Corrosion is one of those "equal opportunity" hazards that affects all industries indiscriminately, to the tune of billions of dollars annually in repair and replacement costs. Some types of corrosion are readily apparent, such as rusting of unprotected plain carbon steel tanks and piping. But when corrosion occurs in hidden places as it so often does, it may go undetected, sometimes with catastrophic results. Two common and costly examples, hidden from view under insulation, are non-uniform attack of plain carbon steels and stress corrosion cracking of 300 series stainless steels.
Because corrosion under insulation occurs out of sight, it frequently is out of mind until a leak occurs, producing a release. Personnel, the environment, plant process uptime and system integrity all may be impacted adversely, as a result.
CUI happens when water is allowed to enter an insulated system or component and contact the underlying surface. This water can originate as rain (CUI is predominant in high humidity, high rainfall coastal areas), run-off from equipment washdowns, deluge systems tests, condensation from temperature cycling and leakage from aqueous process systems. A minute low pressure stream leak from a steam tracer tube fitting or valve stem packing under insulation can cause all sorts of problems. Furthermore, corrosion and cracking are accelerated by corrosive salts such as chlorides which are leached from insulation materials by intruded water.
We know how to prevent CUI. It requires a fundamental systems approach, starting with a good protective coating suitable for aqueous immersion service, such as a catalyzed epoxy-coal tar or epoxy-phenolic, on the bare metal substrate. This is followed by installation of dry insulation under dry conditions, all wrapped with protective metal or non-metallic jacketing designed to exclude water, not collect it. Finally, the system must be monitored and maintained to keep it dry, corriosion-free and thermally efficient. Don't expect caulked seams to stay tight and resilient year after year. Don't expect to keep water out if you drill access holes in insulation for ultrasonic wall thickness measurements, and forget to seal the holes. I think you'll agree, the need for monitoring and maintenance can't be over emphasized.
Unfortunately, there's a lot of insulated process piping and equipment in our chemical pants and refineries which never received immersion grade protective coatings before insulation was applied, and the unsulation system was poorly designed/installed/maintained, and trouble in the form of corrosion/cracking is brewing. So it behooves us to find the trouble spots before process fluid leaks and spills and releases occur, especially the hazardous ones. After all, we are charged with keeping the processes inside the pipes, tanks and pressure vessels and not out in the environment. Here's where NDE and awareness training can rise to the occasion.
Source:https://inspectioneering.com/journal/1995-05-01/56/i-wonder-what-s-going-on-under
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