Corrosion potential profiles

Figure 7-60. Corrosion potential profile as measured during cathodic delamination on iron in the presence and in the absence of oxygen in the atmosphere (Leng et al., 1999c).

Figure 1. Schematic representation of potential profile and charge distribution across an anodic oxide film of thickness S on aluminum (a) hypothetical situation in the absence of any current (b) in the presence of an anodic current caused by corrosion or by an external source. RE, reference electrode to which the potential of aluminum is referred.

OSHA PEL TWA 1 mg(Fe)/m3 ACGIH TLV TWA 1 mg(Fe)/m3 DOT CLASSIFICATION 8 Label Corrosive SAFETY PROFILE Poison by ingestion and intravenous routes. Experimental reproductive effects. Corrosive. Probably an eye, skin, and mucous membrane irritant. Mutation data reported. Reacts with water to produce toxic and corrosive fumes. Catalyzes potentially explosive polymerization of ethylene oxide, chlorine + monomers (e.g., styrene). Forms shock-sensitive explosive mixtures with some metals (e.g., potassium, sodium). Violent reaction with allyl chloride. When heated to decomposition it emits highly toxic fumes of HCl. 

All protein solutions shifted the corrosion potential (ia=ic) in the negative direction due to the adsorption process. For albumin and globulin systems, an enhancement in the anodic current density was observed at potentials positive to the passivation potentials. However, fibrinogen solutions exhibited an inhibition in the polarization profiles. 

The most severe limitation of the sacrificial anode CP technique is the small driving force, which restricts its use to conductive environments, short current throws and, marine use apart from, wellpower supply, the danger of overprotection, the difficulty of achieving a satisfactory potential profile over a complex shape and the possibility of improper connection causing corrosion of the structure intended to be protected are the major disadvantages of the impressed current CP technique. 

Fig. 26 Typical two-dimensional potential profiles on a polymer-covered zinc substrate as measured with the SKP in -(95% relative humidity) for different corrosion times fas indicated) with 0.5 M NaCi at the defect [83]. (Reprinted with permission.)...

Fig. 27 Potential profiles as measured with SKP on partly delaminated polymer-coated zinc sample after a change of the atmosphere from air to pure nitrogen corrosion time before the change 2710 min corrosion times since change as indicated [83]. The Nernst equations explain the local potentials in the absence of oxygen. (Reprinted with permission.)...

In this case, the reaction proceeds to the right-hand side, because sodium can dissolve in molten sodium chloride and also distill out of the system. The practical consequence of these processes is that the Nernst electrode potential for the alkali or alkaline earth metal-metal ion couple will be shifted in the positive direction as a result of the metal activity being less than unity, its normally defined standard state. Hence, the overlap of the current-electrode potential profiles for the conjugate pair of processes contributing to the overall corrosion reaction becomes more probable, and corrosion occurs. 

FIG. 2—Potential profile showing galvanic corrosion activity [47]. (Reprinted from Corrosion. Copyright by NACE International. All Rights Reserved by NACE reprinted with permission.)... 

Regarding carbon corrosion. Reiser et al. have proposed a one-dimensional static (steady-state) model devoted to the calculation of the PEMFC electrode/ electrolyte electrostatic potential profiles when the anode is partially exposed to air, in order to define the conditions that could be favourable for the cathode... 

In addition, the phenomenon of potential attenuation has been theoretically included as a cmde approximation as predicted by the Charge Disk and Uhlig models. Both models exhibit similar potential profiles. RnaUy, sets of formulae used in cathodic protection are given as mathematical recipes for practical purposes in the field of corrosion prevention. 

The original clamshell device was first introduced in 1989. It consisted of a double umbrella composed of four stainless steel arms covered with a polyester (Dacron ) meshwork. Each arm had a single springed hinge (see Fig. IS.S). - At the time of its introduction, double-umbrella devices were attractive because they offered the potential for minimal blood-flow pattern distortion due to their low profile. Additionally, since they possessed low alloy content, their corrosive potential was minimal. This device was deployed using an IIF sheath, which was smaller than the sheaths used by other devices at the time, but larger than those employed by the current Amplatzer Septal Occluder. ... 

SIMS studies of these same films, however, did add useful information. The film thicknesses were too low to be able to use a high-current Cs primary beam for depth profiling. An Ar beam was used instead at low currents, which would not have introduced many chemical perturbations into the film. The results of such profiling are shown in Figs. 19a-c for samples of alloy A after exposures in pH 10 solution at the corrosion potential for periods of 3, 12, and 24 h. The intensities of the oxide secondary ions NiO" and CuO" are shown as functions of equivalent sputter time. The depths profiled in these instances were so shallow that it was impossible to gauge them by profilometry thus only the product of sputter time and current density is given on the abscissa. As before, the RSFs for the two ions are considered to be approximately equal. 

Two recent studies have used surface techniques to examine the protective layers formed in various oxidation conditions and related the nature of the oxide to the corrosion rates in specific environments. Hultquist and Leygraf (43, 44) have examined crevice corrosion resistance in a NaCl solution at room temperatures while Baer (45) has looked at higher temperature (800°C) air oxidation. Oxygen potential, or partial pressure, is an important parameter in both studies where it was found that controlled oxidation can tailor the Cr profile in the alloy and surface oxide for maximum corrosion protection. Specific goals in each experiment were oxide characterization and comparison to corrosion rates... 

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