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.