Construction + Civil Engineering

Cortec Corporation is committed to providing world-class corrosion solutions for industries across the globe, with the company recently supplying its MCI^® technology to help preserve the famous Zagreb Cathedral.

Zagreb Cathedral is the tallest and one of the most beautiful buildings in Croatia – attracting thousands of tourists worldwide. Its construction dates back to 1093 with continued enrichment of the cathedral by famous architects during the following centuries.

Reconstruction of the cathedral in the late 1800s was led by Hermann Bollé, who brought the cathedral to its most recent architectural form – in which it stood until the earthquake on 22^nd March 2020, which damaged the cathedral’s southern spire. 

Over the last 30 years, extensive restoration work has also been undertaken on the cathedral, with ongoing repairs to this day. During reconstruction work on the south tower of the cathedral in 2012, damaged steel joints were found surrounding the tower – 10cm (4 inches) below the surface at approximately every 3m between the 1^st and 25^th rows. Most of the joints were only partially exposed in order to replace the surface layer of stone on the belltower, while the back of the joints remained embedded in stone and lime mortar. The joints were covered with a layer of rust and in drainage areas corroded all the way through the cross-section. 

In order to define the optimal solution for maintaining or improving the mechanical resistance and structural stability of the tower, the Faculty of Mechanical Engineering and Naval Architecture of Zagreb was called in to examine the joints. At their laboratory, they performed experiments on steel joints removed from the cathedral. They recommended doing the following:

• Remove corrosion from accessible joint connections.
• Apply corrosion protection to accessible joint connections.
• Strengthen the joint connections where damage had occurred.

It was suggested that a minimal range of intrusion be used to keep the mechanical resistance and stability of the tower structure at their existing level – while keeping costs at a minimum. Cortec’s CorrVerter^® MCI^® Rust Primer was recommended for corrosion protection. CorrVerter^® is a water-based product that quickly converts rust into a protective layer – and is capable of penetrating into corroded surfaces. It contains a novel chemical chelating agent that modifies surface rust into a hydrophobic passive layer. 

During application, a metal brush was used to remove loose rust from the joints. Then, two layers of CorrVerter^® MCI^® Rust Primer coating were applied directly onto the metal. A brush was used for CorrVerter^® MCI^® application on smaller metal joint surfaces, while spray application was used for larger areas. The first coat was applied at a thickness of 100 microns. A second coat was applied at a thickness of 75 microns. During application, the coating temperature was 13°C. The joints were then reinforced with steel fishplates that were welded onto the joints and also protected with CorrVerter^® MCI^® Rust Primer. The final step was to replace the stones around the joints. With the help of a skilled team and good project management, the entire project was completed successfully with minimal cost and intrusion as specified. The coating penetrated into the metal and stopped further advancement of the corrosion process.