How do you verify the integrity of a clad layer in a pressure vessel after fabrication?

Clad layer integrity in a pressure vessel is verified through a combination of non-destructive and destructive testing methods applied after fabrication. Ultrasonic testing is the primary tool for detecting disbonding, while visual inspection, dimensional checks, and metallurgical tests confirm the overall quality of the cladding. The specific combination of methods depends on the cladding material, the applicable design code, and the criticality of the application. The sections below address each verification method in detail.

What methods are used to detect disbonding in a clad layer?

Disbonding in a clad layer is detected primarily through ultrasonic testing, supplemented by hammer testing for accessible surfaces and, in some cases, acoustic emission monitoring during pressure testing. These methods identify areas where the cladding has lost metallurgical or mechanical contact with the base material, which can compromise corrosion protection and structural performance.

The most widely applied approach is pulse-echo ultrasonic testing, which sends sound waves through the vessel wall and identifies reflections that indicate a lack of bond between the cladding and the substrate. For more complex geometries or thicker cladding, phased array ultrasonic testing (PAUT) offers improved resolution and coverage.

Hammer testing, though simple, remains a practical first-pass method on accessible flat or mildly curved surfaces. A change in acoustic response when tapping the surface can indicate delamination beneath. It is not a substitute for ultrasonic methods but is useful for rapid screening during fabrication.

How does ultrasonic testing verify clad layer bond quality?

Ultrasonic testing verifies clad layer bond quality by measuring the acoustic transmission across the bond interface between the cladding material and the base metal. A well-bonded interface transmits sound energy with minimal reflection, while a disbonded or poorly bonded area reflects the signal back, registering as an indication on the instrument display.

In practice, the inspector calibrates the equipment using reference blocks that replicate the actual material combination and thickness of the vessel being tested. This ensures that the equipment response to a known disbond is well understood before scanning the actual component.

Phased array ultrasonic testing is increasingly used for clad vessel inspection because it allows multiple beam angles to be applied simultaneously, improving detection of disbonds at oblique orientations and providing a more complete map of bond quality across large surface areas. Automated scanning systems are particularly valuable for large-diameter vessels where manual coverage would be inconsistent.

What role do visual and dimensional inspection play after cladding?

Visual and dimensional inspection after cladding confirm surface quality, weld appearance, and that the cladding meets the specified thickness and coverage requirements. While they cannot detect subsurface disbonding, they identify surface defects, weld discontinuities, and areas of concern that warrant further non-destructive examination.

Visual inspection of the clad surface checks for cracks, pitting, lack of fusion at clad weld seams, and any mechanical damage introduced during fabrication. For explosion-bonded or roll-bonded cladding, the inspector examines edge conditions and transitions where the cladding meets nozzle welds or other attachments.

Dimensional checks confirm that the cladding thickness meets the minimum specified value across the full surface. Ultrasonic thickness gauging is typically used for this purpose, providing point measurements at defined grid intervals. In applications where the corrosion allowance is tight, consistent cladding thickness is directly linked to the vessel’s design life.

Which destructive tests confirm cladding metallurgical integrity?

Destructive tests that confirm cladding metallurgical integrity include shear bond strength testing, bend testing, and metallographic examination. These tests are performed on representative test coupons produced alongside the actual vessel and are used to qualify the cladding procedure rather than test the finished component directly.

Key destructive test methods include:

Shear bond strength testing: measures the force required to separate the cladding from the base material, confirming that the bond meets the minimum strength requirement specified in the applicable standard
Side bend and face bend testing: applied to welded clad joints to verify that the bond remains intact under plastic deformation, with no cracking or separation at the interface
Metallographic examination: cross-sections of the bond zone are prepared and examined under magnification to assess grain structure, interface quality, and the presence of any intermetallic phases or voids
Hardness testing: performed across the bond zone to identify unacceptable hardening that could indicate metallurgical incompatibility or improper heat treatment

These tests are integral to the cladding procedure qualification process and provide documented evidence that the bonding method produces a metallurgically sound result under the defined fabrication conditions.

How do inspection requirements differ by cladding material?

Inspection requirements differ by cladding material because different metals bond differently to carbon steel substrates, respond differently to heat input during welding, and present different risks in service. Titanium, zirconium, tantalum, and nickel alloys such as Hastelloy and Inconel each require tailored inspection approaches that reflect their metallurgical characteristics.

Titanium cladding, for example, is sensitive to contamination during welding and requires careful visual and surface inspection to detect discolouration that may indicate oxidation. Zirconium and tantalum are similarly reactive and demand strict atmospheric controls during fabrication, with post-weld inspection focused on surface condition and bond continuity.

Nickel alloy claddings applied by weld overlay require inspection of each deposited layer for cracking, porosity, and dilution. Ferrite content measurement is sometimes required for austenitic stainless steel overlays to verify that the chemistry of the deposited weld metal falls within acceptable limits.

The base metal and cladding combination also influences the ultrasonic inspection technique. Acoustic velocity differences between materials affect calibration requirements, and some combinations require specialised transducer configurations to achieve reliable bond detection.

What standards and codes govern clad vessel inspection after fabrication?

Clad pressure vessel inspection after fabrication is governed primarily by ASME Section VIII (Division 1 and Division 2), the European Pressure Equipment Directive (PED) and its harmonised standards, and material-specific standards such as ASTM A263, A264, and A265 for roll-bonded clad plate. The applicable code depends on the vessel’s design basis, the country of installation, and any customer or project specifications.

ASME Section VIII provides requirements for the examination of clad vessels, including the extent of ultrasonic examination required for different cladding types and the acceptance criteria for bond quality. ASME Section IX governs the qualification of welding and cladding procedures, including the destructive tests required to validate a weld overlay or bonding process.

Under the PED, harmonised standards such as EN 13445 address the design and inspection of unfired pressure vessels, with specific provisions for clad construction. These requirements define the non-destructive examination methods, coverage, and acceptance criteria that apply to clad vessels placed on the European market.

Project specifications from EPC contractors or technology owners frequently impose additional requirements beyond the code minimum, particularly for vessels in corrosive or high-criticality service. These may include 100% ultrasonic bond scanning, third-party inspection witness points, and extended documentation requirements for traceability of the cladding material and process.

How Coek supports clad layer verification in custom pressure vessels

Coek Engineering manufactures custom clad pressure vessels for technically demanding applications where bond integrity directly affects equipment reliability and service life. With more than 30 years of experience in clad construction, Coek applies a structured approach to inspection and verification that covers the full fabrication sequence.

Coek’s approach to clad layer integrity includes:

Cladding procedure qualification with full destructive testing before production begins
Ultrasonic bond testing across the clad surface, with phased array methods applied where geometry or material combinations require it
Material-specific inspection protocols for titanium, zirconium, tantalum, Hastelloy, Inconel, and other demanding cladding materials
Compliance with ASME, PED, and project-specific inspection requirements, including third-party witness and documentation
In-house mechanical engineering to ensure that inspection access and testability are considered during the design phase

If you are specifying or sourcing a clad pressure vessel and want to discuss inspection requirements, material selection, or fabrication approach, contact the Coek team to speak with an engineer directly.

Frequently Asked Questions

How early in the fabrication process should clad layer inspection be planned?

Inspection planning should begin at the design stage, not after fabrication is complete. Access for ultrasonic scanning, probe placement on complex geometries, and the location of nozzle attachments all affect inspection feasibility and should be considered during engineering. Addressing these factors early prevents situations where critical bond areas become inaccessible after assembly, which can force costly rework or result in uninspected zones.

What are the most common causes of clad layer disbonding during fabrication?

The most common causes include excessive heat input during welding, improper pre-weld or post-weld heat treatment, surface contamination at the bond interface prior to bonding, and mechanical stress introduced during forming or fit-up. For weld overlay cladding, dilution control is critical — too much base metal mixing into the deposited layer can compromise both corrosion resistance and bond integrity. Identifying and controlling these variables during procedure qualification is far more effective than relying solely on post-fabrication inspection to catch problems.

Can a pressure vessel with localised disbonding still be accepted for service?

In some cases, yes — acceptance depends on the size, location, and extent of the disbonded area relative to the acceptance criteria defined in the applicable code or project specification. ASME Section VIII and other standards define maximum allowable disbond sizes and require engineering evaluation for indications that exceed screening thresholds. However, disbonding near welds, nozzles, or high-stress regions is typically treated more conservatively, and repair or rejection may be required regardless of size if the location presents a service risk.

What is the difference between weld overlay cladding and explosion-bonded cladding from an inspection standpoint?

Weld overlay cladding is deposited layer by layer and must be inspected for inter-run cracking, porosity, dilution, and ferrite content in addition to bond quality. Explosion-bonded clad plate arrives as a pre-bonded material and is typically inspected using ultrasonic methods to confirm bond continuity before and after vessel fabrication. Explosion-bonded cladding generally produces a more uniform bond over large flat areas, while weld overlay offers greater flexibility for complex geometries but requires more rigorous layer-by-layer process control and inspection.

How is cladding integrity verified after a pressure vessel has been in service for several years?

In-service inspection of clad vessels typically follows a risk-based inspection (RBI) programme that identifies the inspection intervals, methods, and coverage required based on the service environment and degradation mechanisms. Ultrasonic thickness gauging monitors corrosion of the cladding layer, while bond integrity scanning may be repeated if process upsets, thermal cycling, or mechanical events suggest that disbonding may have progressed. Fitness-for-service assessments under standards such as API 579 can be used to evaluate whether detected disbonds or thickness loss allow continued operation within defined safety margins.

Is ferrite content measurement always required for stainless steel weld overlay cladding?

Ferrite content measurement is not universally mandatory but is frequently specified for austenitic stainless steel overlays, particularly in hydrogen service or cryogenic applications where ferrite levels affect susceptibility to hydrogen embrittlement or low-temperature toughness. Project specifications and technology licensors in refinery and petrochemical applications commonly impose ferrite limits — typically between 3 and 8 FN — and require measurement using a calibrated ferritescope on the finished overlay surface. When in doubt, it is worth clarifying ferrite requirements with the end user or technology owner before procedure qualification.

What documentation should be expected from a fabricator after clad vessel inspection is complete?

A complete inspection documentation package should include the cladding procedure qualification records, material test reports for the cladding material, ultrasonic bond testing reports with scan maps or coverage records, thickness measurement records, and any non-conformance reports with associated disposition. For vessels built to ASME Section VIII, a Manufacturer’s Data Report (MDR) is required, and third-party inspection certificates should accompany it where a notified body or authorised inspector was involved. Traceability from the raw cladding material through to the finished inspection result is essential for high-criticality applications and should be confirmed with the fabricator before the order is placed.