Atmospheric corrosion moisture

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether mral, urban or marine. 

There are many special factors controlling atmospheric bimetallic corrosion that entitle it to separate treatment. The electrolyte in atmospheric corrosion consists of a thin condensed film of moisture containing any soluble contaminants in the atmosphere such as acid fumes from industrial atmospheres and chlorides from marine atmospheres. This type of electrolyte has two characteristics which are summarised in a paper by Rosenfel d . 

In principle, cathodic protection can be used for a variety of applications where a metal is immersed in an aqueous solution of an electrolyte, which can range from relatively pure water to soils and to dilute solutions of acids. Whether the method is applicable will depend on many factors and, in particular, economics — protection of steel immersed in a highly acid solution is theoretically feasible but too costly to be practicable. It should be emphasised that as the method is electrochemical both the structure to be protected and the anode used for protection must be in both metallic and electrolytic contact. Cathodic protection cannot therefore be applied for controlling atmospheric corrosion, since it is not feasible to immerse an anode in a thin condensed film of moisture or in droplets of rain water. 

Paint for structural steelwork is required mainly to prevent corrosion in the presence of moisture. In an industrial atmosphere this moisture may carry acids and in a marine atmosphere this moisture may carry chlorides. Paint is therefore required to prevent contact between steel and corrosive electrolytes, and to stifle corrosion, should it arise as a result of mechanical damage or breakdown of the coating through age and exposure. 

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... 

Titanium metal is very highly resistant to corrosion. It is unaffected by atmospheric air, moisture and sea water, allowing many of its industrial applications. The metal burns in air at about 1,200°C incandescently forming titanium dioxide Ti02. The metal also burns on contact with liquid oxygen. 

Corrosion in other environments such as organic media and gaseous atmospheres are discussed in the literature. Corrosion in organic media is dependant upon the viscosity and the presence of other chemical reagents. Corrosion in gaseous atmospheres is similar to atmospheric corrosion in requiring moisture and the formation of an electrolyte, which in turn can cause corrosion. The corrosion rate will vary with the type of gas such as N02 or S02 present. 

Silicate glasses resist corrosive elfcets of atmosphere, water, aqueous solutions as well as other reagents. They generally resist acidic media better than alkaline media. However, chemical durability depends considerably on glass composition glass of unsuitable composition will lose its gloss and become coated with a white layer under the effect of atmosphere containing moisture. In many instances, chemical durability is the main criterion in the choice of a suitable material. 

Corrosion inhibitors The conditions that influence the onset of corrosion are the entrainment of atmospheric oxygen, moisture from the combustion of fuel, and stop-start running coupled with temperature cycling. In the marine diesel engine, the problem is exacerbated by contamination with fortuitous saline. Corrosion inhibitors are added specifically to cope with this electrochemical process. These additives operate by creating a physical barrier, in the form of a dense hydrophobic, monolayer of chemisorbed surfactant molecules, which prevent access of the water and oxygen to the metal surface. 

Electrochemical corrosion may occur in aqueous electrolytes, gas atmosphere in the presence of moisture on the metal surface (atmospheric corrosion), or as soil corrosion (Fig. 2.1). Corrosive failure may also occur due to electrocorrosion, which is caused by an external electric current. 

Atmospheric corrosion is electrochemical corrosion in a system that consists of a metallic material, corrosion products and possibly other deposits, a surface layer of water (often more or less polluted), and the atmosphere. The general cathodic reaction is reduction of oxygen, which diffuses through the surface layer of water and deposits. As shown in Section 6.2.5, the thickness of the water film may have a large effect, but it is more familiar to relate atmospheric corrosion to other parameters. The main factors usually determining the accumulated corrosion effect are time of wetness, composition of surface electrolyte, and temperature. Figure 8.1 shows the result of corrosion under conditions implying frequent condensation of moisture in a relatively clean environment (humid, warm air in contact with cold metal). 

In other respects, corrosion in soils resembles atmospheric corrosion in that observed rates, although usually higher than in the atmosphere, vary to a marked degree with the type of soil. A metal may perform satisfactorily in some parts of the country, but not elsewhere, because of specific differences in soil composition, pH, moisture content, and so on. For example, a cast iron water pipe may last 50 years in New England soil, but only 20 years in the more corrosive soil of southern California. 

Corrosion is classified, according to the medium that the metal or alloy is exposed to (Table 14.1), as wet and dry corrosion. Corrosion of metals immersed in an aqueous medium is an example of wet corrosion, which proceeds electrochemically. Atmospheric corrosion also belongs to the class of wet corrosion, since it is caused by moisture deposited on the metal surface. Another division of wet corrosion is the degradation of metals and alloys in nonaqueous media by chemical pathways. 

Atmospheric corrosion is influenced by a number of meteorological factors of which humidity is the most important. This is because the electrochemical corrosion process on metals requires moisture. 

Atmospheric corrosion is an electrochemical process with the electrolyte being a thin layer of moisture on the metal surface. The composition of the electrolyte depends on the deposition rates of the air pollutants and varies with the wetting conditions. The factors influencing the corrosivity of atmospheres are gases in the atmosphere, critical humidity and dust content. Two rural environments can differ widely in average yearly rainfall and temperature and can have different corrosive... 

Effect of Temperature. Temperature plays an important role in atmospheric corrosion. There is normal increase in corrosion activity which can theoretically double for each 10° increase in temperature. As the ambient temperature drops during the evening, metallic surfaces tend to remain warmer than the humid air surrounding them and do not allow condensation until some time after the dew point has been reached. As the temperature begins to rise in the surrounding air, the lagging temperature of the metal structures will tend to make them act as condensers, maintaining a film of moisture on their surfaces [60-63]. 

Also, FeS04 is also hygroscopic in nature which enhances the atmospheric corrosion rate by attracting moisture from the atmosphere. 

Weathering steel is used to protect stmctures from atmospheric corrosion in specific environments. Rust layers on mild steel are not protective and are permeable to air and moisture. Protective impervious rust coatings are supposed to form on weathering steel with time and due to high initial cost and sometimes unfavourable environments this steel cannot be used. Microscopic observation of rust layers on weathering steel reveals the two phases in layers parallel to the steel surface. The inner phase extends up to outer surface if exposure periods are longer than 5 years, and then it becomes the only component of the rust coating. In mild... 

The control of atmospheric corrosion is due to the formation of films comprising basic salts, notably carbonate (Tables 2.16 and 2.17). The most widely accepted formula is 3Zn(OH)2 2ZnC03, which may be written Zn5(0H)6(C03)2- Environmental conditions that interfere with the formation of such films, or conditions that lead to the formation of soluble films, may cause quite rapid attack on zinc. One most important factor affecting the corrosion of zinc in the atmosphere is the duration and frequency of moisture contact. 

Atmospheric corrosion of zinc is roughly proportional to the time of wetness in a particular location, a point emphasized by Mikhailovskii et al. (1986) for areas of the former Soviet Union, provided the nature and quantity of environmental pollution do not change a high relative humidity, which can cause condensation, increases corrosion. Rain obviously increases time of wetness, but it can have an indirect beneficial effect by removing corrosive materials. In practice, time of wetness is often taken as the time when relative humidity (RH) exceeds 80% and the temperature is above 0°C. Thin layers of solutions (except acids) are more corrosive than bulk solutions (Mansfield and Tsai, 1979). The general consensus is that the significance of atmospheric humidity in the corrosion of zinc is related to the conditions that may cause condensation of moisture on the metal surface and to the frequency and duration of the moisture contact. 

The fundamentals of atmospheric corrosion were restated by Goodwin (1991), who said that zinc has good corrosion resistance in neutral and mildly alkaline environments, but the protective film can be destroyed by acids. Evaporation of moisture is believed to allow very strong acids to react with the normally passive surface film for brief periods of time. 

The vinyl ester resins are the most corrosion resistant of any of the monolithic surfacing systems, and they are also the most expensive and difficult to install. They are used when extremely corrosive conditions are present. The finished flooring is vulnerable to hydrostatic pressure and vapor moisture transmission. Refer to Table 18.5 for their resistance to atmospheric corrosion and Table 18.10 for their resistance to selected corrodents. 

Atmospheric corrosion typically occurs only during a small percentage of the total exposure time, typically when the surfaces are wet fix>m precipitation, dew, fog, or when hygroscopic compounds on the surfitce absorb moisture during periods of high humidity. For this reason, atmospheric corrosion occurs slowly, and long exposure durations are usually necessary. Most tests typically involve minimum durations of 1 to 3 years, with 20 years maximum exposure not unusual. Tests of shorter duration than one year not only have so litde corrosion as to make evaluation of mass loss difficult, but also suffer from the effects of seasonal variation of corrosion rate. 

Since relative humidity plays such a key role in the corrosiveness of many environments, it is always desirable to monitor the interrelated humidity factors temperature, humidity, and dewpoint temperature. Since reliable commercial equipment is widely available, it will not be discussed further. Closely related to dewpoint is time-of-wetness (TOW), which is measured by monitoring the resistance between oppositely biased electrical conductors as a function of relative humidity. Bias can be applied through an external power source [72]. Alternatively, adjacent metal conductors can be selected to have substantially different corrosion potentials [73]. Above a critical level of relative hiunidity, the test specimen will adsorb a sufficient amoimt of moisture to produce a sharply lower resistance between conductors. The fraction of time of lowered resistance is commonly referred to as the time-of-wetness. It is one useful measure of the corrosivity of an environment. Such measurements were popular in the 1960s and 1970s. More recently, the preferred measurement, due to ease of use, is fraction of time the dewpoint is reached. A procedure for measuring time-of-wetness is contained in ASTM G 84, Standard Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting Conditions as in Atmospheric Corrosion Testing. 

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