Coatings, Cathodic Protection Check Cold-Climate Corrosion.
Corrosion is the deterioration of a material or its properties, caused by a reaction within its environment. Wood, plastics, ceramics and metals are substances gradually destroyed by corrosion, which is measured by changes in weight, dimension or mechanical properties, such as tensile strength( the greatest longitudinal stress a substance can bear without tearing apart) or ductility (capability to be molded into a new form).
Corrosion causes about $1 billion a year in damages in Alaska, according to Lyle Perrigo, director of the U.S. Arctic Research Commission's Alaska Office. That cost could be as high as 6 percent of the value of goods and services produced in Alaska, but Perrigo said there are ways to cut losses.
"About 1 percent could be saved by using cathodic protection, by using the right kind of building materials or by using the right kinds of inhibitors. Just because you have high (corrosion rates, that doesn't mean you can't cope with it (corrosion)," Perrigo said. "We can paint more frequently, use better coatings, better designs--these are not insurmountable problems. We do not need to reinvent the wheel. We just need to figure out where it is going."
Internal environments include the interiors of oil and water pipelines, and mechanical systems. Fluids transported, or carried for lubrication, in these systems are corrosive, but corrosion in these environments can be controlled using proper materials, designs and chemical inhibitors.
"Internal corrosion is big business," according to Christopher Dash, a corrosion engineer with Phillips Alaska Inc. "Oil, water, gas and solids can eat pipes at phenomenal rates. With unprotected pipes, we're talking inches a year. And most pipes are not inches thick."
The pipes Dash is concerned about carry crude oil from drill sites to gathering sites and ultimately into the trans-Alaska oil pipeline. The pipes are 24 inches in diameter and have walls three-eighths-of-an-inch thick. They carry crude mixed with seawater (which is pumped into oil-rich reservoirs to build pressure and force the oil out of the ground), gasses and solids.
Preventing the corrosive concoction of oil, water, gas and solids from damaging these pipes is a function of chemical inhibitors. There are about a half dozen different classifications of inhibitors, each with different applications. For use inside the crude-carrying pipes, Phillips Alaska Inc. uses an inhibitor classified as a surfactant, which finds its way to the space between the flowing substances and the inner walls of the pipes to prevent corrosion.
While chemical inhibitors successfully inhibit corrosion, Phillips monitors the effects and extent of corrosion on pipeline interiors with sophisticated sensors.
"We use a lot of ultrasonic techniques to measure pipe-wall thickness. We use a series of ultrasonic pulses that go into little radio transmitters so we can monitor what is happening at any given point, Dash said. "We cannot measure the entire pipe, so we measure areas that may be corroded and we watch them."
Inline inspection is another method of monitoring pipe-wall thickness. This is accomplished by "smart pigging," Dash said. A pig is a cylindrical device that is inserted into a pipeline to clean the pipeline wall and monitor the internal condition of the pipeline. It uses an odometer to determine the location of problem areas and is capable of storing the information it gathers.
Fuel storage tanks are also affected by internal corrosion. Phillips has not had any significant leakage from its storage tanks, but Dash said the tanks are monitored. To prevent corrosion, tanks are painted inside and outside with corrosion-resistant coatings.
External environments fall into three categories, atmospheric, buried and sub-merged. Controlling corrosion in these environments is accomplished by using anti-corrosion designs, suitable building materials, protective coatings and cathodic protection.
External corrosion is also an issue on Phillips Alaska Inc. pipelines. With most of its pipelines elevated on racks above the tundra, atmospheric corrosion (attacks by airborne contaminants) and under-insulation corrosion are important concerns.
When pipelines are constructed, sections of pipe are welded together. These joints are insulated, but joint integrity can be compromised at the insulated joints by water seeping under the insulation, causing under-insulation corrosion.
This form of external corrosion takes 10 years to 20 years to cause significant damage, but is very costly. Under-insulation damage costs about $5 million a year for Phillips' Kuparuk site, Dash said.
While it may take more than a decade for under-insulation corrosion to cause problems, there are examples of external corrosion that are detected much quicker.
On the Aleutian Islands, an improperly fastened, coated metal panel on a building will show signs of corrosion in as few as three years. If the fasteners used to attach these metal panels to building frames are not properly used, even coated panels have a difficult time withstanding the detrimental effects of salt-laden atmosphere of the Aleutian Chain, Perrigo said. Saltwater can seep under the fasteners and underneath coatings, causing paints to bubble or blister. When water evaporates, a salty film is left behind and over time becomes more concentrated and more corrosive to metals and building panels.
In submerged environments in Alaska, corrosion occurs at some of the highest rates anywhere. Corrosion rates typically decline as water temperatures decrease, but the opposite is true in oxygen-rich waters of Alaska. Offshore oil and gas rigs in Cook Inlet have some of the highest corrosion rates in the world, Perrigo said.
"Every 10-degree Celsius drop reduces the corrosion rate, but oxygen propels corrosion," Perrigo said. "Cold seawater carries 50 percent more oxygen than warm seawater."
Highly oxygenated, cold seawater is largely responsible for corrosive damage in Alaska's marine environments, but abrasion from glacial silt, twice-daily tidal shifts and the action of ice floes contribute to the high rate of corrosion on Cook Inlet drilling rigs. Components of these rigs corrode six times faster than their counterparts in the Gulf of Mexico.
Cathodic Protection Keeps Cold
Climate Corrosion at Bay
To combat corrosion in marine environments, allowances are made for corrosion during construction of docks, piers and offshore petroleum-production platforms, and special coatings are applied to metallic surfaces. In submerged or buried marine environments, cathodic protection is the most effective corrosion inhibitor.
The simplest form of cathodic protection is coating structures in need of safeguarding with specialized paint, according to Civil Engineer John C. Daley, with Tryck Nyman Hayes Inc.
"A good coating is the first line of defense," he said.
Cathodic protection prevents a structure, such as a buried underground storage tank, from corroding by making it the cathode of a galvanic cell, Daley said. Cathodes are electrodes that practically do not corrode. Anodes are electrodes that oxidize and corrode. When an anode is attached to a cathode, the anode corrodes instead of the cathode.
In systems using passive cathodic protection, a magnesium plate (the anode) is welded to an underground tank (the cathode) and buried near the tank. The effect is that the buried plate corrodes while the storage tank does not. In this case, the integrity of the anode is sacrificed for that of the cathode.
In active cathodic protection, a direct current power supply is attached between the anode and the cathode, with the positive terminal connected to the anode, which corrodes as it discharges the current.
In marine environments the same principles are applied, using aluminum and zinc plates as anodes, which are suspended in seawater.
"Corrosion ends up being a really significant cost to a lot of structures," Daley said. "It's not sexy, but it is an up-front problem."
Shining a Northern Light on Corrosion
NACE International's Northern Area Western Region Conference
February 26-28, Hilton Anchorage
When corrosion-minded engineers from Alaska, Argentina, Australia, Canada, Norway, Russia, Sweden, Uruguay and the Lower 48 converge on the Hilton Anchorage hotel in February, cold climate corrosion is going to be a hot topic.
This month's conference is the first NACE International meeting to be held in Alaska since 1996. That first meeting was held in Anchorage and drew some 218 attendees who presented 75 scholarly papers.
NACE International (formerly known as the National Association of Corrosion Engineers) divides the globe into eight geographic regions, with Alaska falling into the Western Region's Northern Area. NACE International has more than 16,000 members whose mission is to inform the world about corrosion control.
"We want to make a contribution," said Lyle Perrigo, director of the Alaska Office of the Arctic Research Commission. "We want to work on defining the cost of corrosion in Alaska. The cost of corrosion may be as high as 6 percent of the total value of goods and services produced in Alaska. And every time you have to work on a pipeline in the winter the costs escalate."
There are about 60 papers on corrosion-related topics slated for presentation during the conference's eight scheduled sessions. Engineers from Alaska will play a prominent role in the conference, presenting more than half of the papers.
Of all the papers being presented, 14 of them will address issues related to corrosion in cold climates.
There is also a session titled Cold Climate Corrosion. "That's our biggest session," said Christopher Dash, NACE International's Northern Area technical program chair.
Other sessions planned for the conference include Oil and Gas Chemical Treatments, Coatings and Linings, Inline Inspection Pigging, Cathodic Protection, Computers and Corrosion, Materials and Metallurgy, Infrastructure Corrosion, Pulp and Paper Mill Corrosion and Marine Corrosion Control.
NACE International's Northern Area is also sponsoring a photo contest to focus attention on corrosion problems. Photos may be submitted in the following categories: Impact of Corrosion on Aesthetics, Effects of Corrosion on Materials, and Effects of Cold Climates on Corrosion Control. For information, contact Lyle Perrigo at firstname.lastname@example.org.
The conference also will provide those in the corrosion-control industry the opportunity to display their products or services in a trade show at the Hilton Anchorage. NACE International is still looking for financial sponsors for the technical conference and exhibitor space is still available. For details, call Chuck Clark at Polar Supply in Anchorage at 563-5000.
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|Publication:||Alaska Business Monthly|
|Date:||Feb 1, 2001|
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