The Adhesion of Protective Coatings to Novel Trivalent Cr Metal / Cr Oxide Coatings Used for Packaging Applications
Authors: Dodd Matthew , Jewell Eifion , Wint Natalie , de Vooys Arnoud 
Published: November 2022

When manufacturing canned goods, the aim is to create a shelf-stable product that will keep for anywhere between one and five years (under ambient storage conditions). It is for this reason that steel, which offers desirable mechanical properties, is often used. However, steel is known to undergo corrosion thus impacting final product quality. It is therefore common for packaging substrate materials to have a protective metallic coating which is further coated with a lacquer to shield both the substrate material from the cans contents and vice-versa.

One example of a coated packaging product is Electrolytic Chromium Coated Steel (ECCS) which is typically used for lacquered applications [1]. This chrome coating is deposited from a chromic acid bath containing Cr (VI) and results in a steel substrate covered in a bi-layer chromium metal (~20 nm) /chromium (hydr)oxide (~7 nm) layer. This substrate is then overcoated with lacquer which is usually epoxy based.

Once the can is lacquered and filled, a retort process is used to pressure cook its contents. This is undertaken at 121 °C at a pressure of 1 bar for 90 minutes. This retort process has proven to be detrimental to the adhesion of protective lacquers applied to the substrate material which serve to prevent corrosion caused by the cans contents [2,3]. At elevated temperatures and pressures, the lacquers exhibit porosity which creates an ion pathway for the cans chemical contents to migrate through the coating and interact with the substrate. This interaction initiates localised corrosion on the surface causing de-adhesion of the lacquers through localised anodic metal dissolution.

However, both Cr (VI) and Bisphenol-A, a chemical commonly added to epoxy lacquers to enhance thermal and chemical stability, have been identified as substances of concern by REACH - a regulation associated with the restriction of chemicals that are harmful to human health. Thus, a dual issue exists of finding a suitable substrate replacement for ECCS and investigating the adhesion of lacquers to this substrate. Coatings made with Trivalent Chromium Coating Technology (TCCT®) are currently being developed as a potential replacement for ECCS. TCCT® production has developed to consist of a two-electrolyte process. Various issues including coating in-homogeneity have previously been reported [4]. Additionally, lacquers to which Bisphenol-A have not intentionally been added (BPANI) have been manufactured.

To ensure that TCCT® substrates are successful, it is important that they perform comparably to existing products such as ECCS. For this reason, a range of TCCT® substrates varying in Cr (III) oxide coating weight (1.8 mg.m-2 to 23.35 mg.m2) have been experimentally analysed for their lacquer adhesion (both epoxy-phenolic and BPANI) in simulated industrial retort conditions, using scratch testing. They have also been analysed for their surface electrochemical activity utilising the scanning vibrating electrode technique (SVET) which is a technique designed to spatially resolve areas of dissimilar electrochemical activity.

A Cr (III) oxide coating weight of approximately 8-10 mg.m2 was found to be critical to the adhesion of the lacquers. In addition, the SVET scans illustrated a pronounced rate of increase in anodic activity on the substrates coated with smaller amounts of Cr (III) oxide. Indeed, higher coating weights exhibited a lower total anodic and cathodic activity alluding to the insulating and inert nature of Cr (III) oxide.

[1] B. Boelen, H. den Hartog, H. van der Weijde, Product performance of polymer coated packaging steel, study of the mechanism of defect growth in cans, Prog. Org. Coatings. 50 (2004) 40–46. https://doi.org/10.1016/J.PORGCOAT.2003.09.011.

[2] A. Allman, E. Jewell, A. de Vooys, R. Hayes, Inter-layer Adhesion Performance of Steel Packaging Materials for Food Cans Under Retort Conditions, J. Packag. Technol. Res. 2 (2018) 115–124. https://doi.org/10.1007/s41783-018-0033-6.

[3] A. Allman, E. Jewell, A. Vooys, R. Hayes, H.N. McMurray, E. Jewell, R. Hayes, Food packaging simulant failure mechanisms in next generation steel packaging, Packag. Technol. Sci. 32 (2019) 1–15. https://doi.org/10.1002/pts.2448.

[4] A. Allman, J. Whiteside, E. Jewell, C. Griffiths, N. McMurray, A. de Vooys, Surface modification of Cr(III) packaging substrates for enhanced adhesion via citric acid processing, Surfaces and Interfaces. 20 (2020) 100545. https://doi.org/10.1016/J.SURFIN.2020.100545.