Time of wetness

In addition to the above, there are other factors that affect the corrosion rate, including initial exposure conditions, sample mass, orientation, extent of sheltering, wind velocity, nature of the corrosion products formed, and pollutants present (both known and unknown). 

The term time of wetness refers to the length of time during which the metal surface is covered by a film of water that renders significant atmospheric corrosion possible. The actual time of wetness will vary with climatic conditions at the location. It is dependent on the relative humidity of the atmosphere (being greatest when the relative humidity is 80%), the temperature of the air and the metal surface above 32°F/0°C, the duration and frequency of rain, fog, dew, and melting snow, as well as the hours of sunshine and wind speed. 

Moisture on the surface of the metal resulting from high humidity of the atmosphere and the chemical and physical properties of the corrosion products produces an adsorption layer of water. Phase layers of water are the result of rain, fog, wet or melting snow, or dew formed by condensation on cold metallic surfaces. 

The total time of wetness (Tt ) may be divided into the periods when the metal is moistened due to adsorption of water vapor on the surface (Taw) and the periods when the surface is covered by a phase layer of water (Tph) resulting from rain, fog, dew, or wet or melting snow  

It is difficult to distinguish experimentally between the two categories of time of wetness because there is no sharp boimdary. 

Various methods have been developed for measuring many of the factors that influence atmospheric corrosion. The quantity and composition of pollutants in the atmosphere, the amount collected on surfaces under a variety of conditions, and the variation of these with time have been determined. Temperature, RH, wind direction and velocity, solar radiation, and amount of rainfall are easily recorded. Not so easily determined are dwelling time of wetness (TOW), and the surface contamination by corrosive agents such as sulfur dioxide and chlorides. However, methods for these determinations have been developed and are in use at various test stations. By monitoring these factors and relating them to corrosion rates, a better understanding of atmospheric corrosion can be obtained. 

Time of wetness is an estimated parameter based on the length of time when the relative humidity is greater than 80 percent at a temperature greater than 0°C. It can be expressed as the hours or days per year or the annual percentage of time. 

A method of measuring the TOW has been developed by Sereda and correlated with the corrosion rates encountered in the atmosphere [13]. The moisture sensing elements in this sensor are manufactured by plating and selective etching of thin films of appropriate anode (copper) and cathode (gold) materials in an interlaced pattern on a thin nonconductive substrate (Fig. 9.23). When moisture condenses on the sensor it activates the cell, producing a small voltage (0 to 100 mV) across a 10 Q resistor. 

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Wetness of a metal surface The lime of wetness of the metal surface is an exceedingly complex, composite variable. It determines the duration of the electrochemical corrosion process. Firstly it involves a consideration of all the means by which an electrolyte solution can form in contact with the metal surface. Secondly, the conditions under which this solution is stable with respect to the ambient atmosphere must be considered, and finally the rate of evaporation of the solution when atmospheric conditions change to make its existence unstable. Attempts have been made to measure directly the time of wetness , but these have tended to use metals forming non-bulky corrosion products (see Section 20.1). The literature is very sparse on the r61e of insoluble corrosion products in extending the time of wetness, but considerable differences in moisture desorption rates are found for rusted steels of slightly differing alloy content, e.g. mild steel and Cor-Ten. 

The atmospheric corrosion data in Table 4.34 (and also Table 13.8) is related to historic environments. Current use in the industrial areas listed with acidic pollution would show much lower corrosion rates as the corrosion of zinc in the atmosphere is essentially related to the SOj content (and the time of wetness) and in many countries the sulphurous pollution has been greatly reduced in the past 20 years. Zinc also benefits from rainwater washing to remove corrosive poultices thus, although initial corrosion rates are usually not very different on upper and lower surfaces, the latter tend —with time—to become encrusted with corrosion products and deposits and these are not always protective. 

Many factors influence the corrosion of metals in the atmosphere, including the natural phenomena that make up the vagaries of climate and weather. Of these, the feature of greatest importance is moisture in its various forms, since, other factors apart, the amount of corrosion that takes place is largely a question of whether and how long a period the surface of the metal is wetted ( time of wetness ). 

On the Concept of Time of Wetness and Its Interaction with Contaminants... 

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). 

The calculation of Time of Wetness established in ISO 9223 should be revised based on new results obtained in outdoor and indoor conditions in tropical humid marine climate. Some proposals are made to improve the estimation of TOW, taking into account changes in its nature depending on outdoor or indoor exposure, linear relationship between time and TOW, the effect of rain, and the role of contaminants and air temperature. 

Taking into account the electrochemical nature of the atmospheric corrosion process it is absolutely necessary to use the concept of Time of Wetness (TOW). It is a concept commonly used in atmospheric corrosion of metallic materials and refers to the time when the metal is sufficiently wet for corrosion reaction to occur, that is, when an electrolyte is present in the metallic surface. Under the particular characteristics of atmospheric corrosion there are time periods where corrosion could not occur due to the absence of an electrolyte in the metallic surface. The lowest outdoor TOW values are observed in the desert regions, as also in the Antarctic and Arctic regions. Atmospheric corrosion rates of metals at these climatic conditions are also very low and in the case of cold regions, the increase of temperature leads to the increase of TOW and corrosion rate [11], In principle, TOW is a parameter that depends upon both the climatic conditions and in the characteristics of the metallic surface. 

Water adsorption on silver surfaces exposed into a ventilated shed in an urban-rural site of Cuba was studied [13] using quartz resonators covered with a silver layer. It was determined that in these indoor conditions water adsorption significantly diminishes when air temperature increases over 25°C at relative humidity ranges of 80-90% and 90-100%. All these results confirm the idea that an upper limit of temperature should be established for the estimation of time of wetness. 

Corrosion rate is a function of time of wetness, considered as the time during which corrosion occurs, but in general it should not be a linear function because corrosion rate changes with time. There are different factors influencing, for example, the protective properties of the corrosion products, the increase or decrease of the acceleration caused by contaminants, increase or decrease of the thickness and conductivity of the electrolyte layer,... 

T0W-IS05 25 = Time of wetness estimated according to ISO-9223 for air temperatures between 5 and 25°C. Lower temperatures are not considered because they are almost nonexistent in Cuban climate. 

The suggestion of dividing the time of wetness into three different contribution parts was made in order to get a more quantitative approach step to study the atmospheric corrosion process. The influence of time and quantity of rain is very important for characterizing differences between indoor and outdoor corrosion. It can be observed that in all cases the variable TOW 25-35 is affected by a negative sign, indicating a diminishing of corrosion rate... 

A calculation of time of wetness according to ISO 9223, considered as the time when relative humidity is over 80% and air temperature over 0°C is presented on figure 5. 

In other words, the concept of time of wetness (established in ISO 9223) shows limitations, because the nature and the changes occurred in the electrolyte formed on the metallic surface have clear differences between exposure conditions. There is a significant difference between outdoor time of wetness and sheltered or indoor TOW. 

Figure 5. Time of wetness at different exposure conditions and types of atmospheres calculated according to ISO 9223 for test stations of the western side of the Cuban Isle.

As it can be observed, in the storehouse where there is determined the maximum time of wetness, it is reported the lower corrosion rate. It is explained based in the fact that in this case TOW consists mainly in air humidity without the contribution of any precipitation or condensation (no rain, no dew, no significant condensation). It is a demonstration that the estimation of time of wetness according to ISO 9223 presents limitations. 

The value of steel corrosion rate ratio for Viriato and Quivican stations at six months of exposure (1154.67/108.37 = 10.65) is between Cl-DR and chloride concentration in corrosion products ratio and is higher than sulfate. There should be rapid changes in chloride concentrations due to liquid precipitations and arrival of aerosol particles. Changes in sulfate concentration should be lower than for chloride. The corrosion rate in coastal zones should depend mainly on a sum of time of wetness at different chloride and sulfate concentrations. 

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... 

From these considerations, it follows that in the case of formation of the layer of a chemical compound under conditions of simultaneous dissolution of a solid in a liquid, the shape of the layer thickness-time dependence is rather complicated. An evolution of this dependence in the course of interaction of initial substances from the moment of their contact to the establishment of equilibrium in the A-B system will now be analysed in its most general features. It should be noted that the time of wetting the solid surface by the liquid phase will not be taken into account, i.e. this process is assumed to be instantaneous. 

The time-of-wetness t, the amount of pollution P and the amount of chloride S are defined to estimate the corrosion rates of carbon steel, zinc, copper and aluminum and the resulting values for an exposure duration were obtained. Use of the different degrees of time of wetness, various values of pollution along with different amounts of contaminant such as chloride, the estimated corrosion rates13 of steel, zinc, copper and aluminum for exposure duration of one year are given in Table 2.2. 

It is also mandatory to observe the principles of replication, randomization and blocking.22 24 Replication overcomes the problems associated with attrition of samples. Nonuniformity of time-of-wetness on samples is minimized by randomization of sample mounting location. In the absence of randomization it is possible to create statistical blocks and processed statistically. 

Table 2.2 Time-of-wetness, pollution, chloride, estimated corrosion rates...

ASTM G84, Standard Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting Conditions as in Atmospheric Corrosion Testing, West Conshohoken, PA, 1993. 

The four important areas of application of carbon steels are (i) atmospheric corrosion (ii) corrosion in fresh water (iii) corrosion in seawater and (iv) corrosion in soils. The atmospheric corrosion of steel is caused by major environmental factors such as (i) time of wetness as defined by ISO 9223-1992 (ii) sulfur dioxide in the atmosphere due to the combustion of fossil fuels and (iii) chloride carried by the wind from sea. The equations for corrosion rates of carbon steel by multiple regression analysis have been obtained.1... 

The "time of wetness of a surface will depend upon its geometry and presence or absence of surface deposits. It will also depend on how long the relative humidity remains above the critical level. In warmer climates, this is most likely to occur at late evening and through to early morning. 

In outdoor atmospheres dry deposition of S-pol-lutants and especially of S0 is of greatest importance. Dose response functions describing corrosion as function of SOp and time of wetness are available for some materrals as steel and zinc. 

The results from lengthy exposure periods are of course more relevant for practical purposes e.g. for classification of the corrosivity of atmosphere on a given location or for cost-benefit analysis. In this case the corrosion rate may be assessed from yearly mean values of the concentration of pollutants and from the time-of-wetness class estimated from meteorological measurements. 

Also for zinc dose-response functions obtained in the temperate climatic zone show the dominating influence of SO on inland sites. The inclusion of time of wetness in the linear model did not further significantly improve the correlation. The following dose-response functions have been obtained in the previously mentioned investigation (FIG. 2) ... 

Dose-response functions describing the corrosion as function of SO and time of wetness are today available for steel and zinc. In extensive areas of the temperate climate zone the corrosion loss may be described by SO pollution solely. 

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