Microcell corrosion

Mixed inhibitors may simultaneously affect both anodic and cathodic processes. A mixed inhibitor is usually more desirable because its effect is all-encompassing, covering corrosion resulting from chloride attack as well as that due to microcells on the metal surface. Mixed inhibitors contain molecules in which electron density distribution causes the inhibitor to be attracted to both anodic and cathodic sites. They are aromatic or olefinic molecules with both proton-forming and electron acceptor functional group such as NH2 or SH, as in... 

Areas of fundamental electrochemistry that are particularly relevant to the manufacture of microelectronic components include the sciences of semiconductor electrochemistry, ion transport, corrosion, plating, microcells (of 1 to 20 pm dimension), photoetching, and photoelectroplating. 

The most relevant areas of fundamental electrochemistry are modeling of microcells and interfacial corrosion. 

Different aspects of pitting corrosion can be studied by varying the diameter of die microcell. 

The role of the environment is twofold on the one hand it helps the kinetics of a corrosion process by providing the electroactive species, on the other hand, it attenuates its intensity by hindering the motion of the ions that transport the current necessary for the overall process between the anodic and cathodic microcells. 

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... 

For carbonation the concrete is generally dryer (otherwise the CO2 does not penetrate far). Corrosion therefore tends to be on a microcell level with apparently continuous corrosion along the reinforcing steel for carbonated concrete. 

The second issue is how representative are macrocell currents of the true corrosion currents in the steel. The microcell currents may be more important than macrocell current flows. In a comparison with linear... 

Raupach, M. and Gulikers, J. (1998). Determination of Corrosion Rates Based on Macrocell and Microcell Models - General Principles and Influencing Parameters, in EUROCORR 98 - Event No. 221. Utrecht, The Netherlands. 

Local versus general corrosion (macrocells versus microcells)... 

The second issue is how representative macrocell currents are of the true corrosion currents in the steel. The microcell currents may be more important than macrocell current flows. In a comparison with linear polarization (Berke ef aL, 1990) the macrocell technique underestimated the corrosion rate sometimes by an order of magnitude. As this was using the ASTM prism technique, it should be considered the most accurate use of the macrocell technique, so if it is an order of magnitude out, field use of macrocell techniques is probably even le.ss accurate. 

The settled dust may promote corrosion by absorbing sulfur dioxide and water vapor from the air. Hygroscopic salts such as chlorides or sulfates form a corrosive electrolyte on the surface. Carbonaceous particles can start the corrosion process by forming cathodes in microcells with a steel surface. 

Like GBC, LC is caused by the dissolution of one element in an alloy and the formation of leailike scale exfoliation. Some cast irons and brasses show flakelike corrosion products. The corrosion is due to microcells between varying compositions of an alloy. 

On disbonded areas under the protective coating, corrosion of the metal may occur as a result of the action of corrosion micro-and macrocells and stray currents. In microcells, oxidation and reduction reactions may proceed locally in the area of the defect and the disbonded surface. In the macrocell, the oxidation reaction takes place in the defect and the reduction reaction in another place in the structure, causing cathodic disbonding. Corrosion caused by stray currents is similar to corrosion in the macrocell. The oxidation reaction occurs in such a case in the area of the defect or on the disbonded... 

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