Localized corrosion penetration rate

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 ... 

The number of corrosion spots increases with time, but the maximum penetration rate remains roughly constant locally and with time. The penetration rate corresponds to a Gaussian distribution curve [18]. 

Pitting is a form of localized corrosion in which part of a metal surface (perhaps 1 per cent of the exposed area) is attacked. Rates of pitting penetration can be very high type 316 stainless steel in warm seawater can suffer pit penetration rates of 10 mm per year. This is a natural... 

A metal corroding at this rate would lose 100 nm of thickness per year.2 On the other end of the spectrum, measurements of reaction rates of several to hundreds of A/cm2 are needed in some transient studies of localized corrosion. A dissolution rate of 100 A/cm2 corresponds to a penetration rate of 1 km/s. Fortunately for modern society, such penetration rates last in practice for far less than one second Thus modern instrumentation allows the measurement of dissolution rates over more than 10 orders of magnitude with accuracy on the order of a few percent. 

Localized corrosion is the direct result of the breakdown of passivity at discrete sites on the material surface. As was stated above, once passivity is established on a surface, one might expect either that it would remain passive or that a complete activation of the surface would occur. However, what is often observed in practice is the appearance of discrete areas of attack that begin to corrode actively while the vast majority of the surface remains passive. These isolated regions of attack are more than mere annoyances the local penetration rates can be on the order of 10 mpy or higher, leading to rapid perforation of any reasonably sized container. Since the original intent in using passive materials (e.g., CRAs) in any application is to exploit their low dissolution rates, localized corrosion can be a major operational problem. 

Pitting is the term given to very localized corrosion that forms pits in the metal surface. If a material is liable to pitting, penetration can occur prematurely, and corrosion rate data are not a reliable guide to the equipment life. 

In review, consider a mixed electrode at which one net reaction is the transfer of metal to the solution as metal ions, and the other net reaction is the reduction of chemical species in the solution such as H+, 02, Fe3+, or N02 on the metal surface. For purposes of the present discussion, no attempt is made to define the individual sites for the anodic (net oxidation) and cathodic (net reduction) reactions. They may be homogeneously distributed, resulting in uniform corrosion, or segregated, resulting in localized corrosion. In the latter case, the cathode-to-anode area ratio is of practical importance in determining the rate of penetration at anodic areas. 

The calculation of a local corrosion rate (penetration rate) is intrinsically difficult because the area of the localized attack is not known [39]. 

A coating that is cathodic, compared to the metallic substrate subject to general uniform corrosion, may be inherently dangerous, because the presence of defects in the coating would promote the localized dissolution of the base material with a penetration rate that is higher when the exposed surface area is small, that is, when the density of defects is low. 

Experience shows that at least duplicate test specimens should be exposed in each test. Under laboratory tests, corrosion rates of duplicate specimens are usually within 10 % of each other, when the attack is uniform. Occasional exceptions, in which a large difference is observed, can occur under conditions of borderline passivity of alloys that depend on a passive film for their resistance to corrosion. If the rate difference exceeds 10 %, re-testing should be considered, unless it is observed that localized attack is predominant. Corrosion rates are calculated assuming a uniform loss of metal, and therefore when specimens are attacked non-uniformly, the calculated corrosion rates indicate only the relative severity of attack and should not be used to predict the performance of an alloy to the test solution. In such cases, weight loss per unit of surface area may be used to avoid implying a uniform penetration rate. 

Patchy biofilms and localized colonization give rise to localized corrosion reactions and to anodes and cathodes that are fixed in space and stable in time [6S] as opposed to the randomly spaced smd mobile oxidation and reduction reactions required by uniform corrosion. Under these conditions, the cedcirlated polarization resistance vsdue is correct but the anode and cathode aresrs are unknown, so that one does not know how to determine the cirrrent density (i.e., the penetration rate). Adding to the level of... 

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