Atmospheric corrosion aqueous layer

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether mral, urban or marine. 

Rust formed by atmospheric corrosion is often voluminous (Fig. 18.4) and visually appears as loose orange-brown or black masses. This type of rust is always a mixture of phases and frequently consists of two layers - magnetite at the iron/rust interface (as a result of reduced oxygen supply) with an outer layer of loose lepidocrocite and/ or goethite. Hematite is formed during high temperature aqueous corrosion and is also found in the passive layer (which forms at room temperature). 

Silicate glasses resist corrosive elfcets of atmosphere, water, aqueous solutions as well as other reagents. They generally resist acidic media better than alkaline media. However, chemical durability depends considerably on glass composition glass of unsuitable composition will lose its gloss and become coated with a white layer under the effect of atmosphere containing moisture. In many instances, chemical durability is the main criterion in the choice of a suitable material. 

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). 

Fig. 4 Important steps during initial stages of S02-induced atmospheric corrosion formation of protons and bisulfite ions in the aqueous layer (a), ligand exchange between a surface hydroxyl group and a bisulfite ion and subsequent weakening of adjacent bonds through proton-bonded neighbors (b), detachment of metal surface center that enters the aqueous phase as a hydrated metal anion-complex (c). (Reproduced with permission from Ref [7].)...

Atmospheric corrosion rates proceed through three stages or periods the induction period, the transition period, and the stationary period. During the induction period the metal is covered with a spontaneously formed oxide and the aqueous layer. This oxide provides some degree of protection, depending on the metal and the aggressiveness of the atmosphere. During the transition period the oxide layer transforms into a fully developed layer of... 

The atmospheric corrosion rate is influenced by the nature of the specific corrosion products formed, some of which have a greater protective ability than others. The composition of the corrosion products formed is dependent on the specific dissolved metal ions and the access to anions dissolved in the aqueous layer. 

Damp atmospheric corrosion occurs when water is present on the surface of the metal as an aqueous phase layer as caused in humidity, dew, or fog. Wet atmospheric corrosion occurs when bulk water is present, such as rain. 

Gaseous constituents of the atmosphere dissolve in the aqueous layers formed. Corrosive attack is generally found in areas where water adsorption is favored, permitting easy dissolution of the gaseous molecules such as SO2 and NO2. The properties of wet atmospheric corrosion are approached when the aqueous films are greater than approximately three monolayers. At this point the relative humidity is close to the critical relative humidity. At values above the critical relative humidity, atmospheric corrosion rates increase appreciably, whereas below this value atmospheric corrosion is negligible. The critical relative humidity varies for different metals and pollutants. 

Wet atmospheric corrosion results from repeated wet and dry cycles, the presence of pollutants, and the formation of an aqueous layer in which the atmospheric pollutants dissolve. The wet cycles result from dew, fog, rain, or snow. In many cases the dew, fog, rain, or snow may already contain the dissolved corrodent, which then deposits on the surface. 

In outdoor exposure conditions subject to wet-dry cycles, the actual concentration of most corrosion-stimulating gases under many conditions is not at equilibrium between the gas in the atmosphere and the same gas in the aqueous layer. Even so, thermodynamic considerations have been used for predicting the formation of different corrosion end-products and their stability. Figure 3.1 is a schematic illustration of processes occurring at the aqueous layer. 

Oxygen, because of its ability to accept electrons and its involvement in chemical transformations of the atmosphere, is particularly important to atmospheric corrosion. Other materials present in the troposphere that play a part in atmospheric corrosion are water and carbon dioxide. Water acts as an electrolyte and carbon dioxide, which has a concentration of approximately 330 ppm and is highly soluble in water, contributes to the acidity of the aqueous layer. 

Zinc is a relatively base metal. Atmospheric corrosion of zinc starts with the instantaneous formation of a thin film of zinc hydroxide, which may occur in different crystal structures, and the subsequent formation of basic zinc carbonate Zn,(CO02(OH)6. The pH of the aqueous layer controls the stability of initial corrosion products and results in the dissolution of Zn. ... 

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]. 

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. 

Under most atmospheric corrosion conditions, the anode reaction rather than the cathode reaction is observed to be the rate-limiting step [2]. Upon evaporation of the aqueous layer, a film of corrosion products—consisting of metal hydroxides or metal oxyhydroxides—may precipitate. With repeated condensation-evaporation cycles, this film usually hinders the transport of ions through the corrosion product or the transport of Me from the anodic site. Hence, the anodic reaction rate is lowered and, thereby, the atmospheric corrosion rate. 

The incorporation of atmospheric species into the aqueous layer may occur through either dry or wet deposition. In dry deposition there is no involvement of any precipitation, whereas wet deposition requires, e.g., rain, dew, fog, or snow for atmospheric pollutants to deposit. Indoors or in highly polluted areas close to emission sources, dry deposition is considered to be dominating but the relative importance of wet deposition may be difficult to establish because of the incidental nature of precipitation. Controlled field studies combined with extensive laboratory exposures have been undertaken within the National Acid Precipitation Assessment Program (NAPAP) to explore the relative contribution of wet and dry deposition to increased corrosion rates of a number of metals [45]. 

A useful parameter is the dry deposition velocity, which is defined as the ratio of deposition rate or surface flux per time unit of any gaseous compound and the concentration of the same compound in the atmosphere [46]. The concept of dry deposition velocity of SO2 and its relevance to atmospheric corrosion rates is well established [47]. By examining data based on both field and laboratory exposures, it can be concluded that the factors controlling dry deposition fall into aerodynamic processes and surface processes. Aerodynamic processes are connected with the actual depletion of the gaseous constituent (e.g., SO2) in the atmospheric region next to the aqueous phase and the ability of the system to mix new SO2 into this region. This ability depends on, for instance, the actual wind speed, type of wind flow, and shape of sample. Surface processes, on the other hand, are connected with the ability of the aqueous layer to accommodate SO2. This ability increases with the thickness of the aqueous layer and, hence, with the relative humidity, the pH of the solution (as discussed earlier), and the alkalinity of the solid surface. 

Silver exhibits a corrosion behavior which hardly resembles that of any of the other metals described. Its unique behavior is to a large extent governed by the existence of Ag upon dissolution of silver into the aqueous layer. Ag2S is the most abundant component of the corrosion products formed. AgCl can form in environments with high chloride content, whereas no oxides, nitrates, sulfates, or carbonates have been reported in coimection with atmospheric exposure. Silver exhibits corrosion rates comparable to those of aluminum, lower than those of zinc, and much lower than those of iron. 

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