Apart from salinity, another variable being tested in the experiment is the aeration, more specifically the presence of dissolved oxygen. Oxygen, being a major ingredient in order for corrosion to occur, can consequently affect the rate of corrosion. In the following experiment, both the magnesium and zinc are being exposed to solution instead of air. However, as oxygen is present in water (in a comparatively small proportion compared to air) in varying amounts, the corrosion of a substance in that water will differ depending on the level of dissolved oxygen. The dissolved oxygen content present in water is generally approximately 0.001%, while the oxygen content of air is approximately 21%. In marine atmospheres, the spray and waves ensures that the dissolved oxygen content is slightly higher than average (On corrosion in a marine atmosphere, 1996). Dissolved oxygen is an important factor in determining the amount of corrosion, as oxygen can contribute to destroying the hydrogen film surrounding many metals. The precise amount of hydrogen film removed and at what rate is determined by the water temperature, salinity and pressure. In this experiment, salinity is the only variable which is being tested that could have an effect on the gas solubility. As salinity increases, gas solubility decreases. As previously mentioned, the presence of sodium chloride at a corrosion site is theorised to increase the corrosion rate. Therefore, in order for the greatest rate of corrosion to occur, the salinity must be in a suitable proportion with the content of oxygen in the solution (NACE Resource centre, n/d). Magnesium and Zinc, although both metals, have differences in properties which causes corrosion, and consequently corrosion rate, to differ. Magnesium, when exposed to air, forms an impenetrable oxide which protects the rest of the metal from corrosion. However, when this oxide is scrubbed off the metal begins to corrode again. Once this oxide is removed, chloride ions are able to actively increase corrosion rate. The equation for the corrosion of Magnesium in salt solution is: 2Mg+ O_2→2MgO (in the presence of sodium chloride and water) While this reaction is taking place, it is important to note that dissolved oxygen does not play as much of a role in the corrosion rate of Magnesium as with other metals (Magnesium Corrosion, n/d). Zinc is slightly more corrosion resistant then magnesium, as it is position lower on the standard electrode potential activity series. The equation for the corrosion of Zinc in salt solution is: 2Zn+O_2→2ZnO (in the presence of sodium chloride and water) The density of each metal is also integral in determining the corrosion rate. The Zinc oxide produced from the reaction between zinc and oxygen has a density of 5.61g/cm3, while the magnesium oxide has a density of 3.58g/cm3. Therefore, it could be assumed that Magnesium corrodes at a faster rate because it is less dense. In terms of a marine context, ship architects and builders would have to ensure that the metal used had an appropriate balance of both buoyancy and density in order to reduce corrosion and avoid sinking the boat.
Discussion
In this extended experimental investigation, it was aimed to stimulate a marine environment by calculating the corrosion rate for both magnesium and zinc in different concentrations of sodium chloride solutions and varying aeration levels. In order to calculate the corrosion rate, the density and area was also recorded. The corrosion rates were then compared over both differing concentrations and in stagnant and aerated solution, as well as between the magnesium and zinc in order to create conclusions.
The first test was conducted to discover the corrosion rate of magnesium in stagnant solution, with each test tube containing a different concentration of NaCl (as calculated in the results section). Three separate recordings of the weight of magnesium were noted, each at least one day apart, in order to find how much of the metal had corroded. Along with this, the density and volume were also noted in order to calculate the corrosion rate, using the formula noted in the results and calculations section. As shown by graph 1, as the concentration of sodium chloride increased, as did the corrosion rate for each strip of magnesium. This result is supported by theory and was predicted in the hypothesis, as according to the thesis written by Katie Webster of Boston University, the chloride ions in the sodium chloride solution have the ability to negate the passive film surrounding the magnesium and consequently allow it to form crevices in the metal which act as the anode and allow the metal to be oxidised. Assuming that the chloride ions and metal volume exist in a proportional relationship, it can be assumed that the more chlorides ions present in the solution, the greater the corrosion rate of the magnesium. However, due to the nature of the trendline present on the first graph, it is unlikely that a proportional relationship exists. The corrosion rate, while still increasing, begins to flatten out after the concentration of sodium chloride in the solution reaches 0.16 mol/L. This indicates that while the magnesium is still corroding, the rate at which it continues to corrode is decreasing. This is due to the small amount of metal placed in the solution, and at the time the final measurement for the corresponding NaCl concentration was recorded the magnesium had almost completely corroded away; hence the plateau of the trendline.
The third test conducted was aimed at identifying the corrosion rate of zinc in stagnant solution, with the same concentrations of NaCl being used as the test for magnesium in sodium chloride solution. However, instead of receiving a gradual trend which showed the corrosion rate of the zinc in the highest concentrated solution as having the greatest corrosion rate, this result was the lowest, and on average all results were less than magnesium. Although there were not enough tests conducted in order to fully explain this occurrence, it can be tentatively stated that this unpredicted result was a result of lack of time spanning the experiment and human error. Due to the fact that
