WonderQuest:  On the web since 1997...      

Rust eats axles, water seeks its level, plants reflect green light

Q: What does rust do to weaken metal, particularly steel? All the exposed surface area of my car axle is covered with rust. How much damage is it doing and can it weaken the axle to the point of failure? —Chris, Key Largo, Florida

Never drive on an ocean beach as this rusty car attests. [Rust Check Centre]

A: Iron or iron-containing metal rusts when, in the presence of water, it combines with oxygen to form a powdery or scaly reddish-brown material (hydrated ferric oxide, aka rust). One area of the metal has a positive charge and another negative. Water acts as a conductor that allows current to flow between these two areas. The metal absorbs oxygen from the water, gives up an electron, and forms rust.

Salt water acts as a much better conductor than water. So it speeds rusting. Warm temperatures also hasten the rust reaction. Consequently, rust occurs fast in the warm, salty air of Key Largo—off the tip of Florida. No wonder you’re concerned.

Corrosion weakens metal because it changes the metal surface from a hard durable material to powdery flaky rust. As rust eats through the metal, it weakens the system (axle, in your case) and will cause failure eventually. How much damage is it doing? Much. The immediate problem is the axle bearings. If they rust, they will fail. When the bearings fail, it can damage or break the axle shaft and even cause a wheel to fall off, says Jim Kerr, auto tech for the Canadian Driver.

Inspect wheel bearings at least yearly, repack the wheel bearings with quality wheel bearing grease, and replace the axle seals. Rusty spots, grooves, pits in the metal, and blue or black spots indicate problems. Replace the bearings if you see any of these signs.

You can prevent rust or slow its progress. The easiest preventive measure is to wash your car monthly at a jet wash that sprays the underside of the car and the inside of the wheel well. Salty water (either from salt applied to snowy winter roads or from salty ocean air) speeds rusting. So, wash it off.

Stopping rust, once it’s started, is more challenging. You need a wire brush, goggles, gloves, axle stands, and a big can of good fluid-film corrosion inhibitor. It’s a several-day job and hard work: scraping off every scrap of loose paint or rust under your car and then painting the clean rust-free surface.

"I’d suggest Corrosion Block [fluid-film corrosion inhibitor], sold through West Marine all over the States," says Bruce Hector of Rust Check Centre in Kingston, Ontario, Canada. "Never drive on an ocean beach. One exposure to salt in a warm climate is as bad as a whole winter here in the rust-belt."

Further Surfing:

WonderQuest: Why stainless steel is stainless

Morris Minor Auto Club: Rust prevention–how to stop rust

Rust Check Centre

Q: If you have a 1 inch pipe 20 feet tall and a 4 inch pipe 20 feet tall filled with standing water will the downward pressure be the same or different at the bottom of each pipe and why? Brent, Indiana

The 4-inch and 2-inch pipes are connected but no water flows and the two water levels are the same because the pressure is the same. [April Holladay]

A: The liquid pressure is the same for any given depth below the surface, regardless of the shape of the containing vessel and regardless how much water is in the tank.

Why is simple. The pressure depends only on the density and the depth. The cross-section area of the pipe doesn’t matter. The old adage, "water seeks its own level" explains it.

Assume the 4-inch pipe and the 1-inch pipe are connected at the bottom such that water can flow from one to another as illustrated in the figure. Suppose the 4-inch pipe had greater pressure 20 feet down than the 1-inch one. Then the pressure at the bottom of the larger pipe would push the water towards the lesser pressure of the smaller pipe. Let’s see what happens. We would expect the greater pressure at the bottom of the larger pipe to push water out of the top of the smaller one. However, this doesn’t happen. The two water levels are the same because the pressure is the same.

Water seeks its own level and will go uphill if it has to. We can fill a garden hose with water and hold both ends up to demonstrate this. The water at each end is at the same level.

Ancient Roman aqueduct builders knew this. When they faced a deep valley, they used pressurized pipes. The water traveled down one valley side in watertight pipes (like our garden hose). Water pressure forces (siphons) the water up the other side and water exits at almost the same level that it entered.

The Romans aren't the only ones to use siphon aqueducts.  "They are used on modern systems, too," says G. Donald Bain at the University of California. 

The Jawbone Siphon on the Los Angeles Owens Valley Aqueduct (built in 1912-13). The canyon is 1000 feet and the siphon pipe is 7,096 feet long. [G. Donald Bain, Geography Computing Facility, University of California, Berkley]

Further Surfing:

PBS, NOVA: "Roman" manual on how to construct an aqueduct

G. Donald Bain, University of California, Berkley: Geo-images project

Q: Why are trees and grass green instead of red or blue or black? I have learned more in this one page of questions and answers than I have in my life. —Lawrence

Plants use red and blue light and reflect the green. [US Fish and Wildlife Service]

A: Trees and grass (and all plants) are green because they use a green dye (called chlorophyll) to produce sugar. In fact, when we "get a ‘grass stain’, it’s actually a chlorophyll stain," says Art Cameron, horticulturist at Michigan State University. During the summer, tree leaves and grass absorb light energy and, through a series of reactions, convert that energy to a more stable form — namely, carbohydrates. The conversion process is called photosynthesis. Plants store the sugars and starches to fuel growth.

Chlorophyll is a light-catcher pigment. It absorbs blue, violet, red, and orange light and reflects green. That’s why leaves look green. We don’t see red, or blue because chlorophyll keeps those colors for the plant. Blue light fosters growth. Red light promotes flowering and also stretching so a plant can get higher than the rest to grab more red-rich sunlight.

Fungi (for example, mushrooms) don’t have chlorophyll, don’t produce food via photosynthesis, and aren’t green. Instead, they feed on mostly dead or decaying organic matter, like leaf litter. But then, fungi aren’t plants. They belong to their own kingdom (the Fungi Kingdom).

Further Surfing:

The Flying Turtle: Plants and photosynthesis

WonderQuest: Plant colors

Art Cameron, University of Michigan: Light and plant growth

(Answered Aug. 1, 2003)