MTRL 456 Homework Assignment 03 Spring 2025

Assignment Brief

Provide neat, orderly concise yet complete answers to each of the following questions. Partial or full credit can only be awarded if the route to the answer is clear. Place final quantitative answers in a box for full credit.  

1. (a) Calculate the corrosion potential and penetration rate of an iron pipe carrying 0.5N sulfuric acid at 0.2 m/s at 25 °C. Assume that the entire iron surface acts as the cathode, Tafel slopes are ±0.1 V/decade, [Fe2+]=10−6M, and the exchange current densities for Fe2+/Fe and for hydrogen on iron are 10−4and 10−3A.m−2, respectively. The density and Molar mass of iron are 7.78 g.cm−3and 55.8 g.mol−1, respectively [8 pts]. (b) Sketch the theoretical polarization curves (Evans diagrams) for the cathodic and anodic half-cell reactions on the same diagram and show the corrosion potential and corrosion current density are the same as what you calculated in section a [6pts].

2. During the hot-dip galvanizing process, a steel tank is cleaned and immersed in molten zinc at 450 °C, forming a zinc layer approximately 0.15 mm thick. Calculate the time required for the zinc coating to remove completely in a deaerated acid solution at pH 4. Data: EZn2+ /Zn= −0.8 V, i0,Zn= 10 μA.cm−2, i0,H= 10−3μA/cm−2, βa= 0.08 V, βc = 0.12 V, T = 25 °C [8pts]. 

3. A steel component is immersed in an acidic solution with pH=4 and a hydrogen diffusion coefficient of 510−5cm2.s−1.

• Calculate the thickness of the diffusion layer (in μm) for the depletion of hydrogen ions on the steel surface if the limiting current is 104mA.m−2under stationary conditions [3 pts].
• Assume that upon stirring the solution, the diffusion layer thickness is reduced by a factor of 10, however, no change in the corrosion rate is observed. Briefly explain why this occurs [2 pt].

 4. Calculate the overpotential that causes the cathodic current density of hydrogen evolution on a flat platinum electrode immersed in an acid solution to reach 6,000 μA.cm−2. Assume a hydrogen exchange current density of 100 μA.cm−2and a symmetry factor of 0.50 at room temperature (25 °C) [3 pt].