Page 185 - CW E-Magazine (14-5-2024)

Special Report

Advances in protective coatings and conventional

metal coatings for corrosion protection

INTRODUCTION A coating of a corrosion-resistant DR. S.K. CHAKRAVORTY
onstruction metals are selected metal on a corrosion-prone substrate Consultant (Plant Engineering)
based on their mechanical pro- can be formed by various methods. The Email: chakravorty4410@gmail.com
Cperties and machineability at choice of coating material and selection
a low price, although, they should also of an application method are determined posed to increase the longevity of
be corrosion resistant. Very seldom can by the end use. metallic structures and enhance their
these properties be met in the same corrosion resistance. Among them,
material. This is where coatings come into It is essential to understand the metal coatings stand out as one of the
play. By applying an appropriate coat- advantages and disadvantages of each most effective and convenient protection
ing, the base metal with the good metal coating type to select the one best methods. There are numerous methods
mechanical properties can be utilized while for a particular application. Metallic for coating metallic surfaces, each with
the appropriate coating provides corro- coatings protect other metals by provid- its own set of limitations and benefi ts.
sion protection. The majority of coatings ing a combination of barrier and galvanic In the following sections we will take a
are applied on external surfaces to pro- protection. They offer a form of pro- detailed look at some of the most common
tect the metal from natural atmospheric tection unmatched in its sheer number types of metal coatings, and discuss their
corrosion, and atmospheric pollution. of applications and cost-effectiveness. suitability for various applications(2).
On occasion, it may also be necessary to
provide protection from accidental spills Understanding the corrosion process How metal coatings protect surfaces
and/or splashes. In some instances, coat- in metals and structures
ings are applied internally in vessels for Corrosion is a natural electrochemical The logic behind metal coatings is
corrosion resistance(1). There are three reaction that converts a refi ned metal to create an inert (non-reactive) barrier
different classes of coatings: organic; to a more chemically stable state. For around the metallic object being pro-
inorganic; and metallic. In this article corrosion to occur, three components tected to prevent it from reacting with
advances in protective coatings and must be present: an anode (in this air and moisture. Metallic coatings
various conventional metal coatings for case, the iron found in steel); a cathode offer two main types of protection: barrier
corrosion resistance are discussed. (oxygen); and an electrolyte solution protection and, in specifi c cases, galvanic
(such as the moisture found in air). protection.
Metallic coatings are applied to metal
substrates for several purposes – im- During the corrosion process, the Barrier protection
proved corrosion resistance, wear resis- iron in the steel undergoes an oxidation When a metallic coating such as zinc
tance, and appearance. Of primary con- reaction due to the presence of oxygen is applied to steel, it dries and hardens,
cern is corrosion resistance. By providing to form hydrated iron (III) oxide, com- forming an impervious barrier that pre-
a barrier between the substrate and the monly known as rust. Unlike mild cor- vents moisture intrusion. This removes
environment or by cathodically protect- rosion in aluminum, which is benefi cial one of the essential components needed for
ing the substrate, metallic coatings protect because the oxide provides a protective corrosion to occur. Without the electrolyte
the substrate from corrosion. Coatings layer that prevents further corrosion, rust (moisture/water), oxidation cannot occur
of chromium, copper, and nickel pro- is brittle and easily fl akes away, expos- and therefore rust cannot form(3).
vide increased wear resistance and good ing more of the metal to the atmosphere
corrosion resistance. However, these noble and resulting in further corrosion and Another important aspect of barrier
metals make the combination of the degradation. This continuous corrosion protection is called corrosion fi lm pro-
substrate (mostly steel or an aluminium eventually leads to the loss of material tection. As mentioned previously, some
alloy) with the protective layer sensitive thickness, decreased material strength, metals, such as aluminium, react with
to galvanically induced local corrosion. formation of perforation and reduced oxygen to form a protective oxide fi lm
Non-noble metallic layers such as zinc or service life. on its surface. This oxide fi lm is resilient
cadmium provide good cathodic protec- and fi rmly adheres to the surface of the
tion, but show poor wear resistance. Multiple solutions have been pro- aluminium to prevent moisture intru-

Chemical Weekly May 14, 2024 185

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