Corrosion intensity

Linear polarization instruments provide an instantaneous corrosion-rate data, by utilizing polarization phenomena. These instruments are commercially available as two-electrode Corrater and three electrode Pairmeter (Figure 4-472). The instruments are portable, with probes that can be utilized at several locations in the drilling fluid circulatory systems. In both Corrater and Pairmeter, the technique involves monitoring electrical potential of one of the electrodes with respect to one of the other electrodes as a small electrical current is applied. The amount of applied current necessary to change potential (no more than 10 to 20 mV) is proportional to corrosion intensity. The electronic meter converts the amount of current to read out a number that represents the corrosion rate in mpy. Before recording the data, sufficient time should be allowed for the electrodes to reach equilibrium with the environment. The corrosion-rate reading obtained by these instruments is due to corrosion of the probe element at that instant [184]. 

The general features described above for the metal dusting corrosion of Inconel 600 can also be extended to the other Ni based alloys except that the corrosion intensity decreases with increase of Cr content. The rate of corrosion is also a strong function of temperature. The maximum local metal dusting rate is plotted as a function of temperature in Fig.5. It is interesting to note that a maximum in... 

Faraday s law is the connecting relationship between the corrosion current density, icorr = Icorr/Aa, and other expressions of corrosion rate, such as corrosion intensity (Cl), in units of mass-loss per unit area per unit time, and corrosion penetration rate (CPR) in units of loss-in-dimension perpendicular to the corroding surface per unit time. 

From Icorr, the total amount of corrosion can be calculated from Faraday s law, and by dividing Icorr by the corroding area, the corrosion current density and hence the corrosion intensity or corrosion penetration rate is determined. Thus, the intersection of the extrapolated Tafel line with E = Ecorr gives an experimentally determined value for Icorr. 

After icorr is evaluated by any one of the foregoing methods, use of one of the Faraday-law expressions (Table 6.2 and Chapter 4) leads to either the average corrosion intensity (Cl) or average corrosion penetration rate (CPR). If the corrosion process is uniform, these average values relate directly to the uniform surface dissolution rate. If, on the other hand, the corrosion process is localized, the actual corrosion intensity and corrosion penetration rate at local areas can be orders of magnitude greater than the average values. 

If localized corrosion is occurring (e.g., pitting corrosion), and the experimentally determined value of Icorr is divided by the total specimen area, A, to determine icorr, then Faraday s law is used to calculate the corrosion intensity, Cl, or corrosion penetration rate, CPR, why are the resultant values to be regarded as minimum values (i.e., the actual local values will be considerably higher) ... 

Why should the specimen surface be carefully examined for localized corrosion after an electrochemical corrosion-rate test before calculating the corrosion intensity or corrosion penetration rate based on the total exposed area of the specimen, the experimentally determined corrosion current, and Faraday s law ... 

CHEMICAL PROPERTIES stable under ordinary conditions of use and storage tends to decompose on prolonged storage or on contact with moisture reacts with strong oxidants hydrolyzed by alkali unstable to lime non-corrosive intense mass spectral peaks readily dispersible decomposes above 180 C (356"F) FP (unknown) LFL/UFL (unknown) ... 

Figure 9.8 Regimes of interaction of erosion and corrosion as erosive and corrosive intensities are varied.

Once Rp is known, the corrosion rate can be evaluated using the Stern-Geary equation. Polarization resistance and corrosion current are determined from the current measured close to the corrosion potential. Polarization resistance can be determined with minimum system perturbation with linear polarization resistance or by using EIS. Experimentally determined potential ranges that indicate expected iron corrosion intensity for different-measured corrosion potentials are shown in Fig. 12.3. 

In most eases there is a zone of less noble material in/at the grain boundaries, which acts as an anode, while the other parts of the surface form the cathode. The area ratio between the cathode and the anode is very large, and the corrosion intensity can therefore be high. 

By now, comprehensive studies have been performed and reliable industrial experience has been gained on material corrosion in sodium. Corrosion intensity in sodium is significantly lower than that in water or lead-based coolants [5.9], In sodium, as well as in the other liquid metals, corrosion rate depends on many factors (temperature level, coolant velocity, impurity content, temperature difference, time, etc.). When evaluating corrosion rate, the major part of researchers took into account only the most contributing factors. Empirical equations for corrosion rate were most commonly derived for 316 steel at the coolant velocity of > 4 m/s and oxygen content of < 10 ppm. The most reliable results were obtained in [5.10, 5.11] for corrosion rate K, mg/cm h, that can be expressed as follows ... 

Development of an environmental corrosion intensity factor that facilitates quantification of the acceleration provided by test conditions and enables prediction of performance, based on exposure time, in any combination of field environments. 

Lifetime modeling and prediction, design for specific corrosion properties, quantitative environmental corrosion Intensity factor / / / / / /... 

Released electrons are usually consumed in the reduction of dissolved oxygen. The rate of such a conjugated process depends on different factors, such as the oxygen concentration, the acidity of the medium, the copper surface state, and so on. Corrosion intensity can be also suppressed by inhibitors. 

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