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explainer: Answers to your questions about the news.

My Pipeline's CorrodedBut I thought oil prevented rust.

By Daniel Engber
Updated Tuesday, Aug. 8, 2006, at 6:34 PM ET

Download the MP3 audio version of this story here, or sign up for The Explainer's free daily podcast on iTunes.

Severely corroded pipelines will force BP to shut down its oil field at Prudhoe Bay, Alaska, company officials said on Sunday. After a major spill in March, the company discovered stretches where the pipe had lost more than 70 percent of its mass to corrosion. Why does oil corrode a pipeline?

It has water in it. The crude oil that flows from Prudhoe Bay isn't the pure stuff we might use to stave off rust in our cars. When an oil company pumps crude out of the ground, it can also pump out a hot mixture of water, carbon dioxide, sulfur, and microorganisms. (Crude oil with lots of sulfur in it is called "sour," as opposed to "sweet.") If enough of these contaminants collect in a steel pipe, they'll work together to eat away at its inner surface.

Water is the main concern. If there's only a little bit of water in the oil, or if the oil is flowing fast enough, you generally won't have a problem. That's because the water will be dispersed in tiny droplets that are suspended in the flow. These suspended droplets won't react much with the steel surface of the pipes.

Problems can arise in stretches of pipeline that run at lower pressures. In these spots, water droplets can coalesce and fall out of the oil flow. They might inundate globs of sand or dirt that have also fallen out of the crude-oil mix and form a watery sludge on the edge of the pipe. Once a watery muck forms in one part of the pipe, the natural process of corrosion speeds up. The crude can also serve as a breeding ground for anaerobic bacteria, which form slimy, sulfur-producing colonies on the inside of the pipe.

To make matters worse, crude oil comes out of the ground hot, and the pipelines are insulated to maintain those high temperatures. Oil flows better when it's hot, but heat also exacerbates corrosion within the pipes.

Sometimes the oil companies introduce corrosive bacteria, water, or gas into reservoirs themselves. When an oil field gets old, it starts to lose pressure, and it becomes harder to draw out the remaining crude. Engineers can try to add pressure underground by putting in seawater or carbon dioxide. While this increases the field's output, it also sours the product and leads to more pipeline corrosion.

Bonus Explainer: What can the oil companies do to stave off internal pipe corrosion? Very little. It would be far too expensive to build pipelines out of stainless steel, and there's not much you can do to protect the inner surface of a carbon steel tube from water damage. The companies can paint the outer surface with a protective coating, but there's no way to do that on the inside. (You could paint the inner surface of pipe sections before welding them together, but the assembly process would destroy the coating.) Companies can flush their pipelines with various chemicals that create a mildly protective film on the inner surfaces. They can also wash them with a biocide to kill off the sulfur-producing bacteria.

Bonus Bonus Explainer: How do the oil companies test for corrosion in the pipes? They can use ultrasound to measure the thickness of the pipe walls. They can also run corrosion tests by inserting "coupons," or 3-inch strips of metal, into the pipe at various points. After six months or so, they remove the coupons and check to see how much they've corroded. The best way to measure corrosion is with a "smart pig," a bullet-shaped sensor that flies through the pipe measuring the shape of its inner surface.

Got a question about today's news? Ask the Explainer.

Explainer thanks Srdjan Nesic of the Institute for Corrosion and Multiphase Flow Technology at Ohio University, Steve Nikolakakos of Russell Corrosion Consultants Inc., and Van Cosby and Dennis Gerson of IBM. Thanks also to reader Lee Goodman for asking the question.