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What You Should Know About Rust.

"It's only a little surface rust..."

• Unfortunately, when it comes to polished surfaces, the expression "surface rust" is misleading. The term may be appropriate, for example, when sheet metal (such as a car body panel) has corroded only superficially and has not rusted completely through.

• Rust never forms on the surface—it forms from the surface itself.
• Rust is simply the corrosion of the material itself—the transformation of the non-stainless steel particles at the surface of the tool under the combined effect of moisture and oxygen. During this process, the material becomes porous and expands in volume. The corroded areas therefore protrude above the surrounding surface, giving the impression that the rust is merely sitting on top of it. In reality, however, material has already been lost beneath those areas.
• A rust spot consists of thousands—or even millions—of tiny rust "spots", which are in fact microscopic pits whose depth is generally of the same order of magnitude as their diameter.

• This is why
• although the rust itself can easily be removed or wiped away because of its loose, porous structure, the damage left behind between the unaffected areas of the surface remains.
• To eliminate these defects, material must be removed from the entire surface down to the bottom of the deepest corrosion pit. Only then can the surface be polished again.
• The appropriate repair method for a rust-damaged surface must always be determined individually, taking into account the size of the affected area, the depth of the corrosion pits, the geometry of the surface and its dimensional tolerances.
• Depending on the depth of the damage and the geometry of the surface, restoring the original shape and surface quality may require remachining, provided that the dimensional tolerances still permit it.
• If the required dimensions can no longer be maintained within tolerance, the only possible solutions are complete removal of the corrosion, rebuilding the surface by welding followed by remachining, or replacing the component.