Argentum Solutions, Inc.
Sterling guidance on corrosion and materials degradation
| Potential-pH Diagrams |
 |
|
| Intelligent Tools |
 |
|
 |
|
 |
|
| Corrosion Calculator |
 |
|
| Corrosion Economics Estimator |
 |
|
|
David C. Silverman |
||||||||||||||||||
|
Case Study-Titanium/chloride under acidic conditions Two questions arise when discussing a chemical reaction. The first is, "Can the desired product form as indicated by the reaction steps?" The second is, "How fast will the desired product form and what other paths might occur that decrease the reaction selectivity?" As mentioned above, thermodynamics defines the most stable state of a system. Kinetics defines the most likely path and the speed along that path to reach the final state which may or may not be the most thermodynamically stable state. Thermodynamics, therefore, addresses the first question. Kinetics, therefore, addresses the second question. In much the same way, the potential-pH diagram can be used to first estimate the most stable state of the corrosion product as a function of pH, potential, and temperature. Kinetic experiments address how quickly that state (if it formed) would be reached and, from a corrosion standpoint, if that state is one suggesting metal loss.If all species important to the corrosion process under consideration have been included, then how to interface the calculated diagram with the real world process becomes the only impediment to successful use. As mentioned in the section Background on the Potential/pH (Pourbaix) Diagram. the steady state corrosion potential and measured pH may sometimes be used as surrogate coordinates for the equilibrium potential and hydrogen ion activity to define the state of the system. This assumption means that combining the diagrams with rather simple measurements can enable the corrosion practitioner to estimate the most stable state for the particular system. More importantly, the diagrams can be used to provide guidance as to what change in corrosion potential or pH could move the system from definite corrosion as defined by a metal-containing ion being the most stable state to possible passivity as defined by a metal oxide being the most stable state. That particular usage is demonstrated in this case study. This particular case study involves the following questions. First, can Grade II titanium withstand a process stream of extremely high acidity (low pH) and a high concentration of sodium chloride almost at saturation at an elevated temperature of 75oC? Second, if titanium cannot withstand this environment, can the process be modified so that Grade II titanium can be used? The complete details can be found in D. C. Silverman, "Derivation and Application Of EMF pH Diagrams", in Electrochemical Techniques for Corrosion Engineering, R. Baboian, ed., p. 117, NACE, Houston, 1987 1  (592k),.
The only a priori information available at the time was titanium was known to
corrode catastrophically at some undefined low pH. The concept was to interface
the potential-pH diagram with simple corrosion potential and pH measurements to
estimate that low pH range and guide the direction of further experiments to define
more appropriate environmental conditions.
The separate "Metal State Diagram" and "Solution State Diagram" as calculated by THERMEXPERT are shown in these figures, For the experiments to demonstrate the possibility, the acidity of the solutions was created using a combination of sodium chloride and hydrochloric acid at several molalities. Though the abscissa has a label of pH, it actually is the negative logarithm of the hydrogen ion activity. The pH as measured by a pH electrode might not accurately reflect the hydrogen ion activity at hydrochloric acid activities of 10 to 100. The discussion in the paper indicates that the activity coefficient was calculated from the known molality of hydrochloric acid and the activity coefficient correlation developed by R. W. Potter and M. A. J. Clynne, Chem. Eng. Data, 25, 50 (1980). Other reasonable procedures could have been used for estimating the activity coefficient for hydrochloric acid in the presence of high concentrations of sodium chloride. The hydrogen ion activity was estimated by multiplying this activity coefficient by the molality. The corrosion potential was monitored for about 15 hours in a solution under a continuous nitrogen purge. The (pH,corrosion potential) coordinates indicate that the stable state of titanium is either a titanium ion possibly associated with chloride or the solid oxide TiO2. The form of the ion at low pH depends on the activity chosen for chloride ion. The possible uncertainty in this value means that some titanium ion might exist as Ti+3. Thus, both TiCl+2 and Ti+3 were included in the combined diagram. The separated diagrams reflect a very high chloride activity. The implication is that though titanium metal itself is thermodynamically unstable relative to oxidized species in this environment, the most thermodynamically stable product of the reaction is a strong function of pH. The minimum free energy state of titanium (in the absence of oxygen which only forms freely above the upper parallel line) is an ion at the lower pH values. Such a product, if it formed, would imply little or no corrosion resistance. the minimum free energy state of titanium at pH values above about 0 is titanium dioxide, an oxidized solid that does have passivating capability if it formed. Immersion tests were run at various pH values to verify if the implications of the diagram would be observed. The potential-pH diagram provided the guidance as to where the experiments should focus. The following table shows the results. Once again, pH values below 0 have been converted from millivolt measurements.
| ||||||||||||||||||
|
Previous Page: Generation of a diagram - Iron in water Next Page: Case Study-Complexing Agents-Iron/iminodiacetic acid 1 © NACE International publication and year shown in citation above. All rights reserved. Displayed with permission from NACE International, Houston, TX (http://www.nace.org). Published in Corrosion, in the month and year shown in the citation above. | ||||||||||||||||||
|
David C. Silverman, Ph.D. - Primary Consultant |
||||||||||||||||||
){kind=link}
){kind=link}
){kind=link}