The problem?
A beverage manufacturer uses aluminum cans coated internally with a polymer varnish to protect the contents (a carbonated beverage) from the aluminum and prevent any chemical interaction or metallic taste.
The customer approached FILAB because it is committed to continuous improvement and is seeking to optimize the thickness of this coating. Insufficient coating creates defects (pores, discontinuities) and risks of premature corrosion. Conversely, excessive coating represents a costly waste of raw materials.
To address this issue, FILAB developed a comprehensive assessment of the integrity of the packaging, including its chemical composition, physical thickness, and electrochemical resistance.
Issues and comprehensive analytical approach
The customer's initial request to optimize their product quickly evolved into a comprehensive assessment of packaging integrity, encompassing chemical composition, physical thickness, and electrochemical resistance.
Our laboratory has developed a comprehensive analysis protocol combining electrochemistry and materials characterization to answer the following questions:
- What is the minimum thickness that guarantees the absence of critical defects?
- Does contact with the opening tab (made of a different shade of aluminum) introduce a risk of galvanic coupling?
- How do the coating and metal behave after accelerated aging?
Materials and corrosion expertise services
Our laboratory has developed a comprehensive analysis protocol combining electrochemistry and material characterization to ensure packaging integrity.
To evaluate the integrity of the coating, the analysis began with electrochemical impedance spectroscopy (EIS). This non-destructive technique made it possible to measure the electrical impedance of the coating. A high-quality varnish acts as a perfect capacitor with very high impedance, while any drop in impedance reveals and quantifies microscopic defects and pores.
To understand the physical structure of the coating, we performed micrographing and metallography, allowing us to accurately measure the thickness of the varnish and characterize the metal phases of the can. The corrosion rate (LSV) was measured to establish the kinetics of degradation of the bare metal. This data was supplemented by accelerated aging tests using salt spray (BSN) simulating storage conditions to predict the product's lifespan.
The analysis then focused on electrochemical interactions. Knowing that the pull tab is often a different color from the body of the can, we studied galvanic coupling. This measurement made it possible to quantify the corrosion current that is created when these two metals come into contact in the presence of the beverage (the electrolyte), which poses a major risk to the integrity of the container.
Finally, for surface characterization and chemical analysis, several techniques were used.
- The ICP (Inductively Coupled Plasma) was used to verify the exact composition of the aluminum alloy.
- The SEM (Scanning Electron Microscope), coupled with XPS (X-ray Photoelectron Spectroscopy), made it possible to analyze the chemical nature of the oxides formed on the surface.
- Finally, XRD (X-ray diffraction) identified the crystallographic phase of these oxides and any corrosion deposits, providing a complete picture of the degradation phenomena.
Conclusion of the stud
Thanks to an integrated approach combining electrochemistry (EIS, galvanic coupling) and material characterization (micrography, ICP, XRD), the laboratory was able to accurately map the integrity of the coating and metal assembly.
The study by EIS was essential, as it enabled the parameters of the coating process to be correlated with the presence of invisible defects (pores), leading to the identification of the optimal settings. This optimization generated substantial savings in raw materials while ensuring continuous protection.
More importantly, the expertise validated two fundamental points: the absence of critical defects in the varnish and the elimination of risks of chemical interaction or galvanic coupling between the can and the pull tab.
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Frequently asked questions
How can the thickness of a can’s internal coating be optimised without compromising its protective function?
Optimisation combines thickness measurement with an assessment of coating integrity and the packaging’s electrochemical interactions. FILAB cross-referenced these analysis to identify defects and compare coating conditions. The aim was to avoid insufficient deposition while limiting excess material, in accordance with the requirements of the can under study.
What is electrochemical impedance spectroscopy used for when examining can varnish?
Electrochemical impedance spectroscopy, or EIS, provides information about the electrical behavior of the coating/metal system under the measurement conditions. In this study, it made it possible to establish a link between varnishing parameters and defects such as pores. Its interpretation was cross-referenced with the other packaging characterization results.
Why complement EIS with a coating thickness measurement?
Thickness measurement provides physical information that complements the coating’s electrochemical response. FILAB used micrographic observations to characterise the varnish and metallic phases of the can. Combining the data helps distinguish between a deposition issue, a continuity problem and the behaviour of the protective system. Discover coating thickness measurements.
Why study the galvanic coupling between a can and its tab?
The body of the can and its tab may be made from different metal grades. Their electrical contact in the presence of the beverage may therefore require an assessment of electrochemical interactions. FILAB studied this coupling in addition to coating integrity, to examine a risk associated with the packaging’s metal assembly.
What did combining chemical, surface and corrosion analysis contribute to this study?
ICP helped verify the alloy composition, while SEM, XPS and XRD provided information on the surfaces, oxides and deposits. These results complemented EIS and corrosion testing. Cross-referencing them helped guide varnishing settings and the assessment of the integrity of the packaging under study.