Atmospheric corrosion outdoors

TOW-ISO does not take into account the washing and cleaning effect of rain, a very important aspect in atmospheric corrosion outdoors. 

The complex interaction between the metal and the environment ranges from the atmospheric region over the thin aqueous layer to the solid surface region. Hence, the next three sections are devoted to various reactions and other phenomena in each of the regions involved. They are followed by two sections that deal with selected aspects of atmospheric corrosion outdoors and indoors, respectively. 

Table 3.4 Effect of the surface condition at the time of exposure on the atmospheric corrosion of mild steel (BISRA) (5 years outdoor exposure at Sheffield)...

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. 

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. 

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. 

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... 

L. Veleva, M. A. Alpuches-Aviles, Outdoor atmospheric corrosion, ASTM, STP 1421, H.E. Townsend, Ed. American Society for Testing and Materials International, West Conshochoken, PA, 2002. 

Atmospheric Corrosion. The aluminum-based alloys in general are corrosion resistant to outdoor exposure with the exception of copper-bearing alloys. The Alclad alloys gave the best performance. The loss in tensile strength has been used as a measure of corrosivity and the loss of 1-2% of tensile strength over a period of 1 yr and in particular a loss of 17% was observed with 2017T alloy in 1 yr of outdoor exposure. 

Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. 

The influence of NO on atmospheric corrosion, especially under outdoor conditions, as so far been investigated only on a limited scale. A number of studies is, however, at present ongoing or planned. 

A recent laboratory study (1 ) has shown that a combination of NO2 and SOp at 90% RH causes rapid corrosion of copper compared to the influence of the two gases when present alone, see FIG. 6. These results indicate that the synergistic effect of NO and SO may be of importance for atmospheric corrosion of copper outdoors ... 

Outdoor Tests. One standard protocol for atmospheric corrosion testing is to mount small rectangular plates (1.6 x 2.4 cm) on a test rack at about 30 from horizontal, about 3 feet off the ground, usually facing south (Figure 4). The test plates are held off the rack by porcelain insulators, and although strictly speaking a new boundary layer should form on each plate, there may be some positional differences in corrosion rate due to turbulence created by the plates first encountered by the wind flow. 

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 )...

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... 

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. 

ISO CORRAG [23], the International Organization for Standardization (ISO) has implemented a classification system for evaluating atmospheric corrosivity and on the basis of variables that are fairly easy to obtain [24, 25]. This ISO classification has found several applications, for example, to predict the long-term corrosion behavior in different environments and to evaluate the effect of protective coatings. It contains two principally different approaches of assessing the corrosivity of any outdoor atmospheric environment. The first is based on exposure of standard specimens of steel, copper, zinc, and aluminum for one year whereby the corrosion effect is measured through mass loss measurements. One of five measured corrosivity classes... 

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

An alternative method to inhibit atmospheric corrosion, in particular in outdoor applications, is to add corrosion inhibitors to surface coatings, especially paint films. Here the inhibitor reduces corrosion reactions, resulting from the residual permeability of the polymer film (see Chapter 5.4, this volume). 

In wet atmospheres, nickel initially forms NiO and (NiOH)2 [35,36]. Nickel sulfates are present as corrosion products on the surface in outdoor exposures [37].Jouenefatmospheric corrosion of nickel in industrial, urban, and rural atmospheres. Nickel corrodes through a pitting corrosion process. The highest corrosion rates were observed in industrial areas. The corrosion products were mainly sulfates, chlorides, and n ligible amounts of nitrates surrounded by carbonate species. The pitting corrosion process occurs in two steps on nickel surfaces exposed to an outdoor atmosphere, as shown in Fig. 10.9 [38]. 

S. Jouen, M. Jean, B. Hannoyer, Atmospheric corrosion of nickel in various outdoor environments, Corros. Sci. 46 (2004) 499-514. 

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

Ethylene-tetrafluoroethylene has outstanding resistance to sunlight, ozone, and weather. This feature, coupled with its wide range of corrosion resistance, makes the material particularly suitable for outdoor applications subject to atmospheric corrosion. 

TABLE 9.1. Atmospheric Corrosive Gases in Outdoor Urban Environments [1 ]... 

Townsend, H.E. Estimating the atmospheric corrosion resistance of weathering steels in outdoor atmospheric corrosion, STP 1421, pp. 292-300. ASTM, West Conshohocken (2002)... 

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