Corrosion potential values

The specific AC conductivity values show a generally decreasing value with decreasing substrate corrosion, with one exception the novolac epoxy cured with an aromatic/cycloaliphatic amine. This is one of the coatings that also did not fit into the trend for the corrosion potential values. 

Table 5.5 summarizes the corrosion potential values, corr. corrosion currents, and estimated corrosion rates of Ni-Zn-P, Zn-Ni alloys and Cd, polarization curves shown in Fig. 5.6. 

Table 6.8 Corrosion Potential Values of Selected Coatings Determined in 0.5 M H3BO3 Solution at 25 °C [62]...

Figure 10 signifies the electrochemical analysis (Tafel plot) of MS panels coated with neat alkyd resin, and coated with 2 and 4 wt % loading of ZMP nanocontainer incorporated in alkyd resin. Above analysis was carried out in 5 wt % aqueous NaCl solution at room temperature. The Tafel plot is plotted as log (current density) as a function of applied potential. In Tafel plot analysis current density is measured in corrosion process for simultaneous redox reactions occurs at the surface of cathode and anode of MS plate. Icorr i.e. corrosion current density and Ecorr i.e. corrosion potential, values were found from the Tafel plot analysis. It is observed that corrosion current... 

Experience indicates that the corrosion potential value of buried copper cable correlates with its corrosion behavior (Table 3). A potential value of -0.50 V or more negative, versus a C11/CUSO4 reference electrode, indicates full cathodic protection of this metal, typically provided by a connection to a steel structure. 

ASTM C 876 Describes a technique to measure the corrosion potential of steel in concrete. In the past researchers have used absolute corrosion potential values to give probabilities of the existence of corrosion and to estimate corrosion rates. This is no longer the case and current thoughts are that the contours are an indication of corrosion activity for larger structures, but absolute corrosion rates must be determined by some other method. 

Since pitting tends to be initiated at relatively high corrosion potential values, higher redox potentials increase the localized corrosion risk. Common grades of stainless steel and even the very highly alloyed versions are certainly not immune to MIC. 

Table 3.1 Typical corrosion potential values for common engineering alloys...

Table 3.2 Typical corrosion potential values [17] for common magnesium second phases (after 2h in de-aerated 5% NaCI solution saturated with Mg(OH)2 (pH 10.5))...

TABLE 6.8 Significance of Rebar Corrosion Potential Values (ASTM C876)... 

A range of corrosion potential values in which acceptable corrosion resistance is achieved for a particular purpose. 

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

An interesting field of application is the protection of tantalum against hydrogen embrittlement by electrical connection to platinum metals. The reduction in hydrogen overvoltage and the shift of the free corrosion potential to more positive values apparently leads to a reduced coverage by adsorbed hydrogen and thereby lower absorption [43] (see Sections 2.1 and 2.3.4). 

These values are roughly constant across a range of electrolyte environments except where noted but the variations between alloys, heat treatment conditions, etc. creates a range for each metal. For some metals such as iron and steel the range is low ( 100 mV), but for lead, nickel, stainless steels a range is given. The corrosion potential is reported with respect to the saturated calomel reference electrode. 

These data have been obtained by anodic polarisation work and might therefore be more relevant when cathodic reduction of oxygen takes place that can increase the corrosion potential to high positive values. 

In-vitro fretting corrosion experiments have been performed using weight loss , polarisation and corrosion potential measurements. Thull and Schaldach have performed corrosion potential versus time measurements in-vitro in a joint simulator with different values of applied load. Brown and Simpson have performed similar studies with screw/plate fretting. Both studies found larger shifts towards active potentials with larger loads . 

Variations in pH promoted increases in corrosion potentials from acid pH levels to neutral pH thereafter, however, corrosion potentials were lowered in alkaline solutions to more active values. Decreasing pH caused a lowering of breakdown potentials in the presence of Cl and an increase in the current densities for passivation. 

V (SHE) and the corrosion potential of Zn approximates to this value in sea water, but is more positive in fresh waters, particularly at elevated temperatures. 

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