Hydrogen Concentration Distribution in Cladding Structures and Its Effect on Delamination Fracture

Literature Overview

This paper by Xu Ying, Yuan Leping, Lin Dongliang, Lin Jianhong, Wang Zhengdong, and Wu Dongdi, published in the Chinese Journal of Corrosion and Protection in 1995, addresses one of the most critical yet often overlooked failure mechanisms in cladded and weld-overlay structures: hydrogen-induced delamination. The study originates from Shanghai Jiao Tong University and East China University of Science and Technology, both institutions with strong traditions in materials science and chemical engineering. The research was conducted at a time when hydrogen damage in clad systems was still poorly quantified, making this work particularly valuable for engineers who must now design and inspect such components under modern standards such as NB/T 47002 and ASME VIII Division 1.

Core Technical Content

The fundamental problem investigated is the accumulation of hydrogen at the interface between the base metal and the overlay layer during the cladding process, particularly electroslag welding (ESW) overlay and submerged arc welding (SAW) overlay. Hydrogen enters the weld zone from multiple sources: moisture in the flux, hydrogen content in the electrode coating, surface contamination on the base plate, and the decomposition of hydrogen-containing compounds in the atmosphere. During the thermal cycle of multi-pass cladding, hydrogen diffuses preferentially toward the cooler interface region, where it becomes trapped at microstructural defects, inclusions, and the metallurgical boundary.

The authors conducted systematic hydrogen concentration measurements using gas chromatography on samples extracted at various depths from the base metal through the interface into the overlay layer. The results demonstrated that hydrogen concentration peaked at or near the clad-base interface, reaching values significantly higher than those in the bulk base metal or the fully solidified overlay. This interface concentration gradient creates a localized zone of hydrogen embrittlement susceptibility that can trigger interfacial delamination under tensile or cyclic loading.

Hydrogen Source Analysis and Quantification

Understanding hydrogen ingress is essential for any practical mitigation strategy. The following table summarizes the primary hydrogen sources and their approximate contributions in a typical multi-layer ESW cladding process:

The total hydrogen content in the weld metal and heat-affected zone can be expressed as the sum of absorbed hydrogen from all sources. The critical threshold for hydrogen-induced cracking in carbon steel base metals is generally accepted as 1.5-2.0 mL/100 g Fe, while in the interface region of a clad structure, even lower concentrations can initiate damage due to the constraint imposed by the dissimilar metallurgical bond.

Delamination Mechanism

The delamination fracture mechanism identified in this study follows a well-defined sequence. First, hydrogen atoms accumulate at the interface during welding, particularly at the last few overlay passes where the thermal input is highest and the base metal is most susceptible to hydrogen pickup. Second, during post-weld cooling, hydrogen migrates toward the cooler interface zone due to the concentration gradient. Third, hydrogen atoms recombine at microvoids, inclusions, and grain boundaries to form molecular hydrogen, creating internal pressure. Fourth, under applied stress—whether residual stress from welding, service loading, or thermal cycling—these pressurized microvoids coalesce into interfacial cracks.

The fracture surface examination revealed a mixed-mode failure characteristic, with regions of intergranular fracture in the base metal near the interface, transgranular fracture in the overlay, and cleavage features at the very interface. This mixed morphology is diagnostic of hydrogen-assisted cracking and distinguishes it from purely mechanical delamination, which would show more uniform transgranular features.

Engineering Implications and Mitigation Strategies

The practical significance of this research cannot be overstated. For engineers designing cladded pressure vessels, hydrogen-induced delamination represents a latent failure mode that may not be detected by conventional radiographic testing (RT) because the interface cracks are parallel to the primary stress direction and thus nearly invisible to projection-based methods. Ultrasonic testing (UT) with specific interface scanning techniques is far more effective, and this paper implicitly supports the requirement in NB/T 4730 for interface UT inspection of cladded components.

The following mitigation measures are directly supported by the findings of this paper:

1. Preheat the base plate to 200-250°C before cladding to reduce hydrogen pickup during the initial passes.
2. Use low-hydrogen fluxes and ensure thorough drying per manufacturer recommendations.
3. Apply interpass temperature control to prevent excessive hydrogen accumulation between passes.
4. Perform post-weld baking at 200-250°C for a duration proportional to wall thickness (typically 1 hour per 25 mm of thickness) to allow hydrogen diffusion out of the interface region.
5. Implement UT interface scanning per NB/T 4730 Part 3 or ASME V Article 4 for all critical clad interfaces.

Key Questions and Reflections

Several questions arise from this study that remain relevant to current practice. First, the paper does not address the effect of overlay layer thickness on hydrogen distribution; modern engineering often employs thicker overlay layers for severe corrosion environments, and it is reasonable to expect that thicker layers create larger thermal gradients and potentially different hydrogen profiles. Second, the study focuses on ESW overlay, but modern practices increasingly use PTA cladding and laser cladding, which have fundamentally different thermal cycles and hydrogen ingress mechanisms. Third, the paper does not quantify the effect of cooling rate on hydrogen trapping at the interface, which is particularly important for high-strength base materials where martensitic transformation can occur.

This 1995 paper remains a foundational reference for understanding hydrogen-related failures in cladded systems. Its methodology—combining quantitative hydrogen analysis with fracture mechanics interpretation—set a standard for subsequent research in this area. Engineers working on cladded pressure vessels today should treat this paper as a mandatory reading when evaluating interface integrity, particularly for components operating in hydrogen-containing environments such as hydrogenation reactors and high-pressure hydrogen storage vessels.

Study Insights and Practical Recommendations

The most important insight from this literature is that hydrogen-induced delamination is not a defect that can be eliminated by inspection alone; it must be prevented through process control. The PDCA cycle applies directly: Plan by selecting low-hydrogen consumables and establishing a hydrogen control procedure; Do by implementing preheating, interpass temperature monitoring, and post-weld baking; Check by performing hydrogen hot-short-test measurements and UT interface scanning; Act by adjusting process parameters based on inspection results. This systematic approach, rooted in the fundamental research presented in this paper, provides a robust framework for ensuring the long-term integrity of cladded structures in demanding service environments.