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BROMOCO's participation in the Corten Steel Experiment in Basha-Franklin's London 25 Hanover Square architectural design

25 Hanover Square is a landmark development in the prestigious Mayfair Conservation Area in London, adjacent to the SOHO area. The intersection of traditional and contemporary architectural elements defines the unique aesthetic of the site's complex. The project was led by Basha-Franklin and brought together a number of collaborators, including BROMOCO, renowned for their innovative use of restoration and protective coatings for building materials.

25 Hanover Square
Basha-Franklin has designed a seven-storey office building at 25 Hanover Square in London’s Mayfair Conservation Area.

The design intent for 25 Hanover Square was to create a building that respects the historical context of its surroundings while introducing contemporary elements that speak to the future of urban architecture. Basha-Franklin envisioned a high-performance, low-carbon approach that maximizes our client's investment. The proposals for 25 Hanover Square aimed to restore the building’s relationship with the Square while achieving the highest possible standards for design and sustainability. This vision required a delicate balance between heritage conservation and modern innovation, a challenge that Basha-Franklin approached with creativity and sensitivity.


One of the outstanding features of the project was the use of Corten steel with Portland stone. Corten steel typically takes several months to weather in a satisfactory level of patina. Therefore in common building practice, Corten steel will be placed on the building for the lengthy weathering process. In London's rainy weather conditions, runoff from the early stages of weathering can seep into other facade materials and cause permanent staining. This will result in the envisaged aesthetic effect of Corten steel paired with Portland stone not being realised.

BROMOCO has played an absolutely crucial role in reducing weathering time and avoiding runoff from Corten steel, bringing our expertise in surface treatment of building materials to the table. Our team worked closely with Basha-Franklin from the early stages of the project and applied CT-7571 Corten Steel Sealer to the supplied models and materials to ensure the desired appearance and performance was achieved. The results of eleven months of model testing, which did not reveal any negative effects, confirmed the feasibility of the solution, and BROMOCO's involvement was crucial in solving the problem and coordinating the different materials.

Challenges and Solutions

As above belongs, the interface between Corten steel and Portland stone required meticulous detailing. As the design solution allowed for gaps or hidden drainage channels to drain excess water, in addition to preventing staining due to run-off, it was also necessary to prevent galvanic corrosion of the Corten steel with other materials that would affect the service life. BROMOCO solved these problems by carrying out several experiments and adapting the surface treatment solution to prevent any possible contamination or corrosion from occurring.

At the same time, the application of the XL-08 Corten Steel Weathering Accelerator will allow Corten steel to achieve a patina in just a few days that would otherwise take months to achieve, a significant saving in project time and a consequent change in construction logic. Engineers can consider fully sealing the Corten steel after it has achieved a satisfactory patina, avoiding the disadvantage of only being able to seal and protect one side of the outer surface. This is also an important solution to avoid staining runoff from water flowing over the back of the Corten steel.


The testing phase at 25 Hanover Square has been instrumental in validating the design and material choices before actual construction begins. The mock-up, which prominently features the corten steel and Portland stone combination, successfully demonstrated the feasibility and visual appeal of the proposed façade. BROMOCO's treatment solutions proved effective in rapidly achieving the desired patina on the corten steel and ensuring its protection from environmental elements.

Client and Community Reception

The response to the mock-up has been overwhelmingly positive. The client expressed confidence in the project's direction, particularly impressed by the high-quality finishes and innovative material solutions provided by BROMOCO. The local community and architectural reviewers have also shown appreciation for the project's commitment to blending traditional and modern elements, as well as its sustainability focus. This positive reception during the testing phase bodes well for the project's future implementation.

25 Hanover Square Project: Conclusion

The collaboration between BROMOCO and Basha-Franklin on the 25 Hanover Square model test exemplifies the importance of thorough preparation and innovation in architectural projects. The successful testing phase underscores BROMOCO's crucial role in material treatment and application, ensuring that the corten steel and Portland stone will perform as intended in the final construction. Our weathering accelerator and Corten steel sealing and protective coatings have proven the design solution to be viable, providing a solid foundation for the actual building process. The project begins in early 2024.


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