Corrosion prevention coating effectiveness

SACI [Crompton Knowles]. TMfora rust-, and corrosion-preventive product effective in coatings as thin as 0.5 mil. 

Safety No year goes by without some widely used chemical being declared suspect on toxicity grounds. The paint industry has responded rapidly to eliminate toxic chemicals from coatings or to show how they can be used safely in an industrial environment. Examples are the elimination of specific ether-alcohol solvents and the introduction of air-fed hoods for spraying isocyanates. Of particular interest in corrosion prevention is the current pressure to eliminate chromate pigments. Currently there are no equally effective alternatives and the emphasis has had to be on safe usage. The search for replacements continues. 

Sometimes the need to be environmentally acceptable may lead to new problems. For instance, ozone was suggested to replace biocides with no data available on the performance in the chlorination of water (60). Corrosion control techniques can have both favorable as well as ill effects and hence one has to exert balanced judgment before embarking on a corrosion prevention method. Organotin antifouling coatings on ships were effective, but they polluted the seawater and hence were banned from further use. The use of cadmium as a sacrificial anode is restricted because of its toxicity. Large amounts of zinc are used to protect steel platforms in the sheltered and shallow waters of the sea, and the effects of zinc on the contamination of waters are not known. 

Conversion coatings. Some metals can be treated by oxidizing agents, preferentially chromate solutions, to prepare an oxide film saturated by chromate ions. This is one of the most effective corrosion preventing treatments of metal surfaces and is applied to aluminum, copper, and zinc. Chromate ions are known for toxic properties. Pohtical decisions have banned this surface treatment for the future. So far no alternative corrosion protecting treatment of similar effectiveness has been developed. 

Kowalski, D Ueda, M., and Ohtsuka, T. (2008) The effect of ultrasonic irradiation during electropolymerization of polypyrrole on corrosion prevention of the coated steel. Corros. Scl., 50, 286-291. 

One of the most effective methods to prevent corrosion is to isolate the substrate from corrosive environments by means of a corrosion-resistant coating. In this way, ILs have been extensively studied and used as electrolyte during the electrochemical deposition of metals to obtain metallic protective films against corrosiOT [3-5]. But some ionic liquids can also form a corrosion-resistant thin film over a metallic substrate in absence of metal salt, as corrosion inhibitors. 

Additional corrosion may occur where the supply of oxygen at interfaces between joined components is limited. This is not bimetallic corrosion, but its effect at bimetallic joints can be as detrimental as true bimetallic corrosion. However, no crevice corrosion occurred in couples of zinc-aluminum alloys and polyethylene in 10-year atmosphere tests (Noranda, personal communication) nor around nylon bolt heads. Jointing compounds are useful in preventing crevice corrosion as well as bimetallic corrosion. Also, stressed parts of the surface tend to be anodic to unstressed parts, but this effect is not usually of practical significance with zinc and zinc-coated steel. 

Whereas corrosion-protection coatings are sup>-posed to be dense and not swell from water intake, other coatings must intentionally be porous. For example, the gas turbine engines, such as those in airplanes, can be made more efficient by increasing the temperature inside the combustion chamber. However, the turbine blades are rotating at a high velocity and high temperature, and consequently the metal surface of the blades may oxidize, and some deformation may occur because of creep. To prevent these detrimental effects, a 0.4-millimeter thin, porous layer of yttrium-stabilized zirconia (a type of... 

Seven primary corrosion prevention methods are available to reduce the corrosion of metals material selection, electrical isolation, electrical bonding, environmental modification, inhibitors, coatings, and cathodic protection. Corrosion prevention methods are often used in conjunction with one another. Coatings are used to reduce current requirements in cathodic protection. Zinc is used as a cathodic protection system in coatings. The effects of two or more corrosion prevention systems can be synergistic or antisyner-gistic. Two effects are synergistic if the combination of the two is more effective than the sum effect of the two. 

Rhodium is used mainly in platinum alloys the rhodium imparts additional corrosion resistance and good high-temperature characteristics. A rhodium coating on steel or brass is effective for corrosion prevention, especially in sea water. The coating must be very thin to justify its cost. Rhodium has very high optical reflectance and has been used in optical equipment, especially where tarnish might be a problem. Sometimes, it is coated on to silver... 

Enhancing Crevice Corrosion Resistance. Several effective strategies for preventing titanium alloy crevice corrosion and smeared iron pitting are alloying titanium, predous metal surface treatments, metallic coatings, thermal oxidation, noble alloy contact, and surface... 

The individual galvanic effect of metallic coatings should be evaluated before their application in a design for corrosion prevention (see Figure 9.26). 

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