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7.3 Combating Corrosion
A corrosion inhibitor is a chemical compound when added to a fluid or gas, decreases the corrosion rate of the metal or alloy. The effectiveness (corrosion inhibition efficiency) of the corrosion inhibitor is a function of many factors like: fluid composition, quantity of water and flow regime. If the correct inhibitor and quantity is selected then is possible to achieve high efficiency up to 90-99%. Some of the mechanisms of its effect are formation of passivation layer which is a thin film on the surface of the material that stops access of the corrosive substance to the metal, inhibiting either the oxidation/reduction part of the redox corrosion system (cathodic/anodic inhibitors), or scavenging the dissolved oxygen. Some example of corrosion inhibitors are hexamine, phenylenediamine, sodium nitrite, cinnamaldehyde, condensation products of aldehydes and amines/imines, chromates, nitrites, phosphates, hydrazine, ascorbic acid and others. The suitability of any given chemical for a task in hand depends on many factors like the material of the system they have to act in, the nature of the substances they are added into and their operating temperature.
Some metals are more intrinsically resistant to the corrosion than others, either due to the fundamental nature of the electrochemical processes involved or due to the details of how the reaction products form. If a more susceptible material is used, many techniques can be applied during an item's manufacture and use to protect the construction materials from damage.
Painting, plating and enamel application are the most common anti-corrosion treatments. They work by providing a barrier of corrosion-resistant material between damaging environment and often cheaper, tougher and/or easier-to-process structural material. Aside from cosmetic and manufacturing issues, there are trade offs in the mechanical flexibility versus resistance to the abrasion and high temperature. Platings usually fail only in small sections and if plating is more noble than the substrate, a galvanic couple will cause any exposed area to corrode much more rapidly than the unplated surface would and thus for this reason, it is often wise to plate with a more active metal such as zinc or cadmium.
If environment is controlled especially in recirculating systems,then corrosion inhibitors can often be added to it. These form an electrically insulating and/or chemically impermeable coating on the exposed metal surfaces, to suppress the electrochemical reactions. Such methods obviously make system less sensitive to the scratches or defects in the coating, since extra inhibitors can be made available wherever the metal becomes exposed. Chemicals that inhibit corrosion include some of salts in the hard water, chromates, phosphates and a wide range of the specially-designed chemicals that resemble long-chain organic molecules with ionic end groups that is the surfactants. Other preventive measures involve the use of protective coatings and modification of environment. Some trace impurities(in very less amount) can significantly reduce rate of corrosion and can be added in low concentration to surrounding medium. Paint is the most common coating used to slow the rate of atmospheric corrosion and many other materials, such as plastics, ceramics, rubbers, and even electroplated metals, can be used as protective coatings. The corrosion resistance of a metal can be greatly increased by proper choice of the alloys, for example, aluminum added to brass will increase its corrosion resistance.
Various Protective Measures:
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