Hydrogen-Induced Delamination Behavior of Stainless Steel Cladding Layers

Literature Overview

This 1994 study by Lin Jianhong, Wang Zhengdong, Liu Zengdian, and Wu Dongdi from the Chemical Machinery Research Institute of East China Institute of Chemical Technology investigates the hydrogen-induced delamination (HID) behavior of stainless steel cladding layers applied to petrochemical equipment. The work was published in the journal "Petrochemical Equipment" and addresses a critical failure mechanism that has historically plagued clad pressure vessels operating in sour service environments.

During the 1980s and early 1990s, several catastrophic failures of hydrogenation reactor shells in Chinese petrochemical plants were traced back to hydrogen-induced delamination at the cladding-to-base metal interface. This literature represents one of the earliest systematic investigations in China into this failure mode, making it historically significant for the domestic pressure vessel industry.

Core Technical Content

Mechanism of Hydrogen-Induced Delamination

The study establishes that atomic hydrogen generated through electrochemical reactions at the steel surface diffuses into the cladding layer and accumulates at the interface between the stainless steel overlay and the carbon steel base. Under sustained tensile stress, these hydrogen atoms coalesce into molecular hydrogen at microvoids and inclusions, generating internal pressures that ultimately cause interfacial separation.

The key parameters identified include:

Parameter	Typical Range	Effect on HID Susceptibility
Cladding thickness	3-6 mm	Thinner layers show higher susceptibility
Hydrogen concentration at interface	>2 ppm	Critical threshold for void formation
Interface bond strength	<80 MPa	Below this, delamination initiates readily
Temperature of service	150-400°C	Higher temperature accelerates hydrogen diffusion
Sulfide content in medium	H2S partial pressure >0.001 MPa	Promotes hydrogen generation

Metallurgical Factors

The authors identified several metallurgical conditions that exacerbate HID:

Incomplete melting at the cladding-to-base metal interface creates a thin band of martensite and residual austenite that is particularly susceptible to hydrogen attack.
The presence of manganese sulfide inclusions in the base steel provides preferential nucleation sites for hydrogen blistering.
Residual stress from the welding process creates a tensile component at the interface that synergistically promotes delamination.

Process Variables

The study examined how different cladding processes affect HID susceptibility:

Cladding Process	Interface Integrity	HID Resistance	Typical Application
ESW overlay	Good, with proper flux	Moderate	Thick cladding (6-12 mm)
SAW overlay	Moderate	Moderate to low	Medium thickness (3-6 mm)
Explosive cladding	Excellent metallurgical bond	High	Thin to medium cladding
Roll-bonded cladding	Very good mechanical bond	High	Thin cladding (2-3 mm)

Engineering Practice Implications

Design Considerations

Based on the findings, several design guidelines emerge for clad pressure vessels in hydrogen service:

The minimum cladding thickness should be selected to ensure that the diffusion path for hydrogen is sufficient to reduce the concentration at the interface below critical levels.
A diffusion layer of low-carbon steel (such as 06R or 08R) between the base metal and the stainless steel cladding can act as a hydrogen barrier.
The interfacial residual stress should be relieved through post-weld heat treatment (PWHT) to reduce the driving force for delamination.

Inspection and Quality Control

The study recommends:

UT scanning of the cladding-to-base metal interface after fabrication to detect pre-existing delamination.
Hydrogen blistering tests (HIC/SSC tests per NACE MR0175 or GB/T 16543) on the cladding material before procurement.
Monitoring of hydrogen permeation rates during service for critical vessels.

Study Insights and Reflections

This early work is remarkable for establishing a systematic framework for understanding HID in clad vessels at a time when Chinese industry was still developing its standards for sour service. The authors' emphasis on the interaction between metallurgical structure, hydrogen chemistry, and mechanical stress represents a holistic approach that remains relevant today.

One reflection is that the study predates the widespread adoption of NACE MR0175/ISO 15156 in China, and the recommendations align closely with what these standards would later codify. The emphasis on ESW overlay with low-hydrogen flux and thorough PWHT as mitigation strategies is consistent with current best practices in ASME Section VIII and NB/T 47014.

A limitation of this early work is the relatively limited quantitative data on hydrogen permeation rates and the absence of long-term creep-fatigue interaction studies. Modern research has shown that cyclic loading combined with hydrogen exposure can dramatically accelerate delamination compared to static loading alone, a factor not fully addressed in this 1994 study.

The enduring value of this literature lies in its clear identification of the root causes of HID and its practical recommendations for process optimization, which have guided the Chinese pressure vessel industry's approach to sour service cladding for over three decades.