Atmospheric corrosion REACH

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). 

Most known procedures of this group of methods are called oxidation and sulfuration tests. In the former case a metal test specimen wrapped in a VCI film material is placed in a desiccator (about 10 1 in capacity). The internal atmosphere reaches 100% RH using 20 cm of water. The desiccator is blocked up and is placed in a 50° C constant-temperature tank or in normal-temperature room to promote the growth of rust. In sulfuration test the desiccators about 2.5 1 capacity are used. After having adjusted the inside atmosphere to reach 93% RH using 10 cm of a saturated solution of Na2S04, a test metallic strip wrapped in inhibited film material is placed inside. The tests are continued until a corrosive phenomenon is observed. 

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). 

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. 

Effect of Temperature. Temperature plays an important role in atmospheric corrosion. There is normal increase in corrosion activity which can theoretically double for each 10° increase in temperature. As the ambient temperature drops during the evening, metallic surfaces tend to remain warmer than the humid air surrounding them and do not allow condensation until some time after the dew point has been reached. As the temperature begins to rise in the surrounding air, the lagging temperature of the metal structures will tend to make them act as condensers, maintaining a film of moisture on their surfaces [60-63]. 

The relative humidity is the most critical parameter for atmospheric corrosion, because it determines whether condensation can take place. At the metal surface, condensed water forms an electrolyte with the salts deposited from pollutants and thus permits electrochemical reactions to take place. In principle, condensation of water occurs when the relative humidity reaches 100%. However, in practice it takes place often at lower values of relative humidity ... 

Reactions (8.29) and (8.30) are responsible for the phenomenon of acid rain in polluted areas, the pH of rainwater and of fog can reach relatively low values, typically between 4 and 5. At the surface of exposed steel, the pH remains higher (5 to 7), because the reaction (8.31) consumes sulfuric acid. This explains the fact that proton reduction (2 H" + 2e H2) is less important for the atmospheric corrosion of steel than oxygen reduction. 

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. 

While pits formed on aluminum immersed in a chloride solution can reach an appreciable size and depth (Chapter 7), those formed under atmospheric corrosion remain generally shallow, because periodic drying favors repassivation. According to Table 8.24, the maximum depth of the pits on aluminum alloys exposed to a marine atmosphere for a period of 20 years, did not exceed 260 /an. Although this attack has a negative impact on the visual aspect of a structure, it rarely impedes its function. 

Since relative humidity plays such a key role in the corrosiveness of many environments, it is always desirable to monitor the interrelated humidity factors temperature, humidity, and dewpoint temperature. Since reliable commercial equipment is widely available, it will not be discussed further. Closely related to dewpoint is time-of-wetness (TOW), which is measured by monitoring the resistance between oppositely biased electrical conductors as a function of relative humidity. Bias can be applied through an external power source [72]. Alternatively, adjacent metal conductors can be selected to have substantially different corrosion potentials [73]. Above a critical level of relative hiunidity, the test specimen will adsorb a sufficient amoimt of moisture to produce a sharply lower resistance between conductors. The fraction of time of lowered resistance is commonly referred to as the time-of-wetness. It is one useful measure of the corrosivity of an environment. Such measurements were popular in the 1960s and 1970s. More recently, the preferred measurement, due to ease of use, is fraction of time the dewpoint is reached. A procedure for measuring time-of-wetness is contained in ASTM G 84, Standard Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting Conditions as in Atmospheric Corrosion Testing. 

The crucial SO2 effect in zinc atmospheric corrosion has been corroborated through a large number of studies [4, 5, 11-13]. When the critical relative humidity is reached, SO2 adsorbed in the humidity layer that forms on zinc is transformed into sulfite and, subsequently, into sulfate. The hmnidity layer is acidified by the oxidation of SO2 to sulfate, which greatly accelerates the rate of corrosion. Haynie and Upham [14] proposed a simple linear function to correlate SO2 concentration with the corrosion rate of zinc. In subsequent investigations these equations have been adjusted and the eontribution of other variables has been analyzed [15-18]. 

These protective films tend to reduce the corrosion rate with time. Eventually the corrosion rate reaches a steady state and changes very little on further exposure. This is a characteristic of all metals and alloys. The average atmospheric corrosion rate of various metals in mils/year are shown below ... 

In the previously described work, low levels of lead were found in the rust layer near the paint-rust interface, within 30 tm of the rust-paint interface. Thomas suggests that because lead salts do not appear to reach the metal substrate to inhibit the anodic reaction, it is possible that lead acts within the rust layer to slow down atmospheric corrosion by interfering with the cathodic reaction (i.e., by inhibiting the cathodic reduction of existing rust [principally FeOOH to magnetite]) [33], This presumably would suppress the anodic dissolution of iron because that reaction ought to be balanced by the cathodic reaction. No conclusive proof for or against this theory has been offered. 

Relative humidity is defined as the ratio of the quantity of water vapor present in the atmosphere to the saturation quantity at a given temperature, and it is expressed as percent. A fundamental requirement for atmospheric corrosion processes is the presence of a thin film electrolyte that can form on metallic surfaces when exposed to a critical level of humidity. While this film is almost invisible, the corrosive contaminants it contains are known to reach relatively high concentrations, especially under conditions of alternate wetting and drying. 

Atomospheric corrosion is the result of interaction between a material—mostly a metal—and its atmospheric environment. When exposed to atmospheres at room temperature with virtually no humidity present, most metals spontaneously form a solid oxide film. If the oxide is stable, the growth rate ceases and the oxide reaches a maximum thickness of 1 to 5 nm (1 nm = 1(T m). Atmospheric corrosion frequently occurs in the presence of a thin aqueous layer that forms on the oxidized metal under ambient exposure conditions the layer may vary from monomolecular thickness to clearly visible water films. Hence, atmospheric corrosion can be said to fall into two categories damp atmospheric corrosion, which requires the presence of water vapor and traces of pollutants, and wet atmospheric corrosion, which requires rain or other forms of bulk water together with pollutants [3]. 

Under certain conditions, it will be impossible for the metal and the melt to come to equilibrium and continuous corrosion will occur (case 2) this is often the case when metals are in contact with molten salts in practice. There are two main possibilities first, the redox potential of the melt may be prevented from falling, either because it is in contact with an external oxidising environment (such as an air atmosphere) or because the conditions cause the products of its reduction to be continually removed (e.g. distillation of metallic sodium and condensation on to a colder part of the system) second, the electrode potential of the metal may be prevented from rising (for instance, if the corrosion product of the metal is volatile). In addition, equilibrium may not be possible when there is a temperature gradient in the system or when alloys are involved, but these cases will be considered in detail later. Rates of corrosion under conditions where equilibrium cannot be reached are controlled by diffusion and interphase mass transfer of oxidising species and/or corrosion products geometry of the system will be a determining factor. 

Industrial atmospheres usually accelerate the corrosion of zinc. When heavy mists and dews occur in these areas, they are contaminated with considerable amounts of acid substances such as sulphur dioxide, and the film of moisture covering the metal can be quite acid and can have a pH as low as 3. Under these conditions the zinc is dissolved but, as the corrosion proceeds, the pH rises, and when it has reached a sufficiently high level basic salts are once more formed and provide further protection for the metal. These are usually the basic carbonate but may sometimes be a basic sulphate. As soon as the pH of the moisture film falls again, owing to the solution of acid gases, the protective film dissolves and renewed attack on the metal occurs. Hudson and Stanners conducted tests at various locations in order to determine the effect of atmospheric pollution on the rate of corrosion of steel and zinc. Their figures for zinc are given in Table 4.34 and clearly show the effect which industrial contamination has on the corrosion rate. 

The hardness of such coatings may reach a maximum of about 400 Hy as compared with approximately 50 Hy for a soft gold deposit. A series of corrosion studies in industrial and marine atmospheres by Baker" has indicated that the protective value of hard gold coatings is comparable with that of the pure metal, and that a thickness of only 0-002 5 mm gives good protection to copper base alloys during exposure for six months. 

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%. 

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