control techniques to mitigate corrosion. Use of inhibitors, material selection, suitable design, coatings, cathodic/anodic protection, and electrochemical protection are some of them used extensively [1, 2, 3, 4, 5].

Among these available techniques, application of inhibitors is easy to apply, and cheap and environmentally benign. The corrosion rate is reduced by the presence of corrosion inhibitors. Corrosion inhibitors incorporate themselves to corrosion product films in such way so as to increase the film’s capacity to prevent corrosion. According to NACE, an inhibitor is defined as “a substance which retards corrosion when added to an environment in small concentration” [NACE (1965)] and recent ISO definition of an inhibitor is “a chemical substance which decreases the corrosion rate when present in the corrosion system at a suitable concentration without significantly changing the concentration of any other corrosive agent” [ISO (1986)]. Inhibitors may also be defined on electrochemical basis as substances that reduce the rates of either or both of partial anodic oxidation and cathodic reduction reaction. The corrosive media can be the acidic solutions, alkaline solutions, or the neutral media. Apart from them, gases such as carbon dioxide (CO₂) and hydrogen sulfide (H₂S) present in the solutions can make it more corrosive for the metals. Several organic inhibitors are being used, but due to the regulations and policies, they cannot be applied in higher concentrations to mitigate corrosion. So, as a replacement several organic/inorganic inhibitors were developed by means of the green principles in order to get the less toxic inhibitors that can be used in higher concentrations. Several degradable bio-polymers, plant extracts, ionic liquids, bio-macromolecules, proteins, and drugs were used by different research teams all over the world [6, 7, 8, 9, 10].

Stainless steels are used in industries due to their good resistance toward corrosion and high mechanical strength. Several joints and parts are made up of stainless steels in order to reduce corrosion. Iron (Fe), chromium (Cr), and nickel (Ni) are the base elements that are present in the stainless steels. As soon as the stainless steel is exposed to a corrosive media, a thin layer of chromium oxide is formed on the surface of stainless steel due to passivation. The thickness of the film can vary from metal to alloy and from acidic to alkaline medium. The presence of this oxide layer on the metal surface keeps it intact from the reactive environment, and therefore, the steel remains protected from corrosion. The layer is inactive and will not react with any harsh environments being self-repairing in nature. The ability of stainless steel and other alloys to resist corrosion is increased through the process of passivation, which is a chemical treatment. Passivation can also be applied to other metals. There are various advantages of using passivated equipment and systems, including the following: Passivation removes surface contaminants. The process of passivation makes materials more resistant to corrosion. Any steel with 10.5% of chromium and 1.2% of carbon can be termed as stainless steel. The stainless steel can be classified into five types: a) austenitic, b) ferritic, c) martensitic, d) duplex, and e) precipitation hardening. Out of these five steels, the first two (austenitic and ferritic) account for almost 95% of the total steel applications. And, among these two austenitic steels account for 75% of the applications in the market. Stainless steels consist of chromium and widely due to its corrosion resistant and heat-resistant properties [11].

The stainless steel consists of various compositions with iron (Fe) as a base element. It is combined with chromium (Cr) and nickel (Ni) to produce 304 stainless steels. Due to the presence of harsh environments and dynamic conditions, these steels can still undergo corrosion. It is easily affected by the pH, temperature, flow velocity, pressure/load, heat treatment, welding, and stress. Little changes in design, mechanical strength, and resistance properties can help in mitigation of corrosion. If sulfur (S) or selenium (Se) is added, then 303 SS or 303 Se SS can be prepared. If Cr is increased and Ni is reduced, the duplex stainless steel can be prepared. The addition of copper (Cu), aluminum (Al), and titanium (Ti) can lead to the formation of precipitation hardening stainless steels. The addition of Mn and N in 304 stainless steels can form 201 and 202 type stainless steel. If Cr is lowered in 304 SS, then it can lead to the formation of martensitic 403, 410, and 420 type stainless steels. The addition of molybdenum (Mo) in 304 type SS can form 316 and 317 type stainless steels. So all these modifications and heat treatment can be done to obtain the desired stainless steels for the laboratory and industrial usage. All these can be prepared or purchased following the AISI or any other international standards [12].

The book depicts the different studies done by research groups on the stainless steels. It includes the working mechanisms, chemical reactions, and surface characterizations to produce a clear picture and help the readers to develop a good understanding of the experimental situation. This book provides an overview of problems encountered in stainless steels due to corrosion and their control. Stainless steels are almost an essential component in most of the industries, and the satisfactory working of these systems is affected by corrosion, scale formation, and fouling of equipment. Apart from that the usage of traditional and new corrosion monitoring techniques will help readers to gather all the required information at one place. Most of the compiled work is new and not reported elsewhere. It will serve as a good reference for those interested in corrosion studies of stainless steels in different corrosive solutions. The book is useful for professionals, researchers, graduate, and undergraduate students. Moreover, it would be of immense importance to the professionals undertaking specialized short-term courses in this field.