Dissolved aqueous corrosion

Aqueous corrosion can occur even when the metallic object to be protected is ostensibly not immersed in water, if the relative humidity of the atmosphere exceeds 60%. In that case, a film of water will in fact be present on the metal surface. Further, if sulfur dioxide is present in the air, corrosion in the thin film of water will be greatly accelerated, partly because the acidity of the dissolved SO2 facilitates the oxygen absorption reaction... 

In contrast to aqueous corrosion typically involving loss of electrons from the dissolving metal, liquid metal corrosion is generally considered to proceed by simple solution mechanisms. The principal variables affecting corrosion in a liquid metal system are temperature or temperature range or cycling, elements present, area-to-volume ratio, purity, flow velocity, surface condition, and microstructure. In reactor applications, the neutron flux may be an additional factor. In combination, these variables produce enough complexity so that in the present state of the art, it is rarely possible to make confident predictions about the performance of a previously untried systan. Empirical tests are usually required. 

In this case, the units of concentration, on which the values of/j and depend, must correspond to that of the standard state. In the study of aqueous corrosion and electrochemistry, the concentration of dissolved species is most often expressed in... 

In the case of aqueous corrosion, one must often consider dissolved ionic species. Unfortunately, their activities are rarely known. Indeed, because cations cannot be separated from the anions, the activity of an individual ion is not measurable. By defining the mean ionic activity, a measurable entity, one can get around this problem. 

The material removal mechanism of the CMP process was relatively well explained by the previous scientists. The material removal mechanism of dielectric CMP is further well explained by Cook in his paper pubhshed in 1990 [7]. It was explained that the rate of mass transportation during glass pohshing is determined by five factors the rate of water diffusion into the glass surface, the dissolution of the glass under the applied load, the adsorption rate of the dissolved material onto the abrasive surface, the redeposition of the dissolved material onto the surface of the work piece, and the aqueous corrosion between particle impacts. Water diffuses into sUoxane bonding (Si—O—Si) and the diffusion rate is controlled by multiple process conditions such as pressure or temperature. This hydrated oxide surface is removed by an abrasion process. The indentation process by each abrasive was modeled by Hertzian contact and their contact stress was calculated from the theory of elasticity. 

Aqueous corrosion of iron Fe) in H2SO4 solution and of Zn in diluted H2SO4 solution are examples of uniform attack since Fe and Zn can dissolve (oxidize) at a uniform rate according to the following anodic and cathodic reactions, respectively. 

As a consequence of exposure to the aqueous layer, the oxygen-containing film can dissolve— the corrosion rate being normally controlled by the rate of dissolution of the film formed [55], As mentioned in the preceding section, the dissolution rate of many oxides and other minerals is acid dependent and can be written in the form ... 

On a metal surface exposed to atmosphere, only a limited quantity of water and dissolved ions are present, whereas the access to oxygen present in the air is unUmited [1]. Corrosion products are formed close to the metal surface, unlike the case in aqueous corrosion, and they may prevent further corrosion by acting as a physical barrier between the metal surface and environment, particularly if they are insoluble as in the case of copper or lead. The following is a simplified mechanism of aqueous corrosion of iron (Fig. 4.2) ... 

For aqueous corrosion remove dissolved 0, increase the pH (for steels), add inhibitors. 

STM has been used to study adsorption on surfaces as it relates to corrosion phenomena [6, 7]. Sulfiir is a well known corrosion agent and is often found in air (SO2, FI2S) and in aqueous solution as dissolved anions ( HSO7) or dissolved gas (FI2S). By studying the interaction of sulfur with surfaces, insights can be gained into... 

Corrosion. Ammonium bifluoride dissolves in aqueous solutions to yield the acidic bifluoride ion the pH of a 5% solution is 3.5. In most cases, NH4HF2 solutions react readily with surface oxide coatings on metals thus NH4HF2 is used in pickling solutions (see Metal surface treatments). Many plastics, such as polyethylene, polypropylene, unplasticized PVC, and carbon brick, are resistant to attack by ammonium bifluoride. 

Electrowinning from Aqueous Solutions. Electrowinriing is the recovery of a metal by electrochemical reduction of one of its compounds dissolved in a suitable electrolyte. Various types of solutions can be used, but sulfuric acid and sulfate solutions are preferred because these are less corrosive than others and the reagents are fairly cheap. From an electrochemical viewpoint, the high mobiUty of the hydrogen ion leads to high conductivity and low ohmic losses, and the sulfate ion is electrochemicaHy inert under normal conditions. 

Corrosion inhibitors are substances which slow down or prevent corrosion when added to an environment in which a metal usually corrodes. Corrosion inhibitors are usually added to a system in small amounts either continuously or intermittently. The effectiveness of corrosion inhibitors is partiy dependent on the metals or alloys to be protected as well as the severity of the environment. For example, the main factors which must be considered before apphcation of a corrosion inhibitor to an aqueous system are the compatibility of the inhibitor and the metal(s), the salt concentration, the pH, the dissolved oxygen concentration, and the concentration of interfering species such as chlorides or metal cations. In addition, many inhibitors, most notably chromates, are toxic and environmental regulations limit use. Attention is now being given to the development of more environmentally compatible inhibitors (37). 

Attack associated with nonuniformity of the aqueous environments at a surface is called concentration cell corrosion. Corrosion occurs when the environment near the metal surface differs from region to region. These differences create anodes and cathodes (regions differing in electrochemical potential). Local-action corrosion cells are established, and anodic areas lose metal by corrosion. Shielded areas are particularly susceptible to attack, as they often act as anodes (Fig. 2.1). Differences in concentration of dissolved ions such as hydrogen, oxygen, chloride, sulfate, etc. eventually develop between shielded and nearby regions. 

Fig. 4-1 Electrochemical partial and subsequent reactions in corrosion in aqueous media with and without dissolved salt.

The deleterious effect of sulfur dioxide and sulfites in domestic water is increased corrosivity owing to the lowered pH. However, oxidation of sulfite to sulfate in aqueous solutions uses dissolved oxygen, and lliis may retard corrosion. While llte oxichition of sulfite and sulfiirous acid to sulfate and sulfuric acid in the atmosphere is an environmental concern, this reaction is too... 

Oxygen dissolved in aqueous solutions, even in very low concentrations, is a leading cause of corrosion problems (i.e., pitting) in drilling. Its presence also accelerates the corrosion rate of other corrodents such as hydrogen sulfide and carbon dioxide. Oxygen plays a dual role both as a cathodic depolarizer and an anodic polarizer or passivator. Within a certain range of concentration the... 

The corrosion conditions can be different at the fluid line from the bulk condition. Aqueous liquids have a concave meniscus, which creates a thin film of liquid on the vessel wall immediately above the liquid line. Some corrosion processes, particularly the diffusion of dissolved gases, are more rapid in these conditions. Additionally, the concentration of dissolved gases is highest near the liquid surface, especially when agitation is poor. Locally high corrosion rates can therefore occur at the liquid line, leading to thinning in a line around the vessel. This effect is reduced if the liquid level in the vessel varies with time. Any corrosion tests undertaken as part of the materials selection procedure should take this effect into account. 

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. 

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