Rust experiment, different coatings

Jan 20, 1999
My daughter is working on a science project in which the finish was ground off roofing disks and they were not coated or coated with various substances and immersed in a saltwater solution for 17 days. I have posted pictures at the links indicated, if anyone is interested. They are rather large, so load times may be slow, unless you have a fast connection. The numbers given below, in parentheses after the various products, are readings taken at the end with as much rust rubbed off as possible with a paper towel. The small section on the edge is the original coating that was not ground off. The ones that were ground off on a belt sander (120 grit) and only exposed to air are shinier than shown by the picture.

The odd part of the experiment were the un-coated disks. Once the disks were rubbed off they had no red rust. The metal had turned gray or in other words developed a patina, but no red rust. A .5 was used to indicate this change. As can be seen from the numbers, they did better than most of the coated disks. Can anyone explain this?

The reason for the rinsing and then rubbing was to get rid of rust that had just accumulated in the water and come from the edges, but was not necessarily from the surface of the disks. Rusted edges did not count.

Rust Preventative Used from bottom left and around counterclockwise, i.e., bottom left to right, up to next row, top left to right and then last one on top.

None used (.5, .5 .5), Renaissance Micro-Crystalline Wax (4, 4, 4), Sentry Solutions Marine Tuf-Cloth (1, 2, 1), Rust Free Silicone (.5, 1, .5), White Lightning Lubricant (2, 3, 1), Mineral Oil(1, 2, 3), Ballistol (3, 3, 4), Auto Wax (3, 2, 3), Vaseline (3, 4, 3), Paint( 0, 1, 0), Roofing disks ground off and exposed only to air (0, 0, 0).

Rust overview of experiment and products used:

Rust rinsed off:

Rust rubbed off:

Observations were made using the following scale: Measure the rust each day and record the amount of rust, using the following scale:
0 = no rust
1 = any visible rust
2 = some spotting with rust
3 = mild rust, more spotting than 2
4 = moderate amount of rust, more spotting than 3
5 = patches of continuos rust
6 = heavy rusting
7 = heavy rusting with pitting
what are the roofing disks made from and are they galvinized?.....second question--did you also try fresh water as well as open air?......the journal called guntests has run some great experiments/proof sessions with various coatings for firearms in the past and recently....the results were suprising and it has made me change coatings completely...feverdoc
Thank you for your inquiries. The discs appear tinned on one side and painted on the other. We had tossed a disc in saltwater prior to running the experiment and it basically only rusted around the edges. We then ground the painted side off with 120 grit belt sander leaving a small portion of the paint coating. The paint coating was left for comparison, but mainly since it did not make any difference, because that is where the Levr-Lock (sp?) pliers held the disk. We removed all traces of coating and in a couple of cases were vigorous enough to overly thin and/or "burn" the metal.

I short, I believe we removed any traces of whatever coating may have been there, even if it was galvanized under the paint. It was shiny like bare steel after grinding. Whatever the discs are made of, they DO RUST, as there was a lot of rust in the water. I find it hard to believe the coatings would be thicker than the base metal. Of course, since I do not have a good explanation, who knows!

I do feel the experiment was appropriate, in that when coating/lubricating an object such as a knife, it is common that either areas might be missed, the coating imperfectly applied,or the coating might be abraded, especially on the edge. Thus, if the coatings attracted rust from the edge, which was more difficult to coat, or from wherever, that would not be good.

Do you have any of the guntest articles available, suggestions as to which dates to look for, etc.? I know it is trouble to look things like that up, as I have done it myself, but we would appreciate it, especially if there is are good discusions or explanations. My email is available in my profile, if you need it. Thanks again!
guntests volume xii, number 7 (july 2000) ran an article on rust preventives on pages 27 thru was an ok test (acceptable) that was run in the spring time outside in the dry mountain air in the meadow of some idaho mountain range...that climate proved to be more corrosive than one would think..i also have a "package of gun maintenance articles" that guntests offered about 4 or 5 years ago and one of the articles was soley on rust preventives and was done in the very harsh climate of the beach and was a very well done study and was an eye opener as to what worked and what didnt work for rust prevention...i have that set of articles in storage in a very large storage bin and have no idea where they might be buried in there but i am sure you can contact gun tests at (this is the editor todd woodard's email address) and he/they can help you with that particular also might want to call the back issue place for them at 203-661-6111..hope this helps....if you want a copy of the july 2000 me your address and i will ship you out a pirated copy early next week for your personal

[uncoated disks]

As can be seen from the numbers, they did better than most of the coated disks. Can anyone explain this?

Iron can oxidize to form what is commonly called rust, a red powder that breaks apart easily. However it can also form a black oxide that is very stable and hard. You can wipe off the red oxide, the black one you have to sand off.

The red rust is very harmful as it is very weak and not durable at all and so it breaks/flakes off readily and thus weakens the material rapidly. The black oxide is often intentionally created to act as an inhibitor against the formation of the red oxide. It doesn't do a lot in and of itself, but it absorbs protectants better than the bare steel.

It seems that your found that the protectants seemed to increase the formation of the black oxide (Fe3O4) over the red (hydrated Fe2O3). This could be because of changes in conductivity or simply protection from the enviroment slowing down contact with the oxiding elements. Highly elevated temperatures also promote the black oxide over the red, but I doubt that is the case here.

donald... pirated article headed your way....check the snail mail box in a few days....feverdoc