Question

1) How does the coating in metals prevent a) corrosion b) stop flamability c) wear d) UV - radiation

Answer 1

Coating of metals is a necessary aspect of protecting metals for long time endurance.For metal parts exhibiting dynamic motion are often well maintained by coating the metals.Metal coating is nothing but providing a layer on the surface of metal for not getting damaged due to environmental factors and wear and tear.These metal coatings may vary on different factors like type of metal,damage to be protected from,cost,level of thickness,aesthetic look etc.

Coating of metals to prevent Corrosion: Corrosion is a natural phenomenon caused due to environmental factors like mositure,dust,metal contact with fluid(air or water),formation of nonmetallic scale or film(also known as oxidation) usually oxides,hydroxides and sulphides.Corrosion is a negative effect.It is electrochemical and usually begins at surface.The most common form of corrosion is rust (iron oxide layer) formed on the ferrous metals.The various forms of corrosion are Uniform attack,Galvanic corrosion,crevice corrosion,Pitting,Intergranular corrosion,erosion corrosion,stress corrosion etc.

Corrosion Prevention:

a). Inhibitors: These are the substances usually added in relatively low concentrations to the environment which helps in decreasing corrosiveness.These inhibitors depends on type of metal,alloy,corrosion type.There are several mechanisms that may account for the effectiveness of inhibitors. Some react with and virtually eliminate a chemically active species in the solution.

b). Cathodic Protection: One of the most effective means of corrosion prevention is cathodic protection. Cathodic protection simply involves supplying, from an external source, electrons
to the metal to be protected, making it a cathode; the reaction above is thus forced in the reverse (or reduction) direction. One cathodic protection technique employs a galvanic couple which is that the metal to be protected is electrically connected to another metal that is more reactive in the particular environment.

c). Coatings: Coatings for corrosion are usually paint coatings tends to operate both as a functional repair (By inhibitors to stop corrosion) as well as appearance repair (Void filling for scratches by altering ruptured spots). Coating is a mixture of ingredients such as pigments, fillers, modifiers within a host matrix (polymer binder) which is applied to a substrate as a liquid (aqueous or non-aqueous) but which subsequently transforms into a solid. Most common pigmented coatings are paints,which require durability and serviceabilityand Printing Inks,inks require brightness of colour and suitability for their method of application. Mechanism behind the coatings is simple that this mixture of coating forms a protective layer on the metal surface such that there is no contact with the surrounding environment inorder to protect from exposure to environment.

Coating of metals to prevent flammability: Flamability is the capability of a metal to catch fire,burn, ignite,combust due to the sensitivity of corrosive metals to environments that are rich in oxygen.Metals involving flammability usually include sodium, potassium, uranium, lithium, plutonium and calcium, magnesium and titanium. The metals that invles fire are usually occurs in fire industry. These are also known as fire metals or Class D fire metals

The best and only recommended way to extinguish fire from fire metals is to use a dry powder fire extinguisher as coating mixed with pigments and inhibitors. This works by extinguishing the fire, and therefore the oxygen within it, and also absorbing the heat contained within the fire, eventually leading to its extinction. Powder fire extinguishers will also stop the burning metal or powder from spreading, reducing the spread and resulting devastation of the fire. Metal fires are often one of the most hazardous and extremely dangerous to use water as metals such as sodium will react further with the water, acting as an stimulate to combustion.

Coating of metals to prevent Wear :  Wear of a metal is defined as the removal or deformation or detoriation of metal surface with contact with other metal or surface or environment. Wear mechanism usually involve abrasion (The hard particles penetrate into the softer surface under the normal contact pressure. When a tangential motion is imposed, the materials in the softer surface is removed.This is most common in manufacturing process of machining), adhesion (When two contact surfaces undergo relative movement, tearing must take place at the weaker junction), fatigue and oxidation or other tribo-chemical.

Resistance to wear can be improved by cold spray particle decompostion.In which  depositing materials in the solid state by spraying on to the metal surface to be protected. This method depends on various factors such as  impact velocity, initial particle temperature, friction coefficient, and materials combination. By varying these parameters we obtain suitable sprays for metal protection. The most commonly used particles are copper and Aluminium are usually considered. Other types of methods involving by utilising nanocomposite for better and efficient way of protection to the metals.

Coating of metals to prevent UV Radiation : Sol–gel derived films exhibit a higher potential for prevention of UV radiation.It is environmentally unfriendly. Inorganic sol–gel derived films offer good adhesion between metals and organic paint. However, they cannot provide enough corrosion protection due to UV radiation.A special comibination of composites are efficient to prevent from radiation.For further efficient coatings use of nanocomposites are desirable.By Incorporating nanoparticles in the hybrid sol–gel systems there is an increase in the corrosion protection properties due to lower porosity and lower cracking potential along with that of the mechanical properties.