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Name: Adam
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 2001 - 2002


Question:
I am doing a science project about the oxidation process of several kinds of metals such as Iron, Copper, Aluminum... but I need to know the types of oxidation agents I can find for the experiment. My goal is to find out whether or not the metals rust, if they do, to what extent. So I need to compare the level of rusting, is there a scale of that I can refer to? If not, how do people usually describe the level of rusting of metals? Also, I know that making a protective layer on metals can prevent them from rusting, so I wonder if I can coat the metals myself by using some simple substances to make the project more interesting.


Replies:



Replies:
First, you should use the word "corrosion" instead of "rust" because "rust" refers to the oxidation of iron-containing metals. It does not usually apply to the attack on other metals.

Second, standard ways of evaluating rust are: visual qualitative observation (not very quantitative), or weight change (usually weight loss). This simple method entails taking metal panels (use rectangular panels about 3 cm by 9 cm). Drill a small hole in the end of each panel. Clean them with dish soap and water, rinse with distilled water and dry them with acetone. This gets rid of any residual oil. From then on only handle the panels with gloves, or by the edges.

Third, immerse each panel completely in a corrosive solution (see below) with a piece of nylon monofilament fishing string tied through each hole, so you can remove the panels from the solutions. After a certain time remove the panel from the solution, rinse it with distilled/deionized water using a plastic kitchen scrubber pad to remove any loose scale. You do not want this to be very aggressive, because you don't want to remove any metal, only loose stuff. It is important that EACH SYSTEM BE IN A SEPARATE GLASS CONTAINER. Otherwise, you could set up a galvanic cell which could really confuse your interpretation. Galvanic corrosion is a whole other topic for study.

Fourth, corrosion will involve the following factors:
1. the type of metal.
2. the composition of the corrosion solution.
3. the pH of the solution.
4. the temperature of the solution.
5. the time interval of immersion.
6. the presence/absence of protective coatings of various types.

You will have to decide on what combinations of these variables you want to use. In all cases the amount of solution should be large enough that its composition does not change appreciable during the course of the testing. A better way may be to replace the solution with fresh solution every time you remove the panel for weighing (Yeah, I like this method better). Common solutions are: HCl @ pH about 2 with/without NaCl @ about 3%, NaOH @ about pH=10, distilled water alone, household bleach. There are many combinations that you could choose, but don't use substances that are volatile like ammonia for example. You can decide visually how often you want to weigh the panels, and you may decide that certain panels are either destroyed (or not affected at all say after 7 days) and remove and replace them with other tests.

Plot weight loss vs. time, log(weight loss) vs. time. If the corrosion is zero order the former plot will be a straight line; if the process is first order the latter plot will be a straight line. Your teacher can help you with how to "crunch" the numbers.

Vince Calder


Adam,

In a technical sense of the word, only iron-based metals "rust." Other metals are said to have "oxidized." In your example, iron rusts; copper and aluminum form oxides. Use a chemistry reference source to look up the relative reactivities of copper and aluminum as well as the properties of their oxides.

Iron oxide (rust) is rather porous. This means that oxygen can pass into and through the oxide film to reach the unoxidized underlying metal. As a result, gradually over time, iron will completely rust away. Aluminum, on the other hand forms a non-porous oxide film that prevents oxygen from penetrating to the base metal.

Because of the variable rate at which iron oxidizes -- depending on temperature, the oxygen content of the air in contact with the metal, moisture levels, presence or absence of salt contamination, presence or absence of paint and/or other protective film/s -- I know of no scale that one might use to assess the extent of rusting on a particular specimen of metal.

I suggest you use iron-based metal in your study and -- rather than to complicate things unnecessarily -- simply mention the oxidative properties of copper and aluminum. Steel wool oxidizes easily because it offers a large surface area for attack. Get a few pads of steel wool, wash them thoroughly on detergent solution, dry them thoroughly and store them in a sealed jar away from air. Use small samples of the wool and be sure to test the oxidation rate of damp and dry wool as a control. Weigh samples in a pre-weighed, corked test tube and then expose them to air. As they oxidize, the specimens will appear to gain weight -- that's because iron is combining with oxygen to make an oxide that weighs more than the parent metal.

Use a little ingenuity and you can figure out how to coat the specimens with various protective substances -- taking into account the weight of the protectant -- and you should be able to measure the effectiveness of each.

Regards,
ProfHoff 401



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