Composite manufacturing and testing ENG 364 Materials Engineering Laboratory PRAC 3 – Corrosion rate and galvanic series measurements. Aim · To measure corrosion rate for metallic materials · To...

Composite manufacturing and testing
ENG 364 Materials Engineering Laboratory
PRAC 3 – Co
osion rate and galvanic series measurements.
Aim
· To measure co
osion rate for metallic materials
· To develop a galvanic series for engineering metals in sea wate
· To understand how to use the galvanic series in co
osion protection
Theory
Co
osion is the loss or degradation of materials due to electrochemical reactions. The co
osion rate of metals is determined by their ability to “give up” electrons. The co
oding metal is the anode and it undergoes oxidation. For example:
Fe XXXXXXXXXXFe+2 + 2e        (1)
Zn XXXXXXXXXXZn+2 + 2e        (2)
Al XXXXXXXXXXAl+3 + 3e        (3)
On the other hand, for co
osion to occur there should be a cathodic reaction where the electrons generated at the anode are accepted. In aqueous co
osion, the cathodic reactions are:
Hydrogen evolution                     2H+ + 2e     H2    (4)
Oxygen evolution (acid solution)            O2 + 4H+ + 4e      XXXXXXXXXX2H2O    (5)
Oxygen reduction (neutral or basic solutions)    O2 + H2O+ 4e XXXXXXXXXX4OH-    (6)
IMPORTANT:
Stopping either the cathodic or the anodic reaction will inhibit co
osion. Mechanism of co
osion protection by paint or inhibitors is based on this idea.
By reversing the flow of cu
ent, i.e. supplying electrons to a co
oding structure, co
osion can be stopped or reduced considerably.
Uniform co
osion
Metals undergo both localised and uniform co
osion. Surface of metal is uniformly co
oded in presence of co
osive electrolyte. Co
osion rate is measured by weight loss, and rates are expressed as the loss of thickness of components. In engineering loss of thickness means loss of load bearing area and that is, loss of load bearing ability. An example of uniform co
osion would be roof tops of most houses which are made of galvanised iron sheets.
Galvanic co
osion
Metals when immersed in an electrolyte assume an electrical potential depending on its propensity to co
ode. A metal that has a higher positive potential becomes a more noble metal. For example, gold is a noble metal and has a high positive potential in most environments. When a noble is connected to a less noble metal, the less noble metal will likely undergo more co
osion. This co
osion is higher than when it is not connected to a more noble metal. There are exceptions to this especially in the case of metals such as stainless steels, titanium and aluminium. By measuring the potentials in a given solution against a reference electrode, it is possible to construct a galvanic series.
Procedure
I. Co
osion rate determination (must prepare this a week before you make measurements)
The co
osion rate is determined using the equation below.
Co
osion rate (mm/year) = 87.6 X (w / DAT) (Equation. 7)
w – weight loss in milligrams
D – metal density in g/cm3
A – area of sample in cm2
T – Time of exposure in hours
Laboratory equipment:
· Commercial bleach
· Beakers
· Digital balance
· Tweezers
· Gloves
· Safety glasses
· Apron (
ing your own as bleach might discolour your clothing)
· Iron nails (without galvanising)
· Galvanised iron nails
1. Estimate the surface area of the nails. Record values in Table 1.
2. Measure the weight of each of the nails on a digital balance. Record three significant figures. Do not touch the nails directly but use the tweezers.
3. You must wear gloves when handling bleach or the nails. Fill a beaker with sufficient amount of commercial bleach. Immerse the two nails in the beaker. Record time of immersion.
4. Leave nails immersed for a week. Describe the physical reaction on the surface of the nails while immersed (register changes in physical appearance and colour)
5. After a week carefully remove the nails with the tweezers from the beaker, and rinse them thoroughly with water. Note the time of removal. Dry them immediately with air hose. You must wear gloves
6. Again, measure the weight of nails, and record values in Table 1.
7. Record changes on their surface.
8. Calculate the co
osion rate of the nails using Equation 7.
Table 1
    Specimen
    Surface area (cm2)
    Initial weight (g)
    Final weight (g)
    Weight loss (mg)
    Iron nail
    
    
    
    
    Galvanized iron nail
    
    
    
    
II. Construct galvanic series
Laboratory equipment:
· 3.5% NaCl electrolyte about 250ml
· Multimeter
· Beakers
· Connectors with crocodile clips
· Stainless steel
· Aluminium
· Copper (or
ass)
· Titanium
· Magnesium
· Galvanised steel nails
· Steel nails
· Ag/AgCl reference electrode
1. Fill a beaker with 250 ml of sodium chloride solution.
2. Immerse the reference electrode in to the solution.
3. Now immerse one metal in to the solution; start with steel nail and progress to other metals. Keep the metal and the reference electrode away from each other. Do not let the solution touch the electrical contacts.
4. Using a multimeter measure the voltage between the reference electrode and the metal.as shown in Fig.1
5. Make a table of the voltages measured and construct a galvanic series.
Voltmete
Reference Electrode
Metal
3.5% NaCl solution
Fig.1 Set up for measuring the voltage of a metal immersed in an electrolyte
Questions
1. What is the significance of co
osion rate and how will you incorporate this in structural design?
2. Calculate the total cu
ent requirement for a year to protect the steel nails from co
oding based on the chemical formula in equation 1.
3. If the surface area of a more noble metal is doubled what will happen to the co
osion rate when connected to a less noble metal? Explain why.
4. Based on the galvanic series Discuss 3 ways in which galvanic co
osion can be prevented. Explain with schematic diagrams.
5. Using the voltages measured for each metal, show where the voltages are placed on Pou
aix diagrams of metals. pH of the 3.5% solution is 7.
Report: Not more than 4 pages
The report should contain:
· Results, charts, discussion including comparison to theory, predictions and e
o
Conclusions and suggestions for improvement
Marking Scheme
    
Item
    
Marks allocated
    Results, charts
    
25
    Conclusions and suggestions for improvement
    15
    Answers to questions
    60
    Total
    100

Experimental data for lab 4: Co
osion
Measurement of co
osion rates was ca
ied out using steel nails (one galvanised, one not) suspended in a bleach solution. The nails were measured in all dimensions, weighed, and placed in bleach for exactly three days. After this period they were removed from the bleach, cleaned and dried and weighed again. The co
osion rate is required to measure as a function of lost mass, density, surface area and time.
The dimensions and surfaces of the nails are detailed in Figure A1. The surface area of each nail is required to estimate in the following way:
· Area 1 – the cylindrical surface of the nail shaft
· Area 2 – the (approximately) cylindrical surface of the nail head
· Area 3 – the circular surface of the nail head
· Area 4 – the annular surface of the underside of the nail head
· Area 5 – one-quarter of the area of the nail’s point, approximated as a triangle
· Figure A2 shows how Area 5 was approximated from several i
egular surfaces
Observation of the nails after removing them from bleach after three days (72 hours) period:
    Specimen
    Initial weight (g)
    Final weight (g)
    Iron nail (Non Galvanised)
    2.42
    2.22
    Galvanized iron nail
    2.67
    2.57
Galvanic Table Measurements:
The voltages for the Galvanic table were measured against the Ag-AgCl reference electrode. Each material’s place in the Galvanic table is independent of the reference electrode used, so voltages are given as measured in the table below:

Lavf XXXXXXXXXX