Snow Belt - how fast is your car oxidizing?

[Kestas]

Originally Posted By: ecotourist
I hear comments about cheaper and better steel quite often. Old Volvos were said to be rust resistant because they were made of "good Swedish steel". At one point Japanese cars were said to rust because they were made of poor quality steel (with the second assertion that the steel was of poor quality because it was made from scrap steel - as if that would make a difference).

Calling any steel experts. Is rust resistance because of differences in steel quality or because of better coatings and more attention to design?

And if "better quality steel" is more rust resistant, what makes some steel better than others?

Humbleness aside, I am a steel expert. Plus, I've studied corrosion cases for the automotive industry.

There is no such thing as one steel being better than the other. Automotive bodies are typically made of 1008 steel, which is about as simple a steel as you can get before you have pure iron.

The differences are in the coatings. There are zinc, or zinc-rich coatings typically applied, phosphate, E-coat primer, color coat, and clear coat. I've studies dozens of varieties of zinc coatings. Some better, some worse. The presence and quality of these coatings will have an impact on corrosion resistance.

Even the best of coatings can be breached by a rock leaving a chip in the paint, which exposes the metal and continues with the onset of corrosion.

 

[Kestas]

Originally Posted By: Johnny2Bad
With modern two- and three-coat paints (incorporating clearcoat) it seems to be a non-issue. I would say my personal experience is any vehicle made in the 21st century seems to have solved the problem. Before that, well, paint is expensive, so maybe only premium vehicles slayed that dragon. Basecoat/Clearcoat formulas were introduced by Sikkens in the 80's, now everybody makes it.

Unprotected steel surfaces are going to oxidize (rust), but iron oxide is actually a protectant. If the panel thickness is high enough it won't penetrate (such as a frame member on a body-on-frame truck). With thin metals, combined with water (and can be simple humidity) and any corrosion accelerant (say, road salt) you will have a problem eventually.

There's a lot of misinformation in this post.

As I mentioned in my previous post, paint can be breached with rock chips, allowing corrosion to take a toe hold. No paint coating is immune to it.

Once steel rusts, the iron oxide does not provide a layer of protection. Unlike aluminum oxide on aluminum, iron oxide is considered a "non-coherent oxide." That's why you see heavier rust flaking off the surface. It doesn't matter how thick the metal is, it'll eventually corrode completely away in a corrosive environment. One example is the Titanic. It is losing metal to corrosion at a rate of one ton each day. With time, all steel in the ship will be gone.