Pitting corrosion time dependence

Electrochemical corrosion is understood to include all corrosion processes that can be influenced electrically. This is the case for all the types of corrosion described in this handbook and means that data on corrosion velocities (e.g., removal rate, penetration rate in pitting corrosion, or rate of pit formation, time to failure of stressed specimens in stress corrosion) are dependent on the potential U [5]. Potential can be altered by chemical action (influence of a redox system) or by electrical factors (electric currents), thereby reducing or enhancing the corrosion. Thus exact knowledge of the dependence of corrosion on potential is the basic hypothesis for the concept of electrochemical corrosion protection processes. 

Microbiocides may be toxic to humans therefore, care must be taken when used. When selecting the microbiocide, the field engineer can obtain pertinent information on chemicals from the service company providing the chemicals. The microbiocide selected must be compatible with the system in which it is being used. Some chemicals such as quaternary amines have dual functions one as microbiocides and the other as film-forming corrosion inhibitors. Insufficient concentrations of this type of chemical may not be enough to coat the whole surface of metal and can cause pitting corrosion. The selection must also depend on chemicals that can produce the desired control in minimum time limits and... 

Secondly, under certain conditions copper may suffer intense localised pitting corrosion, leading sometimes to perforation of the tube, in quite a short time. This form of attack is not common and depends on a combination of unusual circumstances, one of which is the possession by the tube of a fairly, but not entirely, continuous film or scale that is cathodic to the copper pipe in the supply water this can set up corrosion at the small anodes of bare copper exposed at faults or cracks in the film. Carbon films give rise to such corrosion, but since 1950, when the importance of carbon films was... 

Aluminum Foil. Studies of various foods wrapped in aluminum foil show that food products to which aluminum offers only fair resistance cause little or no corrosion when the foil is in contact with a nonmetallic object (glass, plastic, ceramic, etc.) The reactions, when found, are essentially chemical, and the effect on the foil is insignificant. However, when the same foods are wrapped or covered with foil that is in contact with another metallic object (steel, tinplate, silver, etc.), an electrochemical or galvanic reaction occurs with aluminum acting as the sacrificial anode. In such cases, there is pitting corrosion of the foil, and the severity of the attack depends primarily on the food composition and the exposure time and temperature. Results obtained with various foods cov-... 

This phenomenon, however, is not difficult to understand in view of the mechanism of dissolution under such conditions. Since the number of active sites increases linearly with current density and these sites are characterized by a film structure (or thickness or both) different from that at the OCP, one could expect corresponding increases in the corrosion rate. However, as was mentioned earlier, the active surface area in the pits increases with time, and hence one should expect the corrosion rate to increase correspondingly. Therefore, since the effect is not time dependent, one... 

Conversely, for several aluminium alloys, pit initiation can be accepted under many circumstances. This is so because numerous pits are usually formed, and the oxide is insulating and has therefore low cathodic ability, so that the corrosion rate is under cathodic control. However, if the cathodic reaction can occur on a different metal because of a galvanic connection or for instance deposition of Cu on the aluminium surface, the pitting rate may be very high. Since we in other respects can accept pit initiation, the time dependence of pit growth and pit depths is important, and we shall consider this more quantitatively. 

It has long been known that pitting corrosion develops in the presence of chloride. Chloride ions Cl are adsorbed on the natural oxide film [5], followed by the rupture of the film at weak points, with formation of microcracks that are a few nanometres wide. Many pits are initiated within a very short time, up to lO cm . Their density depends on the alloy from 10" cm on 1199 to 10 ° cm on an alloy containing 4% copper. 

Uniform and pitting-type corrosion of various materials (carbon steels, stainless steels, aluminum, etc.) could be characterized in terms of noise properties of the systems fluctuation amplitudes in the time domain and spectral power (frequency dependence of power) of the fluctuations. Under-film corrosion of metals having protective nonmetallic coatings could also be characterized. Thus, corrosion research was enriched by a new and sufficiently correct method of looking at various aspects of the action of corrosive media on metals. 

Corrosion control of metal surfaces depends on the formation and maintenance of a protective corrosion inhibitor film on the exposed metal surface. This protective film may be established during normal application of a corrosion inhibitor program however, there will be some lag time before the film is completely built up. Metal surfaces that are exposed to the cooling water before the film is completed may become candidates for accelerated corrosion during the initial system operation. Normally, localized corrosion or pitting is common during these early stages of operation. 

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