A type 316 stainless steel tube and sheet in a heat exchanger of a seawater reverse osmosis (SWRO) desalination plant is subject to crevice corrosion.
Corrosion, one of the most common forms of equipment degradation, is split into two major categories: crevice corrosion and corrosion under insulation. Any equipment that handles high-volume electrolytes and has gaps or contact areas is susceptible to both types. Knowing how to recognize and prevent both types is invaluable.
Stainless steel is normally protected by a thin layer of oxide film. But under highly acidic or high-temperature conditions, the film breaks down and the stainless steel becomes vulnerable to corrosion.
Steel is especially vulnerable at gaps or contact points between parts, under gaskets, washers, fastener heads, threads, lap joints, clamps or seals, inside cracks and seams, or in spaces that have accumulated corrodent deposits. These "crevices” tend to be wide enough to permit passage of corrodents, but too narrow to allow for fluid movement, which leads to the buildup of deposits.
Crevices tend to develop their own local chemistry that can be significantly different from the chemistry of the bulk fluid. For example, in a neutral pH solution, crevice pH can drop to 2, which is highly acidic and corrosive to most metals and alloys. Note:"pitting" is a term used for extremely localized attack that result in holes in metals, and is commonly used to mean the same thing.
What to Do About It
The easiest way to prevent this kind of pipeline damage is to design crevices out of the system. For example, using welded butt joints instead of riveted or bolted joints helps eliminate potential points of corrosion. Solid, non-absorbent gaskets like Teflon are a safe choice, and higher-weight alloys provide increased resistance. Continuous welding and soldering are also viable options.
Corrosion Under Insulation
The simplest way to think of crevice corrosion is as internal process that occurs at crevices within a system. Corrosion under insulation, on the other hand, is external corrosion. CUI (as it is sometimes called) occurs on pipes and equipment carrying high-temperature fluids. These pipes are first coated in high temp coatings, then coated a second time in insulation that lowers external temperatures and protects personnel from being burned. However, this insulation is exactly what causes CUI degradation.
When naturally-occurring moisture becomes trapped between the hot pipe and the layer of insulation, rusting and corrosion are the result. Moisture typically enters through cracks or holes in the insulatory layer, and CUI is often unnoticed because the insulation itself obscures the damage.
What To Do About It
The best way to prevent corrosion under insulation is to take steps to ensure that insulation is 100% enclosed. Joints, terminations, and irregularities along the pipe must be meticulously sealed and checked.
Still, even with the best quality control, moisture may still find its way past insulation. In other words, there are almost no methods that can guarantee CUI prevention, but its ravaging effects can be limited with superior insulation and scheduled coatings maintenance.
Mark Tool & Rubber has been installing insulation on high temp pipes and equipment for over 60 years and counting. Our state-of-the-art SplashTRON® pipeline coating bonds permanently to metal and can be expected to protect against corrosion for the life of the platform. It’s the best way to prevent CUI, and is used by the leading producers of oil, gas and transmission companies. We also carry our MG-2400 non-drying coating for moderate temperatures and cold ferrous applications.