Conversion factors corrosion rate

Corrosion Rate by CBD Somewhat similarly to the Tafel extrapolation method, the corrosion rate is found by intersecting the extrapolation of the linear poi tion of the second cathodic curve with the equihbrium stable corrosion potential. The intersection corrosion current is converted to a corrosion rate (mils penetration per year [mpy], 0.001 in/y) by use of a conversion factor (based upon Faraday s law, the electrochemical equivalent of the metal, its valence and gram atomic weight). For 13 alloys, this conversion factor ranges from 0.42 for nickel to 0.67 for Hastelloy B or C. For a qmck determination, 0.5 is used for most Fe, Cr, Ni, Mo, and Co alloy studies. Generally, the accuracy of the corrosion rate calculation is dependent upon the degree of linearity of the second cathodic curve when it is less than... 

Commercial instruments are generally calibrated directly in corrosion rate units and conversion factors are utilised for probes of metals other than that for which the meter is calibrated [29]. Some instruments also have data capture facilities for "unmanned monitoring. Probes may consist of from two to four elements of which at least one is of the material under test. The higher the solution resistance, the larger the number of elements in the probe, the extra elements are used to assess and nullify the effects of solution resistance. 

Activation polarization is because of a rate-controlling step within the corrosion reaction(s) at either the cathode or anode sites. An example of this can be seen with the H /H2 conversion reaction. The first step of this process, 2H+ + 2e 2H, takes place at a rapid pace. The second part of this reaction, 2H H2, occurs more slowly and can become a rate-controlling factor. 

Table 5.1 summarizes the relationship of commonly used corrosion rate units and their expressions. The conversion factors are given in Table 5.2 and Table 5.3 [16]. The current density equivalent to a corrosion rate of 1 g/m day is given in Table 5.4 [16].

Hydrogen sulfide corrosion rates depend on many factors, one of which is the conductivity of the electrolyte. Rates increase as the conductivity increases conversely, the rates decrease as the electrolyte conductivity decreases. If an electrolyte is used that has essentially no conductivity, the corrosion process would be reduced to very low levels. This is the case when using an oil mud as the drilling fluid. See Table 18.3 for the effects of HjS corrosion on sample metal rings. 

Sulfur oxides and other corrosive species are brought to react with the zinc surface in two ways dry deposition and wet deposition. Sulfur dioxide has been observed to deposit on a dry surface of galvanized steel panels until a monolayer of SO2 formed (Maato, 1982). In either case, the sulfur dioxide that deposits on the surface of the zinc forms sulfurous or other strong acids, which react with the film of zinc oxide, hydroxide, or basic carbonate to form zinc sulfate. The conversion of sulfur dioxide to sulfur-based acids may be catalyzed by nitrogen compounds in the air—usually referred to collectively as NQt compounds—and it is believed that this factor may affect corrosion rates in practice. The acids partially destroy the Film of corrosion products, which will then re-form from the underlying metal, so causing continuous corrosion by an amount equivalent to the film dissolved, hence to the amount of sulfur dioxide absorbed. Above about 85% RH, corrosion rates increase further—probably as a result of the formation of basic zinc sulfates. 

Table 1.4 indicates the conversion factors between units that are frequently used for measuring corrosion rate. 

Table 1.4 Frequently used units for measuring corrosion rate. To obtain the units indicated in the first row, one multiplies the unit in the first column by the corresponding conversion factor.

It is worth mentioning that at 0.6 M NaCI concentration for 46 days, the corrosion rate for CS and WS were of 3l00 xm per year and 3686 xm per year, respectively. For comparison purposes, it is possible to see that the performance of both steels in such aggressive environments of high chloride content is very poor, but it is even three to five times worse than in the case of total immersion tests. These corrosion rates seem to be inversely related to the iron conversion factor, which is lower (about 0.21) in the immersion tests than in the dry-wet tests (about 0.80). The different behaviors should be related to the type of iron phases, their characteristics, and their relative amounts. 

In addition, electrochemical measurements require a critical conversion factor between the measured electrical current and the corrosion rate of the alloy. These conversion factors are variable. The factor depends on the valency of the corrosion reaction, on how each element that makes up an alloy corrodes individually in the environment, and on the empirically determined Tafel slope for the corrosion mechanism (see ASTM G 102, Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements). 

Electrochemical tests are often preferred to mass loss studies as a method of measuring uniform corrosion. Such tests are preferred because in theory they (1) provide a realtime measurement of the metaUic corrosion rate, (2) can provide time-corrosion rate data on a single coupon, and (3) are rapid to perform. Disadvantages of the electrochemical tests include the requirements for comparatively expensive equipment (versus mass loss tests) and higher levels of technical expertise for data analysis. Fmthermore, the data reduction requires the use of conversion-"constants," factors applied to the results of the electrical measurements to convert the data to a corrosion rate. These constants ... 

Because corrosion is so uniform, corrosion rates for materials are often expressed in terms of metal thickness loss per unit time. The rate of uniform attack is reported in various units. One common expression is mils per year (mpy) sometimes millimeters per year is used. In the United States, it is generally reported in inches penetration per year (ipy) and milligrams per square decimeter per day (mdd). To convert from ipy to mpy, multiply the ipy value by 1000 (i.e., 0.1 in. X1000 = 100 mpy). Conversion of ipy to mdd or vice versa requires knowledge of the metal density. Conversion factors are given in Table 1.1. 

The corrosion current itself can be either estimated by using specialized electrochemical methods or by using weight-loss data and a conversion chart (Table 3.1) based on Faraday s principle. Table 3.1 provides the conversion factors between commonly used corrosion rate units for all metals and Table 3.2 describes these conversion factors adapted to iron or steel (Fe) for which n = 2, M = 55.85 g/ mol and d = 7.88 g cm... 

M = atomic weight (g - mol ) n = number of electrons involved C = constant which includes F and any other conversion factor for units, for instance, C = 0.129 when corrosion rate is in mpy, 3.27 when in mm/year and 0.00327 when units are in mm /year. 

The relationship between the magnitude of current (current density, p,A/cm ) and the rate of penetration of corrosion (mpy) is important. The rate of corrosion of different metals and alloys can be equated with 1 p,A/cm of current generated by a corroding metal as shown in Table 3.4. For instance, a conversion rate of 0.540 mpy for 316 stainless steel and 0.52 mpy for type 304 stainless steel corresponds to 1 xA/cm of current. Such factors are called electrochemical conversion factors. They can be calculated for any desired alloy or metal as shown below. 

Sufur rapidly oxidizes to sulfur dioxide in the combustion chamber, Some of this sulfur dioxide undergoes further oxidation to produce the very corrosive gas sulfur trioxide. The conversion rate is fairly low and is dependent on several factors, including ... 

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