Metals processing corrosive atmospheres

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 is the process whereby a metal deteriorates. Corrosion caused by atmospheric oxygen is a widespread and costly problem. About one-quarter of the steel produced in the United States, for example, goes into replacing corroded iron at a cost of billions of dollars annually. Iron corrodes when it reacts with atmospheric oxygen and water to form iron oxide trihydrate, which is the naturally occurring reddish-brown substance you know as rust, shown in Figure 11.17 ... 

A dominant problem for all synthesis gas processes is the metal-dusting corrosion (MDC) phenomenon. Further improvement of the reforming technology requires a minimization of MDC. Aging of flie alloys and defects in the metal oxide layer as well as the gas atmosphere might explain this corrosion effect. A possibility for MDC prevention is the displacement of CO by a purge gas [28]. 

By their designation, inhibited films can be subdivided into those protecting metals against electrochemical or microbiological corrosion. As a rule, under service conditions, corrosion processes, whether atmospheric, soil or water, go hand in hand. A number of Cl used as modifiers of polymer films combine the properties of inhibitors of both electrochemical and microbiological types of corrosion [7,54]. 

It should be underlined in conclusion that inhibited polymer films are an efficient rust-inhibiting means of protecting hardware and metal structures against atmospheric, hydrosulfuric, acidic, microbiological and other types of corrosion. The numerous film types and their production processes fit any customer s preference in line with the chosen preservation method, transportation or storage conditions and anticorrosion terms. 

Relative humidity leads to the formation of a thin surface film on a metal when exposed to rain, fog, or dew formation. Xu et al. [52] monitored the dew formation process with a specially designed experimental arrangement. The results indicated that dust on the metal surface facUitates dew formation and increases atmospheric corrosion. Atmospheric corrosion requires both a thin film as well as some type of contaminant to initiate. The process of dew formation occurs much more rapidly in the presence of salts on the metal surface. Figures 10.13 and 10.14 compare the development and advancement of the dewing process on a clean and dust-contaminated surface, respectively [52]. Dew formation was monitored on mild steel (i) before dewing (ii) after 2 min (iii) after 4 min (iv) after 8 min and (v) after surfece drying (ambient temperature 13 °C, relative humidity 69). If the d.c. current for... 

Because this is an electrochemical process, an electrolyte must be present on the surface of the metal for corrosion to occur. In the absence of moisture, which is the common electrolyte associated with atmospheric corrosion, metals corrode at a negligible rate. For example, carbon steel parts left in the desert remain bright and tamish-free over long periods. Also, in climates where the air temperature is below the freezing point of water or of aqueous condensation on the metal surface, rusting is negligible because ice is a poor conductor and does not function effectively as an electrolyte. 

The atmospheric corrosion of Mg alloys is a complex process which results from the interaction between a metal and its atmospheric environment. A prerequisite for atmospheric corrosion is the presence of a water layer on the surface. The thickness of the water layer varies considerably with the climatic conditions and may range from monomolecular thickness to clearly visible water hlms. The formation of an aqueous layer occurs in humid air by adsorption on the hydroxylated oxide present on most metal surfaces exposed to ambient conditions. The thickness of the reversible adsorbed water him varies with the relative humidity (RH). Table 7.1 shows the approximate number of water monolayers on a metal surface at 25 and steady state conditions (1). Thicker aqueous hlms can also form in the atmospheric environment by condensation, precipitation or water absorption by hygroscopic substances on the surface. 

A fundamental requirement for electrochemical corrosion processes is the presence of an electrolyte. Thin-film invisible electrolytes tend to form on metallic surfaces under atmospheric exposure conditions after a certain critical humidity level is reached. It has been shown that for iron, the critical humidity is 60 percent in an atmosphere free of sulfur dioxide. The critical humidity level is not constant and depends on the corroding material, the tendency of corrosion products and surface deposits to absorb moisture, and the presence of atmospheric pollutants. 

The reason for their extreme resistance can be explained as follows. Ceramics are compounds between metallic and non-metallic elements. Corrosion products are also ceramics. Hence, ceramic may be thought of as having already been corroded [2]. Contrary to the corrosion of metals, corrosion of ceramics, if at all they corrode, involves chemical dissolution. Metals corrode by electrochemical processes. Another form of corrosion is oxidation. Oxidation takes place in an oxidizing environment. In this respect, the oxidation resistance of materials is to be considered. When we consider ceramics for application in an oxidizing environment, the nonoxide ceramics may not be that resistant. Members of the ceramic spectrum of borides, carbides, nitrides, silicides, and so on tend to get oxidized when exposed to an oxidizing atmosphere. Herein, oxide ceramics are the most stable. In general, ceramics are more stable than metals in terms of their oxidation. 

Chlorides occur as particulate matter (e.g., NaCl, CaCl or MgClj) mainly in marine atmosphere. These salts are hygroscopic and promote the electrochemical process of atmospheric corrosion by favoring electrolyte formation at low values of relative humidity. H S is extremely reactive and reacts with most technical metals, such as copper, nickel and iron. 

I. S. Cole, W. D. Ganther, J. D. Sinclair, D. Lau, and D. A. Paterson, A study of the wetting of metal surfaces in order to understand the processes controlling atmospheric corrosion. 

Thin-layer cells are experimental devices of interest for simulation of various localized corrosion processes. For example, a thin moisture film covers the metal surface during atmospheric corrosion, and the thin-layer geometry is also relevant to crevice corrosion, and corrosion under delaminated protective films. Thin-layer cells are usually achieved by confining a thin electrolyte layer (the thickness... 

Aqueous environments will range from very thin condensed films of moisture to bulk solutions, and will include natural environments such as the atmosphere, natural waters, soils, body fluids, etc. as well as chemicals and food products. However, since environments are dealt with fully in Chapter 2, this discussion will be confined to simple chemical solutions, whose behaviour can be more readily interpreted in terms of fundamental physicochemical principles, and additional factors will have to be considered in interpreting the behaviour of metals in more complex environments. For example, iron will corrode rapidly in oxygenated water, but only very slowly when oxygen is absent however, in an anaerobic water containing sulphate-reducing bacteria, rapid corrosion occurs, and the mechanism of the process clearly involves the specific action of the bacteria see Section 2.6). 

The hydrogen evolution reaction (h.e.r.) and the oxygen reduction reaction (equations 1.11 and 1.12) are the two most important cathodic processes in the corrosion of metals, and this is due to the fact that hydrogen ions and water molecules are invariably present in aqueous solution, and since most aqueous solutions are in contact with the atmosphere, dissolved oxygen molecules will normally be present. 

Oxygen from the atmosphere, dissolved in the electrolyte solution provides the cathode reactant in the corrosion process. Since the electrolyte solution is in the form of thin films or droplets, diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Moreover, convection currents within these thin films of solution may play a part in further decreasing concentration polarisation of this cathodic process . Oxygen may also oxidise soluble corrosion products to less soluble ones which form more or less protective barriers to further corrosion, e.g. the oxidation of ferrous species to the less soluble ferric forms in the rusting of iron and steel. 

Wetness of a metal surface The lime of wetness of the metal surface is an exceedingly complex, composite variable. It determines the duration of the electrochemical corrosion process. Firstly it involves a consideration of all the means by which an electrolyte solution can form in contact with the metal surface. Secondly, the conditions under which this solution is stable with respect to the ambient atmosphere must be considered, and finally the rate of evaporation of the solution when atmospheric conditions change to make its existence unstable. Attempts have been made to measure directly the time of wetness , but these have tended to use metals forming non-bulky corrosion products (see Section 20.1). The literature is very sparse on the r61e of insoluble corrosion products in extending the time of wetness, but considerable differences in moisture desorption rates are found for rusted steels of slightly differing alloy content, e.g. mild steel and Cor-Ten. 

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