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Volume 16, Number 4July/August 1965

In This Issue

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Day and night current flows into the ground—an electrical shield against…

An Enemy Below

Written by Daniel Da Cruz

Next to rats, Public Enemy Number One in the United States is corrosion, which eats away industrial and public facilities at the rate of one billion dollars a year—an amount well above the combined annual budgets of Lebanon, Syria, Jordan, Sudan and Iraq.

Corrosion is a subtle force. It doesn't destroy wholesale, like fire or flood, but gnaws slowly at hidden places until one day, suddenly, an engine sputters to a stop, a cable snaps, or a pipeline springs a leak.

Corrosion and rust are commonly used as interchangeable terms, but the scientist makes an important distinction between them: rust is a purely chemical action, which involves the combination of atoms of oxygen and atoms of iron into molecules of iron oxide. Corrosion, on the other hand, is an electro-chemical reaction, which involves the movement of metal particles from one point to another.

"The distinction is not, as you might imagine, semantic nit-picking," points out Fahd M. Maasry, planning analyst and former corrosion engineer of the Trans-Arabian Pipe Line Company—Tapline—which has pioneered in corrosion prevention techniques. "In dry desert air, rust is not a significant problem. It's true that sulfur and salt water impurities in crude oil could theoretically cause rusting within the pipe, but their quantity is small and in the absence of air they actually do no harm whatsoever.

"Corrosion is another matter entirely. It costs Tapline, to cite but one example, $500,000 a year to keep corrosion at arm's length. Yet, if we didn't, the ultimate price tag of neglect would be astronomical—nothing less than the replacement of all the buried portions of the pipeline between the start of the line in Saudi Arabia and Sidon, in Lebanon—and that's nearly 380 miles."

Corrosion of buried metal is the result of an electric current flowing from the metal into the adjacent soil, carrying with it minute particles of the metal. When enough particles have been carried off, a hole appears in the metal. This loss of metal is what is called corrosion. And roughly this is how it comes about: the electric current, which is composed of minuscule particles (electrons) moves from a high point (the anode) to a low point (the cathode), much like water seeking its own level. For the current to flow, two conditions are required: differences in electrical potential between the two points (one high, the other low) and a bridge or path (called an electrolyte) by which the particles (electrons) can move from one point (the high point) to another (the low point).

With respect to an oil pipeline buried in the ground the process of corrosion is somewhat difficult to visualize. Two parts of the same pipe—sometimes only a fraction of an inch apart—may have different potentials due to minute differences—such as impurities in the metal, or even just scratches on the pipe's surface. When those two points are bridged by an electrolyte—in this case damp sand or soil—an electric current is generated. This current flows from the high-potential point on the pipe (the anode), through the soil (the electrolyte), to the low-potential point on the pipe (the cathode). It then flows back through the pipe wall to the high point, forming a complete circuit. The flow of this current causes the pipe to corrode at the anodic point. To put it another way, the flow carries away metal particles from one point and deposits them at another. The millions of galvanic cells created by these combinations of iron pipe and damp soil are individually so weak as to be virtually undetectable to all but the most sensitive instruments. The difference in potential at any two adjacent points in the pipe wouldn't produce a tingle in a nervous gnat, and the resistivity of the soils surrounding the buried portions of Tapline ranges from 400,000 to 100,000,000 times that of copper, so that the tiny particles move through it as if wading through glue.

Even so, the infinitesimal direct current generated over the years steadily robs the pipe of tiny particles of iron at the anode, so that a pit is gradually formed on the pipe surface. The day comes at last when that tiny pit at the anode has gone straight through the pipe wall, and oil, moving through the pipe under high pressure, spurts out into the surrounding soil.

"Any high school physics student," Fahd Maasry observes, "can tell you how to prevent corrosion on a pipeline; wrap it with a good insulator to shield it from the electrolytic soil, thus breaking the electrical circuit and the flow of ions. And he would be one hundred per cent right. Before Tapline's pipe was put into its ditch and covered, some 16 years ago, it received an application of petrolastic primer, a coat of asphalt, a wrapper of glass floss to add strength, and finally an outer wrapper of asbestos felt.

"But, even that was not enough. No insulator will last forever, especially one encircling buried 30-inch and 31-inch pipe such as Tapline uses. Pipe expansion and contraction in the desert and the extremes of hot and cold wear off the insulation in some places, while at other points various acid or alkaline soil constituents rot it. And as soon as the pipe is bared, corrosion begins."

Well aware that the attrition of time would one day imperil the pipe, Tapline engineers took out an insurance policy in the form of a well-proven technique called "cathodic protection." Reasoning that the trouble in buried pipe is entirely due to those points on its surface that function as anodes, and thus corrode, electrical engineers decided that if the whole pipeline could be made into one huge cathode, the outward flow of iron ions would stop and pitting would be arrested. This was done by burying "anodes" made of conducting material, such as old steel rails or graphite, some distance from the pipeline and connecting them to the positive terminal of a direct current source, such as a direct current generator, and connecting the negative terminal of the power source to the pipeline. Current was then sent into the sacrificial anode from which it flowed through the soil and into the pipeline, through the pipe wall to the cable connecting the pipe to the power source, and into the negative terminal of the generator. This formed a complete electrical circuit. With the whole pipe a cathode, particles of metal (from the sacrificial anode) were now deposited on bare spots rather than drawn off from them.

Instead of stringing a power line the length of the pipeline—a prohibitively expensive operation—Tapline derives its direct current for cathodic protection from rectifiers at each of the pump stations from Turaif to Sidon, and from 25 unattended cathodic-protection stations posted at regular intervals between. Each station has two diesel generators rated at 6.5 kilowatts with outputs of up to 162 amperes at 40 volts (equivalent to 8.7 horsepower), which run on crude oil piped directly from the pipeline. The total power applied to the entire pipeline is equal to 270 horsepower. To keep the current flowing toward the pipeline, the soil-electrolyte at the anodes must, naturally, be kept damp. In the driest areas this means that the ground anode bed must be watered as often as twice a month in the blazing hot summer; this in turn requires the dispatch of a tank truck of water from the nearest well, which may be as far as 90 miles away.

A similar system guards the submarine facilities at Sidon, where the oil is transferred from pipeline to tanker. Each underwater pipe is protected by a 600-foot length of steel rail laid on the sea bottom parallel to it, and receiving electric energy from a shore-based rectifier. The rail lasts up to five years. Buoys, on the other hand, are protected by aluminum anodes bolted on the buoy below the water line, with a rubber insulator between. The bolts become the conductor, the sea water the electrolyte, and the combination of these elements produces the required flow of electrons, just as in a battery.

Though theoretically a perfect system to forestall corrosion, in practice cathodic protection is not foolproof. Three layers of protective asphalt, glass floss and asbestos may with time lose their bond to the pipe, permitting moisture and oxygen to reach the pipe's surface. Because the protective coatings are insulated, the protective current of the cathodic protection system cannot reach the pipe surface and so corrosive activity begins. This type of corrosion is referred to as "shielded" corrosion because the corrosive cell is shielded from the effect of the protective current by the enveloping coating around it. Sections of the pipe affected by shielded corrosion are constantly being unearthed and their coatings replaced at considerable cost.

"As expensive as cathodic protection and pipe reconditioning may seem," Engineer Maasry says, "the alternative—doing nothing—would be corporate suicide. You see, iron corrodes at the rate of 20 pounds per ampere-year (the current consumed by a 100-watt bulb burning one year). Now remember that we apply approximately 2,000 amperes to the line to neutralize the line-generated currents..." Mr. Maasry fiddled a moment with his slide rule "... the tiny pits caused by corrosion—literally millions of holes—could add up to a maximum of 40,000 pounds of steel a year. If we failed to take countermeasures, the pipeline in a few years would look very much like a gigantic colander—dandy for draining spaghetti, but of very little use in transporting oil."

Daniel da Cruz writes regularly on technical, cultural and historical subjects for Aramco World.

This article appeared on pages 12-13 of the July/August 1965 print edition of Saudi Aramco World.


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