How does paint protect from rust

[Danijel Gorupec]

Hi,

 

How does paint protect steel from rusting? I was thinking that the paint cover physically prevents water from contacting steel. Hovewer, I noticed (on my car, unfortunately) paint bubles indicating rust below them. But the paint cover is still undamaged and I don't know how can rust form below seemingly healthy paint.

 

Could be that paint is not the perfect cover. Probably not. It is only slowing down the rusting process... What confuses me is that painted steel can do good for years (without rust signs), but once it starts rusting it seems that the paint doesn't slow the process any more. Why?

Edited September 5, 2011 by Danijel Gorupec

[dragonstar57]

as I understand it the paint prevents oxygen from reaching the surface of the steel and once the seal is not perfectly air tight it no longer prevents the oxidization as well as when the seal was air tight. the one thing i hear often that is not correct is that rust causes more rust when in fact oxygen and an conductive materials cause the oxidation that forms rust

*edit*

with the exception of rust making more metal exposed to oxidizing elements

Edited September 5, 2011 by dragonstar57

[TonyMcC]

Sometimes the rust forms under the paint because the corrosion started on the inside surface and has actually penetrated the metal allowing moisture and air to work under the paint.

[StringJunky]

as I understand it the paint prevents oxygen from reaching the surface of the steel and once the seal is not perfectly air tight it no longer prevents the oxidization as well as when the seal was air tight. the one thing i hear often that is not correct is that rust causes more rust when in fact oxygen and an conductive materials cause the oxidation that forms rust

 

The physical structure of rust, being open as opposed to densely compact, allows the ingress of more oxygen, moisture and electrolytes allowing it to spread...I think this is what people when they say rust causes more rust. Take another scenario with lead and it's oxide...once the oxide, which is dense and compact is sufficient to be impermeable to moisture and air, oxidation ceases and is therefore self-limiting unlike iron rust formation which continuously exposes new oxidizible surfaces until exhausted.

Edited September 5, 2011 by StringJunky

[Danijel Gorupec]

The physical structure of rust, being open as opposed to densely compact, allows the ingress of more oxygen, moisture and electrolytes allowing it to spread...I think this is what people when they say rust causes more rust.

 

This seems logical for non-painted steel... However, if rust is sealed with paint, this effect shuld be much smaller (limited by paint permeability). But it seems that sealing rust with paint does not slow the rusting process.

[dragonstar57]

This seems logical for non-painted steel... However, if rust is sealed with paint, this effect shuld be much smaller (limited by paint permeability). But it seems that sealing rust with paint does not slow the rusting process.

metal expands when it rusts so any rust that may occur under the paint would damage the protective layer of paint

Edited September 6, 2011 by dragonstar57

[CaptainPanic]

This seems logical for non-painted steel... However, if rust is sealed with paint, this effect shuld be much smaller (limited by paint permeability). But it seems that sealing rust with paint does not slow the rusting process.

If there is a microscopic hole in the paint, the flow of oxygen or water through that hole (even if it's only diffusion) is so large compared to the rate at which the oxygen is consumed that it is not a limiting factor anymore.

 

In other words, at some point and at a particular place, the permeability of the paint is just not the bottleneck anymore. The formation of rust proceeds at maximum speed.

 

Of course, the paint can be damaged in a particular spot, and still be a protective layer in another place.

[Newbies_Kid]

This form of corrosion is blistering. My dad's car also have same problem. Blistering occur due to adhesion problem between paint and steel. A very small space gap between paint and steel is enough to initiate rust.