|Neutralizing Metal Corrosion -
It starts with a benign-looking bubble and grows, like the malignancy it is,
into a metallic carnivore with the potential to consume a Hummer and then go
looking for dessert. It's rust, the bane of anything metal and one of the
greatest challenges for auto restorers.
|The Creep |
No one is truly safe from rust. It first rears its bumpy head in regions with
high rainfall and urban areas where salt is used on icy streets. But, even
high humidity or beach communities in relatively arid Southern California feel
the sting of rust.
To most vehicle owners, rust is annoying at best, and at worst it's a total
automotive meltdown. For those hearty souls in the midst of restoring a
classic that's suffering from long years of neglect, the bubble patch on the
surface may be the symptom of terminal problems, like the proverbial tip of
the boat-sinking iceberg.
No More Metal
The conventional approach to rust is to keep poking at it until you hit
uncontaminated metal. In the most serious cases, that just doesn't happen. You
scrape and sand and finally see the daylight through the metal. That's when
it's time to turn to companies that specialize in providing replacement
sheetmetal for body panels and floor pans. That's the extreme case, and we'll
deal with installing panel replacements in a following article. For now, we're
going to take a more optimistic approach: rust can be stopped, despite Neil
Young's baleful song that "Rust Never Sleeps."
In any discussion of corrosion, it's important to understand exactly what it
is, in scientific terms. The vehicle's steel is a combination of iron, metal
impurities and negatively charged free electrons that are attracted to the
iron atoms under normal conditions. All that changes when moisture is
introduced—moisture in the form of actual water or simple humidity. Through
electrolysis, the electrons abandon the iron and head straight for the metal
impurities, forming rust.
Rust treatments, generally, start with elbow grease, removing the bubbles and
pock marks with grinders, wire brushes, sandpaper or sand blasting, and taking
the affected area down to bare metal. The more deep-seated the rust, the more
aggressive the tools.
There are also rust paints available that seal or encapsulate the rust. These
products are highly flammable and include toxic chemicals that require
professional application. Acids can also dissolve rust, leaving a thin oxide
coating on the surface. Like the paints, acid rust removal necessitates
special breathing equipment. For high-end restorations, metal panels (or the
whole car) can be submerged in a hot bath of caustic soda, stripping all the
paint down to the metal. The panel is then treated in a tank of alkaline
solution and the rust removed electrolytically.
Prior to these methods, the first "professional applicators" of the oldest
method of rust treatment were probably blacksmiths. To protect their tools,
the smithies would coat them with oil and heat them in their forges, not
unlike the method of seasoning a cast-iron skillet. This process creates a
hard coating, called magnetite, on the metal surface. This coating is
chemically inert and will not react to oxygen or moisture. The path from the
blacksmith shop to auto parts counter ran through the mining, construction and
agricultural industries as scientists attempted to solve the problem of
preserving metals exposed to the elements.
For modern automotive applications, rust converters come in the form of
primers designed for use directly on the rusty surface, with no scraping,
grinding, sanding or blasting. In fact, the product has to be applied to rust
in order to work. The two components of converters are tannin and an organic
polymer. Tannin, a water-soluble natural product derived from a variety of
plants, reacts with iron oxide changing it to iron tannate, a stable
blue/black corrosion product. The polymers provide the protective primer layer.