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CORROSION Corrosion is technically, oxidation of metals, initiated by the reaction of Oxygen with the Metal in presence of moisture. It is also known as electrochemical degradation of metals and alloys due to reaction with their environment. It is accelerated by the presence of acids or bases (Alkalis) Oxides are formed in case of all the metals, but they are most distinctly visible in case of Iron and Steel, in form of flaking and pitting. The reason being that, the Oxide in Iron does not adhere to the surface of Iron and Steel, whereas in case of Aluminum and Zinc, they form a tough oxide coating, which strongly bonds to the surface of the metal, thus, preventing surface from exposures to Oxygen and Corrosion. However, up till now, the most familiar example of corrosion is the rusting of Iron, a complexed chemical reaction in which, Iron combines with Oxygen and Water to form hydrated Iron oxide. This hydrated forms of Iron oxide is porous & brittle and this we call rust in the common language. PREVENTION OF CORROSIONNormally, there are three methods, by which, Corrosion can be prevented.
Polymeric coatings are the most economical, but few of the coatings are costlier like Epoxy and PU coatings. They protect Iron till any crack appears in the coatings. Besides Iron, other non-ferrous metals like, Aluminum and Copper also Corrode. But, there is a general feeling that, Aluminum does not corrode. In fact, Aluminum corrodes faster than Iron and gives a thin, continuous transparent layer of Oxide on the surface of the metal and protects it from further corrosion. Such thin layer formed is visible in many places in form of spots in aluminum doors and windows. Similar is in case of Lead, Zinc and copper. However, Copper is less inactive metal and corrodes very slowly by producing a green coloured basic Carbonate of Copper. Such corrosion also appears on Copper Alloys, like Brass and Bronze. Among the Metals, only. Silver reacts with Air, Water and Hydrogen sulfide. But, the degree of corrosion is very very low and is visible in form of Black discolouration, called tarnishing, produced by formation of silver sulfide. Since, we are talking of Corrosion, we should not forget that, other than metals, Glass also Corroded by strong Alkaline solutions and even concrete is corroded by sulfide bearing waters. In a nutshell, we can say that, everything brought from nature, tries to go back to the nature in a natural process and we can protect it only through man-made methods, like Hybrid Coating systems, developed by SREECHEM RESINS LIMITED. FACT YOU SHOULD KNOWA. CORROSION IS LIKE CANCER FOR METALS, SPECIALLY FOR IRON The formation of rust ( corrosion ) can occur even at far away from pitting or erosion spot of Iron. Corrosion is like, cancer to the metals. Initial oxidation of iron can be conducted through the metal and the iron ions can diffuse through water layer to another point on the metal surface where Oxygen is available. Technically speaking, this process results in an electrochemical cell in which iron serves as the anode, oxygen gas as the cathode, and the aqueous solution of ions serving as a "salt bridge". The presence of salt greatly enhances the rusting of metals and this is what is more commonly known as Costal area of Sea weather corrosion. B. CORROSION PROTECTION THROUGH COATINGS WHAT ARE COATINGS ? Coating is generally a film of thin layer applied to the base material, called substrate. Various types of coatings commonly used by Industries are metals, alloys, polymer solution and suspensions on various substrates like, metals, plastics, wood, paper leather etc. Coatings may be applied by various methods and the liquid polymers are mostly applied by spraying, calendaring and roller coating. SRL has developed LIQUID PLASTIC LAMINATION coating, which may be applied by any of the above method and has grip on almost all these types of substrates including plastic, which is most difficult to coat. The best way to counter the attack of corrosion is coating and the coating to be affective should have the following processes involved. (1) SURFACE PREPARATION The base surface must be cleaned to remove all dirt, corrosion, contamination and surface should be roughened with proper emery paper, grinding etc. before coating. Bonding strength of any coating depends on proper preparation of the surface to be protected for long term performance of the product. The substrate should be free of oil, grease, salt/chloride and moisture contamination. Specifications call for a white metal finish with 25-62.5 microns anchor profile. Cleanliness is a most important to protect the surface and then only the coatings will perform to its best. (2) SELECTION OF PRIMERS Primer should provide corrosion inhibition and additional barrier between metal & environment. If the printer is good, then it can even make up for the poor quality of paints. Notes : Our TOPCOAT-RED and TOPCOAT-YELLOW are excellent corrosion resistance primers/ undercoates /finishes. (3) SELECTION OF TOPCOAT Multiple coats of proper paints with sufficient thickness built up should be given. The coating should have resistance to temperature, chemical and moisture resistance. Our WHITE KNIGHT is resistance to chemical and moisture and its abrasion resistance is also excellent. (4) APPLICATION INVOLVING CATHODIC PROTECTION The coatings must give resistance to cathodic disbonding and for this, we recommend TOPCOAT-4045 over all the coatings. TOPCOAT is also UV resistant. C. WHY ALL PAINTS DO NOT HAVE GRIP ON GALVANIZED STEELS The most difficult job while painting galvanized zinc is due to poor initial adhesion of coatings and poor wettability to the paint of the zinc substrates. This makes the coatings brittle due to weathering stresses such as temperature change and wetting/drying cycles. In a nutshell, zinc ions together with organic acids in the paint forms a zinc soap, which in turn break down the cured paints. D. SEAWATER CORROSION Seawater corrosion is one of the very important phenomena, which results in to losses due to corrosion running in hundreds of crores of rupees. The major constituents of sea water, which cause corrosions are Chlorides, Sulphates and Sodium. The other factors that contribute to the cause of corrosion are given below. (1) DISSOLVED OXYGEN The normal saturation for dissolved oxygen in seawater is about 6-8 ppm at 25-30 Deg. C. The pH of seawater ranges between 7.7 to 8.3 due to buffering effects of carbonate salts. In deep water, the pH is found in the range of 3-4, produced by bacterial action. (2) LOCALIZED VARIATIONS The temperature decreases with the depth of the seawater and may vary from 5 Degree C. at 1000 meters depth, here Dissolved Oxygen may come down to 1 ppm due to bacterial activity. (3) CHLORINATION EFFECTS Sodium Chloride presents in the water vapour near sea is the major cause of corrosion in metals and the degree of corrosion is more intense because of high humidity & temperature above 21 degree C. (4) TEMPERATURE EFFECTS Temperature plays an important role in seawater corrosion and every 10°C raise in temperature doubles the corrosion rate. (5) SALT PRECIPITATION, DEPOSITS, BIO-FOULING AND SEDIMENTS EFFECTS |
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