Sample Questions in Envi Degradation

CHEM3562/3 Corrosion Science/Environmental Degradation of Materials  An Introduction to the Principles and Prevention of Corrosion Tutorial #2 – for week #9: 16-20/03/2015 1.

24 g of zinc metal are dissolved in a 1 M HCl solution. How many coulombs have have been produced by the anodic process?

2.

A sheet of carbon steel one meter wide by three meters long has lost 40 g to corrosion over the past six months. Convert that mass loss to a penetration rate of the steel in mm units. What would be the total corrosion current associated with such a corrosion rate? (carbon steel density = 7.8 g/cm 3)

3.

An electrochemical measurement shows a uniform corrosion current density for a nickel base alloy 400 of 23.2 µA/cm2. Calculate the equivalent corrosion rate in mpy for this alloy. (The equivalent weight of nickel base alloy is 30.12 g/mol and the density is 8.84 g/cm 3)

4.

The Figure below represents a simplified version of the Pourbaix diagram of Zink in water. .

e

When Zinc corrodes in water, the following species can be present over the complete range of pH and potential: H+, H2, O2, OH-, H2O, Zn, Zn2+, Zn(OH)2 and ZnO22-! Relevant E° values are: Zn/Zn2+, – 0.76 V; Zn/Zn(OH)2, -0.439 V; Zn/ ZnO22-, +0.441 V; H2/H+, 0 V; O2/H2O, 1.226 V (all values measured against SHE). (a) (b) (c) (d) (e)

5.

What do the dotted lines 1, 2 mean? Write out the five reaction equations that describe the inter-conversions between species. Describe how the lines that are labelled with a-e can be obtained. Relate the above reactions to the individual lines? What can Pourbaix diagrams be used for and what are their major limitations? Define the terms “domain of immunity”, “domain of passivity” and “domain of corrosion” as they are used within Pourbaix diagram and, based on this, discuss the stability of zinc metal in water at 25oC across the range of pH. Calculate the penetration rate in mpy that is equivalent to 1 µA/cm2 for a titanium alloy containing 5% aluminum and 2.5% tin. (Relevant atomic weights are: titanium: 47.867, aluminum: 26.982, and tin 118.710 (in g/mol); densities are: titanium: 4.51, aluminum: 2.70 and tin: 7.30 (in g/cm 3)).

6.

What is the relation between the overpotential and standard potential of an electrochemical reaction?

7.

Mercury is sometimes referred to as a “high overvoltage” electrode surface and platinum as a “low overvoltage” electrode surface. Justify this terminology, using the hydrogen reduction reaction, 2H+ + 2e- → H2, as an example.

8.

Describe a simple method to verify if an electrochemical reaction is limited by a concentration polarization effect.

9.

Using the mixed potential theory and appropriate polarization diagrams (Tafel plots), determine the effect of the following parameters on the corrosion potential and corrosion rate of a Metal M corroding to dissolved M+ in an acid solution: (a) increasing the exchange current density i 0 of the anodic reaction (b) increasing the exchange current density i 0 of the cathodic reaction (c) increasing the concentration of dissolved H+ (d) increasing the Tafel constant of the anodic reaction.

10.

Assume that the corrosion of a metal M is controlled completely by activation polarization for both anodic and cathodic reactions. What is the effect of solution stirring on the corrosion rate and corrosion potential?

A. M. Wilson (2015)