Applications for authorisation AFA075-01 - Sodium dichromate

Overview

Public consultation on alternatives

Reference: AFA075-01

Applicant: Tata Steel UK Ltd

Application type: Review report

Substance name: Sodium dichromate

• EC Number: 234-190-3
• CAS Number: 10588-01-9, 7789-12-0

Proposed use: The use of sodium dichromate for the passivation of electrolytic tinplate (ETP).

Broad information on use applied for (conditions of use and function):

The Applicant uses sodium dichromate at its site in Llanelli, Wales (the Trostre Works) for the passivation of electrolytic tin-plated steel (ETP, also known as tinplate). The Applicant is the only manufacturer of ETP for packaging applications in the UK. Sodium dichromate is used to apply a very thin chromium-based conversion layer to the tin-plated steel strip. This passivation step stabilises the freshly deposited tin surface and prevents uncontrolled oxidation during storage, transport and subsequent processing. The resulting steel product is free of hexavalent chromium and is supplied to the Applicant’s customers for the manufacture of food and beverage cans, aerosol containers and other forms of metal packaging.

Sodium dichromate is supplied to the site in aqueous solution and is used to prepare and maintain the passivation electrolyte within a closed, automated and highly controlled industrial process. During passivation, hexavalent chromium present in the electrolyte is reduced at the steel surface, forming a layer of metallic chromium and chromium (III) oxide on both sides of the tin-plated strip. Operators are involved in specific tasks including receipt and storage of the substance, sampling and analysis of the passivation solution, routine operational checks, maintenance activities and waste management. Comprehensive technical, organisational and administrative risk management measures are implemented to minimise exposure to workers and releases to the environment, as described in the Chemical Safety Report.

Passivation using sodium dichromate plays a critical and irreplaceable role in the manufacture of ETP for packaging applications. The controlled chromium-based conversion layer formed during passivation performs several essential functions. It prevents excessive growth of tin oxide on the surface of the steel, which would otherwise occur rapidly after tin plating and would lead to surface discolouration, impaired performance and variable product quality. The passivated surface also provides a stable and reproducible interface that ensures reliable adhesion of subsequent organic coatings, such as lacquers, which are widely applied by downstream users. In addition, the passivation layer contributes to the corrosion and chemical resistance of the steel when exposed to packaging contents, supporting the integrity, safety and shelf life of packaged goods, particularly food and beverage products.

Eight overarching requirements have been identified for evaluating suitable alternatives to sodium dichromate in the electrolytic process used to form metallic chromium (Cr(0)) and trivalent chromium (Cr(III)) layers on steel:

1. Corrosion resistance of the steel plate prior to use
2. Adhesion, i.e. a suitable surface for organic coating application
3. Chemical resistance to the contents of the packaging
4. Compatibility with downstream processes, including heat resistance and formability
5. Suitability for food contact (in compliance with food contact regulations)
6. Visual properties, including aesthetics, brightness, and overall appearance
7. Scalability and stability of the production process for high-volume manufacturing
8. Sustainability, in particular, the ability to recycle

The Applicant has assessed a wide range of potential alternatives to sodium dichromate for the passivation of ETP. While a chromium-free passivation alternative (CFPA) has been identified as the most promising option, it does not currently meet all necessary technical and performance requirements for the full range of the Applicant’s products and applications. In particular, it has not yet been demonstrated to deliver equivalent corrosion resistance, coating adhesion and robustness in combination with emerging bisphenol-A-free lacquer systems now required for food contact applications. As a result, no technically and economically feasible alternative is currently available that would allow continued manufacture of ETP meeting customer and regulatory requirements.

Additional information on the use applied for and the production process is given in the Chemical Safety Report (CSR) and Analysis of Alternatives (AoA). Operational Conditions (OCs) and Risk Management Measures (RMMs) are described in the CSR and OC/RMM summary table.

Information on key functionalities of sodium dichromate for plating is given in the AoA.

List of descriptors:

• Life cycle stage: IS (Use at industrial sites)
• Sector of end use (SU): 15 (Manufacture of fabricated metal products, except machinery and equipment)
• Environmental release category (ERC): 5 Use at industrial site leading to inclusion into/onto article
• Process category (PROC): 1, 8b, 9, 13, 28, 0
• Product category (PC): 14 (Metal surface treatment products)
• Article category related to subsequent service life (AC): N/A
• Technical Function: Plating agent

Annual tonnage used: 10 - 20 tonnes/year

Review period requested: 10 years

How to submit comments

Provide some basic information on possible alternative(s) in the online survey below but more detailed information should be provided in document(s) prepared using the commenting templates in the related section below. A “public version” should always be completed and will be published after the consultation has closed. If you want to include confidential information in your submission, please additionally complete a “confidential version” and submit both versions as attachments.

The consultation lasts for 8 weeks (unless otherwise specified) and closes at 23:59 London time (BST).