Atmospheric corrosion electrolyte

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. 

There are many special factors controlling atmospheric bimetallic corrosion that entitle it to separate treatment. The electrolyte in atmospheric corrosion consists of a thin condensed film of moisture containing any soluble contaminants in the atmosphere such as acid fumes from industrial atmospheres and chlorides from marine atmospheres. This type of electrolyte has two characteristics which are summarised in a paper by Rosenfel d . 

However, in this section emphasis is placed upon damp and wet atmospheric corrosion which are characterised by the presence of a thin, invisible film of electrolyte solution on the metal surface (damp type) or by visible deposits of dew, rain, sea-spray, etc. (wet type). In these categories may be placed the rusting of iron and steel (both types involved), white rusting of zinc (wet type) and the formation of patinae on copper and its alloys (both types). 

In principle, cathodic protection can be used for a variety of applications where a metal is immersed in an aqueous solution of an electrolyte, which can range from relatively pure water to soils and to dilute solutions of acids. Whether the method is applicable will depend on many factors and, in particular, economics — protection of steel immersed in a highly acid solution is theoretically feasible but too costly to be practicable. It should be emphasised that as the method is electrochemical both the structure to be protected and the anode used for protection must be in both metallic and electrolytic contact. Cathodic protection cannot therefore be applied for controlling atmospheric corrosion, since it is not feasible to immerse an anode in a thin condensed film of moisture or in droplets of rain water. 

Paint for structural steelwork is required mainly to prevent corrosion in the presence of moisture. In an industrial atmosphere this moisture may carry acids and in a marine atmosphere this moisture may carry chlorides. Paint is therefore required to prevent contact between steel and corrosive electrolytes, and to stifle corrosion, should it arise as a result of mechanical damage or breakdown of the coating through age and exposure. 

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. 

The definition of TOW presented on ISO standard 9223 is the following The period during which a metallic surface is covered by adsorptive and/or liquid films of electrolyte that are capable of causing atmospheric corrosion .. In addition, the new document ISO WD/9223... 

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],... 

Almost all tests carried out to study the starting process of atmospheric corrosion have been performed in a surface without corrosion products however, in real conditions, the metal is covered with corrosion products after a given time and these products begin to play its role as retarders of the corrosion process in almost all cases. Corrosion products acts as a barrier for oxygen and contaminants diffusion, the free area for the occurrence of the corrosion is lower however, the formation of the surface electrolyte is enhanced. Only in very polluted areas the corrosion products accelerate the corrosion process. Water adsorption isoterms were determined to corrosion products formed in Cuban natural atmospheres[21]. Sorption properties of corrosion products (taking into account their salt content-usually hygroscopics) determine the possibilities of surface adsorption and the possibility of development of corrosion process... 

Corrosion in other environments such as organic media and gaseous atmospheres are discussed in the literature. Corrosion in organic media is dependant upon the viscosity and the presence of other chemical reagents. Corrosion in gaseous atmospheres is similar to atmospheric corrosion in requiring moisture and the formation of an electrolyte, which in turn can cause corrosion. The corrosion rate will vary with the type of gas such as N02 or S02 present. 

It has been detected that a phase chemisorbed protective layer is formed on the steel sample surfaces. A protective layer similar in chemical composition is formed on samples exposed to a tetrazole-containing electrolyte (0.05 M aqua solution of Na2S04), simulating the condensed medium during atmospheric corrosion. 

The droplet cell. Fig. 2(d), has uniform current distribution and shrunken dimensions that allow resistive electrolytes to be used [5]. This approach was developed for the use of pure water as an electrolyte as a means to mimic atmospheric corrosion, but it can be used with any electrolyte. An area of a flat sample is exposed through a hole in a piece of protective tape. Electroplater s tape is a very resistant tape with good adhesion that is useful for this and other masking applications in corrosion. If the hole in the tape is made with a round punch, the same punch can be used to make circular dots from pieces of filter paper. One such dot is placed securely into the exposed hole. A small (typically 10-20 gl) droplet of soluhon is placed on the filter paper using a calibrated pipette. This wet filter paper acts as the electrolyte. A piece of woven Pt mesh is placed on top of the wet filter paper, and a reference electrode is held against the back of the Pt counterelectrode. As mentioned, the small dimensions allow the use of even very pure water. This simulates atmospheric corrosion, in which a thin water layer forms on the surface. As in atmospheric corrosion, soluble species on the sample surface and pollutant gases in the air are dissolved into the water droplet, which provides some conductivity. This technique has been used... 

Electrochemical corrosion may occur in aqueous electrolytes, gas atmosphere in the presence of moisture on the metal surface (atmospheric corrosion), or as soil corrosion (Fig. 2.1). Corrosive failure may also occur due to electrocorrosion, which is caused by an external electric current. 

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]. 

When steel or iron is exposed to an atmospheric environment, a thin layer of magnetite, Fe304, is formed, covered by a layer of FeOOH. Atmospheric oxygen then penetrates though the almost water-free, porous outer layer of FeOOH and oxidizes the magnetite to hydrated ferric oxide, Fe203, or FeOOH. The presence of Fe " in the electrolyte initiates the precipitation of various corrosion products. The electrochemical mechanism of atmospheric corrosion of iron suggested by Evans is briefly summarized in this chapter [8]. 

The pollutants include SO2, nitrogen oxides, chlorides, and phosphates. All gases in the Troposphere (Ne, Kr, He, and Xe) do not participate in atmospheric corrosion [11]. Only oxygen acts as an oxidizer in a cathodic reaction. The presence of CO2 in the electrolyte ( 300 ppm) decreases pH and increases the corrosion rate of metals. 

The model for atmospheric corrosion tmder high chloride concentration su ested by Kamimura et al. [32] is based on the separation of cathode and anode sites under the rust and the thin electrolyte. The pH at the anode compartment is affected by the chloride ion concentration and decreased to 1.5 by the hydrolysis of ferric ions and the formation of P-FeOOH. Chloride ions accumrrlate at the anode site and initiate the oxidation of ferrous ions to ferric ions. Accumulated chloride ions increase ferric ion solubility in the electrolyte and accelerate the hydrolysis of ferric ions, causing the pH at the anode to decrease. Low pH at the metal-electrolyte interface accelerated the formation of P-FeOOH. The atmospheric corrosion process is summarized as follows ... 

Experimental measurements indicated that the change in the thickness of the electrolyte affects the mass transport of oxygen, hydration ofdissolved metal ions, and accumulation of corrosion products [1,54—56]. Dubuisson et al. [57] investigated the atmospheric corrosion of galvanized steel in a micrometric electrolytic droplet containing sulfate and chloride. The measurements were performed in an electrochemical microcell through controlled... 

M. Stratmann, H. Streckel, On the atmospheric corrosion of metals which are covered with thin electrolyte layers—II. Experimental results, Corros. Sci. 30 (1990) 697—714. 

G. A. El-Mahdy, Advanced laboratory study on the atmospheric corrosion of zinc under thin electrolyte layers. Corrosion 59 (2003) 505—510. 

E. Dubuisson, P. Lavie, F. Dalard,J.P. Caire, S. Szunerits, Study of atmospheric corrosion of galvarrized steel in a micrometric electrolytic droplet, Electrochem. Commrm. 8 (2006) 911—915. 

Atmospheric corrosion is electrochemical corrosion in a system that consists of a metallic material, corrosion products and possibly other deposits, a surface layer of water (often more or less polluted), and the atmosphere. The general cathodic reaction is reduction of oxygen, which diffuses through the surface layer of water and deposits. As shown in Section 6.2.5, the thickness of the water film may have a large effect, but it is more familiar to relate atmospheric corrosion to other parameters. The main factors usually determining the accumulated corrosion effect are time of wetness, composition of surface electrolyte, and temperature. Figure 8.1 shows the result of corrosion under conditions implying frequent condensation of moisture in a relatively clean environment (humid, warm air in contact with cold metal). 

The parameters that determine time of wetness and composition of surface electrolyte have been surveyed by Kucera and Mattson [8.1]. They present also a thorough description of the mechanism, with thermodynamic and kinetic aspects of corrosion on various materials. For instance, they consider potential-pH diagrams as a useful thermodynamic basis for understanding atmospheric corrosion. 

Hot corrosion is the accelerated oxidation of a material at elevated temperatures induced by a thin film of fused salt deposit [36]. It is called hot corrosion because, being caused by a thin electrolyte film, it shares some similarities with aqueous atmospheric corrosion, in which corrosion is commonly controlled by the diffusion of oxygen to the metal surface. In hot corrosion, the soluble oxidant is SO3 (8207") in the fused salt. 

Atmospheric corrosion is an electrochemical process with the electrolyte being a thin layer of moisture on the metal surface. The composition of the electrolyte depends on the deposition rates of the air pollutants and varies with the wetting conditions. The factors influencing the corrosivity of atmospheres are gases in the atmosphere, critical humidity and dust content. Two rural environments can differ widely in average yearly rainfall and temperature and can have different corrosive... 

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