engine and drivetrain Just curious...almost OT

[auggiemize]

I made a reply to gazanik, about eight posts down about the Polaris car that had the engine fire, as I had one previously on my old truck; question is this; what does happen to steel when there is a fire. I've heard that it changes from a molecular standpoint relative to paint, and its propensity to not bond any more. Those of you that are smarter than me help educate me on what happens, and what changes occur, performance of said steel, etc. Before I paint the hood on the truck again, and it peels off again....Thanks, Dave V. in NC

[dubois]

Here is my educated guess:

There are two main ways to bond paint to metal:

Chemical etching, like using an etch primer, which is mostly phosphoric acid that "burns" and etches itself onto the metal, creating a first bond, where subsequent coats of primer can be applied.

Physical bonding, most commonly known as sanding. This works because you are in effect increasing the surface area of contact per square inch, because all the grooves created (you have to visual that 10x in your brain)

There are probably other ways, but these are the main ones used in a paint shop.

When metal is exposed to fire, is probably exposed to oxygen due to the combustion, creating FeO2, commonly known as rust, which is not a good "substrate" for either etching or sanding, unless is removed by sandblasting or other methods.

If you can reach shiny metal, you can paint over it, the paint is not interested in the molecular level of the material, and doesn't know its past history.

[Kidasters]

From a metallurgical standpoint - if you heat the steel hot enough - it will change it's crystal structure. The simplest way to think about it is when a blacksmith heats something up really hot, and then throws it in water really quick - you change the crystalline structure of the steel and then quench it really quick to freeze the structure in place. That's the whole reason "Tempered steel" works - it's stronger material than the starter material. The problem is that when you do this - you can also make the metal more brittle.

So - I think dubois is right - if you can get it back down to bare metal, you are probably OK. If the steel has changed, it may be more brittle in those areas, but probably not something that is going to really matter. Unless it warped out of shape....

Ken