Partial Topic report/ Discussion

Topic Report
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

Results, error calculations and corrosion rate

Table 1- Corrosion Rate of Magnesium in stagnant solution Test Tube Number Amount of NaCl in Tube (g) Weight of Magnesium strip (g) weights used to estimate weight loss Dissolved Oxygen content (mg/L) Density (cm3) Volume (cm3) Corrosion Rate (mmpy) 1 1.0 Original: 0.010 Test 1: 0.015 Test 2: 0.008 7.4 3.3 0.003 12.3 2 0.5 Original: 0.033 Test 1: 0.017 Test 2: 0.007 6.8 11.0 0.003 8.3 3 0.3 Original: 0.016 Test 1: 0.013 Test 2: 0.007 6.6 5.3 0.003 5.7 Table 2- Corrosion Rate of Magnesium in aerated solution Test Tube Number Amount of NaCl in Tube (g) Weight of Magnesium strip (g) weights used to estimate weight loss Dissolved Oxygen content (mg/L) Density (cm3) Volume (cm3) Corrosion Rate (mmpy) 1 1.0 Original: 0.007 Test 1: 0.003 Test 2: nil 7.9 2.3 0.003 52.9 2 0.5 Original: 0.012 Test 1: 0.005 Test 2: nil 6.5 4.0 0.003 30.4 3 0.3 Original: 0.012 Test 1: 0.005 Test 2: nil 7.8 4.0 0.003 30.4 Table 3- Corrosion Rate of Zinc in stagnant solution Test Tube Number Amount of NaCl in Tube (g) Weight of Zinc strip (g) weights used to estimate weight loss Dissolved Oxygen content (mg/L) Density (cm3) Volume (cm3) Corrosion Rate (mmpy) 1 1.0 Original: 0.221 Test 1: 0.220 Test 2: 0.218 6.1 12.3 0.018 2.0 2 0.5 Original: 0.190 Test 1: 0.191 Test 2: 0.189 6.0 10.6 0.018 2.3 3 0.3 Original: 0.211 Test 1: 0.211 Test 2: 0.210 5.9 11.7 0.018 2.1 Table 4- Corrosion Rate of Zinc in aerated solution Test Tube Number Amount of NaCl in Tube (g) Weight of Zinc strip (g) weights used to estimate weight loss Dissolved Oxygen content (mg/L) Density (cm3) Volume (cm3) Corrosion Rate (mmpy) 1 1.0 Original: 0.284 Test 1: 0.282 Test 2: 0.281 9.6 15.8 0.018 7.7 2 0.5 Original: 0.258 Test 1: 0.258 Test 2: 0.256 9.5 14.3 0.018 4.3 3 0.3 Original: 0.280 Test 1: 0.281 Test 2: 0.280 7.8 15.6 0.018 *Not enough change to even make an estimate as to the amount of corrosion Calculations: Error and Uncertainty- Table 1: % uncertainty for weight of magnesium strip (test 1): % uncertainty= 0.05/0.01 × 100=500% % uncertainty for weight of magnesium strip (test 2): % uncertainty= 0.05/0.033 × 100=151.5% % uncertainty for weight of magnesium strip (test 3): % uncertainty= 0.05/0.016 × 100=312.5% Table 2: % uncertainty for weight of magnesium strip (aerated-test 1): % uncertainty= 0.05/0.007 × 100=714.3% % uncertainty for weight of magnesium strip (aerated-test 2): % uncertainty= 0.05/0.012 × 100=416.7% % uncertainty for weight of magnesium strip (aerated-test 3): % uncertainty= 0.05/0.012 × 100=416.7% Table 3: % uncertainty for weight of zinc strip (test 1): % uncertainty= 0.05/0.221 × 100=22.6% % uncertainty for weight of zinc strip (test 2): % uncertainty= 0.05/0.190 × 100=26.3% % uncertainty for weight of zinc strip (test 3): % uncertainty= 0.05/0.211 × 100=23.7% Table 4: % uncertainty for weight of zinc strip (aerated)(test 1): % uncertainty= 0.05/0.284 × 100=17.6% % uncertainty for weight of zinc strip (aerated)(test 2): % uncertainty= 0.05/0.258 × 100=19.4% % uncertainty for weight of zinc strip (aerated)(test 3): % uncertainty= 0.05/0.280 × 100=17.8% Corrosion Rate: All corrosion rates were calculated based on the formula; mmpy=87.6 × W/DAT where W=weight loss in grams; A= area of metal;D=metal density; T=time exposed

Topic Report Draft

Introduction- This extended experimental investigation is aimed at investigating the scientific principle of corrosion rate, both in regards to magnesium and zinc. In order to examine the concept thoroughly, the variables of salt and aeration will be altered and the different effects noted. Magnesium is a highly versatile, reactive metal which reacts easily with water to create hydrogen bubbles in solution. When in contact with air, an impenetrable oxide is formed which protects the rest of the metal from corrosion. Zinc, however, is less reactive than magnesium due to its lower standard electrode potential, though acts as a strong reducing agent. Corrosion rate is known to increase when salt concentration is increased, as the water holds a greater capacity to carry a charge due to its increased amount of electrons in solution. Aeration is also thought to increase corrosion rate (in comparison to metals in stagnant solutions), as oxygen is the main component in corrosion. If there are increased oxygen levels, then there will be increased corrosion. Discussion- Corrosion is generally known as any degradation of materials through chemical methods, which is commonly with some reference to metals. There are two very broad areas of corrosion, the first being known as Direct Chemical Attack and the second Galvanic Corrosion. Galvanic corrosion occurs when two metals with dissimilar properties are placed in each other’s presence with a connection of an electrolyte. In this situation, the Galvanic series can be referred to in order to determine which metal becomes cathodic and anodic if a reaction should take place. In the following experiment, however, the Direct Chemical Attack (or generalised) area of corrosion will be investigated. The Direct Chemical Attack method refers to when metals are exposed to their environment, commonly in terms of moisture and air. In this situation, the metals are not reacting in cells; rather any part of the metal exposed to that environment should react accordingly. In order for corrosion to occur, an oxidation-reduction reaction must occur, generally with respect to anodes and cathodes. An oxidation-reduction reaction can only occur when there is two equal half reactions, one oxidising and one reducing. This is to ensure that any electrons being transferred have somewhere to go. - how does corrosion occur (reactions) As previously mentioned, the presence of sodium chloride at a corrosion site is theorised to increase the corrosion rate. Therefore, it could be assumed that an increase in in the sodium chloride concentration would correspond with an increase in the rate of corrosion of a particular substance. - Salt concentration effect 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%. However, moisture is obviously a major factor in the increased corrosion rate in wet or humid environments. -aeration effect (dissolved oxygen) Magnesium and Zinc, although both metals, have differences in properties which causes corrosion, and consequently corrosion rate, to differ. - difference between magnesium and zinc - density, SA

Hypothesis, Aim and revised equipment/method

Aim- To investigate the effect of salt concentration on corrosion rate of magnesium and zinc in stagnant and aerated solutions. Hypothesis- If the concentration of salt in solution is increased, then the corrosion rate for both magnesium and zinc will increase. If the salt concentration remains the same, though the solution is aerated, then the corrosion rate will increase again for both metals. Materials- 50g Sodium Chloride 20cm Magnesium Strips 20cm Zinc Strips Metal Scour Dissolved Oxygen Probe Marking Pen Distilled water Sensitive weight scales Test Tube Rack 12 x Test tubes Mortar and Pestle Procedure- 1) A test tube rack was obtained and three of the test tubes were numbered and placed chronologically in the rack. A table was drawn to record the results, with the numbers used to indicate each test tube. 2) 10mL of distilled water was added to each test tube, making sure not to disturb the water more than absolutely necessary. 3) Using the mortar and pestle, the NaCl was crushed into a consistent fine powder. 1g of the powdered sodium chloride was placed into test tube number 1, 0.5g into test tube number 2 and 3g into test tube number 3. Each test tube was swirled slightly to dissolve. 4) The dissolved oxygen content was then measured for each of the solutions in the test tubes and the results recorded. 5) 3 x 1cm long magnesium strips were scoured in order to remove the oxide present, before being weighed and measure (length, width and depth) and added to each of the three test tubes respectively, remembering to place the correctly weighing strip in the corresponding tube. Leave for three days, recording as many observations as possible. 3) Repeat above steps with zinc. 4) Repeat previous tests for both Zinc and Magnesium, though before doing so ensure water is aerated. Add required amounts of salt before testing dissolved oxygen levels. 5) EXTENSION: If there is an appropriate amount of time, add both magnesium and zinc to the same test tube and compare any differences in corrosion rate.

Method/Equipment

Equipment: 50g NaCl Distilled water Sensitive weight scales Dissolved Oxygen probe Test tube rack Test tubes 20cm Magnesium 20g Zinc Method: 1) Obtain test tube rack containing three separate test tubes. Add 20mL distilled water to each test tube, before placing 1, 3 and 7g of NaCl in each and swirling to dissolve. Measure Dissolved oxygen content. 2) Scour oxide off 3 x 1cm long strips of magnesium. Record weight and calculate density and area of metal. Place each strip into one each of the three test tubes, and leave for one day. 3) Repeat above two steps with Zinc, and considering the metal is in crystal format, use exact weight of the magnesium strips in each test tube. 4) Repeat previous tests for both Zinc and Magnesium, though before doing so ensure water is aerated. Add required amounts of salt before testing dissolved oxygen levels.

Research

http://www.electrochemsci.org/papers/vol7/7054235.pdf (Good site about the Mg in stagnant and aerated solutions) Mg is very reactive metal because of which, its free element is not naturally found on earth, though once produced, it is coated in a thin layer of its oxide. The formation of this layer on the surface of Mg and its alloys does not provide full protection against corrosion, especially in chloride containing environments. This occurs as the Cl‒ ions from the surrounding environment penetrate the oxide layer and reach the surface of Mg then react with the metal substrate. The high corrosion susceptibility is also expected as a result of the high active potential of Mg. The presence of impurities and second phases within Mg alloys can be considered as active cathodic sites, which accelerate the local galvanic corrosion of the alloy matrix [11]. The presence of impurities such as iron, copper and nickel in low amounts increases the corrosion resistant of the Mg alloy. These elements if present in high contents will act as active cathodes with small hydrogen over voltage and result in dissolution of the Mg matrix [12-14]. This explains why the corrosion and corrosion protection of Mg in different aggressive electrolytes have been reported [15-19]. The objective of the current work is to compare between the effects of naturally aerated stagnant Arabian Gulf seawater (AGS) and 3.5% sodium chloride (3.5% NaCl) solutions on the corrosion of magnesium after its immersion for 1 hour and 6 days. The study was carried out using weight-loss immersion test for 600 hours along with SEM/EDX investigations and variety of electrochemical measurements such as polarization (CPP), current time (PCT) and electrochemical impedance spectroscopy. It is well known that AGS is a complex mixture of inorganic salts, dissolved gases, suspended solids, organic matter and organisms [20,21]. It has inorganic salts that include Na+, Mg2+, K+, Ca2+, and Sr2+ as well as very high concentrations of chloride (24090 mg/L), and sulfate (3384 mg/L) ions. AGS also has HCO3-, Br-, and F-, with total dissolved solids (TDS) of 43800 mg/L [21]. The presence of such high TDS concentration represents a very corrosive medium. Oxygen content also has a marked effect on corrosivity of seawater. At the same time, 3.5% NaCl solution has been also reported to simulate the percentage of chloride ion in the seawater and to be an aggressive medium towards many metals and alloys [22-41]. The weight loss (Δm, mg.cm−2) and the corrosion rate (KCorr, mg.cm−2.h−1) over the exposure period were calculated according to the previous studies as follows [40-43]: ) 1(A int fin m m m ) 2(A t int fin Corr m m K SEE WEBPAGE FOR PROPER FORMULAE Where, mint is the initial weight before immersion, mfin is the final weight after exposure to the test solution, A is the total surface area, 54.0 cm2, and t is the time of exposure in hours.

Fleshing out idea

General Aim (altered)- The effect of salt concentration on corrosion rate of magnesium and zinc in stagnant and aerated solutions.

                - Possible extension; Compare results to different metals such as steel and aluminum.

How to measure corrosion rate:

The rate of corrosion is the speed at which a metal deteriorates in a specific environment

In order to calculate the rate of corrosion, the following information must be collected:

•Weight loss (the decrease in metal weight during the reference time period)

•Density (density of the metal)

•Area (total initial surface area of the metal piece)

•Time (the length of the reference time period)

To do-

-       

   Identify topic and central concept of interest

-          Refine ideas to generate a hypothesis

-          Complete risk assessment for equipment/materials list

Components to measure:

-          Concentration of salt (three different levels)

-          Corrosion rate, comparably, of magnesium and zinc (see page 365/371)

-          Difference in rate in stagnant and aerated solutions. Dissolved oxygen (aerated/stagnant solutions)

Possible quantitative measurements:

-          Corrosion rate in aerated/stagnant solutions

-          Compare standard electrode potentials with results for magnesium and zinc

-          Graph amount of oxygen with rate of corrosion, then compare for both metals.

-          Concentration of salt and level of oxygen vs corrosion rate

Magnesium, before I use it, will already have oxidized to a certain degree depending on the amount of oxygen the strip has been exposed to. Therefore, there will be a protective layer of black surrounding the metal, preventing any further oxidation to occur. Before I begin my experiment, I need to scrub this oxidized layer off to ensure further oxidation in the salt water has occurred.

RISK ASSESSMENT:*Corrosion is faster when salt is present in the water

http://avstop.com/maint/corrosion/ch2.html

203. FACTORS INFLUENCING CORROSION.
a. Some factors which influence metal corrosion and the rate of corrosion are the:
(1) Type of metal;
(2) Heat treatment and grain direction;
(3) Presence of a dissimilar, less corrodible metal (galvanic corrosion);
(4) Anode and cathode surface areas (in galvanic corrosion);
(5) Temperature;
(6) Presence of electrolytes (hard water, salt water, battery fluids, etc.);
(7) Availability of oxygen;
(8) Presence of different concentrations of the same electrolyte;
(9) Presence of biological organisms;
(10) Mechanical stress on the corroding metal; and
(11) Time of exposure to a corrosive environment.
(2) Oxygen concentration cells.
The solution in contact with the metal surface will normally contain dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse into the solution, thereby creating a difference in oxygen concentration between two points. Typical locations of oxygen concentration cells are under either metallic or nonmetallic deposits on the metal surface and under faying surfaces such as riveted lap joints. Oxygen cells can also develop under gaskets, wood, rubber, and other materials in contact with the metal surface. Corrosion will occur at the area of low oxygen concentration (anode) as illustrated in Figure 2-6. Alloys, such as stainless steel, which owe their corrosion resistance to surface passivity, are particularly susceptible to this type of crevice corrosion.
c. Salts.
Most salt solutions are good electrolytes and can promote corrosive attack. Some stainless steel alloys are resistant to attack by salt solutions but aluminum alloys, magnesium alloys, and other steels are extremely vulnerable. Exposure of airframe materials to salts or their solutions is extremely undesirable.