post‐weld treatment, etc.
1.4.6 Stress‐Corrosion Cracking (SCC)
Such a cracking occurs by the simultaneous action of a corrodent and sustained tensile stress. This bars the corrosion‐less sections, intercrystalline or trans‐crystalline corrosion, which might destroy an alloy without any stress. It is accompanied with hydrogen embrittlement. It might be a conjoint action of a susceptible material, a specific chemical species, and tensile stress. Sedriks and Turnbull reviewed the standard SCC testing [19–20]. Time‐consuming techniques, bulky specimens, and expensiveness limit the usage of SCC monitoring techniques. Stress corrosion cracking might be prevented by avoiding chemical species that causes it, controlling hardness and stress, using un‐crackable materials specific to environment and temperature/potential control of operation.
1.4.7 Filiform Corrosion
On steel, aluminum, aircraft structures in humidity, flanges, beverage cans, gaskets, and weld zones, this type of corrosion can be detected. Irregular hairlines, sometimes corrosion products filaments present below coatings of paint, rubber, lacquer, tin, silver, enamel, and paper, develop. Material is not lost significantly, but the surface deteriorates. Copper, stainless steel, and titanium alloys are unsusceptible to this attack.
1.4.8 Erosion Corrosion
Rapidly flowing electrolyte and turbulence cause erosion of the metal. The main culprits for turbulence are the pits within a pipeline. Turbulence finally causes a pipeline to have leakage. The velocity of the flowing electrolyte and the physical action of it moving against the surface causes metallic loss at an accelerated rate. Erosion is common occurrence in constriction areas like pump impellers and inlet ends. Erosion can be tackled by less turbulent fluid movement, low velocity of flow, using corrosion resistant pipeline materials, inhibitors, etc.
1.4.9 Fretting Corrosion
A slight oscillatory slip between two surfaces in contact causes fretting corrosion. Bolted/riveted parts are made such that they do not slip or oscillate, which fails in the presence of fluctuation of pressure and vibration. Fretting can be prevented by regular inspection and maintenance of the lubrication.
1.4.10 Exfoliation
At the elongated grain boundaries, the corrosion products present cause the metal to be forced away from the material and form layer‐like look, and this is called exfoliation. Also known as lamellar, layered, and stratified corrosion, it proceeds along selected subsurfaces. If the grain boundary attack is severe, it is visible; otherwise a microscope conducts the microstructure examination. Alloys of aluminum are most susceptible to exfoliation. This can be controlled using coatings, heat treatment to control precipitate distribution, and exfoliation‐resistant aluminum alloy.
1.4.11 Dealloying
Dealloying is selective corrosion of solid solution of alloy also known as leaching/selective attack/parting. Dealloying can be manifested in various categories like decobaltification (selective leaching of cobalt from cobalt‐base alloys), decarburization (selective loss of carbon from the surface layer), dezincification (selective leaching of zinc from zinc‐containing alloys), denickelification (selective leaching of nickel from nickel‐containing alloys), and graphitic corrosion (gray cast iron in which the metallic constituents are selectively leached). Dealloying might be prevented by selecting more resistant alloys, controlling the selective leaching, sacrificial anode/cathodic protection.
1.4.12 Corrosion Fatigue
Corrosion and cyclic stress when occur simultaneously result in cracks. This is corrosion fatigue. Rapidly fluctuating stress below the tensile strength usually are causative agents. The metallic fatigue strength decreases in corrosive electrolyte. It can be prevented by using high‐performance alloys resistant to corrosion fatigue and by using coatings and inhibitors delaying the crack initiation.
1.5 Common Methods of Corrosion Control
Corrosion control is applying the principles of engineering to limit corrosion economically. Each preventive measure bears its own complexities and specificity. Basically, the idea is to detect the mechanism and causative agents of the degradation and reduce them or completely prevent them from occurring. Let us have a look on some of them as given below.
1.5.1 Materials Selection and Design
There is no all‐noble and completely corrosion‐resistant metal, but a careful selection might increase the longevity of the metal component. Factors influencing the materials selection are resistance to degradation, test data and design availability, cost, mechanical properties, availability, compatibility with other components, maintainability, life expectancy, appearance, and reliability. Availability, inexpensiveness, and easy fabrication make carbon steel a favorable material for selection [21]. In the petrochemical plants, highly corrosive catalysts and solvents are usually encountered, so stainless steel is best option [22]. Duplex stainless steels are used in pressure vessels, storage tanks, and heat exchangers owing to their good mechanical properties, high resistance to chloride stress corrosion cracking, good erosion and wear resistance, and low thermal expansion [23]. For seawater service, duplex stainless steels of higher molybdenum content (e.g. Zeron 100) have been developed [24]. Appropriate system design is crucial for efficient corrosion control. Numerous factors like materials selection, geometry for drainage, process and construction parameters, avoiding or sealing of crevices, avoidance or electrical separation of dissimilar metals, operating lifetime, and maintenance and inspection requirements are involved.
1.5.2 Coatings
Coatings are generally good option to insulate the metals from exterior aggressive environments. They extend a lengthy protection in wider spectrum of corrosive conditions, atmospheric to aqueous electrolyte solution. Although they provide no structural strength, yet they protect the strength and integrity of a structure. Their function is that of a physical barrier preventing electrolytic attack on metal. Organic coatings like paints, resins, lacquers, and varnishes are the most popular protective coatings. Metallic coating (noble or cathodic and sacrificial or anodic) is also used for corrosion control.
1.5.3 Cathodic Protection (CP)
A metal is completely converted to a cathode to protect it against corrosion. CP is implemented by driving the potential to a negative region/stabilized metal region. Either an external power supply changes the amount of charge on the metal surface or a more reactive metal is converted to a sacrificial anode. The principle involved in CP is to potentially let the metallic article or structure attain corrosion immunity. A stable and unreactive metal is impossible to corrode. This method might be expensive as electricity is consumed, and the extra metals are involved. Cathodic protection can be attained by coupling a given structure (like Fe) with a reactive metal like zinc or magnesium or by impressing a direct current between an inert anode and the metal to be immunized.
1.5.4 Anodic Protection
Based on phenomenon of passivity, anodic protection can control the corrosion in an electrochemical cell. Metal is kept in a passive state; surface is connected as an anode to an inert cathode in the corrosion cell. Anodic protection is used to protect metals that exhibit passivation in environments; when the current density in a corroding structure is much higher than the current density of
