Atmospheric corrosion prevention

The reason for the use of zinc as a power-impressed rather than a sacrificial anode is that the high concrete resistivity limits the current output, and a higher driving voltage than that provided by the e.m.f. between zinc and steel in concrete is used to provide the necessary current output. No cementitious overlay is required, although it may be advisable to paint the top surface of the sprayed zinc to prevent atmospheric corrosion of the zinc anode. 

Atmospheric corrosion can be prevented by using volatile inhibitors which need not be applied directly to the surfaces to be protected. Most such inhibitors are amine nitrites, benzoates, chromates, etc. They are mainly used with ferrous metals. There is still some disagreement as to the mechanism of action. Clearly, any moisture that condenses must be converted to an inhibitive solution. There is no doubt that the widely used volatile inhibitors are effective in aqueous solutions containing moderate... 

Dust (especially from industrial activities) and salt spray will also exacerbate atmospheric corrosion (Section 16.4). In enclosed industrial premises, atmospheric corrosion could be minimized by preventing noxious emissions, filtering the air to remove particulate matter, and scrubbing the air with water to remove SO2 and other objectionable gases, although the humidity should itself be kept as low as possible (e.g., steam leaks should not be tolerated). On the global scale, however, the cost to the public of atmospheric corrosion could be substantially reduced by sharply limiting SO2 and, to a lesser extent, NO. emissions from power plants, smelters, automobiles, and other industrial functions. This is an aspect of the acid rain threat (Chapter 8) that is usually overlooked. 

From a chemical viewpoint, the interior of a crystal and its surface can be looked upon as if they were different individuals, comparable to two modifications of a substance or to a metal which is under mechanical stresses. The phenomenon of stress corrosion reveals that the part under tension differs in its chemical reactivity from the part under compression. Different modifications of silica can have very different reactivities. Therefore, it must be expected that the surface film can affect the apparent stability of a solid. Indeed, the surface structures of some hydrated salts are in equilibrium with the ambient atmosphere and prevent these crystals from losing water (efflorescing), even if the bulk of the crystal has a higher water vapor pressure than the atmosphere. The formation of a fresh highly asymmetrical surface may also cause substances to detonate. 

Steel, stainless steel, and copper-free steel alloys are the preferred materials of construction for mercaptan service. In particular, stainless steel should be used for any vessel or line that is to be open and exposed to air frequently. Aluminum is also suitable for mercaptan use provided the pressure rating of aluminum equipment or piping is sufficient to meet the pressure requirements of the application. Iron or carbon steel is less acceptable than stainless steels or aluminum although it can be used if appropriate measures are taken to condition the iron or carbon steel equipment before putting it into service. Allowing a small amount of mercaptan to stand in it for a period of time and subsequently keeping the equipment under a dry, inert atmosphere can prevent corrosion of the equipment. The hazard in using iron or carbon steel is the formation of iron-sulfur complexes, which are pyrophoric... 

All of these compounds are bactericidal and the ancients knew that they helped to prevent wounds from festering. Accordingly, Achilles has been represented in ancient pictures as scraping the rust or oxidation products from his bronze sword or spear into the wound of Telephusf. These oxidation products are frequently but incorrectly called verdigris, which latter is really a basic acetate of copper, and is not produced by ordinary atmospheric corrosion of the metal or its alloys. 

For nongalvanized steel, cosmetic corrosion generally involves a cathodic delamination mechanism the surface under the paint becomes cathodic and the surface exposed in the hole becomes anodic. To slow down or prevent atmospheric corrosion, it is therefore important that the siuface treatment be a good cathodic inhibitor in the finished product. The phosphate layer increases corrosion resistance by limiting the available free surface for the cathodic reaction. In general, the activity of the free surface is further reduced by passivating posttreatments or by the deposition of amorphous phosphate films between the crystals. 

Steel bridges corrode on exposure of steel to atmospheric conditions. The extent of corrosion is greatly enhanced because of marine (salt spray) exposure and other corrosive industrial environments. The only corrosion preventive method is to provide a barrier coating such as paint. 

The shiny appearance, low-weight per volume, favorable mechanical properties such as material strength, ease of forming, and handling are some favorable features of aluminum and hence its use in the food industry. In addition, aluminum has a better corrosion resistance than carbon steel as it readily forms a protective film that prevents further atmospheric corrosion. Aluminum is also lighter than stainless steel and hence its use in beverage cans. 

The most important methods for preventing atmospheric corrosion are 1) to select a corrosion-resistant structural material, 2) to use a corrosion-resistant coating (metallic, inorganic, organic or composite), and 3) to close internal cavities in order to avoid access of humid air, or conversely, 4) to make sure that there is ample ventilation and drainage, and possibly 5) to supply oil, grease, or special substances such as tectyle to the cavities. 

As mentioned above, the impregnation of other inorganic materials, such as rock (U.S. Bureau of Mines, 1971) and statuary (Gauri, 1970) has received some attention. In these cases, stabilization is the desired goal, whether to prevent the falling or rocks in a mine, or to prevent atmospheric corrosion of irreplaceable works of art. 

To protect the metal against atmospheric corrosion, one can block the active adsorption centers by chelating agents and/or prevent diffusion of oxidative... 

Copper is extremely resistant to atmospheric corrosion as it rapidly forms a complex green hydrated copper(I) oxide/carbonate film that prevents further corrosion. Copper is very suitable for underground services as it is extremely resistant to attack. Corrosion troubles occur when copper and steel pipes are joined together, because copper induces corrosion in steel objects joined to it by electrochemical action. 

Keep cylinder valves closed except when the cylinder is being used. Closing the valve isolates the cylinder s contents from the surrounding atmosphere and prevents corrosion and contamination of the valve. 

To retard weathering discoloration and improve atmospheric corrosion resistance, in addition to preventing wet storage staining, chromate passivation procedures have been developed. 

German, G. (1978). Corrosion prevention with thermal-sprayed zinc and aluminum coatings. Proc. Anniversary Symposium, Atmospheric Factors Affecting the Corrosion of Engineering Metals, ASTM STP 646. ASTM, Philadelphia, pp. 74-82. 

Atmospheric corrosion requires the presence of humidity to form an electrolyte (Chapter 8). Preventing accumulation of humidity in certain spots by clever design therefore greatly reduces the risk of atmospheric corrosion. This aspect is particularly important for stmetures in civil engineering and architecture that are periodically exposed to rainwater. In a similar fashion, in the chemical industry, it is important to be able to completely empty containers when they are not in use. Figure 12.1 shows how the design of a reactor can facilitate its efficient drainage. 

Zinc coatings resist atmospheric corrosion by forming protective films consisting of basic salts, notably carbonate. The most widely accepted formula is 3Zn(0H)2-2ZnC03. Environmental conditions that prevent the formation of such films, or conditions that lead to the formation of soluble films, may cause rapid attack on the zinc. 

Although atmospheric corrosion has been recognized as a major cause of metal degradation for many years, it has been an ongoing challenge to make accmrate predictions of the rate of atmospheric corrosion and the effectiveness of preventive measures. 

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