What does a Pourbaix diagram tell you?
A Pourbaix diagram is a plot of the equilibrium potential of electrochemical reactions against pH. It shows how corrosion mechanisms can be examined as a function of factors such as pH, temperature and the concentrations of reacting species.
What are the limitations of Pourbaix diagrams?
Limitations of Pourbaix diagrams: The diagrams provide no information about the kinetic parameters of the corrosion reactions. The diagrams consider pure metals and aqueous solutions at standard conditions (temperature 298K, pressure 1 bar, ion concentration 10-6M).
How do you plot a Pourbaix diagram?
To plot a Pourbaix diagram the relevant Nernst equations are used. As the Nernst equation is derived entirely from thermodynamics, the Pourbaix diagram can be used to determine which species is thermodynamically stable at a given E and pH. It gives no information about the kinetics of the corrosion process.
What is immunity in Pourbaix diagram?
A simplified Pourbaix diagram indicates regions of “immunity”, “corrosion” and “passivity”, instead of the stable species. They thus give a guide to the stability of a particular metal in a specific environment. Immunity means that the metal is not attacked, while corrosion shows that general attack will occur.
What is the use of Pourbaix diagram in corrosion engineering?
Pourbaix diagrams are useful in predicting the spontaneous direction of electrochemical reactions, identifying the corrosion products and predicting the changes in environment in terms of potential and pH that result in high or low corrosive attack [8].
What is the difference between pH and EH?
pH is commonly said to have a possible range from 1 to 14, but highly acidic solutions can have pH = -1 (10N) and even -2. At 25°C, pH + pOH =14. Eh is a measure of the redox (oxidation-reduction) state of a solution or, more exactly, its solutes.
What is passivity in Pourbaix diagram?
Pourbaix diagrams provide information on three states of metals: Corrosion: active state. Passivity: forming passive layers inhibiting the corrosion process on the surface of the metal. Immunity: thermodynamic stability.
What is the Y axis on a Pourbaix diagram?
Pourbaix diagrams depict the most stable species or phase of an element in aqueous solution as a function of potential (on the y-axis) and pH (on the x-axis).
What does EH mean in chemistry?
Oxidation/Reduction Potential
Oxidation/Reduction Potential (Eh) Metal ions have strong and weak oxidizing/reducing potential. In order to evaluate redox reactions, the redox potential of a solution is measured and reported as Eh.
How do you measure eh?
Eh is measured with a platinum electrode (5 × 5 mm) and an Ag/AgCl reference electrode coupled to a voltmeter, preferably with high input resistance. 4.
What does a positive Eh value mean?
Higher Eh positive values suggest that reaction is more likely to occur spontaneously without the need of extra energy. The Eh value gives an indication of how likely a given redox reaction will occur.
What is passivation layer?
Passivation is a widely-used metal finishing process to prevent corrosion. In stainless steel, the passivation process uses nitric acid or citric acid to remove free iron from the surface. The chemical treatment leads to a protective oxide layer that is less likely to chemically react with air and cause corrosion.