Preliminary Research

First, I explored using a textbook displacement reaction using a more reactive metal e.g aluminium or magnesium. I quickly realised this wouldn’t work because the salt and metal doing the displacement would need to be in solution and that would complicate the process. Finally, after deliberating I settled on starting with acids because they could at least dissolve some of the rust and get it into solution and I could worry about displacement from there.

When deciding on which acids to use I wasn’t originally sure which ones would have what effect so I conducted a few tests to see which ones had what effect. Unfortunately, I only have the data for 0.05M HNO₃ (Nitric Acid) and 0.05M CH₃COOH (Acetic Acid/White Vinegar). Either end of an iron rod was immersed in the acid for a period of 24 hours and then removed and rinsed with a metal sponge to remove all residue.

Strong acids were found unsuitable for a number of reasons but notably because, as can be seen in Figure 2 with the visible decrease in rod thickness, they were too corrosive and because they exacerbated the odds of the rod undergoing flash rusting, as seen in area x marked on Figure 2, a name given to the phenomenon where a metal is put in a hyper reactive state often due to humidity or reaction with other highly reactive substances (such as a strong acid) and undergoes oxidation in a matter of minutes rather than the usual day to months durations. Acetic Acid was chosen as the acid of choice for this experiment simply because it was neither too corrosive nor too obscure. An integral part of my DYO was making this accessible to the average person and it’s a lot easier to get one’s hand on White Vinegar (4-6% Acetic Acid) then pure Citric Acid.

Method

The aim of this experiment is to remove the rust from the surface of the metal through the displacement of the oxygen by acetate to form ferric acetate: Fe(CH3COO)₃

Throughout multiple trials, the rods were immersed for varying periods of time of time in 0.05M Acetic Acid in a 250ml beaker and kept on the same trays in the corner of the room devoid of any light and far away from where any other experiments were being held to attempt to control the environment. The results were recorded, photographed and analysed for comparison.

Additionally, rather than simply measuring effectiveness over time I also wanted to measure effectiveness across methods. For each period of time, one end of the rod would be fully immersed while the other would have 1ml drop of the acid put on its surface and spread with the pad of a glove, left to react, and then reapplied once the previous coating evaporated/reacted. Unfortunately, due to not being in class 24/7 I was only able to assess the coating method over a 60 minute period.

I was unable to quantifiably measure the effectiveness of the vinegar in removing the rust from the metal. However, I do have a model for how I would have done so had I had more time.

Using a pure white light source, I would shine it on a part of the rusted iron rod and take a picture using a high definition camera of the resultant gradient displayed on the white piece of paper. Then, using some online program I would extract the HSL values. HSL is an alternative way to the traditional RGB model of measuring colour. Using HSL(see Figure ), I can calculate the relative rust, or less of rust, through the change in saturation of a particular hue (the hue the rust would be coloured) because as the amount of rust decreased, so would the saturation of that hue.

Results

In all trials, the 0.05M acetic acid was determined to have caused a notable deoxidation of the metal and the metal was noted to have returned to a more “pure” looking colour. However, within that deoxidation there was variation. In most cases when the rod was immersed, the deoxidised side was seen to have two distinct results: one slightly dull grey side, and a brighter grey side.

The standard dull grey side was always found to be beneath the vinegar in the beaker and seemed to follow the standard reaction expected: Fe₂O₃ + 6CH₃COOH → 2Fe(CH3COO)₃ + 3H₂O

However, the brighter side was underneath a layer of black sludge and always located on the vinegar-water boundary predominantly on the air side. This led me to believe something about the involvement of some element present in air helped catalyse the reaction. This theory was further supported by the results produced in the immersed vs spread aspect of the experiment. The side which was spread seemed to have a nicer finish, as seen in Figure 7, similar to the one seen in area x on Figure 6.

I currently have no idea what this substance is because there could be any number of elements in the air affecting the reaction (although it is most likely to be oxygen in some way) so for now I will hold off on any speculation.

Conclusion + Where do we go from here?

While unfortunately, I still have not found a way to go back from rusting, because my reaction simply turns the rust into something else which is easier to remove, I have taken the first step by finding a way to remove the rust from the surface of the metal itself. From this experiment I have been able to determine, weak acids are better in this case than strong ones, the displacement of the oxygen in the ferric oxide by acetate is a viable model of disconnecting the salt from the surface of the metal, and some element or compound found in air is affecting the reaction between acetic acid and ferric oxide in such a way that is resulting in a positive result.

Adjustments to experimental design:

The next step would be to design more scientific methods of assessing results e.g massing metals pre and post deoxidation and using the lost weight to calculate what percentage of mass was lost and from there derive what percentage was originally rusted and use that to gain a more concrete value of effectiveness.

Actually implement the light and HSL idea and test viability.

Furthermore, varying the concentration of the acid in use would also be important. While 5% acetic acid clearly works, varying the concentration and recording the amount of time taken to achieve similar results is a logical progression of the project.

Thank you for sticking with me through this though!

Sources

https://www.corrosionpedia.com/definition/992/rust

https://www.corrosionclinic.com/types_of_corrosion/pitting_corrosion.htm

https://www.sciencedirect.com/topics/engineering/pure-iron

https://workingbyhand.wordpress.com/category/rust-removal/

https://www.researchgate.net/post/What-is-the-reaction-between-acetic-acid-and-a-metal-oxide-powder

https://www.learnpick.in/questions/details/24335/why-do-vinegar-and-other-acids-remove-rust#:~:text=Vinegar%20contains%20CH3COOH%20(acetic%20acid,crumbles%20from%20the%20rust%20layer.