Atmospheric corrosion indoors

Extensive outdoor exposure programs have provided evidence that corrosion rates can be interpreted in terms of deposition of primarily SO2 and CE and time of wetness. The history of indoor exposure programs is much shorter. Atmospheric corrosion indoors is, in general, influenced by more constant humidity conditions and lower levels of SO2 and CE, from which follows an increased relative importance of other corrosion stimulants including organic gaseous species and particulate pollutants. 

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. Results of outdoor and indoor corrosion rate and corrosion aggressivity in tropical corrosion test stations of Cuba and Mexico are reported. 

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 presence of water does not only create conditions for the existence of an electrolyte, but it acts as a solvent for the dissolution of contaminants [10], Oxygen plays an important role as oxidant element in the atmospheric corrosion process. The thickness of the water layer determines the oxygen diffusion toward the metallic surface and also the diffusion of the reaction products to the outside interface limited by the atmosphere. Another aspect of ISO definition is that a metallic surface is covered by adsorptive and/or liquid films of electrolyte . According to new results, the presence of adsorptive or liquid films of electrolyte perhaps could be not in the entire metallic surface, but in places where there is formed a central anodic drop due to the existence of hygroscopic particles or substances surrounded by microdrops where the cathodic process takes place. This phenomenon is particularly possible in indoor conditions [15-18],... 

Under indoor conditions, in the same way than outdoors, it is necessary the presence of surface humidity for corrosion to occur due to the electrochemical nature of the atmospheric corrosion process however, in indoor conditions there are no precipitations and the presence of surface water depends mainly on water content in the air and changes in temperature on the surface, as well as the presence of hygroscopic substances on the metallic surface. 

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. 

In ISO 11844-3 [28] it is established that the combination of different parameters is what determines the corrosivity of the atmosphere. Under indoor conditions, corrosion process depends on a more complex number of parameters than outdoors however in the same way than outdoors two types of parameters are proposed ... 

The atmospheric corrosion rate of metals depends mainly on TOW and pollutants however, if the differences in the corrosion process between outdoor and indoor conditions are taken into account, the influence of direct precipitation such as rain is very important for outdoor and negligible for indoor conditions. The acceleration effect of pollutants could change depending on wetness conditions of the surface, so the influence of the rain time and quantity should be very important in determining changes in corrosion rate. 

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... 

Leygraf. Indoor atmospheric corrosion. Proceedings 15th International Corrosion Congreso, Granada, Spain, September 22-27, 2002. 

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). 

NO in combination with SOp has a synergistic corrosion effect especially indoors on electrical contact materials, copper and steel. The influence of acid precipitation may differ for different metals and depends also on the pollution level. The atmospheric corrosion of metals due to acid deposition is in most regions mainly a local problem restricted to areas close to the pollution source. 

Tab. 1 Outdoor and indoor concentration ranges (in ppbv, parts per billion per volume) of selected gaseous air constituents of importance in atmospheric corrosion together with their solubilities in aqueous systems, expressed as Henry s law constant (H, M atm )...

One excellent example of a multiana-lytical laboratory study is the influence of submicron sized particles of ammonium sulfate ((NH4)2S04) on the atmospheric corrosion of selected metals [22]. These particles were aerosolized and deposited under dry conditions on the metal surface, whereby the deposited amounts corresponded to up to 10 years of exposure in indoor locations of USA. By introducing humidity into the exposure... 

Corrosion in natural outdoor or indoor environments is complex because of the influence of many different parameters. Yet, it is possible to grasp, at least qualitatively, the behavior of atmospheric corrosion through consideration of concepts and... 

As pointed out in the introduction, outdoor and indoor atmospheric corrosion... 

A.R. Mendoza, F. Corvo, Outdoor and indoor atmospheric corrosion of non-ferrous metals, Corros. Sci. 42 (2000) 1123-1147. 

TABLE 6—Gaseous and solid materials affecting indoor atmospheric corrosion. 

Because of its flammability and the potential for oxides of sulfur, it is not normally used for most indoor applications. Table 10.2 provides the resistance of sulfur concrete to atmospheric corrosion. 

Atmospheric corrosion poses a problem indoors as well as outdoors. As can be expected, there are obvious differences in outdoor and indoor exposure conditions that lead to a difference between outdoor and indoor corrosion behavior. 

Based on the differences between the indoor and outdoor factors affecting atmospheric corrosion rates, it follows that the corrosion rate of many metals is lower indoors than outdoors. This has been verified by examining... 

These factors do not eliminate the possibility of indoor atmospheric corrosion of materials. Designs must take into account the possibility of indoor atmospheric corrosion. 

Analysis of corrosion products on copper, zinc, nickel, and iron specimens that had been exposed to various benign corrosive indoor atmospheres shows large amounts of infrared bands from carboxylate ions such as formate, acetate, and oxalate. This may be an indication of a stronger influence of carboxylic acids and other organic compounds under benign conditions than in more aggressive atmospheres. 

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