Corrosion penetration rate

Corrosion Rate. The current densities give us an estimate of the rate of the electrochemical reactions involved in corrosion, but they do not give us a direct measurement of the corrosion rate. A simple, empirical expression can be used to quantify the rate of corrosion in terms of a quantity called the corrosion penetration rate (CPR) ... 

A piece of corroded steel plate was found in a submerged ocean vessel. It was estimated that the original area of the plate was 64.5 cm and that approximately 2.6 kg had corroded away during the submersion. Assuming a corrosion penetration rate of 200 mpy for this alloy in seawater, estimate the time of submersion in years. 

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. 

As just substantiated, these numbers apply to unit area (1 m2), and therefore, the values in the right-hand column may be taken as corrosion current densities. The corrosion penetration rate (CPR) can then be calculated from Faraday s law. For iron, CPR ( xm/year) = 1.16 icorr, where icorr is the corrosion current density in mA/m2. 

The corrosion penetration rate for B in the aerated solution in pm/year is calculated as follows (Table 4.1) ... 

Assume the homogeneous corrosion of iron in a deaerated acid solution at pH = 4 (i.e., the anodic and cathodic reactions are occurring uniformly over any unit area). Plot the anodic polarization curve for iron and the cathodic polarization curve for the hydrogen reaction. Estimate Ecorr, icorr, and the corrosion penetration rate in pm/year. Given ... 

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 ... 

Fig. 7.57 Effect of carbon and nickel content on intergranular corrosion penetration rate of 1 8 wt% Cr-base stainless steels. Alloys sensitized for 100 h at 550 °C. Immersion in boiling 65% nitric acid. Pds., periods (48 h) of exposure. Redrawn from Ref 84...

Corrosion rate is also expressed in terms of thickness of metal lost per unit time, known as the corrosion penetration rate (CPR). CPR is given by the following equation ... 

If the current generated by one of the anodic reactions expressed earlier was known, it would be possible to convert this current to an equivalent mass loss or corrosion penetration rate with a very useful relation discovered by Michael Faraday, a nineteenth century pioneer in electrochemistry. Faraday s empirical laws of electrolysis relate the current of an electrochemical reaction to the number of moles of the element being reacted and the number of moles of electrons involved. Supposing that the charge required for such reaction was one electron per molecule, as is the case for the plating or the corrosion attack of silver described respectively in Eqs. (3.11) and (3.12) ... 

In addition to the corrosion penetration rates, one must be mindful of dezincification of brasses and selective attack on some bronzes as well as see of yellow brass or bronze (Fig. 6.46 in ehap. 6). These types of corrosion contribute to the failure of the material in mechanical respects without significant weight changes or losses in thickness. 

Corrosion penetration rate—as a function of spedmen weight loss, density, area, and time of exposure... 

The corrosion rate, or the rate of material removal as a consequence of the chemical action, is an important corrosion parameter. This may be expressed as the corrosion penetration rate (CPR), or the thickness loss of material per unit of time. The formula for this calculation is... 

The following table is a summary of units for the two corrosion penetration rate schemes ... 

Corrosion Rates The rate of corrosion may be expressed as corrosion penetration rate, that is, the thickness loss of material per unit of time CPR may be determined using Equation 17.23. Mils per year and millimeters per year are the common units for this parameter. 

Using the results of Problem 17.13, compute O the corrosion penetration rate, in mpy, for the... 

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