by Cal Chapman
Corrosion control is always a factor in oilfield work. Above-ground tanks, pipelines, and all manners of equipment suffer from corrosion, both internal and external in nature. In South Texas, two interesting physical facts come to dominate external corrosion control needs.
Many areas have shallow soils containing large concentrations of chloride ion. This comes from salt deposited when ancient ocean shorelines ranged across the region. High chloride ion content causes aggressive corrosion rates on unprotected steel. Just as importantly, the sodium and chloride ions, often found with other ions as well, cause the soil’s electrical resistivity to be very low. The low electrical resistance means high corrosion current flows, which again accelerates attack on unprotected steel. Pipelines are buried in this soil. Tanks are set on it.
Corrosion protection for these facilities should always include a combination of good coatings, good installation and inspection practices, and well-designed cathodic protection (CP) systems. Here are some pointers for best practices:
- If constructing bulk above-ground storage tanks (ASTs), put a coating on the bottom plate before field welding begins. This coating will give partial protection to the finished external bottom of tank. This reduces the amount of protective electrical current needed. It lowers capital costs for the CP system, and can greatly reduce a monthly electric bill;
- Avoid putting a plastic leak liner under a bulk tank. This prevents the installation and use of a “remote” impressed-current cathodic protection system on the tank bottom. We all intend for the first CP system to be the only one needed – but it doesn’t necessarily come out that way;
- If high-voltage AC power lines are nearby, avoid setting up pipeline rights-of-way in “close parallel” to the AC lines! With the low soil resistivities, high chloride levels, and good coatings, the AC-induction problem may cause SAFETY ISSUES and aggressive corrosion rates;
- Holiday detection is even more important for coatings inspection on pipelines, tanks, and, really, on any paint job where the metal will be in soil or water contact. Remember, too, that water gets everywhere, sooner or later;
- Coat the bottom of every steel vessel before it is set – not just a paint, but a better-quality and thicker coating! Dehydrator units, separators, bulk tanks all may be set on crushed rock or caliche, but chloride ions, sulfide ions, and other ions will move to the metal – just by diffusion – and cause corrosion attack;
- Does produced water get spilled inside containment? Yes, often it does. That adds chloride and other ions to the tank pad – bad news.
For those not so familiar with corrosion science, here are a few major themes to keep in mind:
- If we just talk about steel, it is not a “naturally occurring” material. Iron ore is dug up, then combined in very hot furnaces with coke and other alloying metals. The resulting steel is then cooled, shaped, and so on;
- A lot of new energy is added in the metal-making process. As soon as this metal is put into the environment, Mother Nature’s electrochemical processes go to work. Their goal is turn steel back into iron ore – rust! The reason? All that energy put into the making of steel is now available to be taken back. One physical law is that everything, sooner or later, will be “taken back” to its lowest stable energy content;
- To prevent or slow down that rusting process, every piece of steel should be coated. No water, either liquid or vapor, should be allowed to contact it. And there are plenty of other chemicals which may drive the corrosion process, chloride ion being one;
- Finally, in addition to the coatings, cathodic protection should be applied. Coatings by themselves are not good enough, because there are always defects or holes in them.
Cal Chapman is co-founder of Chapman Engineering, which began business in fall 1988. He is a licensed “Cathodic Protection Specialist” through the National Association of Corrosion Engineers (NACE, now called NACE International), and is a licensed professional engineer in Texas and New Mexico. Chapman Engineering provides cathodic protection, AC mitigation, coatings and corrosion protection specifications, and other engineering services. The company performs environmental compliance, assessment and remedy services in oilfield, petroleum wholesale, and industrial settings. In-house staff includes engineers, geologists, corrosion technicians and environmental scientists. Please contact Chapman Engineering at 800-375-7747.