Corrosion Under Insulation: Structural Steel & Pipe Coating

This guide breaks down how to protect structural steel and piping across the different exposure zones of a facility, from atmospheric steel down to buried or submerged sections, and shows where corrosion under insulation starts and how to stop it before a specification gets written. New Kent Coatings provides industrial coating services across petrochemical, chemical processing, power generation, and manufacturing facilities, supervised by in-house NACE CIP Level 3 certified inspectors.

How the Zone-of-Protection Framework Determines Your Coating System

Every piece of structural steel and every run of pipe on your site sits in one of three exposure zones, and the coating system has to match the zone it protects, not the zone next to it. Treating a facility as a single uniform environment is the fastest way to end up with a coating that performs in one location and fails in another eighteen months later.

Atmospheric Zone

Atmospheric steel and piping are exposed to UV, weather, and temperature cycling but are not in direct contact with process chemicals or submerged in liquid. This is the largest exposure category on most sites: exterior structural members, pipe racks, and elevated piping runs that see sun and rain but nothing more aggressive. The coating system here has to hold up against thermal cycling and UV breakdown over a service life measured in years, not months.

Splash Zone

Splash zone steel takes intermittent product contact, thermal shock from temperature swings between wet and dry cycles, and occasional immersion during process upsets or cleaning cycles. Steel near loading racks, transfer points, and tank shell exteriors near grade often falls into this category. It is the zone most often under-specified, because it looks like atmospheric exposure on a walkthrough but behaves like a chemical exposure zone under actual operating conditions.

Immersion Zone

Immersion zone steel includes buried piping, submerged structural supports, and soil-side interfaces where steel sits in continuous contact with soil moisture or process liquid. This zone demands the most aggressive coating chemistry on the site, and it is frequently adjacent to containment structures. We cover the coordination between structural steel work and containment systems in our secondary containment coatings guide, since the two are often specified and installed in the same project window.

Why Surface Preparation Has to Match the Coating System, Not Just the Steel

Improper surface preparation accounts for 85 percent of coating failures, and structural steel and piping are not exceptions to that number. A correctly specified coating system applied over an inadequately prepared surface fails on the same timeline as a poorly specified one. The prep standard has to be selected based on the coating system going on top of it, not based on what is fastest to blast.

SSPC Blast Standards by Exposure Zone

Atmospheric coatings on structural steel are typically specified to SSPC-SP 6 Commercial Blast, which removes rust, mill scale, and old coating down to a surface with only light staining remaining. High-performance systems, and anything specified for splash or immersion zone service, require SSPC-SP 10 Near-White Blast or SSPC-SP 5 White Metal Blast, which leave progressively less residual staining and give the primer a cleaner surface to bond to. Specifying SSPC-SP 6 for an immersion-zone application is a common shortcut that shows up as adhesion failure well before the coating’s rated service life.

Why Blast Profile Depth Has to Match the Primer

The surface anchor profile, the depth of the peaks and valleys the blast media creates in the steel, has to fall within the range the primer manufacturer specifies. A profile that is too shallow gives the primer nothing to mechanically key into. A profile that is too deep can leave peaks exposed above the coating’s dry film thickness, creating early corrosion sites at the high points. We prepare structural steel and tanks to the profile depth the coating manufacturer specifies, not the depth that is fastest to reach.

Choosing the Right Coating System for Each Zone

Once the exposure zone and surface preparation are established, coating chemistry selection follows a fairly consistent pattern across structural steel and pipe coating projects:

Atmospheric structural steel: zinc rich primer as the base coat, providing galvanic protection to the steel even if the topcoat is damaged, typically paired with a high-build epoxy intermediate and a polyurethane topcoat for UV stability.
General industrial atmospheric and mild splash exposure: high-build epoxy and polyurethane topcoat systems, balancing chemical resistance with UV and weathering performance.
Chemical splash zones: phenolic epoxy or novolac epoxy systems, formulated to resist the specific chemical classes present in splash and vapor exposure.
Immersion and buried service: 100% solids epoxy or coal tar epoxy systems, built for continuous contact with soil moisture or process liquid without the solvent loss that thinner-film systems experience.

The chemistry decision changes again for tank interiors, where the coating is in sustained contact with a stored commodity rather than intermittent exposure. We break down how commodity type drives that decision in our industrial tank lining guide, which covers the same phenolic, novolac, and 100% solids chemistries from the storage side of the equation.

What Makes Pipe Coating Different From Structural Steel Coating

Pipe coating carries structural steel’s exposure zone considerations plus a set of problems that are specific to piping geometry and insulation systems.

Thermal Movement at Bolted Connections

Bolted connections and flanges experience thermal expansion and contraction independently from the pipe run itself, which puts cyclical stress on the coating film at exactly the points where two dissimilar surfaces meet. A coating system that performs well on a straight run of pipe can crack or disbond at a flange within a fraction of the pipe’s expected service life if the system was not selected with that movement in mind.

Insulation Compatibility and the Transition Zone

Pipe coating under insulation has to be compatible with the insulation material and jacketing system, since some coating chemistries interact poorly with certain insulation types or trap moisture against the pipe surface instead of shedding it. The transition zone, where insulated pipe meets bare pipe or a fitting, is the single highest-risk location on the entire pipe run, because the coating system has to bridge two very different exposure conditions in a few inches of pipe.

What Is Corrosion Under Insulation and Why It’s the Highest-Risk Zone in Your Piping

Corrosion under insulation, often shortened to CUI, is the most damaging and most underaddressed corrosion problem in industrial piping, and understanding how it develops changes how a transition zone gets specified.

What Causes CUI

CUI happens when moisture gets trapped beneath pipe insulation from condensation, rain ingress at damaged jacketing, or a process leak, and then corrodes the pipe exterior with no visible evidence at the insulation surface. The insulation that is supposed to protect the pipe becomes the mechanism that hides active corrosion from routine visual inspection.

Why It Stays Hidden Until the Pipe Wall Is Already Compromised

Corrosion under insulation is usually discovered during a turnaround or after a jacketing repair exposes the pipe underneath, and by that point the wall thickness may already be compromised enough to require pipe replacement rather than a simple recoat. The cost and schedule impact of that discovery is almost always worse than the cost of specifying the transition zone correctly in the first place.

Why the Transition Zone Is Where It Starts

The transition point where insulated pipe meets bare pipe or a fitting is where corrosion under insulation most often starts, because it is where the coating specification most often gets simplified or skipped entirely. A coating system that stops at the edge of the insulation, rather than extending into the transition zone with a chemistry rated for that specific interface, leaves the exact location most vulnerable to moisture ingress uncoated or under-coated.

Get your structural steel and piping systems assessed before you write the spec.

Schedule a Coating Specification Review

When to Overcoat an Existing System and When to Remove It

Facility managers maintaining existing structural steel and piping face a recurring decision: apply a new coat over the existing system, or blast down to bare metal and start over. The answer is not about which option is faster or cheaper. It is determined by the condition of what is already on the steel.

Adhesion testing on the existing coating, confirming it is still mechanically bonded to the substrate rather than just sitting on top of it.
Contamination assessment, checking for soluble salts, oil, or other substrate contamination that would compromise a new coat regardless of surface profile.
Measuring dry film thickness on the existing system, since building additional coats over an already-thick system can exceed the total film build the chemistry was designed to handle.

When all three come back favorable, overcoating extends service life at a fraction of the cost of full removal. When any of them fail, particularly adhesion, overcoating traps the problem under a new layer instead of solving it, and the new coating fails on the same timeline the old one would have.

Why NACE Level 3 Inspection Comes Before the Coating Selection, Not After

Most facility managers think of inspection as the step that happens after the coating goes on, confirming the work was done correctly. NACE Level 3 inspection is more valuable earlier in the process, at the specification stage, where the inspector determines the correct exposure zone classification, coating chemistry, and surface preparation standard before a product is ever selected.

That distinction matters because a contractor who only shows up to inspect finished work has no influence over whether the right system was specified in the first place. New Kent Coatings’ management team holds NACE CIP Level 3 certifications and brings that expertise into structural steel and pipe coating projects across chemical, petrochemical, and manufacturing facilities from the specification stage forward, not after the crew has already mobilized.

Get Your Structural Steel and Piping Specified Before the RFP Goes Out

Whether you’re in Virginia, Kentucky, Ohio, Tennessee, Georgia, Maryland, North Carolina, or South Carolina, reliable structural steel corrosion protection starts with zone classification, matched surface preparation standards, and a transition zone plan for corrosion under insulation. New Kent Coatings’ NACE Level 3 certified team works directly with facility managers during the specification phase, before a contractor mobilizes and before a product gets selected. Contact New Kent Coatings before you finalize the scope, so the spec reflects what your facility actually needs instead of what’s fastest to bid.