Atmospheric corrosion exposure conditions

On figure 7 there is represented Average Chloride Deposition rate determined in Cuba for different types of atmosphere and exposure conditions. The results are very similar to those presented on figure 6, particularly respecting outdoor and ventilated shed conditions. Chloride deposition is determined under shelter, that is why it is assumed the same value for ourdoor and sheltered conditions however, it is very well known, the significant influence of precipitations upon outdoor corrosion and its negligible effect under sheltered conditions. In... 

F. Corvo, J. Rocha. Characterization of environments and their influences on Copper at different atmospheres and exposure conditions in tropical climate, International Corrosion Congress (ICC), Granada, Spain, 2002. 

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)...

The purity of the zinc is unimportant, within wide limits, in determining its life, which is roughly proportional to thickness under any given set of exposure conditions. In the more heavily polluted industrial areas the best results are obtained if zinc is protected by painting, and nowadays there are many suitable primers and painting schemes which can be used to give an extremely useful and long service life under atmospheric corrosion conditions. Primers in common use are calcium plumbate, metallic lead, zinc phosphate and etch primers based on polyvinyl butyral. The latter have proved particularly useful in marine environments, especially under zinc chromate primers . 

Early works were typically carried out using single atmosphere exposure conditions, either air (or moist air) representing the cathode side environment [124-129, 139, 142,144-162] or a reducing atmosphere simulating the anode side environment [124, 125, 127-129, 144, 145], Lately, studies have been also performed to determine the oxidation/corrosion behavior of metal and alloys under dual-atmosphere exposure conditions that closely simulate the interconnect exposure conditions during SOFC operation [154-159], The alloys studied include both Fe-Cr base FSSs and Ni or Ni-Cr base heat-resistant alloys, as well as Cr or Cr base alloys. 

Figure 6.Average corrosion rate (g/m2a) depending on exposure condition and type of atmosphere for test stations located in the western part of the Cuban Isle...

Changes in steel corrosion rate for different exposure conditions and types of atmosphere are better explained based on average deposition rate of Chlorides, as can be observed on figure 5. It can be note that Chloride deposition is higher in coastal stations, as it should be expected, and it is in agreement with a higher weight lost for these stations. 

Rolled or pickled steels corrode faster than machined or polished steels. The mill scale on the sample must be removed before exposure to avoid the possibility of mill scale acting as cathode. The mill scale may initially give protection against atmospheric corrosion, but scale tends to crack and spall under shock conditions, resulting in bare spots on the sample exposed to the atmosphere. The vertical plates are attacked more slowly than plates at 45°. 

The most desirable data are those obtained for the material of interest in the intended conditions of exposure. Such data are not readily available in the literature. Published data on atmospheric corrosion should be used with caution since atmospheric conditions are changing with time, as for example acid rain as a variable factor. Accelerated testing, including electrochemical tests, should have a good link with the natural and practical conditions. Published data should be consulted because they are generally useful. Some published data are mentioned here as examples since they are useful in selecting materials or discussion of case histories ... 

As may be seen from Table I, concentrations of SO2 in the atmosphere are subjected to considerable variations. Moreover, SO2 concentrations used in corrosion tests are larger by several orders of magnitude. Therefore, it becomes highly questionable whether data obtained at high SO2 concentration levels may be also applied to practical exposure conditions. SO2 permeabilities of a series of polymer films like polyethylene, polycarbonate. polyamide 2 gg well as polyacrylate and cellulose triacetate have been shown to depend on SO2 pressure, especially at higher SO2 concentration levels. In the case of organic coatings such data are not available. It is therefore recommended to study SO2 permeability at low SO2 concentrations comparable to practical exposure conditions. 

When the aqueous layer is thin enough (less than 10 pm or so, see below) to permit ample access of oxygen to the metal surface, the anode reaction rather than the cathode reaction is rate limiting. This is the most common situation in atmospheric corrosion [5]. However, the surface and exposure conditions alter over a dry-wet-dry cycle or with extended exposure time and may eventually reach a situation in which the cathode reaction becomes the rate-limiting part (Sect. 3.1.2.3). 

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... 

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. 

Fe-, Cd-, and Sn-based alloys. Chromate solutions are also used to seal coatings formed by anodizing or hydrothermal treatment. CCCs are primarily used to improve adherence of subsequently applied organic coatings, or to impart corrosion resistance under atmospheric exposure conditions. Certain types of CCCs contain labile hexavalent chromium (Cr(VI)), which can be leached from the coating when it is contacted by an attacking solution. This Cr(VI) can be transported... 

Atmospheric corrosion of metals is differentiated from the other forms of corrosion due to exposure of metals to different atmospheres rather than immersion in electrolytes. The spontaneous atmospheric corrosion of materials is controlled by the temperature, the relative humidity, the time of wetness, the pH of the electrolyte, and the presence of contaminants such as chlorides, NH3, SO2, NO2, and acidic fogs. In most cases, the rate equations have hmited validity due to different local atmospheric conditions. Metals spontaneously form a solid metal oxide film when exposed to dry atmospheres. The barrier oxide film reaches a maximum thickness of 2-5 nm [1-6]. The corrosion rate of metals exposed to a wet atmosphere is similar to that observed during immenion in aerated water in the presence of dissolved oxygen. Atmospheric corrosion rates decrease in dry atmospheres with corrosion mechanisms that are different from those in wet atmospheres. 

Figure 10.1 shows initial atmospheric corrosion of iron. The corrosion rate is determined by the alloy and electrolyte composition. Maximum generation of Fe under atmospheric conditions occurs after h of exposure [7]. 

Using ordinary and sulfate-resistant Portland cement to represent differing chloride environments, short-term electrochemical monitoring and SEM were used to characterize corrosion behavior [34]. Steel electrodes attained passivity in mortar with high levels of calcium aluminate, up to 1% wt. chloride. At 1.75% wt. chloride, steel electrodes corrode. All chloride levels resulted in steel corrosion for low levels of calcium aluminate. Pore solution was also impacted by mortar exposure conditions. Atmosphere exposure had a high influence on hydroxide concentration in pore solution but no impact on chloride concentration. Carbonation was also investigated samples in a sealed container had a chloride/hydroxide ratio half that of unsealed samples. 

Cabinet tests consist in tests in which specimens are placed in a testing apparatus where the exposure conditions can be controlled. By means these tests it is possible to examine the performance of materials exposed to corrosive atmospheres that may be encountered in various locations, including near sea or in industrial areas. These tests can include the effects of humidity (RH), salt, corrosive gases (e.g. SO, H S, CO ), temperature and UV radiation. 

The corrosion rate of MS and WS as obtained from humidity and salt spray tests (Table 3.19 and 3.20 respectively) when compared with atmospheric corrosion rate at three sites with different exposures (Table 3.2) show wide gap. This is expected, because at laboratory the exposure is more severe and continuous compared to atmospheric conditions that prevail at the three sites. 

The time of wetness, which depends on climatic conditions, and the concentrations of SO2 and Cr, have been used as criteria for defining five types of atmosphere which differ in their corrosivity [10]. Table 8.17 indicates the average corrosion rate of steel for each of them. We may recall that during the initial phase, which can last several years, the corrosion rate decreases with the exposure time (Figure 8.16). Average rates of atmospheric corrosion given in tables therefore depend on the total exposure time used in the tests. The data of Table 8.17 confirm a decreasing of corrosion rate with time until a steady value is reached after several years. 

The described mechanisms of atmospheric corrosion demonstrate the importance of pollutants and of humidity cycles. Unfortunately, the complexity of the underlying phenomena, makes it difficult to predict atmospheric corrosion behavior Irom accelerated testing. Outdoor exposure tests, which generally require several years, therefore remain popular. Different norms and recommended practices fix in detail the conditions to be applied in atmospheric corrosion tests. The following accelerated test methods are most commonly used in the laboratory ... 

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