CORROSION PROOFING TREATMENTS

Post 367 –  by Gautam Shah 

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Corrosion is degradation of a material due to an electrochemical oxidation process with the environment. The process is more common with metals, but can also cause disintegration of a polymer due to sunlight exposure, or in case of ceramics and stones as seen as efflorescence.

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Stable metals like copper and precious metals like Gold, Silver, Platinum, are less prone to disintegration. Some metals form their own protective cover on the surfaces to prevent, or slow down the corrosion.

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There are several ways to stop or retard the corrosion. One, is to isolate the metal object from environment and other metal objects, and Two, is to constitutionally alter the metal to reduce its vulnerability to rusting. In both the cases the availability of electrons for displacement is reduced. Metals are protected by formation of a barrier. The barrier could be an applique coating like paint or an integrated one like plating or galvanizing. Integrated barriers are also formed by metalizing, surface alloying and ceramic formation. The barrier could be generated by the material itself such as the Patina on bronze. The Barrier could remain on the surface forever, or covered by other coating systems.

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Barrier protection: One of the oldest methods of protecting the metal surface is plating with a metal of stable nature such as Tin, Silver or Gold. Such plating processes were expensive and used for small objects. Post middle ages, metal household objects of iron were covered with a layer of coloured ceramic-glass called enamel. Enamel is inert, and adheres tightly to the steel, protecting it from corrosion while providing attractive appearance. Later chromium plating via electrolytic compounds began to be used as a protective-barrier coating on steel. To get better adherence, the steel is first electroplated with layers of copper or nickel. Today many other types of barrier protections of organic nature such as paints and polymers are used.

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The oxide layer that forms on metals when they are exposed to air also constitutes a protective barrier. Bronze, Stainless steel and aluminum form the most stable and protective of such films. The thickness of the oxide film on aluminum is often increased by making the part function as the anode in an electrolytic cell. This process, called anodizing, enhances the corrosion resistance and makes it easier to colour the surface. The films that form on copper and steel as a result of corrosion (commonly known as tarnish and rust) are somewhat thicker and show a characteristic colour that is often incorporated into the design of the part.

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Galvanic protection: Applique protective films (like paint) on steel are susceptible to being broken at scratches and sharp dents. This occurs in automobiles and other entities, as the applique films have no ability for self-healing. A protection application of zinc metal which has greater capacity to donate electrons then the steel, if forms a prime (first layer on steel) surface, then the objects can be protected from effects of corrosion. This is called galvanic protection. A layer of zinc can be placed on a steel surface by either by hot-dipping or electroplating. Galvanized steel is much more resistant to corrosion than un-galvanized steel. Where a galvanized coating is cut or scratched, the zinc flows in over the exposed area and provides continuous protection. Cadmium can also be deposited for galvanic protection of steel. Hot-dip aluminum-coated steel is used in the exhaust systems of automobiles. At low temperatures its action is galvanic, but at high temperatures the oxidation forms a barrier layer. Galvanic protection is also provided by imposing an electrical potential on steel structures.

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Other coating techniques

Among other methods for applying a metal layer to metal is thermal spray coating, a generic term for processes in which a metal wire is melted by a plasma arc or a flame, atomized, and sprayed onto a surface in an inert gas. A process similar to this is vacuum coating wherein metal is evaporated and deposited as coating in high vacuum. High-temperature bearing super alloy components, such turbines are given oxidation protection by annealing them in a chamber containing volatile aluminum chloride.

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Metal coatings are surface treatments that form the first coatings in a multi coat set. Conversion coatings are of basic two types: phosphating and chromating. These are  temporary, but provide an adequate substrate for subsequent applications.

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Phosphate coatings are used for ferrous, zinc metals, aluminium, tin and cadmium metal surfaces. It is a thin, porous, insulating and adherent application that allows keying of the applied paint film. The electrical inertness of the coating arrests corrosion spread to local spots. Phosphate coatings are applied by immersion, brush application or spraying. Zinc phosphate coatings are smooth and fine-grained treatments, used for reducing the corrosion creep under the paint. Coatings containing manganese phosphate are less widely used as paint pre-treatment because they have a large coarse crystal structure although these heavy coatings are very useful as oil-carriers and have good wear resistance, which is advantageous for engineering components.

Steel structure showing residues of inner coatings

Chromating is formation of a chromium oxide film on the metal surface. It is used to increase corrosion resistance of metals like aluminium, magnesium, tin, zinc and cadmium. It is also used to enhance the tarnish resistance of copper and silver.

Diffusion Coatings are also known as cementation coatings as part of the applied material interacts and forms alloys with the substrate. Cementation coating process is very similar to carburising of iron to produce surface-hardened steel (iron heated with carbon particles for the diffusion to occur). Common processes falling in this category, are: aluminizing (calorising), chromising and Sherardising (zinc cementation coating), siliconising and borating. Processes like hot dip galvanizing, tinning, aluminizing and terneplating also form alloy, but technology is different from diffusion coating. Such alloyed coatings are used where high corrosion and abrasion resistance, in very active environments are needed.

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Metal cladding can cover a metal or other surfaces to form a barrier against corrosion. The thickness of a cladding metal could be few microns (metal leaf) to few millimeters (metal sheets or plates).

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Chromising is the term applied to the formation of a diffusion coating on iron or steel by chromium to produce a surface with enhanced oxidation, corrosion and wear resistance. Gas phase chromising is performed, when the articles heated in a powdered mixture of chromium, alumina or kaolin and an ammonium halide in a hydrogen atmosphere.

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METALS TREATMENTS for CORROSION RESISTANCE

METALS TREATMENTS for CORROSION RESISTANCE

Post 344 ⇒   by Gautam Shah 

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Corrosion is oxidation of metals in reaction with an oxidant such as oxygen. Rusting is formation of iron oxides through gradual destruction of the material by chemical reaction with the environment. Corrosion or degradations as a process of wearing away of the mass also occurs in ceramics (efflorescence of bricks and terracotta), stones, glass and polymers. Corrosion can be local or more widespread affecting the entire surface. Corrosion is a diffusion process and occurs on the exposed surfaces. One of the main methods of preventing such destruction is to isolate the surface from its environment, including the atmosphere and contact with other passive materials.

Glass disease or corrosion

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Bishnupur W Bengal, India Corroded Brick work

Both, Ferrous and Non ferrous metals, have a tendency to corrode that is the atom lose electrons and become ions. This is called an anodic reaction, and for the corrosion process to proceed there must be a corresponding cathodic perceptor that adsorbs the electrons.

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Reasons for corrosion could be:

External (environmental)

Internal (constitutional)

Structural (joint system)

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There are several ways prevent, reduce or terminate the corrosion.

Metals get corroded by the environment. In most cases, to create an ideal environment for surrounding the metal will require a very large and extensive system, and possibly large quantity of energy to maintain it.

Corrosion occurs due to the discharge of electrons. Metal electrons migrate due to electrical potential that builds up amongst different metals and between the metal and an electric conductive substance, of an assembly. If cathodic polarity is imposed by supplying an electric DC current to reverse the direction or by attaching sacrificial anodes such as aluminium or magnesium, whose atomic relationship with steel is such that a current is generated without external assistance (the anodes are earthed and have electrolytic continuity with the structure).

Next course is to isolate the metal from corrosion conducive aspects of environment (such as moisture, water, acidic fumes, chlorides etc.). Coatings and plating provide the required isolation. The most appropriate way would be to place the isolating element as close to the metal surface, as possible, and if feasible even integrate it into the metal itself. Electroplating, metalizing etc., are some of the techniques that are on the verge of integration.

Copper roof on the Minneapolis City Hall, coated with patina

Corrosion can also be controlled by constitutionally upgrading the metal, by one of the following techniques.

Mechanical -stress induction and relief techniques at Normal, Hot and Cold temperatures,

  Chemical -additive and subtractive treatments

  Alloying – solid solution, precipitation, amalgamation.

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