In this article we article we will discuss about the metallic and non-metallic types of coating metals.
Cost is a very important factor while choosing an engineering material. In majority of applications of metals, the importance of the corrosion factor does not warrant the use of expensive corrosion-resistant alloys. Thus, when an easily corrodible alloy like steel is used, some method is used to prolong its life, or improve its appearance by protecting it from environments.
By far the most important protective means is some kind of thin coating. The chief function of such a coating is (except sacrificial coatings such as zinc) to provide an effective barrier between the metal and its environment. The coat itself should have good resistance to corrosion, should be adherent and cover the metal completely.
The coatings are broadly classified as:
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1. Metallic Coatings
2. Non-metallic coatings
Type # 1. Metallic Coatings:
Protecting the metals by metal-coating is an old and very widely used practice as these are more durable, more decorative and provide better protection against corrosion. Two factors are involved in the protection of underlying metal by a metallic coating. One is that it mechanically isolates the metal from corrosive environment.
Second is the galvanic relation of the coated metal and the base metal, and based on this factor, metallic coatings are classified as:
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In such a case, the coating metal is cathodic with respect to the base metal. Coatings of tin, nickel and chromium are cathodic coatings as these are higher than steel in the galvanic series for most service conditions. These coatings provide a physical barrier between the metal and the environment.
When the coating is dense, non-porous, continuous and without discontinuities, corrosive medium does not come in contact with base metal, and the base metal does not corrode. Coating being cathodic does not corrode. Corrosion of the underlying metal occurs at small discontinuities, cracks etc., as it is anodic.
As the anodic area is small, pitting corrosion occurs. This disadvantage is outweighed for some applications (where corrosion is not a severe problem), for the attractive, shinning, lustrous and reflecting appearance such as of nickel and chromium plating on steel, and by the excellent corrosion and fabrication qualities of tin plate for use in food containers. Lots of automobile parts have chromium plating. Silver and gold-plating is done on brasses and bronzes for ornamental purposes.
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The coating metal is anodic to the underlying metal. The anodic coatings provide galvanic protection to the base metal, in addition to acting as a physical barrier between the base metal and the environment. When the coating is dense, non-porous, continuous and adhering, it separates as physical barrier, the environment from the base metal.
At flaws, cracks, discontinuities and pin-holes, the base metal is protected by the galvanic action of the coating, offering a sacrificial type of cathodic protection. Zinc and cadmium, even magnesium are examples of metals that provide galvanic protection to the steels.
The superior properties of cadmium coatings under conditions such as attack by salt spray, or alkalies may justify their high cost. Life of the coatings depends on the nature of environment, as it governs the nature of the corrosion product. The life of the coating primarily depends on the thickness of the coating.
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Galvanised corrugated steel sheets are commonly used for building roofs, shades, cans, containers. Nuts and bolts are also galvanised. Anode coatings, because of their continuous corrosion have poor appearances because of poor lustre and reflectivity. To avoid this poor appearance particularly of hub-caps and wheel-trims of automobiles, double-coating, first anodic coating and then cathodic coating over the anodic coating is given.
Anodic coating takes care of corrosion, while cathodic coating maintains good lustre and reflectivity. In such cases, steel is first flash-coated with copper and buffing is then done. Next a pure nickel-coat followed by bright-nickel-coat containing sulphur is applied. Finally, the steel is given a thin chromium coating which is exposed to the environment.
Bright nickel coat containing sulphur invariably produces 108 cracks/mt2 in chromium coat, but it is anodic to both pure nickel and pure chromium coats, and gets corroded sacrificially. Presence of large number and large area of cracks does not cause pitting corrosion.
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Some metals like tin or cadmium which form a galvanic couple with iron may, reverse their polarity when the corrosive environment changes. This is a clearly established fact using Nernst equation, so that anodic metal as per electrochemical series becomes cathodic, and vice versa.
Tin is cathodic to iron. Inside the sealed tin-plate-containers containing the food stuff, air had been removed before sealing. Certain constituents of food stuff combine chemically with Sn2+-ions to form soluble tin-tin complexes, which drastically lower the activity of Sn2+-ions to make tin more active than iron, but otherwise tin is more noble than iron.
The corrosion products of tin are non-toxic. Once the container is opened, and if base metal iron is exposed (scratches might also be produced during opening the can), it corrodes. The corrosion products of iron are toxic. Before the food stuff is poisoned, it is advisable to transfer the food to some glass or stainless steel container (Such a suggestion is often written on pickle-cans).
Methods of Applying Metallic Coatings:
Some common methods of applying metal coatings are:
i. Electroplating/EIectrodeposition:
The part is made cathode by dipping it in a solution of metal to be plated by passing direct current between the part and another electrode. The coats could be thicker or thin; dull or bright; soft or hard; ductile or brittle, depending on temperature, current density, time and composition of bath. Hard-plating resists erosion corrosion better. (Zn, Ni, Sn, Cr, Cd, Au, Ag, Pt are often electroplated). It gives thin and uniform coat and has become popular. Cadmium and decorative Ni and Cr platings are almost always produced by electroplating.
ii. Metallizing or Flame Spraying:
The metal wire or its powder is fed through a melting-flame (oxy-acetylene) to blow finely divided liquid particles on to the surface. Metal particles solidify in air. Zinc, tin or lead are flame-sprayed. Coatings are porous and non-protective in severe wet corrosive conditions. Tank cars, vessels, bridges, ship-hulls, fabricated steel products, refrigerators are flame-sprayed. This method produces thick coats and any metal can be spray-coated.
iii. Hot-Dipping:
Clean steel plates are immersed in a molten metal bath of low melting-point metals, mainly zinc, tin, lead and aluminium. Galvanised sheet is produced by hot-dipping. Thin coats are difficult to produce.
vi. Vapour Deposition:
In high vacuum chamber, the coating metal is vapourised by heating electrically, and the vapour deposits on the parts to be coated. Steel is vapour-coated by aluminium. As it is an expensive method, critical part such as high-strength parts for missiles and rockets are vapour-coated.
v. Diffusion-Coating or Surface Alloying:
The parts to be coated are packed in powdered solid metal, (or exposed to gaseous atmosphere containing the metal) to be coated. High temperature treatment causes the diffusion and may form an alloy. Sherardizing (zinc), chromizing (chromium), Calorizing or alonizing (aluminium), ferrosilicon (ihrigizing) are common examples.
These coatings are relatively thick and are usually applied to cast, or machined parts such as Holts, valve guides, and turbine buckets. Carbon steel, low alloy steels and stainless steels are alonized. Here, a protective layer of AI2O3 forms on iron-aluminium alloyed surface.
vi. Cladding:
It is the joining of two different metals, or alloys by forming a strong alloy-bond between plates of the two materials. The bond may be formed in many ways, for example by casting one alloy against a solid plate of the other. This thick, composite ingot is then hot worked to reduce it to a useful form such as clad sheet. A 1/8 inch nickel and a 1 inch steel sheet are hot rolled together to produce clad sheet.
The cladding is usually thin than the other material. Production of a corrosion-resistant surface is the principal objective of cladding. For example, Alclad is to provide corrosion barrier to high-strength aluminium alloys with commercially pure aluminium skin on both sides by rolling because the alloy is susceptible to stress corrosion.
Ni, Al, Cu, Ti, stainless steels are used as cladding for steel. It is a very cheap method for protecting against corrosion. Cladding is also used for such purposes as making bimetal-strips for temperature- control devices.
Type # 2. Non-Metallic Coatings:
These coatings can be further classified as:
(a) Inorganic-coatings,
(b) Organic-coatings.
The main inorganic coatings on metals are:
(i) Vitreous-Coatings, or Glassed-Steel:
Steel having glass-lining protects the steel by acting as an efficient barrier. The smooth surface of glass-lining is very easy to clean. Glass-lined steel is used in process-industries in drugs, in wine-industry, brewery, in food-plants, hot water tanks, enamelled cooking utensils, etc.
(ii) Portland Cement or concrete coatings having thickness of few centimeters Find applications in encasing structural steels, concrete-lined-pipes, concrete vessels, and also for fire-proofing.
(iii) Chemical conversion Coating or chemical coating as its formation involves chemical reactions with the metal on which it is formed to produce an adherent and protective corrosion product. Anodic coating on aluminium consists of a layer of aluminium oxide about 10µm thick.
This is produced by anodizing in which the metal is made the anode in an acid (dil. H2SO4 or chromic acid) electrolyte to build up the oxide layer. As these coatings are about 1000 times thicker than the natural oxide film, the surface layer is porous and provides good adherence for paints, organic dyes. The anodized surface can be sealed by exposing to boiling water. Organic-dyes provide pleasing-appearance to find applications for many architectural purposes.
Phosphate coatings (parkerizing, bonderizing) are produced on steels by dipping them in suitable phosphate solutions. Automobile bodies are given phosphalizing. This is used principally as an adherent base for the paint, or a protective oil. Chromate coatings are used widely on magnesium alloys, and to a smaller extent on zinc. These thin coatings are commonly obtained by dipping the metal in a chromate solution.
The protective value of these coatings is a result of mechanical barrier plus the action of the chromate-ions in decreasing the anodic reaction in electrochemical corrosion. The parts are then normally painted. Steels can be given protective oxide-coating and obtain temper colours. These coatings must be treated with petroleum product to avoid rusting.
These are inert-organic thin-barriers applied on the surfaces of metals for corrosion-protection and decoration. Paint is the most widely used protection against corrosion. Paints, varnishes, enamels (dispersion of pigment in a varnish or resin), lacquers (a solvent solution of resin and plasticizers with or without pigment uniformly dispersed) are the organic coatings which protect more metal on tonnage-basis than any other method of preventing corrosion.
Apart from the proper application, three factors must be considered while using organic coat, (1) Surface preparation, (2) Selection of primer (primary coat), (3) Selection of organic coat.
Primers must contain rust-inhibiting pigment such as zinc chromate, zinc dust or red lead (Pb3O4). Red-lead tries to maintain an alkaline, corrosion-preventing environment at the metal surface, and it also acts to make metal passive. Proper choice of nature of coating is important.
Normally, one coat of the paint cannot cover pinholes or defects, thus, multiple-coats are needed to cover them. Thickness is important also because paint deteriorates, or weathers with time. Asphalts and bituminous paints are often used for pipelines and underground tanks. Sometimes a cloth wrapping is employed with the coating for reinforcement. Also used are alkyds, glyptols, red lead, phenolics, lithopones, titanium dioxide paints. Vinyl and epoxy paints find wide applications for corrosion applications.