Chemistry - Painting metal, does it matter if all rust is removed before?

Solution 1:

How exactly is the water broken down into hydrogen and oxygen?

As the acid is formed and the iron dissolved, some of the water will begin to break down into its component pieces -- hydrogen and oxygen.

That's extreme oversimplification. The reaction common for active metals (not including iron) is $$\ce{Mg + 2 H2O -> Mg(OH)2 + H2 ^ }$$

Iron does not react this way. However, it does react with acids, say, with hydrochloric acids $$\ce{Fe + 2 HCl(aq) -> FeCl2(aq) + H2 ^}$$

The reaction is quite slow even for concentrated nonoxidizing acids. However, in presence of oxygen and in diluted acids two subsequent reactions occur, the one noted above is followed by $$\ce{4FeCl2 + 10H2O + O2 = 4Fe(OH)3 v + 8HCl }$$

The second reaction effectively removes iron from solution, significantly fastening the overall reaction. However, even without acid present, even in slightly basic conditions iron slowly reacts with water in presence of free oxygen, forming rust. The process is hindered only at pH above 10 or at absence of free oxygen. Since natural water is usually slightly acidic due to presence of dissolved carbon dioxide, iron always rust on contact with water and atmosphere. This, actually, is a reason why concrete layer over steel frame is regulated: concrete always contains water, and surface layers quickly looses basicity due to reaction with atmospheric carbon dioxide, so steel near surface of concrete quickly rusts, increasing in volume and tearing the concrete from inside.

In the context of automobile repair work, assume a metal surface has rust over it. Does it matter if all of the rust is removed before the metal surface is painted? If it does matter, then why?

Yes, it does. Rust is weak, so any paint over it will hold weak. As long as the paint film over rusty part is broken, water and atmosphere contacts the metal and start to erode it, resulting in more paint film be destroyed. So it is critical to remove all rust from the surface being painted. Abrasive blasting, if you have the equipment, is probably the best way. While some paints may be applied over rust, it should be a very thin layer, and even then I would prefer traditional paint over cleaned surface anyway.

Solution 2:

Most paints will prevent oxygen and water to come into contact with the metal. Usually, rust under paint will allow the paint to break off. Some paints, like Rust-Oleum, can be applied over rust, but most cannot.

Also, with rust, you usually have an uneven surface which shows through the paint and gives an impression of a poorly done paint job.

Solution 3:

Sometimes the best chemistry, is to simply PREVENT the chemistry of rust in the first place.

So in addition to the already well covered chemistry issues above, I think it's very important, that before coating the underlying surface, that that surface is both smooth and solid. Old guys refer to it being 'sound'.

So I've found that one important reason to remove all rust, often by sanding, is to help achieve 'smooth and solid'; in other words not to affect the chemistry of the surface, but to adjust the physical characteristics of the surface.

I've found that in actuality almost all coats of paint have tiny holes in them that show up later. These tiny holes are perhaps from bubbles or debris in the paint as it is applied. A few years goes by and tiny rust spots start to appear, then they get bigger with time.

By carefully preparing the surface each time before it's coated you can minimize the chance for these pin-holes. In other words, by again making sure the surface is smooth, so that dirt from the previous coat don't stick up, and so that the surface is clean to not give any bubbles anything to hang onto.

Multiple coats helps, but only if each new coat is again carefully applied.

BTW, another source of holes in paint where rust starts to form are places where impacts occur, like where rocks hit fenders, or where surfaces connect and expansion and contraction can break the paint joint in time.