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ASTM G ASTM D standar A number in parentheses indicates the year of last reapproval. A superscript epsilon e indicates an editorial change since the last revision or reapproval. Scope 1. Calculation methods for converting corrosion current density values to either mass loss rates or average penetration rates are given for most engineering alloys.
In addition, some guidelines for converting polarization resistance values to corrosion rates are provided. Referenced Documents 2. This practice is intended to provide guidance in calculating mass loss and penetration rates for such alloys. Some typical values of equivalent weights for a variety of metals and alloys are provided. The conversion of these results to either mass loss or penetration rates requires additional electrochemical information. Some approaches for estimating this information are given.
Current edition approved Feb. Published May Originally published as G — Last previous edition G — 89 e1. Corrosion Current Density 4. This is accomplished by dividing the total current by the geometric area of the electrode exposed to the solution. It is assumed that the current distributes uniformly across the area used in this calculation. In the case of galvanic couples, the exposed area of the anodic specimen should be used. This calculation may be expressed as follows: Icor icor 5 A 1 where: icor 5 corrosion current density,?
A, and A 5 exposed specimen area, cm2. Other units may be used in this calculation. In some computerized polarization equipment, this calculation is made automatically after the specimen area is programmed into the computer. A sample calculation is given in Appendix X1.
For pure elements, the equivalent weight is given by: W EW 5 n 2 where: W 5 the atomic weight of the element, and n 5 the number of electrons required to oxidize an atom of the element in the corrosion process, that is, the valence of the element. It is usually assumed that the process of oxidation is uniform and Copyright? If this is not true, then the calculation approach will need to be adjusted to re? In addition, some rationale must be adopted for assigning values of n to the elements in the alloy because many elements exhibit more than one valence value.
Consider a unit mass of alloy oxidized. The electron equivalent for 1 g of an alloy, Q is then: ni? Q 5 Wi 3 where:? Therefore, the alloy equivalent weight, EW, is the reciprocal of this quantity: 1 EW 5 ni? Wi 4 unless a better basis is available.
A sample calculation is given in Appendix X2 1. It is best if an independent technique can be used to establish the proper valence for each alloying element. Sometimes it is possible to analyze the corrosion products and use those results to establish the proper valence. Another approach is to measure or estimate the electrode potential of the corroding surface.
Equilibrium diagrams showing regions of stability of various phases as a function of potential and pH may be created from thermodynamic data. These diagrams are known as Potential-pH Pourbaix diagrams and have been published by several authors 2, 3.
The appropriate diagrams for the various alloying elements can be consulted to estimate the stable valence of each element at the temperature, potential, and pH of the contacting electrolyte that existed during the test.
NOTE 2—Some of the older publications used inaccurate thermodynamic data to construct the diagrams and consequently they are in error. Normally only elements above 1 mass percent in the alloy are included in the calculation. In cases where the actual analysis of an alloy is not available, it is conventional to use the mid-range of the composition speci? B Registered trademark Haynes International. NOTE 2—Mid-range values were assumed for concentrations of alloying elements.
NOTE 3—Only consistent valence groupings were used. NOTE 4— Eq 4 was used to make these calculations. SI unit. Polarization Resistance 5.
Values of 65 and mV are also commonly used. In most programmable potentiodynamic polarization equipment, the current is converted to current density automatically and the resulting plot is of i versus E. This is equivalent to the calculation shown in 4. A sample calculation is given in Appendix X4. The reaction under mixed control will have an apparently larger b value than predicted for an activation control, and a plot of E versus log I will tend to curve to an asymptote parallel to the potential axis.
The estimation of a B value for situations involving mixed control requires more information in general and is beyond the scope of this standard.
In general, Eq 7 and Eq 8 may be used, and the corrosion rate calculated by these two approximations may be used as lower and upper limits of the true rate. NOTE 4—Electrodes exhibiting stable passivity will behave as if the anodic reaction were diffusion limited, except that the passive current density is not affected by agitation. Several approaches have been proposed based on analyses of electrode kinetic models.
See Refs for more information. For simple one electron reactions, K is usually found to be 2. A sample calculation is given in Appendix X5. G 5. The effect of solution resistance is a function of the cell geometry, but the following expression may be used to approximate its magnitude. Rp 5 Ra 2 rl 11 where: Ra 5 the apparent polarization resistance, ohm cm2, r 5 the electrolyte resistivity in ohm cm, l 5 the distance between the specimen electrode and the Luggin probe tip, or the reference electrode in cm, and Rp 5 the true polarization resistance in ohm cm2.
A sample calculation is given in Appendix X6. In this case, the magnitude of the error is proportional to scan rate. The capacitance charging effect will cause the calculated polarization resistance to be in error. Generally, this error is small with modest scan rates In cases where the corrosion potential is within 50 to mV of the reversible potential of the corroding electrode, the electrochemical reactions will occur simultaneously on the electrode surface.
This will cause either the anodic or cathodic b value to appear smaller than the corrosion reaction above. Consequently, the Stern-Geary constant B will be in? In this case, the concentration of the corroding electrode ions is generally of the same magnitude or higher than other ions participating in the corrosion process in the electrolyte surrounding the electrode.
Other redox couples that do not necessarily participate in the corrosion reaction may have similar effects. This is especially true for metals exhibiting passive behavior. Keywords 6. Therefore, the gram equivalents of the dissolved components are given by Eq 3. G X3. France, Jr. Ailor, Ed. Baboian, Ed. The American Society for Testing and Materials takes no position respecting the validity of any patent rights asserted in connection with any item mentioned in this standard.
Users of this standard are expressly advised that determination of the validity of any such patent rights, and the risk of infringement of such rights, are entirely their own responsibility.
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ASTM G102 Standard Practice for Calculation of Corrosion Rates ...
Calculation methods for converting corrosion current density values to either mass loss rates or average penetration rates are given for most engineering alloys. In addition, some guidelines for converting polarization resistance values to corrosion rates are provided. No other units of measurement are included in this standard. Multi-user access to over 3, medical device standards, regulations, expert commentaries and other documents. Learn more about the cookies we use and how to change your settings.
ASTM G102 - 89(2015)E1
Calculation methods for converting corrosion current density values to either mass loss rates or average penetration rates are given for most engineering alloys. In addition, some guidelines for converting polarization resistance values to corrosion rates are provided. No other units of measurement are included in this standard. Although the conversion of these current values into mass loss rates or penetration rates is based on Faraday's Law, the calculations can be complicated for alloys and metals with elements having multiple valence values.
Historical Version s - view previous versions of standard. More G Although the conversion of these current values into mass loss rates or penetration rates is based on Faraday's Law, the calculations can be complicated for alloys and metals with elements having multiple valence values. This practice is intended to provide guidance in calculating mass loss and penetration rates for such alloys.