I want to try and understand "how rust works", and was hoping to start a discussion on it in here.

I was thinking of a dozen different ways to word this in order to get to the point, but I think this first question would be a good place to start...

If you had two identical pieces of metal with the exception that one was rusted and one wasn't, and you finished each one the exact same way. What would happen?

If you had two identical pieces of metal with the exception that one was rusted and one wasn't, and you finished each one the exact same way. What would happen? If both parts were subjected to conditions that support corrosion the piece that was previously corroded will corrode at a much faster rate. Rust can sleep but never dies unless completely removed.

What would you say it is about the corrosion that once started accelerates the process? I'm thinking that since scale is brittle and porous, that it would tend to "wick up" the moisture, holding the elements that start the corrosion in the first place closer to the fresh steel.

Thoughts?

Hydrated iron oxide (AKA Rust) is permeable to both oxygen and water. This is what causes steel to continue to corrode after a layer of rust has already formed. Water and oxygen are the catalysists for rust formation. Paints are okay at slowing this process, but they are not perfect; especially if you get a scratch. You can coat the steel with a layer of zinc (called galvanizing). Zinc, like aluminum forms a protective oxide coating. Even if you get a scratch it still protects the base metal. The reason for this is that the zinc layer has a higher electrode potential (meaning that is will release electrons before the steel will, and this is what starts the whole rusting process). This is very much like the use of zinc anodes on a ships hull.

Let me try and lay this out in generic terms and see if you would agree...

All steel, painted or not will rust. Paints slow the process by blocking (or should I say slowing) the elements that cause rust. Rusted metal continues to rust at a greater rate because the rust allready present acts like a window for the water and oxygen to contact the "good" metal.

What I'm still trying to rationalize is why if you had the two pieces of metal, rusted and not, and painted them, why the rusted metal piece still rusts at a greater rate. I would think provided the rust wasn't holding in any moisture at the time it was painted, that they would both continue to rust at the same rate since the paint would only allow the same amount of air and moisture through to the steel.

??

If you completely sealed both pieces from oxygen the rust will stop. Painted steel will not rust if the paint is providing the seal or barrier. When paint breaks down it becomes porus. Notice how an older paint job accepts and retains wax better than a newer paint job? Rust will only form and grow under paint if there is an opening for oxygen to make contact.

Maybe this is wishful thinking, but I'm not sure I buy the idea that "rust never sleeps" -- that it keeps working, even when the car is in a dry, non-corrosive environment.

For example, we go to great lengths to sandblast and paint our car's frames. But many of them are boxed frames, and it is next to impossible to properly clean and coat the metal inside the frame. Looking in the inspection holes in any boxed frame will tell you that it is rusty in there.

So what is different about surface rust on the underbody or the inside of body panels? Sure,those parts are thinner than the frame, especially if they have been in a wet environment. But if the panels are now kept dry, either uncoated or coated with paint (rust preventative or plain) why would the rust continue to corrode the metal?

Assuming a car is kept in a dry garage and not driven in wet weather, it seems reasonable to expect the body and frame to last many decades?

Unfortunately, our garages are not always as dry as we would like, or we are caught in rainstorms from time to time. To obtain some protection from moisture and further rust, in difficult to reach areas like inside body supports and various nooks and crannies, I've wondered about using one of the spray on products like that made by Lloyd's. It coats the metal with a penetrating oily film. They claim that this protects metal even in locations exposed to almost constant salt water spray. Does anyone have experience with that type of product?
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I guess I will take a stab at this. Rust will sleep if you coat the metal with a low porous material. Regular primer is porous and will allow the metal to rust because water seeps through it and it can breath. That is why you should not just primer and leave it that way. On the dry garage thing. You have moisture in the air (humidity) and when the temp changes the outside varies the inside plus the items in the garage. One changes quicker then the other and causes a dew point. Moisture forms on the items and there is your water. That is why you find rust in areas that are not accessible to rain water. That is the reason that people with big money cars have temp and humid controlled environments.

All good points. I feel the same way that if the car is kept in a dry, climate controlled invironment, exposed or not, the rust probaly won't make any noticeable progression in any of our life times. But you would have to maintain a constant temperature and humidity.

I guess one of the points I am trying to get at is if you were refinishing a panel that was pitted, and you didn't get all of the scale out of the pits, would it really make any difference to the life of the paint job (all of the above points considered)??

You follow me?

Let's all put our 2 cents in. Here's mine. Rusted metal will rust faster than unrusted metal because of one of the by-products of the rusting process - water. Scratch off flakes of rust from any area that isn't allowed to be heated or in direct sunlight and you will find moisture under the flakes. This moisture is an electrolyte. Combine this with O2 and the result is more rust, and more water, and then more rust, and more water. And, life goes on.

Andrew Offutt

Added note - Rust can be made to go dormant as long as it's dried and totally sealed. Scratch it to where O2 and a little humidity can creep in, and "here we go again."

Yes, Andrew is absolutely correct. Water and Oxygen start the rusting process, but once the process is started, one by-product of rust is water itself. So it is very important to remove all rust before coating a panel. And like others have said, it doesn't need to be raining for there to be moisture. Humidity in the air is enough.

Not to put anyone on the spot, but could someone explain how rust actually creates water?

Rust doesn't create water but it will absorb it from the air just like dust. Dust will actually suck or wick moisture for a fairly long distance. Ever see a dusty valve cover suck oil from the oil filler cap all the way across the length of the valvecover? It will do the same with water/moisture. rust is porus, think of it as full of tiny broken openings, scale. Your pinch welds if not fully sealed from dust and dirt and moisture will rot because they will hold the moisture unlike an area that is open and allowed to breathe and dry. This is why door bottoms go bad, window gutters, rocker panels, etc. The newer cars built with adhesives in these areas are blessed with seams that are sealed, but find a skip in the adhesive and there is where the corrosion starts.