Cracking Analysis of Tee Fittings in Continuous Reforming Units

Literature Overview

This failure analysis study by Wang Yu, You Gexin, and Liu Junquan from South China University of Technology, published in Physical Testing and Chemical Analysis (Physical Methods) (2010, Vol. 46, No. 8, pp. 533-536), investigates the cracking mechanism of a tee fitting in a hydrogen-naphtha mixture service within a continuous reforming unit at a petrochemical company. The analysis employs a comprehensive multi-technique approach including macroscopic morphology observation, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), electrode potential measurement, and galvanic current measurement to identify the root cause of failure.

Failure Analysis Methodology and Findings

Multi-Technique Analysis Approach

Material and Environmental Conditions

The tee fitting was fabricated from Cr5Mo steel (a 5% chromium, 0.5% molybdenum heat-resistant alloy commonly used in high-temperature hydrogen service), while the connecting weld was made with a chromium-nickel stainless steel filler metal. This material mismatch created a galvanic couple in the aggressive hydrogen-naphtha mixture environment.

Galvanic Corrosion Mechanism

The root cause of the tee fitting failure was identified as galvanic corrosion superimposed on cavitation erosion and chloride-sulfur corrosion. The 60 mV potential difference between the Cr5Mo tee fitting and the stainless steel weld metal was sufficient to establish a galvanic couple in the corrosive environment. The Cr5Mo tee fitting, having a more negative electrode potential, acted as the anode in the galvanic couple and was preferentially corroded.

Galvanic Corrosion Process

1. The Cr5Mo tee fitting and stainless steel weld metal were electrically connected through the weld joint
2. In the hydrogen-naphtha mixture containing dissolved chlorides and sulfides, an electrochemical cell was established
3. The Cr5Mo tee fitting, being the anode, underwent accelerated oxidation and material loss
4. Concurrent cavitation erosion from fluid flow further accelerated material removal at the weld-adjacent region
5. Progressive wall thinning at the tee fitting near the weld eventually led to mechanical failure and cracking

Engineering Practice and Design Recommendations

Material Compatibility Guidelines

Design and Fabrication Countermeasures

The failure analysis highlights several critical lessons for engineering practice:

1. Material matching: When connecting dissimilar materials in aggressive chemical environments, the potential difference should be minimized. For Cr5Mo equipment, the connecting weld should ideally use a filler metal with a similar electrode potential, or a transition fitting should be used.
2. Isolation measures: If material mismatch is unavoidable, electrical isolation should be provided between the dissimilar materials using insulating gaskets or coatings at the joint interface.
3. Cathodic protection: In hydrogen-containing services with chloride and sulfur contamination, cathodic protection may be considered as a supplementary measure, though this is less common in high-temperature reforming service.
4. Inspection protocols: For existing installations with material mismatch, increased inspection frequency at the weld-adjacent regions of the anodic material is warranted. Ultrasonic wall thickness measurement should be performed at regular intervals to detect progressive thinning.
5. Weld procedure optimization: The welding procedure for dissimilar metal joints should be reviewed to ensure that the weld composition does not create additional galvanic couples. The weld heat-affected zone (HAZ) should be examined for potential sensitization or embrittlement.

Study Reflections and Broader Implications

This case study exemplifies the importance of considering electrochemical compatibility in piping system design, particularly in petrochemical service where multiple corrosion mechanisms can act synergistically. The 60 mV potential difference, while seemingly modest, was sufficient to drive significant galvanic corrosion in the presence of aggressive species (chlorides, sulfides, hydrogen). This underscores the principle that galvanic corrosion risk should be assessed not only based on material nobility rankings but also on the specific environmental conditions and the geometric configuration of the joint.

From a quality control perspective, this failure highlights the need for rigorous material verification and traceability in pipe fitting fabrication and installation. The specification of filler metals for dissimilar metal welds should be governed by standards such as AWS D10.9 (Guide for Welding Dissimilar Steels) or ASME B31.3, which provide guidance on material matching and joint design. The multi-technique failure analysis approach demonstrated in this study - combining macroscopic, microscopic, chemical, and electrochemical methods - represents the gold standard for root cause analysis and should be adopted as best practice in the industry.