Argentum Solutions, Inc.
Sterling guidance on corrosion and materials degradation
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| Corrosion Calculator |
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| Corrosion Economics Estimator |
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David C. Silverman |
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Corrosion Rate From Corrosion Current Appropriate use of electrochemical measurements such as polarization resistance, electrochemical impedance spectroscopy, Tafel extrapolation, and galvanic cell testing ultimately results in the estimation of a corrosion current density. This current is a measure of the corrosion rate. In many practical situations, this number needs to be transformed into a penetration rate to make a practical prediction in industry, to enable comparison with other mass loss derived coupon immersion results, or just be presentable to individuals not familiar with corrosion terminology. This calculator transforms corrosion current into penetration rate.The corrosion rate as a penetration rate is calculated from the corrosion current by the following equation:  (3)where: CR = corrosion rate (mil per year or mm per year depending on units of K) K = constant for converting units icor = corrosion current density (microamp/cm2 or amp/cm2) ρ = alloy density (gram/cm3) EW = alloy equivalent weight (gram/equivalent) The inputs required in the Corrosion Current Conversion Calculator are the values needed for equation (3), the corrosion current density in microamp/cm2 or amp/cm2, the alloy density in gm/cm3, and the alloy equivalent weight in gm/equivalent. Some alloy densities are available in the Table of Selected Alloy Denisities. Some alloy equivalent weights are available in the Table of Equivalent Weights. An Equivalent Weight Estimator can be used for alloys not in the table. The conversion between corrosion current and penetration rate requires using the alloy equivalent weight which is the mass per equivalent. This equivalent weight is given for a pure element by:  (4)where: EW = alloy equivalent weight (gram/equivalent) Matomic = atomic mass of the metal (gram/mole) n = valence of the metal (equivalents/mole) The equivalent weight is more complex in the case of alloys because it must account for all of the elements that are being oxidized and contributing to corrosion. If corrosion does not occur selectively then all metallic components have to be considered at their concentrations in the alloy. If only certain elements participate, then only those elements need be considered. The equivalent weight for the alloy is given by:  (5)where: EW = alloy equivalent weight (gram/equivalent) ni = valance of alloying element "i" (equivalent/mole) fi = mass fraction of alloying element "i" Ai = atomic mass of element "i" (gram/mole) The summation is run over all elements that participate in the reaction. Elements that have a mass fraction of less than 0.01 (less than 1% by weight) can usually be ignored in the calculation and can be ignored in the Equivalent Weight Estimator . When an elemental composition for the alloy in question has not been measured, the best approach is to use a value that is in the middle of the range for that element in the alloy. Use the Equivalent Weight Estimator when the alloy or valences have not been tabulated. The mass percent must be used for the estimator to function properly. The mass fraction is NOT used in this calculator. In other words, if the alloy is 316ss and the chromium composition is 17% by weight, the value to be used for the fraction of chromium (fchromium) is 17. The value to be entered is 17. This approach is taken to make data entry easier since most alloy compositions are given as percentages not mass fractions. See below for information on methods of estimating the valences. Valence assignments for elements having more than one valence state can be tricky. One approach to making the estimate of the proper valence is to measure the corrosion potential of the alloy in the environment and measure the pH. Placing that coordinate on the potential-pH diagram for the element would allow an estimate to be made of the most stable valence under those conditions. That value may or may not be the actual value because (1) the element may behave differently in the alloy and (2) the corrosion potential is at best a steady state potential not an equilibrium potential so the state of the element in the alloy may not be the equilibrium state. But, this approach should allow for a judgment to be made. Sometimes corrosion product analysis will enable a judgment to be made as well. The Table of Selected Equivalent Weights shows examples of a number of combinations for some more common alloys. Use the Equivalent Weight Estimator if the alloy is not included in the table. You, the user, must make the judgment of which valence state to use. This calculator makes the assumption that the methodology used to transform the electrochemical measurement itself into the corrosion current is correct. The tutorial on Electrochemical Impedance Spectroscopy provides some guidelines for estimating corrosion rates from this technique. ASTM G-102 "Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements" provides guidelines for estimating corrosion rates from the polarization resistance technique. Six additional criteria must be fulfilled for the corrosion rate to be estimated from polarization resistance data (e.g. from electrochemical impedance spectroscopy or polarization resistance measurements) using anodic and cathodic Tafel type constants:
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David C. Silverman, Ph.D. - Primary Consultant |