CUI Inspection Singapore — Detecting Corrosion Under Insulation on Pipe Racks

What Is Corrosion Under Insulation?

Corrosion under insulation, universally shortened to CUI, is the corrosion of carbon and low-alloy steel — and the chloride stress-corrosion cracking of austenitic stainless steel — that takes place hidden beneath thermal insulation and its protective cladding. It begins when water penetrates the insulation through damaged jacketing, failed sealant, or condensation, then sits trapped against the pipe or vessel wall. The insulation that keeps heat in also keeps that moisture in, and the metal corrodes in a warm, wet, oxygen-rich pocket that never dries out.

What makes CUI so dangerous is that it is invisible from the outside. The cladding looks intact while the steel beneath is thinning, so the first sign is often a leak, a wet patch, or an outright failure. Because of this, CUI is widely regarded as one of the costliest hidden-corrosion problems in the refining and petrochemical industry and a leading cause of unplanned shutdowns and loss-of-containment events on insulated piping and equipment. It is precisely the kind of degradation that a routine external visual walk-down cannot catch, which is why it needs a dedicated inspection strategy.

Why Singapore's Climate Makes CUI Worse

Singapore is one of the harshest environments in the world for corrosion under insulation. High year-round rainfall and near-saturated humidity mean that once water gets into an insulation system it has little chance to dry out, so insulation stays wet and the metal underneath stays wetted for extended periods. The heavy industrial concentration on Jurong Island — with its dense insulated pipe racks, process columns and interconnecting lines — presents thousands of metres of susceptible piping in exactly this climate.

Operating temperature drives the risk. For carbon and low-alloy steel the CUI-susceptible band runs from roughly minus 4 to 175 degrees Celsius, the range in which water stays liquid against the wall. Lines that cycle in and out of this range, that run intermittently, or that operate below ambient dew point are the worst offenders because they wet and dry repeatedly, concentrating chlorides and accelerating attack. Combine cyclic service with Singapore's rainfall and you have the classic recipe for aggressive, concealed corrosion — which is why insulated lines here deserve proactive CUI screening rather than run-to-failure.

The Detection Challenge and the Methods We Use

The defining problem with CUI is access: the damage is behind insulation and cladding, so you cannot see it, and stripping every metre of insulation to inspect it is slow, expensive and disruptive. The answer is a layered set of screening methods that see through or around the insulation without full removal, reserving strip-out for the few places that genuinely need it.

Where the piping runs high on elevated racks or across hard-to-reach process structures, drone and crawler-assisted access carries cameras and thermography aloft so those sections can be surveyed without scaffolding or extensive rope access, keeping inspectors off height and out of the congested pipe-rack envelope.

The Screen-Then-Verify Workflow

A CUI campaign only works if it is intelligently targeted — the volume of insulated piping in a plant is far too large to inspect blindly. We follow a screen-then-verify workflow that concentrates effort where the risk is real.

• Risk-based prioritisation — susceptible circuits are ranked by service temperature, insulation condition, age and history, so the most CUI-prone lines are screened first.
• PEC and infrared screening — pulsed eddy current maps average wall loss through the cladding while thermography locates wet insulation, together building a picture of where corrosion is likely.
• Mark suspect locations — every flagged zone is logged against a line and location reference, so nothing is lost between screening and follow-up.
• Confirm and size by UT — insulation is removed locally at the flagged hotspots and ultrasonic thickness readings confirm and quantify the remaining wall.
• Integrity assessment and repair — the verified data feeds a fitness-for-service judgement and, where needed, a repair or re-insulation plan.

The result is a defensible dataset that tells the plant's integrity team exactly where the metal is thinning, how much is left, and which lines need attention — without the cost and disruption of blanket insulation removal.

Where CUI Inspection Fits in Your Integrity Programme

Because pulsed eddy current and thermography are online methods, most of a CUI screen can run while the plant is in normal operation, with no shutdown required. That decouples the bulk of the survey from turnaround windows and lets operators build a live CUI picture through the year, reserving shutdown time only for confirmatory strip-out and repair.

CUI screening does not sit in isolation. It is one strand of a broader mechanical integrity programme that also covers UT wall-thickness measurement on tanks and vessels and the wider scope of industrial drone inspection. It supports piping inspection under API 570 and pressure-vessel inspection under API 510, drawing on the CUI-specific guidance of API RP 583, and it pairs naturally with a planned plant shutdown inspection when confirmatory access is scheduled. For operators in the sector, it complements our dedicated oil and gas and refinery inspection services, with thermal imaging providing the wet-insulation screen at its core.

CUI-Susceptible Items We Screen

Not all insulated equipment carries the same CUI risk. Our surveys concentrate on the items where water ingress and susceptible temperatures most often coincide.

Why Choose SG Drone Inspections

We are a specialist industrial inspection provider in Singapore delivering CUI screening data that integrity engineers can act on with confidence.

Frequently Asked Questions