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| Home | Corrosion is just the
pits
In the course of handling used production equipment we become all too familiar with the
effects of corrosion. Some of the lessons learned from 20-year-old equipment are a good
guide to making sure that our newer equipment remains usable for a long time. We see
corrosion on a variety of equipment ranging from pumpjacks to vessels. Pumpjacks are
subject to corrosion on their structures, but also on their gear trains and bearings. On
the other hand, oilfield vessels are in contact with a variety of nasty chemicals brought
up from below. Things we often see in the field are pitting in vessels where the coating has deteriorated; Corrosion on tanks under the paint along the welds; Blisters in pressure vessels; Stress cracking in pipelines, and of course the frequent "rusty bolt on valves and other attachments. I will restrict myself to the types of corrosion we see most often in the field, however if you’re interested in finding out more about the different types of corrosion out there, check out the Corrosion Doctors on www.petroassist.com. The forms of corrosion often seen in the petroleum industry in Canada are Hydrogen Corrosion, Filiform Corrosion, Stress Corrosion Cracking and Galvanic Corrosion. Hydrogen Corrosion I learned about Hydrogen Corrosion from Mr. Hank Andersen of Andersen Quality Assurance and Consulting Inc., when we were inspecting a used treater. Hydrogen may cause a number of problems, and I most often see hydrogen corrosion in pressure vessels. Hydrogen is a small enough molecule that it can migrate through steel and remain inside the material. Hydrogen atoms are formed due to chemical reactions caused elsewhere in the process. Once the single atom meets up with another Hydrogen, it becomes a larger molecule, which can no longer migrate back out of the material. The result is a blister that can form along a vessel and literally split the shell. Hydrogen blistering is controlled by minimizing corrosion in acidic environments, such as water that is acidic due to CO2 or H2S in the gas stream. (Makes wonderful concoctions such as HCO and H2SO4) It is not a problem in neutral or caustic environments or with high-quality steels that have low impurity and inclusion levels. Unfortunately, the older steels may not be as pure as some of the more recently rolled stuff. It is therefore important to especially look hard at older treaters and separators. If you have this problem don’t throw out the vessel. How do you fix it? I’ve seen a 1966 treater that was delivered to the customer with a patch welded in to replace a blistered section. Other fixes are placing a manway or nozzle over the affected area. Either way you’re best off to let an experienced shop with all the ABSA or ASME qualifications handle the repair. Filiform Corrosion This type of corrosion occurs under painted or plated surfaces when moisture gets through the coating. I often see this on pumpjacks on the weld seams, and on tanks, where plates meet. The obvious fix is to make sure the metal is dry prior to coating. The other is to use a coating that resists absorbing water. Certain coatings are more prone to it than others. It is best to stick with a proven paint, and perhaps use a paint that might take a bit longer to dry, as quick dry paints tend to be more susceptible. Coatings with Zinc in them are recommended as they also offer some cathodic protection. (see below) Galvanic Corrosion The Corrosion Doctors define it like this: "Galvanic corrosion refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. The driving force for corrosion is a(n electric) potential difference between the different materials. The less noble material will become the anode of this corrosion cell and tend to corrode at an accelerated rate, compared with the uncoupled condition. The more noble material will act as the cathode in the corrosion cell. A useful application of this type of corrosion is the sacrificial corrosion of zinc, magnesium or aluminum for the cathodic protection of a metallic structure most often made of steel." We often initially determine the shape a treater will be in by the shape the anodes are in. Anodes are usually an easily replaced block of zinc or aluminum designed to corrode in lieu of the shell of the vessel it’s in. If the Anode is well used up, there was obviously quite a bit of galvanic corrosion going on. The good part is that we know the anodes did their job. The anodes are designed to protect the areas of the vessel where the coating has failed, or sometimes replace coatings in environments where coatings will not hold or are just impossible to apply. Galvanic corrosion may happen in a treater where a small chip occurs in the coating. The anode, theoretically, will then provide the Zinc for the galvanic potential to occur, rather than the exposed part of the treater shell. Keep in mind that an anode can only protect that which it can "see". It is therefore important to make sure you have anodes on both sides of baffles and internals if you want good long term protection. Did you ever wonder why some of your Temperature indicators corrode right on the threads, and others don’t? The answer is Galvanic potential. The Galvanic Series will help you determine why some fittings corrode, and others don’t. Galvanic Series (Courtesy Corrosion Doctors)"A galvanic series has been drawn up for metals and alloys in seawater, which shows their relative nobility. The series is based on corrosion potential measurements in seawater only." The basic rule is this: The further apart in this series materials are, the higher the chance of galvanic corrosion between these two materials. Most cathodic or resistant to corrosion
Most anodic or easy to corrode Now you know why Zinc is used for anodes. Stress Corrosion Cracking The NEB is very interested in SCC since a number of pipeline failures occurred. They put out this study: "NEB’s Stress Corrosion Cracking on Canadian Oil and Gas Pipelines" to deal with the problem. Stress corrosion cracking (SCC) happens when steel is subject not only to stress (such as that in a pressured pipeline, or those due to welding performed on the pipe), but also to a corrosive environment. The NEB states that "Stress corrosion cracking on pipelines begins when small cracks develop on the outside surface of the buried pipeline. Since SCC develops slowly, it can exist on pipelines for many years without causing problems. But if a crack becomes large enough, eventually the pipeline will fail and will either leak or rupture." The problems are mainly on older lines built in the 60’s and 70’s. These lines were often coated with polyethylene tape with the good intention of protecting the pipe. Unfortunately the coating can separate and allow water to contact the pipeline. Ironically, the water corrodes the pipeline because the polyethylene is shielding the pipeline from cathodic protection devices. The NEB is systematically addressing the problem "on the most affected pipelines….. for newer pipelines, the use of different coatings…continues to demonstrate great effectiveness.." In the mean time, you can prevent corrosion by monitoring your paint jobs and keeping a good eye on your anodes in your pipelines and vessels. A good inspection by an experienced inspector will identify your problem before it starts costing you real money. |
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