Chrome-free protection against corrosion

Materials World magazine
10 May 2019

A new inhibitor could provide safer and better corrosion protection. Idha Valeur reports.

A new chrome-free smart coating could help protect against corrosion following the EU ban on the most effective inhibitor – hexavalent chromate – in February 2019, because its potential to cause cancer. The smart coating was developed by Hexigone – a startup business made up of researchers and students from Swansea University, UK.

Preventing and treating corrosion costs the world economy US$2.5tln annually. To tackle this, Intelli-ion uses chemical intelligence to prevent metal deterioration.

Chromate’s properties make it imperative to find a replacement quickly. ‘Chromate is highly toxic, carcinogenic and mutagenic,’ Hexigone Inhibitors Ltd CEO and Founder, Dr Patrick Dodds, told Materials World. ‘It can damage your DNA and could affect your children. The chemical is one of the most toxic – with around 78,000 EU workers exposed every year. The EU has now made it illegal to use hexavalent chromate within manufacturing processes, but this has now left EU manufacturers with less corrosion protection than their competitors around the globe.’

In 2016, Dodds, with his fellow researchers, formulated the idea for the new inhibitor during what he described as a ‘creative moment’.

‘We have since worked with our customers and have three product offerings that are suitable as drop-in replacements for coatings manufacturers. The different additives can be utilised in different curing mechanisms, where they have shown to enhance the cure as well as the high level of corrosion inhibition. We are scaling our production and can control the particle size of the product,’ Dodds said.

‘The system works by immobilising the inhibitor on the reservoir backbone, which holds on to the inhibitor until the pre-determined triggered release. In our Intelli-ion product the trigger is corrosive ions. The corrosive ions are sequestered onto the reservoir backbone, while the inhibitor travels to the bare metal or metal-coating interface. The Intelli-ion system allows us to lock the inhibitor onto the pigment system, which enabled us to use inhibitors previously incompatible with coatings.’ Using the university-built scanning Kelvin probe (SKP), the testing showed no initiation of corrosion after four days. Other corrosion inhibitors, including hexavalent chromate, failed within a 24-hour period.

‘After the excellent results on the SKP testing, the more traditional corrosion tests were performed as well as surface analysis techniques. The X-ray photoelectron spectroscopy (XPS) results show the inhibitor to create a nano layer-thick protective film over the bare metal surface, which causes an anodic shift in the free corrosion potential of the underlying metal,’ Dodds said.

‘The inhibitor also forms an insoluble precipitate with any metal ions present to prevent underfilm mass transport of sodium ions to the proceeding corrosion front. At the front, the inhibitor is activated and interferes with the oxygen reduction reaction by causing a physical barrier to oxygen. The traditional corrosion tests show that the inhibitor acts as both an anodic and cathodic inhibitor.’

Comparing Intelli-ion to other chrome-free alternatives, third-party testing shows that Hexigone’s inhibitor increases protection by 10 times. ‘The components are organic and can be acidic or alkaline in nature. The “sense and release” mechanism is different, and we are not an encapsulated product so ultra-fine particle sizes and grinding does not affect the performance,’ Dodds said.

After 4,000 hours of prohesion, humidity and salt spray testing, performed by a global leader in industrial coatings, the Hexigone inhibitor showed 1mm of corrosion compared with the 10mm displayed in the control sample. The test proved it could increase the life span of several end products and prolong the protection window beyond other inhibitors. This will subsequently impact industries such as construction, automotive and aerospace.

What are the next steps?

‘Given the recently reported synergistic benefits seen when using our corrosion inhibitors in conjunction with several others, we are collaborating with selected pigment manufacturers to explore this further,’ Dodds said.

Hexigone has received investment from Development Bank of Wales and former CEO of Spencer Coatings Group, Phil Buck. Hexigone also received an innovation loan from Innovate UK. Through funding and collaboration with 20 industry partners over four continents, the company aims to continue accelerating the scale-up.