Atmospheric corrosion relative humidity

Access to new and more sensitive analytical techniques has resulted in substantial progress in the characterization of corrosion products formed under both laboratory and field exposure conditions. These techniques permit the determination of, e.g., thickness, chemical composition, and atomic structure of corrosion products formed at both early and later stages of exposure. When combined with environmental data, such as deposition rates of corrosion-stimulating atmospheric constituents, relative humidity, temperature, and sunshine hours, the new techniques have resulted in a more comprehensive understanding of the complex processes that govern atmospheric corrosion. In a series of papers, Graedel has summarized the corrosion mechanisms of zinc [62], aluminum [63], copper [18], iron and low-alloy steel [64], and silver [19]. It is beyond the scope of this chapter to provide... 

Load for stress corrosion cracking tests Acidity in immersion tests Amounts of NO2 and SO2 for atmospheric tests Relative humidity for atmospheric tests Impingement velocity in erosion corrosion tests... 

Critical Humidity—the relative humidity (RH) at and above which the atmospheric corrosion rate of a metal increases significantly. 

The main factor in causing filiform corrosion is the relative humidity of the atmosphere, and if this is below 65% (the critical relative humidity for the atmospheric corrosion of most metals, see Section 2.2) it will not occur. As the relative humidity increases the thickness of the filaments increases at 65-80% relative humidity they are very thin, at 80-95% relative humidity they are much wider and at approximately 95% relative humidity they broaden sufficiehtly to form blisters. 

Water vapour is essential to the formation of an electrolyte solution which will support the electrochemical corrosion reactions, and its concentration in the atmosphere is usually expressed in terms of the relative humidity (r.h.). 

The relative humidity of the atmosphere has a large effect on the magnitude of wear", but in a direction opposite to that which is encountered in normal corrosion problems. The increasing wear towards lower humidities is accompanied by severe pitting of the surfaces and, under extremely dry conditions, the oxide debris produced from steel surfaces is jet black. 

Time of wetness (TOW), considered as the time during which the corrosion process occurs, is an important parameter to study the atmospheric corrosion of metals. According to ISO-9223 standard, TOW is approximately the time when relative humidity exceeds 80% and temperature is higher than 0°C. No upper limit for temperature is established. In tropical climates, when temperature reaches values over 25°C, evaporation of water plays an important role and the possibility to establish an upper limit respecting temperature should be analyzed. The concept of TOW assumes the presence on the metallic surface of a water layer however, there are recent reports about the formation of water microdrops during the initial periods of atmospheric corrosion, showing that the idea of the presence of thin uniform water layers is not completely in agreement with the real situation in some cases (particularly indoor exposures). 

Atmospheric corrosion is the most extended type of corrosion in the World. Over the years, several papers have been published in this subject however, most of the research has been made in non-tropical countries and under outdoor conditions. The tropical climate is typical of equatorial and tropical regions and is characterized by permanently high temperatures and relative humidity with considerable precipitation, at least during part of the year. A high corrosion rate of metals is usually reported for this climate. 

Recent reports about the microdroplets formation in the starting periods of atmospheric corrosion [15-18] show that the idea of a thin uniform water layers is not completely in accordance with the reality. It has been observed that when a water drop is on the metallic surface, formed in the place where a salt deposit existed before, microdroplets are formed around this central drop. The cathodic process takes place in these surrounding microdroplets, meanwhile the anodic process takes place in the central drop. This idea is not consistent with the proposal of an uniform water layer on the surface and it is very probable that this situation could be obtained under indoor conditions. It has been determined that microdrops (about 1 micron diameter) clusters are formed around a central drop. An important influence of air relative humidity is reported on microdrops formation. There is a critical value of relative humidity for the formation of microdroplets. Under this value no microdroplets are formed. This value could be considered as the critical relative humidity. This situation is very similar to the process of indoor atmospheric corrosion presence of humid air, deposition of hygroscopic contaminants in the surface, formation of microdrops. Water is necessary for corrosion reaction to occur, but the reaction rate depends on the deposition rate and nature of contaminants. 

Recent reports [30-31] on the use of atmospheric corrosion sensors based on changes in electrical resistance showed that when there were no contaminants [29], in tests of 100-110 h., corrosion rate was zero or insignificant. These sensors can determine changes in metal thickness lower than one nanometer. However, in the presence of 0.08 ppm of S02 or 20 pg/cm2 of NaCl in the system, changes in thickness where always detected over 75% of relative humidity. Corrosion rate was determined at temperatures of 20, 30 and 40°C and the Arrhenius equation was used to calculate the activation energy of the reactions. This method is very similar to the natural conditions. 

It has no sense to calculate TOW-ISO for coastal tropical atmospheres, because in those conditions corrosion process occurs at relative humidity lower than 80%. It has been determined that water adsorption by corrosion products is polymolecular in these conditions. As analogy, in highly polluted atmospheres, corrosion process should proceed at RH lower than 80%, so it has no sense to use TOW-ISO. 

One of the best known examples of electrochemical corrosion is atmospheric rusting. For this to occur, a certain critical relative humidity of between 60-80% or higher (depending upon whether salts are present) is required. At such a relative humidity, every object is covered with a coherent film of water which serves as an electrolyte. Electrochemical corrosion also occurs when an iron object is partly or completely immersed in water. 

The few reported cases concerning other metals, like zinc, aluminum, and magnesium, attest their susceptibility to corrosion due to volatile compounds in the museum environment [271]. Iron is naturally vulnerable to atmospheric corrosion whatever the pollutants, and the conservation of ferrous artifacts implicates a precise control of relative humidity, often requiring a surface protection like varnish, wax, or oil [272]. 

Aqueous corrosion can occur even when the metallic object to be protected is ostensibly not immersed in water, if the relative humidity of the atmosphere exceeds 60%. In that case, a film of water will in fact be present on the metal surface. Further, if sulfur dioxide is present in the air, corrosion in the thin film of water will be greatly accelerated, partly because the acidity of the dissolved SO2 facilitates the oxygen absorption reaction... 

The natural environments—rural, marine and industrial—and some combination of these are of primary concern. Some contaminated atmospheres, such as those containing hydrogen sulfide, ammonia, sulfur dioxide, etc., should be considered rigorously. Relative humidity and its cycling are of major importance for corrosion kinetics and a material s resistance to corrosion. Temperature and pressure are major factors to consider, together with the chemical composition of the medium. 

If corrosion rate is plotted against humidity, then a curve such as that in Fig.3 would be obtained. Here, corrosion rate is low until, over a narrow range of humidity, the rate suddenly begins to increase. This point is termed the critical humidity and its value will depend upon the metal and nature of any dissolved species in the water film. For example, iron in a sulphur dioxide polluted atmosphere will have a critical relative humidity of above 75%, whereas a copper surface polluted with iodide will reach a critical relative humidity of about 35%. 

Coupling of dissimilar metals in the atmosphere may also result in galvanic corrosion. Figure 1.22 can be used to determine the compatibility of metals when exposed to atmospheric conditions that cause corrosion (i.e., when the relative humidity exceeds -50%). 

Direct measurements have not been made, to my knowledge, regarding the lower limit of partial pressure of H2O in air necessary for formation of hydrogen peroxide. One can reason, however, that the limiting partial pressure ought to be the same as that necessary for a metal to corrode. Based on corrosion information, the critical lower limit for the partial pressure is more properly expressed in terms of relative humidity rather than absolute pressure. The critical relative humidity for corrosion is that which allows moisture to condense on the surface of a metal. This value, in turn, depends on the nature and concentration of hygroscopic impurities present both in the atmosphere and on the metal surface. For commercial steels in ordinary urban air, the critical relative humidity is about 50%, but for high purity metals in filtered air, the critical value is undoubtedly much lower. 

On structures exposed to the atmosphere, the corrosion rate can vary from several tens of fim/y to localised values of 1 mm/y as the relative humidity rises from 70 to 95 % and the chloride content increases from 1 % by mass of cement to higher values. These high corrosion rates have been observed in particular on heavily chloride containing structures such as bridge decks, retaining walls and pillars in the Swiss Alps. 

The moisture content in concrete is the main factor controlling the corrosion rate. When concrete is in equiUbrium with the atmosphere, the absorbed water can be correlated to the relative humidity of the environment (Section 2.1.2). Actually, in real structures this condition normally occurs only at the concrete surface. 

Aerosol particles are chemical mixtures of a number of ionic species, including ammonium, sulfate, chloride, nitrate, and hydrogen. The actual chemical mixture of each particle reflects the chemical conditions in which the particle was formed. Once adsorbed on a metal surface, the water-soluble part of each particle acquires water from the atmosphere and deliquesces, whereby it transforms into a concentrated aqueous solution. The ionic constituents that are liberated into the aqueous film may have a significant influence on the atmospheric corrosion processes. Moreover, many particles possess hygroscopic properties and retain water. Lienee, a typical particle may triple its volume when the relative humidity increases from dry to 90%. 

A laboratory test must be designed and performed so that the most important parameters from an atmospheric corrosion perspective are controlled. Reproducibility and the ability to mimic the atmospheric corrosion in different ambient environments are other important criteria of a laboratory test. Parameters to consider in accelerated tests are sample preparations prior to exposure, relative humidity, temperature, exposure time, corrodents, and corrodent delivery rate [1]. 

The temperature is usually kept constant in the range from 20 to 30 °C and within 1 °C of the desired value. Within this range, the temperature dependence on the atmospheric corrosion rate is not so emphasized and the selected temperature value not so critical. If performing laboratory tests that are based on cyclic variations of relative humidity and temperature, it should be remembered that cyclic tests are difficult to reproduce between different experimental setups. 

Each metal behaves in a unique way with respect to atmospheric corrosion properties, and the conclusions drawn from the nickel study cannot necessarily be drawn for other metals. However, if the same or similar corrosion products are formed on a given metal when exposed to a laboratory and a natural atmospheric environment, respectively, the results surest that the same corrosion processes are operating in both exposures. Table 4 displays examples of reported laboratory tests that have generated corrosion products similar to those seen in natural field exposures [13-18]. It appears that certain combinations of two or three corrodents at concentrations below 1 ppmv, together with a proper choice of relative humidity and airflow rate, can generate the corrosion products that are formed in natural field environments. 

Atmospheric corrosion rates are commonly related to a critical relative humidity , above which the corrosion rate increases significantly and below which the rate is insignificant for many practical purposes. Depending on metal and exposure conditions, critical relative humidities have been reported in the range from 50 to 90%. The critical relative humidity is associated with the point of deliquescence of deposited aerosol particles, above which the aerosols rapidly absorb water until a saturated solution is obtained. For a single-phase aerosol, there is a well-defined critical relative humidity, whereas for a mixture of phases (the common situation in natural outdoor environments) the critical relative humidity is lower than those of the single phases. 

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