Corrosion under insulation — known as CUI — is the leading cause of unplanned outages on insulated industrial storage tanks and pressure vessels across Canada. It progresses for years beneath the jacketing, hidden from inspection, until the substrate fails. This page is a reference for procurement, maintenance and operations teams scoping insulated tank capital — what causes CUI, why traditional jacketing systems struggle to prevent it, and what a sealed standing-seam envelope changes.
What CUI Is
CUI is localized corrosion of a metal substrate (typically carbon steel) that initiates beneath the thermal insulation system, almost always driven by moisture trapped between the insulation and the metal surface. Because the corrosion happens out of sight, it is not detected by routine external inspection. By the time a CUI failure surfaces, substantial wall-thickness loss has usually already occurred.
The most aggressive temperature window for CUI on carbon steel is approximately −12 °C to +175 °C — the range in which liquid water can persist against the steel without flashing off. Cycling assets, intermittent service, and outdoor installations in temperate climates (almost every Canadian tank farm) live squarely inside this window.
CUI Temperature Window
Carbon steel: most aggressive between approximately −12 °C and +175 °C. Above and below this range, CUI rates fall significantly because liquid water can no longer persist on the steel surface.
How Moisture Gets In
Traditional jacketing — lapped sheets fastened through the weather face — relies on continuous sealant adhesion to keep water out. Every penetration and every lap is a potential ingress point. Four mechanisms drive moisture into the jacketed system:
- Lap-seam failure. Overlapping sheets depend on sealants and mastic. Sealants age, lose elasticity, and pull away from the steel substrate or the next sheet. Each lap becomes a slow leak.
- Penetrations. Screws, rivets and bolts that fasten the jacket through the weather face create permanent pathways. The sealant around the fastener degrades; the hole remains.
- Vapour drive. Even on a well-sealed system, ambient humidity drives moisture inward through the insulation when temperatures cycle below dewpoint at the metal interface.
- Insulation core that retains moisture. Open-fibre cores (loose mineral wool blanket, unprotected fibreglass batt) hold water once it enters. Wet insulation continues to feed the substrate even after the leak is sealed.
What a Sealed Standing-Seam Envelope Changes
A pre-fabricated standing-seam panel system replaces lap-and-fastener jacketing with a continuous mechanical seam. Adjacent panels are joined by a folded double-lock seam — created on-site by tooling — that locks the panels together without penetrating the weather face. The relevant changes for CUI prevention:
- No fasteners on the sidewall weather face. The most common moisture entry pathway is removed entirely.
- Mechanical seal, not chemical. The double-lock fold creates the seal — there is no sealant adhesion to manage over time.
- Moisture barrier laminated to the interior face. A Surlyn (DuPont) moisture barrier is heat-laminated to the inside of the aluminum jacketing. Moisture cannot contact the jacket interior.
- Insulation core selected to the application. Closed-cell polyisocyanurate (PIR) is the typical choice for cold and cryogenic service because it does not absorb moisture. Mineral wool or fibreglass are used where the operating temperature requires them — and the sealed envelope is what keeps water out of those cores.
- Continuous envelope. The result is a sealed shell around the tank that does not present the conditions CUI needs to start.
Field Implications for Asset Owners
For owners and operators, the practical difference shows up in the inspection and remediation cycle. Traditional jacketing typically requires:
- Periodic re-sealing of seams and fasteners
- Partial re-jacketing where moisture damage has begun
- Substrate inspection — typically destructive removal of insulation — after CUI events are suspected
- Downtime to safely complete the above
A sealed standing-seam envelope is designed to remain in place across a multi-decade service life — without those interventions. The Enerpro Tank Panel System carries a design service life of 25+ years maintenance-free. Lifecycle cost should be compared on that basis, not on first cost alone.
What to Specify for CUI-Resistant Insulated Tanks
When scoping a new tank insulation system — or a re-jacket on an asset with known CUI history — the following are the specification points that matter most:
- Jacketing material: ASTM B209 wrought aluminum (alloys 1100, 3003 or 3105-H14) in 0.016″ to 0.032″ thicknesses, with a heat-laminated interior moisture barrier (Surlyn).
- Seam type: Mechanically folded double-lock standing seam, no fasteners through the weather face.
- Insulation core: Selected to the application — closed-cell PIR for cold/cryogenic, mineral wool for high-temperature process service, fibreglass where specifically called out.
- Fabrication method: Pre-fabricated to exact tank dimensions, including accommodations for nozzles, manways, stairs and other penetrations.
- Installation: Scaffold-free, no welding on tank sidewalls, internal cable-and-clip anchoring.
Where the Enerpro Tank Panel System Fits
The Enerpro Tank Panel System — exclusive to Max Thermal Fabricators, designed and manufactured in Edmonton, Alberta — is built to the specification points above. It is the only pre-fabricated standing-seam tank insulation panel system manufactured in Canada, which means industrial operators across the country source it without cross-border freight, customs delay or currency exposure. We design panels to each tank's exact dimensions and operating temperature, fabricate in-house with CNC plasma cutting and metal forming, and coordinate experienced installation crews.
For a project-specific scope — tank dimensions, operating profile, insulation core selection — contact our team at office@maxfab.ca or call 1-780-717-2956.