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David C. Silverman |
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Case Study-Complexing Agents-Iron/iminodiacetic acid Organic compounds that may have a profound effect on corrosion are those that can react with metals to create metal-organic compound adducts. Corrosion inhibitors and corrosion accelerators are examples. Thermodynamic calculations offer a way to determine if the metal-organic compound adduct is the lowest free energy state as a function of pH and potential. Kinetic experiments are required to determine if the adduct is formed and how its formation affects corrosion, i.e. as a corrosion inhibitor or a corrosion accelerator.An internally consistent thermodynamic data base has to be available before the potential-pH diagram can be calculated and applied to prediction of the effects of such adducts. One procedure that might be used for the development of data such as these has four main steps as shown in this figure  (303k),.
This application involved the need to identify the cause of excessive corrosion of carbon steel when exposed to a certain waste stream. Corrosion rates as high as 1 cm/year were measured in this waste stream in the laboratory. In addition, when this stream was mixed with others, the corrosion rate was found to increase substantially relative to its absence. The question was posed as to what component might be causing this increased corrosion. Attention turned to iminodiacetic acid as the culprit because is was one of only a few components unique to that stream and it was known to form adducts with various metals. The procedure used to interface the diagram with the real world was similar to that described in more detail the section Case Study - Titanium with chloride under acidic conditions. As mentioned on other sections, the diagram is calculated from measured or estimated thermodynamic data and drawn as a function of hydrogen ion activity and equilibrium potential. The real world is usually represented by the measured pH and the corrosion potential. The interfacing is done by placing the measured pH and steady-state corrosion potential on the diagram. The assumptions are that the measured pH value approximates the hydrogen ion activity and the measured steady state corrosion potential approximates the thermodynamic driving force for corrosion product formation. Thermodynamic properties typical for iron were presented in the section Generation of a diagram - Iron in water. Iminodiacetic acid (IDA) has four known ionic forms across the pH range of -2 to 16. These forms are related by acid-base equilibria. The four forms are written in shorthand as H3IDA+, H2IDA, HIDA-, and IDA-2. Though all four forms would be expected to occur across a wide pH range, each would likely be the predominant form when the pH lies between the pK values for each of the acid-base reactions in which it is a member. To simplify the calculation, that species is assumed to be the only one that interacts with iron across that pH range. In other words, the species is assumed to change abruptly at the pH at which pH=pK. For example, the pK of the acid-base reaction between HIDA- and IDA-2 is 9.79 and the pK of the acid base reaction between HIDA- and H2IDA (dissolved and uncharged form) is 2.84 at 25oC. The species HIDA- is assumed to be the predominant species across the pH range of 2.84 to 9.79 shifting to IDA-2 above a pH of 9.79 and to H2IDA below a pH of 2.84. Though this assumption of an abrupt change at the boundary is a simplification, it is reasonably justified by the fact that the concentrations of the other species (H2IDA and IDA-2) decrease very rapidly as the pH moves away from the pK values and into the region of predominance of HIDA-. This table shows the pH range over which each species would be predominant.
The above assumption means that four separate potential-pH diagrams have to be generated for the IDA-iron interaction if the entire pH range of -2 to 16 is to be covered. The form of IDA interacting with iron changes at each boundary. In this particular case, the measured pH was about 8 so the only interaction that was required was that between HIDA- and iron. The waste stream temperature was 50oC. The IDA concentration was about 0.04. All iron-IDA adducts were assumed to be at activities of 10-5 and all other iron-containing species were assumed to be at activities of 10-6. The resulting Metal State Diagram is Note that the above analysis has also been used to examine the effect of pH on the ability of aspartic acid to accelerate or inhibit corrosion of iron. That study can be found in D. J. Kalota and D. C. Silverman, "Behavior of Aspartic Acid as a Corrosion Inhibitor for Steel", Corrosion, Vol. 50, No. 2, p. 138 (1994)1 (439k),.
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David C. Silverman, Ph.D. - Primary Consultant |
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