**2. Corrosion process and inhibition**

Corrosion is a continuous degradation process of a material. As shown in figure 1, the corrosion of a given material system can take place because of two external major components, namely the environment or the electrochemical system (eg: atmosphere, acid or corrosive media), and operating conditions shown by arrows (eg: stress or pressure, erosion and temperature etc.). The process of electrochemical corrosion occurs in multiple steps, where the ions are involved with a media for ionic motion, and at the same time the material involved should be conductive enough to participate in the electron transfer for a mutual charge transfer process due to the ionic motion. During the process of corrosion, the materials can undergo changes into a new form of the material which could be protective or reactive in further process. The driving force for the corrosion is usually the thermodynamic instability of a given material system in the superimposed surroundings and working conditions.

Corrosion of Metal – Oxide Systems 273

prone to corrosion attack because of defects and high energy sites unless they are protected via passivation. Although, chromia cannot be used alone because of brittleness, chromium enhances passivity when alloyed with other metals and alloying elements in stainless steel.

Fig. 2. Schematic for a thin passive layer of chromia (Cr2O3) along the grain boundaries as

layer on their surfaces alone (Jones, 1992), as shown in the schematic in Fig. 3.

As another example, Aluminum (Al) and Al – alloys could be discussed. However, the corrosion resistance of Al and its alloys can be attributed to the formation of passive oxide

Fig. 3. Schematic for a thin passive layer of alumina (Al2O3) on the surface of the Aluminum

There are a number of oxide systems as protective coatings, as well as dispersoids, demonstrating superior performance in terms of corrosion and other properties, which will be reviewed later in this chapter. However, in cyclic operating conditions with temperature fluctuations and wear conditions the oxide layers may not be suitable; as they can break down due to mismatch of the thermal coefficient of expansion (CTE) with underneath phases, or due to wear, or combination of both, and thereby lead to localized pitting, crevice corrosion, etc., of the underlying substrate. In addition, high temperatures can enhance the diffusion rates. To this end, protective coatings with oxide particle embedded systems are

well as on the surface of stainless steel for corrosion protection.

and/or Aluminum alloys for corrosion protection.

There are different types of corrosion that can take place on a material system and they could be uniform type or localized type. In uniform corrosion, as the name suggests, corrosion takes place all over the surface. On the other hand, localized corrosion can be several kinds, such as galvanic corrosion, pitting corrosion, selective attack, stray current corrosion, microbial corrosion, intergranular corrosion, crevice corrosion, thermo galvanic corrosion, corrosion due to fatigue, fretting corrosion, stress corrosion, hydrogen damage etc. (Jones, 1992). For more details on each process, the reader is suggested to refer to any review articles or books on corrosion science.

Fig. 1. Schematic for corrosion of a material system with the components involved in the corrosion process.

Different factors contribute to the corrosion under various situations during the service. For example, the components used in hot sections of gas turbine engines and hypersonic vehicles operate under extremely oxidizing, erosive and high temperature conditions, where a combination of high temperature mechanical strength along with excellent oxidation and corrosion resistance are required. In the applications related to marine and aircraft propulsion systems, quite corrosive and erosive environments exist around the components under different operational temperatures with cyclic nature. Therefore depending on the application, the surroundings and operational conditions vary; and usually high strength and high temperature protective coatings are used to meet the requirements of such harsh operating conditions.

There are different approaches adopted to reduce or slow down the corrosion of a material system depending on the type of application and corrosive environment. The simplest and preferred approach among all of the methods is through the application of protective or non-reactive phases over the material system in the form of coatings, which keeps the material from exposure to the surroundings. A classic example for an oxide layer assisted corrosion resistant alloy is stainless steel, in which the alloying element chromium (Cr) forms an impervious stable oxide layer (Cr2O3, also called chromia) along the grain boundaries and surface, as shown in the schematic in Fig. 2. Usually grain boundaries are

There are different types of corrosion that can take place on a material system and they could be uniform type or localized type. In uniform corrosion, as the name suggests, corrosion takes place all over the surface. On the other hand, localized corrosion can be several kinds, such as galvanic corrosion, pitting corrosion, selective attack, stray current corrosion, microbial corrosion, intergranular corrosion, crevice corrosion, thermo galvanic corrosion, corrosion due to fatigue, fretting corrosion, stress corrosion, hydrogen damage etc. (Jones, 1992). For more details on each process, the reader is suggested to refer to any

Fig. 1. Schematic for corrosion of a material system with the components involved in the

Different factors contribute to the corrosion under various situations during the service. For example, the components used in hot sections of gas turbine engines and hypersonic vehicles operate under extremely oxidizing, erosive and high temperature conditions, where a combination of high temperature mechanical strength along with excellent oxidation and corrosion resistance are required. In the applications related to marine and aircraft propulsion systems, quite corrosive and erosive environments exist around the components under different operational temperatures with cyclic nature. Therefore depending on the application, the surroundings and operational conditions vary; and usually high strength and high temperature protective coatings are used to meet the requirements of such harsh

There are different approaches adopted to reduce or slow down the corrosion of a material system depending on the type of application and corrosive environment. The simplest and preferred approach among all of the methods is through the application of protective or non-reactive phases over the material system in the form of coatings, which keeps the material from exposure to the surroundings. A classic example for an oxide layer assisted corrosion resistant alloy is stainless steel, in which the alloying element chromium (Cr) forms an impervious stable oxide layer (Cr2O3, also called chromia) along the grain boundaries and surface, as shown in the schematic in Fig. 2. Usually grain boundaries are

review articles or books on corrosion science.

corrosion process.

operating conditions.

prone to corrosion attack because of defects and high energy sites unless they are protected via passivation. Although, chromia cannot be used alone because of brittleness, chromium enhances passivity when alloyed with other metals and alloying elements in stainless steel.

Fig. 2. Schematic for a thin passive layer of chromia (Cr2O3) along the grain boundaries as well as on the surface of stainless steel for corrosion protection.

As another example, Aluminum (Al) and Al – alloys could be discussed. However, the corrosion resistance of Al and its alloys can be attributed to the formation of passive oxide layer on their surfaces alone (Jones, 1992), as shown in the schematic in Fig. 3.

Fig. 3. Schematic for a thin passive layer of alumina (Al2O3) on the surface of the Aluminum and/or Aluminum alloys for corrosion protection.

There are a number of oxide systems as protective coatings, as well as dispersoids, demonstrating superior performance in terms of corrosion and other properties, which will be reviewed later in this chapter. However, in cyclic operating conditions with temperature fluctuations and wear conditions the oxide layers may not be suitable; as they can break down due to mismatch of the thermal coefficient of expansion (CTE) with underneath phases, or due to wear, or combination of both, and thereby lead to localized pitting, crevice corrosion, etc., of the underlying substrate. In addition, high temperatures can enhance the diffusion rates. To this end, protective coatings with oxide particle embedded systems are

Corrosion of Metal – Oxide Systems 275

**Spray deposition**: Different number of processes have evolved in this category in which a stream of molten metal droplets are deposited on a substrate to build the matrix layer; and for composites, the oxide particles are co-sprayed to embed them in the matrix layers.

**Reactive formation**: In this approach, selective oxidation of certain phases in the bulk

As listed above there are several approaches available for processing metal – oxide systems, and their corrosion properties are going to be dependent on the processing technique employed too. For example, the processing defects like porosity, improper bonding between the matrix and the oxide dispersoids, and their interfacial properties can influence the corrosion behavior quite extensively. Wetting of the oxide particles becomes a critical factor in some of the processing approaches to deal with the particle - matrix bonding. Fig. 4 shows a schematic for interfacial bonding of the second phase particles with matrix along the grain boundaries and triple junctions. In addition, high temperatures in some of the processing techniques may cause an interfacial reaction between the metal matrix and the dispersed second phase particles, thereby the interfacial stability and its properties play an increasingly important role in the corrosion. It is also possible that the interfaces could become prone to corrosion attack by providing preferential sites. In spray deposition approach splat boundaries, porosity, and distribution of the oxide particles may play an important role in deciding the corrosion properties. Added to that, the microstructures of the composites could also vary from process to process. The effect of some of these parameters on the corrosion of different metal – oxide systems is discussed in brief in the

Fig. 4. Schematic for interfacial bonding of second phase particles at grain boundaries and

structures with exothermic reactions results in the in-situ formation of composites.

following sections.

triple junctions.

found to be more useful. Most of the high temperature coatings and oxide dispersion strengthened (ODS) alloys are embedded with highly stable oxide phases, which can provide mechanical stability as well as enhance the corrosion and oxidation resistance. In addition to the oxide dispersoids, ODS alloys employ alloying elements (eg: Cr, Al etc.) in such a way that the oxide layers are formed on the surfaces as well as at the grain boundaries at high temperatures during the operation, which then act as protective layers from the corrosion point of view. The oxide dispersoids in the ODS alloys can provide mechanical stability with improved creep resistance.

Here, we will touch base on the corrosion phenomenon of oxide layer and oxide particle assisted corrosion behavior of metallic materials at low and high temperature applications with a brief review, and a case study will be presented on the corrosion phenomenon of oxide particle embedded high temperature composite coatings developed by thermal spray technique.
