Basic Corrosion: Recurring Questions & Answers Prof. Garry W. Warren November 2007
Rationale • This presentation provides common examples of recurring questions students pose in developing their proficiency in electrochemistry & corrosion. • Typically these questions recur every year, year after year. • The vast majority of such questions relate to critical basic information covered in the first few weeks, i.e. the foundation upon which the rest of the course depends. • Computer software is a practical way to expose students to these questions.
Recurring Questions • Most recurring questions asked fall into one of the following areas: • Terminology (knowing new terms, e.g. cathode vs. anode) • Thermodynamics (e.g. using the Nernst equation) • Sign conventions (e.g. G = +nFE vs. G = –nFE) • Reference electrodes • Understanding the significance of the cathodic reaction • Understanding “corrosion potentials”
What are the critical terms to know? • Electrochemistry (corrosion) is loaded with interrelated terms somewhat unique to the discipline, for example: Anode Anodic Active Oxidation Oxidation potential EMF series Electrolytic cell Cathode Cathodic Noble Reduction Reduction potential Galvanic series Galvanic cell • Terms on left & right are related, but are NOT equal. • It is best to clearly define all of these from day one!
Critical Terminology (con’t) • On the first day of class students are provided a handout entitled “Important Corrosion Concepts to Remember” defining most of these terms. • That handout is available here: http://bama.ua.edu/~gwarren/ • The explanation of electrolytic cells vs. galvanic cells is best covered after some exposure to the EMF series and Nernst equation (also found at the end of this presentation).
What are the most important things to know about the EMF Series? • Emphasize the title “Standard Reduction Potentials, Eº” or “Standard Oxidation Potentials, Eº” • Each potential is tied to a half cell reaction. • Electrochemical reactions (corrosion) must involve two half cells: one oxidation and one reduction. • The half cell assigned a voltage of zero is the “reference” half cell. • Always include V vs. SHE or V vs. SCE (only then is choice of reference half cell clear).
Is there a connection between G & V? • The connection between Gibbs energy (G) and potential or voltage (V) is given by either: G = –nFE OR G = +nFE • The choice is a convention, either is correct. • Persistent repetition of the text’s choice of –nFEor +nFE is worthwhile for two reasons: • To emphasize that this is the text’s convention • To emphasize the importance of identifying the chosen convention when consulting other texts or references • I prefer using G = –nFE, until students have some experience with the EMF series & the Nernst equation.
Can I connect a sign convention to EMF Series? • Imagine three EMF series (no others are possible!). • The words “oxidation” or “reduction” with respect to ½ cell potentials also indicates selection of –nFE convention. Standard Oxidation Potentials (G = –nFE) Eº (V) Cu = Cu2+ +2e– –0.342 H2 = 2H+ + 2e– 0.0 Ni = Ni2+ +2e– + 0.250 Standard Reduction Potentials (G = –nFE) Eº (V) Cu2+ +2e– = Cu + 0.342 2H+ + 2e– = H20.0 Ni2+ +2e– = Ni – 0.250 Standard Potentials (G = +nFE) Eº (V) Cu2+/Cu + 0.342 H+/H20.0 Ni2+/Ni - 0.250 Values of Eº in these 2 lists are identical. When you reverse the reactions, change sign of Eº. Sign of Eº does NOT change if reactions are reversed, hence the title omits “oxidation” or “reduction”
Why doesn’t the sign of Eº change for +nFE? • Imagine three EMF series (no others are possible). Standard Oxidation Potentials (G = –nFE) Eº (V) Cu = Cu2+ +2e– – 0.342 H2 = 2H+ + 2e– 0.0 Ni = Ni2+ +2e– + 0.250 Standard Reduction Potentials (G = –nFE) Eº (V) Cu2+ +2e– = Cu + 0.342 2H+ + 2e– = H20.0 Ni2+ +2e– = Ni – 0.250 Standard Potentials (G = +nFE) Eº (V) Cu2+/Cu + 0.342 H+/H20.0 Ni2+/Ni –0.250 Values of Eº in these 2 lists are identical. For –nFE, sign of Eº “+” or “–” is chosen to agree with the thermodynamic tendency. For +nFE, sign of Eº is the experimentally observed value of selected ½ cell when connected with H+/H2 half cell, so only one value is ever observed.
Ecorr Software • Computer Aided Instruction (CAI) is a practical way to expose students to recurring questions. • Permits students to work outside class at any time • Allows more class time for other topics • Ecorr software • An introduction to corrosion, electrode potentials & electro-chemical thermodynamics. • Focuses on many recurring corrosion questions via examples and practice problems. • Ecorr is available at MaterialsTechnology@TMS: http://materialstechnology.tms.org/educ/educdigital.asp
What does Ecorr do? • The following screens give a number of examples. • Some previous exposure to thermodynamics is useful • The user interacts with the program in various ways: • Answers to questions or calculations are entered by typing in boxes or by clicking buttons • Clicking on red “hot text” opens popup windows with more information on that term, concept or calculation. • Standard potentials are available in a pull down menu • Menu allows user to navigate to other parts of program • Any screen can be printed.
Can you use potentials to predict reactions? Below is one of several examples addressing this question for standard conditions. Potentials are hot text and remind the user how each was obtained.
What if activities are not unity? First the relation of G to E yields the Nernst equation. Activity, activity coefficient and concentration are defined via hot text popup windows.
What if activities are not unity? After applying the Nernst equation to half cells, several examples for overall reactions are given. Standard potentials are obtained first as shown below.
What if activities are not unity? After obtaining Eº’s the user is led term by term through the Nernst equation to calculate the overall reaction potential. Each box requires user input, and the final answer requires a calculation. There are several other examples similar to the one shown here.
What’s the significance of the reference electrode? In principle any half cell can be selected as a reference, but only some are experimentally convenient. When selected as a reference it is assigned a value of zero volts, e.g. hydrogen or SCE shown below.
How do I convert a potential vs. SCE to another reference electrode? Such conversions are simply adjusting the zero point on the potential scale using the Eº value of the current reference electrode on the “new” scale. Two more examples involving different reference electrodes are given.
What’s the difference between a half cell potential and a corrosion potential? The diagram shows that a corrosion potential is a combination of two half cells, the oxidation of Fe and the reduction of O2?
Why is the cathodic reaction important? Several possible cathodic reactions exist. Knowing which one occurs offers different choices for limiting corrosion. Red numbers reveal popup windows that show how the value was calculated.
How can I determine the cathodic reaction? The decision is a thermodynamic one. Through Nernst eqn calculations the user determines Sum A and Sum B, then selects an answer.
What’s the difference between the Galvanic Series & EMF Series? After giving a definition of each series, the user “measures” the corrosion potential for each metal by clicking & dragging each one into the white box. This shows that Ecorr’s are not single half cells.
What’s the difference between galvanic corrosion and regular corrosion? The difference is demonstrated with a “movie” that places the reduction half cell on the surface of the more noble metal for galvanic corrosion.
Joining dissimilar metals is often necessary, how is galvanic corrosion minimized?
What’s an example of a poor choice of two dissimilar metals? Shown is one example, for Fe and brass. User must enter answers to questions in boxes.
What’s an example of a poor choice of relative areas? Combining stainless and Al is rarely a good choice, but if necessary one option is better than the other. The user must click on the appropriate image to answer.
Sign conventions are really confusing, what are my choices? This section of Ecorr can be omitted if desired. It is probably most useful for advanced study. –nFE = non-IUPAC +nFE = IUPAC
How many possibilities are there? The user can click on each button, work with the same example for each case and compare them. ONLY 4 permutations are possible!
How can I ever remember this? Practice, practice, practice! Using the buttons on this summary screen the user can review any of the four possible permutations.
What’s the difference between an electrolytic cell and a galvanic cell? This question is best answered by comparing one with the other. • Galvanic Cell • Reactions occur spontaneously when connected by a conductor or electrolyte. • Chemical energy is converted to electrical energy. • Examples: AA battery, car battery (when it is being discharged), i.e. nearly every corrosion reaction • Electrolytic Cell • Reactions do not occur without applying an external potential such that Eexternal > Ecell. • Electrical energy is used to cause the desired chemical reaction. • Examples: electroplating of Cu, Au, Ag, car battery (when it is being charged)
Isn’t the anode always negative? Absolutely not! See the two examples below. Never associate the sign of E with “anode” or “cathode.” What is always true is anode = oxidation & cathode = reduction. • Electrolytic Cell • (electrolysis of water) • Galvanic Cell e- - PS + e- O2 H2 - + Zn Cu + - Pt Pt Zn+2 sol’n Cu+2 sol’n ANODE CATHODE ANODE CATHODE 2H2O = O2+2H++2e- 2H++2e- = H2 oxidation reduction Zn = Zn+2+2e- Cu+2+2e- = Cu oxidation reduction
Remember • Corrosion is inevitable. Only under impractical conditions can it be 100% eliminated, but we can reduce or minimize it. • Ecorr program downloads are available through MaterialsTechnology@TMS http://materialstechnology.tms.org/educ/educdigital.asp • Contact information: • Comments are welcome, please reply via MaterialsTechnology@TMS