The problem?
A shipyard is designing a new service vessel with a steel hull and aluminum superstructure to lighten the structure. These two metals are joined using a specific welding technique. The design office is concerned about the risk of accelerated galvanic corrosion at the joint, which will be constantly exposed to seawater.
A failure at this critical point could compromise the structural integrity of the ship. The customer asked FILAB to solve this problem by conducting a galvanic coupling study.
To assess this risk, we conducted a galvanic coupling study in accordance with ASTM G71.
Galvanic corrosion risk assessment
Objective:
The main objective of this study is to provide the shipyard with a quantitative and predictive assessment of the risk of galvanic corrosion associated with the steel hull/aluminum superstructure assembly, in order to ensure the structural integrity of the new service vessel,& nbsp;while avoiding high post-construction maintenance and downtime costs.
Our technical approach
To assess this risk, we conducted a galvanic coupling study in accordance with ASTM G71.
Step 1: Setting up the electrochemical cell
The first step was to prepare the sample. Plates of steel (representing the hull) and aluminum (representing the superstructure) were assembled and electrically connected within an electrochemical cell. The cell was then filled with artificial seawater to accurately replicate the operating environment and initiate the corrosion reaction.
Step 2: Initial measurement and monitoring
Next, we proceeded to the measurement phase. The coupling current and coupling potential were measured continuously. The coupling current is the key indicator of the corrosion rate of the least noble metal (aluminum, in this case), providing a direct reading of the deterioration of the material.
Step 3: Observation of the system's evolution
Finally, the test was maintained over several days. This extended duration made it possible to observe the evolution of the system over time, and more specifically to detect and analyze the possible formation of protective layers (a phenomenon known as passivation) that could modify the kinetics of corrosion.
The benefits of this analysis for the client
The test confirmed that aluminum would behave as a sacrificial anode, corroding preferentially to protect the steel, with an initially very high corrosion rate at the junction.
This data enabled the shipyard to:
• Anticipate a major corrosion phenomenon even before construction began.
• Make informed design decisions by incorporating a cathodic protection system (sacrificial zinc anodes) precisely sized to protect the weld area.
• Ensure the safety and durability of the ship, avoiding extremely high maintenance costs and the risk of downtime.
Conclusion of the stud
The study of galvanic coupling conducted in accordance with ASTM G71 standard has transformed a theoretical uncertainty into critical quantitative data for naval design. Continuous measurements of coupling current and coupling potential confirmed the existence of a strong electrochemical interaction between the steel hull (cathodic) and the aluminum superstructure (anodic) in seawater.
Ultimately, this preventive approach, validated by applied electrochemistry, guarantees the safety and durability of the ship. It has enabled the shipyard not only to technically validate its design, but also to provide assurance of structural reliability to the ship operator, avoiding considerable financial and operational risks.
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Frequently asked questions
Why study the galvanic coupling of a steel/aluminum assembly in a marine environment?
Two metals in electrical contact and exposed to an electrolyte may exhibit galvanic interaction. In the case studied, the junction between a steel hull and an aluminum superstructure was to be exposed to seawater. FILAB assessed this risk before construction to provide data useful for designing the assembly.
How did FILAB reproduce the steel/aluminium couple for the study?
Steel and aluminium plates were electrically connected in a cell containing artificial seawater. The protocol presented is based on ASTM G71. This configuration made it possible to monitor the behaviour of the couple in an environment selected to represent marine exposure of the junction under study.
What information do the coupling current and potential provide?
Monitoring the current and potential makes it possible to study the electrochemical interaction between the two metals over time. In this study, the measurements highlighted anodic behavior for aluminum and cathodic behavior for steel. They therefore documented the risk of preferential dissolution of aluminum in the test configuration.
Why monitor a galvanic coupling test for several days?
The system’s behavior may change with exposure, particularly if surface layers form and modify the reaction kinetics. FILAB maintained the test for several days to monitor this evolution. Observing the system over time complements the initial measurements and helps interpret the behavior of the metallic couple in the selected environment.
What protection approach was selected after studying the naval assembly?
The results led the shipyard to incorporate cathodic protection using zinc sacrificial anodes for the area concerned. The study provided data to guide this design solution. The effectiveness and sizing of a protection system must be assessed for the assembly and the operating conditions for which it is intended.