Zhuojin Pipe Fitting Co., LtdApplication of Automatic Surfacing Technology in High-Sulfur Gas Field Equipment Corrosion Control
Literature Overview
This 2024 paper by Liu Boqiang, Cai Wei, Fan Linyun, and Shao Qing, published in Petrochemical Technology, addresses a critical engineering challenge in the development of high-sulfur gas fields operated by PetroChina Southwest Oil and Gas Field Chuan-Dongbei Operating Branch. The study focuses on how to effectively control corrosion rates in equipment and facilities exposed to high sulfur content environments, and what remedial measures should be applied to equipment that has already suffered corrosion-related defects to ensure safe, stable, and reliable operation. The work was conducted during maintenance operations on gas purification units, making it a highly practical field application study rather than a purely theoretical investigation.
Core Technical Challenges in High-Sulfur Environments
High-sulfur gas fields present a uniquely aggressive corrosion environment for process equipment. The presence of H₂S at concentrations often exceeding 10% of the gas composition creates severe conditions for sulfide stress cracking (SSC), hydrogen-induced cracking (HIC), and general sulfide corrosion. Equipment such as compressors, separators, heat exchangers, and pipeline systems in gas purification units are constantly exposed to wet H₂S environments where the corrosion mechanism shifts from general uniform attack to localized damage forms that are far more dangerous to structural integrity.
The fundamental challenge identified in this work is twofold: first, preventing or slowing the ongoing corrosion rate in active service, and second, restoring equipment that has already developed corrosion defects to a state where continued safe operation is possible. This dual requirement demands not only knowledge of corrosion-resistant materials but also expertise in repair welding and surfacing techniques that can integrate with the existing base metal without introducing new failure modes.
Automatic Surfacing Technology: Process Parameters and Control
The automatic surfacing technology employed in this application is designed to deposit a corrosion-resistant alloy layer onto the surface of carbon steel or low-alloy steel equipment. The key advantage of automatic (mechanized) surfacing over manual methods lies in the consistency and repeatability of process parameters, which is critical when depositing layers that must meet stringent quality requirements for corrosion resistance and mechanical properties.
| Process Parameter | Typical Range | Control Objective |
|---|---|---|
| Welding current | 350–550 A | Ensure adequate melt-through for metallurgical bonding |
| Travel speed | 200–400 mm/min | Control dilution rate to maintain overlay composition |
| Shielding gas flow | 15–25 L/min | Prevent oxidation and porosity in overlay |
| Preheating temperature | 100–200°C | Reduce residual stress and prevent cold cracking |
| Interpass temperature | ≤250°C | Control grain growth and maintain toughness |
| Layer thickness per pass | 1.0–2.5 mm | Ensure uniform coverage and avoid undercut |
The dilution rate between the base metal and the surfacing alloy is perhaps the most critical parameter. In high-sulfur service, the overlay must maintain sufficient chromium and molybdenum content to provide resistance to sulfide corrosion. Dilution exceeding 30% can significantly degrade the corrosion performance of the overlay layer, making it vulnerable to the same attack mechanisms that affected the base metal.
Defect Analysis and Countermeasures
In the maintenance context described in this paper, several common defects were encountered during the inspection and repair of corroded equipment:
- Pitting corrosion: Localized deep pits that reduce wall thickness and serve as stress concentrators for further crack initiation.
- General thinning: Uniform material loss reducing the effective cross-section of pressure boundaries.
- Cracking at corrosion boundaries: Stress corrosion cracks propagating from corrosion damage into the base metal.
- Undercut and porosity in previous repairs: Inadequate prior welding repairs creating weak points.
The countermeasures applied included thorough surface preparation (grinding to bare metal with a minimum 30 mm heat-affected zone removal from existing welds), controlled preheating to manage hydrogen-induced cracking susceptibility, multi-pass surfacing with interpass inspection, and post-weld heat treatment to relieve residual stresses. The automatic surfacing process was programmed to ensure consistent bead profile and overlap, minimizing the risk of interpass defects.
Engineering Practice Integration
The practical value of this study lies in its direct connection to field operations at a major gas field. The authors document not only the technical approach but also the decision-making process for determining when equipment can be repaired versus when replacement is necessary. This involves evaluating remaining wall thickness, the extent of subsurface cracking, and the expected remaining service life.
A key insight from this work is that automatic surfacing provides a reliable repair methodology that can be scaled across multiple pieces of equipment with consistent quality. Unlike manual welding, which depends heavily on individual welder skill and consistency, mechanized surfacing allows for the creation of standardized repair procedures that can be replicated across the operation, ensuring uniform quality regardless of shift or location.
Key Reflections and Study Insights
This paper highlights an important trend in the oil and gas industry: the shift from preventive replacement to condition-based repair and extension of service life. As gas field development costs increase, the economic and operational imperative to extend equipment life becomes paramount. Automatic surfacing technology, when properly applied with appropriate alloy selection and process control, offers a viable pathway to achieve this goal even in the most aggressive corrosion environments.
The study also underscores the importance of post-repair verification. Even when surfacing parameters are well-controlled, the integrity of the repair must be confirmed through non-destructive testing (typically magnetic particle testing for surface defects and ultrasonic testing for subsurface issues) and, where applicable, corrosion testing of the overlay in simulated service conditions.
The experience documented here is particularly relevant for engineers managing aging infrastructure in sour service, where the combination of HIC susceptibility in the base metal and the need for corrosion-resistant overlays creates a complex metallurgical challenge that requires careful process selection and quality control.