CUI Mitigation TSA 2026 — Thermal-Spray Aluminum + Coating Strategy

CUI mitigation and repair: the decision chain from finding to fix

Short answer: CUI mitigation means keeping water and chlorides away from hot or cycling steel under insulation, through design, coatings such as high-performance epoxies or thermal spray aluminum, better insulation and jacketing, and removing insulation that is not needed. CUI repair starts only after inspection has measured the damage: the owner's inspector and engineer decide whether to recoat, repair or replace, then the area is re-examined, recoated and re-insulated to a specification that addresses why water got in.

Most writing on corrosion under insulation stops at detection. The harder question for a plant is what to do next: which locations to fix, how to fix them so the problem does not return in five years, who signs off the repair, and how the repaired area is documented so the next inspection has a baseline. This section walks through that chain. The two key references are AMPP (formerly NACE) SP0198, Control of Corrosion Under Thermal Insulation and Fireproofing Materials, a Systems Approach, currently the 2017 revision, and API RP 583, Corrosion Under Insulation and Fireproofing, whose second edition was published in March 2021. Both are copyrighted; we summarise their approach in our own words, so check the current editions for detail.

Why mitigation is a system, not a coating

SP0198's subtitle, "a systems approach", is the core idea. CUI needs three things: steel at a temperature where water does not boil off quickly, water that gets into the insulation and stays there, and time. Contaminants such as chlorides from the insulation, the atmosphere or process leaks make it worse, and austenitic and duplex stainless steels add the risk of external chloride stress corrosion cracking. No single barrier is reliable for the life of a unit, so mitigation layers several of them: a design that sheds water, jacketing and sealing that keep it out, an insulation material that does not hold it, and a coating on the steel that protects it when the other layers fail, which eventually they do.

That is why a repair that only replaces the damaged insulation and jacketing, without recoating the steel or fixing the water entry point, usually brings the same CUI back to the same location. It is also why the most effective mitigation step is often the one that costs least to maintain: taking insulation off equipment that does not need it.

Step one: is the insulation needed at all?

Insulation is installed for heat conservation, process control, personnel protection or fire protection, and needs change over a unit's life. Lines that were once hot may now run ambient; insulation applied purely to protect people from hot surfaces can sometimes be replaced with guards or cages that keep people away while letting the steel stay dry and visible. API RP 583 and SP0198 both encourage reviewing whether insulation is still required before reinstating it.

Removing unnecessary insulation turns a CUI-susceptible item into one that can be inspected by simple visual examination and protected by a conventional atmospheric coating. The decision belongs to the owner's process, safety and integrity teams together, because personnel protection and process temperature requirements have to be met another way. Where insulation stays, the remaining steps apply.

Coatings under insulation: what the options are

The coating on the steel is the last line of defence and the one that determines how long the repair lasts. Coatings used under insulation have to tolerate the operating temperature range, wet-dry cycling and, for cycling services, thermal shock. The main families referenced in industry guidance are summarised below; the right choice depends on temperature, substrate, application conditions and whether the work is shop or field applied.

Thermal spray aluminum is applied to a joint specification, NACE No. 12/AWS C2.23M/SSPC-CS 23.00, which covers surface preparation, application, thickness measurement, adhesion testing, sealers and topcoats. One industry presentation on CUI design, published through the National Insulation Association, cites typical renewal cycles in the range of 8 to 15 years for liquid-applied coatings and 15 to 30 years for TSA; treat those as indicative, not as a design basis, because actual life depends on service, application quality and how well the insulation system keeps water out.

The practical constraint in repair work is that many CUI repairs are done on operating equipment. Abrasive blasting near live hydrocarbon equipment, and flame or arc spraying of TSA, raise ignition and dust control questions that the site's permit system must address. Some repairs are therefore scheduled into turnarounds, and interim repairs use surface-tolerant systems until the full system can be applied.

Insulation and jacketing choices that reduce CUI

Insulation material matters because some materials absorb and hold water while others do not. Industry guidance distinguishes materials that resist water absorption, such as expanded perlite, from fibrous materials such as mineral wool, which drain differently and should be specified with low leachable chloride content, and from aerogel blankets, whose water behaviour depends on whether they are treated to be hydrophobic. Whatever the material, it should be compatible with the operating temperature and should be specified for leachable chlorides where stainless steel is involved.

Jacketing and sealing do most of the work of keeping water out. API RP 583 and SP0198 both give extensive guidance on jacketing: overlaps oriented to shed water, sealed seams on horizontal runs, proper termination and sealing at nozzles, supports, flanges and other penetrations, and avoidance of horizontal surfaces where water can pond. Damaged jacketing, missing caulk at penetrations and open terminations are the most common entry points found on inspection walkdowns, and fixing them is a maintenance task that should be part of every CUI repair, not a separate project.

Where repeat inspection is expected, removable insulation covers or inspection ports at high-risk locations reduce the cost of the next inspection. They must be designed and sealed so they do not become new water entry points.

Repair after CUI is found: who decides what

Once insulation is removed and the steel is cleaned, the damage is measured. That measurement, not the visual appearance, drives the decision. For pressure vessels under API 510, piping under API 570 and tanks under API 653, the owner's authorized inspector evaluates the remaining thickness against the minimum required thickness and the code's acceptance rules, and repairs and alterations are authorized under the applicable code. NDE contractors produce the measurements; they do not approve repairs.

The common outcomes are:

• Acceptable as is, recoat and reinsulate. Corrosion is shallow and remaining thickness is well above minimum. The area is cleaned, recoated to specification, reinsulated with corrected jacketing, and the thickness readings become the baseline.
• Repair by welding or component replacement. Thinning is below what the code accepts. The repair follows the code's repair rules, with welding procedures, NDE of the repair and pressure testing as the inspector and code require.
• Engineering assessment. Where thinning is localised and the owner wants to know whether the item can continue in service, the owner may commission an engineering assessment, for example a fitness-for-service evaluation under API 579-1/ASME FFS-1. That is an owner-commissioned engineering task, separate from the NDE.
• Temporary repair. Engineered clamps or enclosures may be used under the owner's procedures and the code's provisions for temporary repairs, with a plan for permanent repair.
• Stainless steel cracking. If penetrant testing finds chloride stress corrosion cracking, the response is a repair or replacement decision by the owner's inspector and engineer, plus correction of the chloride source and the insulation system.

Whichever route is taken, the repair should close with a defined set of records: before-repair thickness data, the repair or coating specification applied, quality control results for surface preparation and coating, post-repair thickness readings, and the reinsulation details. Coating quality control, such as dry film thickness, holiday testing and adhesion for TSA, is normally carried out by qualified coating inspectors against the coating specification.

A worked example: CUI on a cycling process line

Consider, qualitatively, an insulated carbon steel line that cycles between ambient and moderate operating temperatures, running across a rack with several pipe supports and two small-bore branch connections. A screening survey flags wet insulation near the supports. Insulation is stripped at those locations and at the branch connections, the steel is cleaned, and the NDE crew performs visual examination and UT thickness readings, then corrosion mapping where the visual shows significant loss.

The results show general shallow corrosion along most of the stripped length, deeper localised loss at one support where the jacketing had been cut for the support shoe and never sealed, and sound steel at the branches. The owner's API 570 inspector evaluates the minimum readings, accepts most areas for recoat, and specifies a pipe section replacement at the support. The engineering team reviews the support detail and changes it so the shoe no longer pierces the jacketing. The coating specification calls for a high-build system suitable for cycling service, applied after the pipe section replacement. The reinsulation uses sealed jacketing with an inspection port at the redesigned support.

The point of the example is the sequence. Measurement came before decisions; the code-authorized inspector made the acceptance decision; the root cause, a cut and unsealed jacket at a support, was fixed rather than simply reinsulated; and the post-repair readings became the baseline for the next inspection interval. Ask us to scope baseline UT for your next CUI repair.

NDE before and after a CUI repair

The inspection methods for finding CUI are covered in our CUI detection guide. For mitigation and repair the NDE has different jobs:

• Before repair: visual examination and UT thickness or corrosion mapping on the cleaned surface to establish the minimum remaining thickness and extent of the damaged area, and penetrant testing on stainless steel where cracking is possible.
• During repair: NDE of repair welds as the code and repair procedure require, for example radiography or UT of butt welds and MT or PT of fillet and attachment welds.
• After repair: UT thickness readings at fixed, recorded locations before reinsulation, so that the next inspection has a true baseline rather than an estimate.

The after-repair readings are the most often skipped and the most valuable. Without them, the next inspection cannot calculate a corrosion rate for the repaired area, and the integrity team has to assume a rate rather than measure one.

Common mistakes in CUI mitigation and repair

• Reinsulating without recoating. New insulation over bare or poorly prepared steel restarts the clock at the same location.
• Fixing the symptom, not the entry point. If damaged jacketing, an unsealed penetration or a support detail let water in, it must be corrected or the CUI returns.
• Reinstating insulation that is no longer needed. Review the need before buying new insulation.
• Using the wrong coating for the temperature. Coating temperature limits and cycling tolerance are product-specific; check them against actual operating conditions, including upsets and steam-out.
• Ignoring stainless steel. Stainless lines need chloride control in the insulation and, where specified, protective foil or coatings; they are not immune.
• No post-repair baseline. Without thickness readings before reinsulation, the next interval is built on assumption.

Regulatory overlay and Canada

CUI management is part of mechanical integrity. For US facilities covered by OSHA process safety management, 29 CFR 1910.119(j) requires inspection and testing following recognised and generally accepted good engineering practice and correction of equipment deficiencies, and API RP 583 and SP0198 are commonly cited as that practice for CUI. EPA's Risk Management Program has parallel mechanical integrity requirements for covered processes. API 510 and API 570 each address CUI inspection of susceptible equipment, and API 653 addresses insulation removal on insulated tanks.

In Canada, pressure equipment integrity management falls under provincial boiler and pressure vessel regulators, such as ABSA in Alberta and TSSA in Ontario, whose integrity management requirements expect owners to identify and manage damage mechanisms including CUI. Confirm the specific requirements with your jurisdiction.

How Atlantis supports CUI mitigation and repair

Atlantis performs the NDE around a CUI campaign: visual examination and UT thickness and corrosion mapping on stripped locations, PT for chloride cracking on stainless steel, NDE of repair welds, and post-repair baseline thickness readings before reinsulation. Our technicians work under an SNT-TC-1A written practice with ASNT Level III oversight, and results go to your API-certified inspector and engineers, who make the acceptance and repair decisions. We do not perform fitness-for-service or risk-based inspection assessments. See CUI inspection services, corrosion mapping and our API RP 583 overview, or request a quote for CUI repair NDE; we reply within 24 hours.

More questions about CUI mitigation and repair

What is CUI mitigation?

CUI mitigation is the set of measures that prevent corrosion under insulation: removing unnecessary insulation, designing and sealing jacketing to keep water out, choosing suitable insulation, and coating the steel with a system rated for the service, such as high-performance epoxies or thermal spray aluminum.

How is CUI repaired?

The insulation is removed and the steel cleaned and measured. The owner's inspector evaluates the remaining thickness under the applicable code, then the area is recoated, repaired by welding or replaced, re-examined, and reinsulated with the water entry point corrected.

Is thermal spray aluminum worth it for CUI?

TSA is widely used for long-life CUI protection on carbon steel, especially in cycling or higher temperature service. It requires high-quality surface preparation and controlled application to NACE No. 12/AWS C2.23M/SSPC-CS 23.00, and field application may need hot-work controls.

What standards cover CUI mitigation?

AMPP (NACE) SP0198, currently the 2017 revision, and API RP 583, second edition 2021, are the main references. API 510 and API 570 address CUI within their inspection requirements, and API 653 covers insulated tank shells.

Who decides whether a CUI-damaged pipe can stay in service?

The owner's API-certified inspector, under the applicable code, using the measured thickness. Where an engineering assessment is needed, the owner commissions it. The NDE contractor provides measurements, not acceptance decisions.

Should we remove insulation instead of replacing it?

Where insulation is no longer needed for process, energy or personnel protection, removing it is often the most effective mitigation. Personnel protection can sometimes be provided with guards instead. The decision needs process and safety review.

How do you prevent CUI on stainless steel?

Use insulation with low leachable chlorides, keep water out with sealed jacketing, and apply a protective barrier such as aluminum foil wrap or a suitable coating where specified. Inspect susceptible areas with PT for chloride stress corrosion cracking.

What NDT is done after a CUI repair?

NDE of any repair welds as required by the code, coating quality checks against the specification, and UT thickness readings at recorded locations before reinsulation to set a baseline for future corrosion rates.

Planning a CUI strip-and-repair campaign? Send us the location list and we will scope the before-and-after NDE for your inspector. Related: piping circuit and CML inspection.

Sources: AMPP SP0198; API RP 583 (API standards catalogue); NACE No. 12/AWS C2.23M/SSPC-CS 23.00; P. Bock, Understanding Issues Surrounding CUI (insulation.org); OSHA 29 CFR 1910.119. Confirm against current editions.