Discovery stops the rot

A solution to the problem of steel corrosion has been cooked up in Washington, US where aluminium is used to create a stronger, cheaper coating. Stuart Nathan reports.

A new method of protecting the surface of steel could cut material costs for metals that need protection from corrosion. Developed in the US at the Pacific Northwest National Laboratory (PNNL), Richland, Washington, it covers relatively low-cost stainless steels with a layer of aluminium that bonds irreversibly with the base metal.

Lead researcher Chuck Henager said the method imparts the anti-corrosive properties of aluminium to steel. Aluminium, although a chemically reactive metal, is useful in anti-corrosive applications; the metal surface reacts with oxygen in the air, creating a layer of aluminium oxide that resists most corrosion. The metal is more expensive and weaker than steel, so combining their properties is an ideal solution.

Henager starts by making the initial coating: a mixture of aluminium powder and a liquid polymer called polysiloxane, dissolved in the organic solvent cyclohexane. Polysiloxane is a preceramic: when heated under the correct conditions, it forms cross-links and becomes a solid ceramic.

The mixture is then applied to the surface of the steel. The coating can be applied by dipping, painting or with a spray-gun. ‘This is one of the advantages of polysiloxane,’ explained Henager. ‘We picked it because it was readily available off the shelf and easy to handle but it’s also very efficient at wetting the steel surface: it gives a smooth, even coating.’

Once applied, the coating is set at low temperature with a ruthenium catalyst. The solidified ceramic holds the aluminium particles close to the steel surface for the next phase – heating in air, argon or nitrogen at 700-900oC. This triggers a reaction between the aluminium and the steel, which forces the metals to diffuse into each other. The result is a layered coating: closest to the surface, corrosion-resistant aluminium oxide; then two layers of iron aluminide, the second containing a larger proportion of iron than the first; and finally the bulk of the stainless steel sheet.

This coating cannot be scraped or flaked off. With the coating, Henager said, the metal can withstand corrosion, oxidation and attack by carbon and sulphur-based compounds. ‘The ceramic also remains on the surface to an extent, and provides even more protection,’ Henager said. ‘One of the reasons behind the choice of preceramic was that we wanted to retain as much ceramic as possible.’

The choice of aluminium powder proved crucial. ‘We ordered one batch with a larger particle size and we found that the coating didn’t go on as easily and didn’t diffuse into the steel as effectively when it was heated,’ Henager said. ‘Choosing the correct flake size turned out to be a balancing act. If you go too small, the powder becomes pyrophoric – it burns fiercely and easily.’

Diffusion coatings have been made before but the PNNL method is the simplest and cheapest version, Henager said. The use of the preceramic matrix allows simple application methods suitable for coating large areas. Because of this, Henager said, the US Navy is interested in the technique for protecting steel hulls from salt water.

The process industries are also interested, as it could cut their materials costs. ‘We’ve had interest from companies that want to make hydrogen from natural gas,’ Henager said. In steam reforming, in which methane and super-heated steam are reacted together, the reaction vessels undergo a process called carburisation, where the steel absorbs carbon and becomes weaker. To avoid this, the vessels are often made from expensive exotic alloys such as Inconel, based on nickel and chrome. ‘Using this coating technique, they could use inexpensive steels instead,’ Henager claimed.

PNNL is now seeking partners to commercialise the process. This, Henager believes, may involve replacing some of the components, particularly the cyclohexane solvents. ‘We might need to find something less volatile; we’d want to switch to the most benign components we can find that are suitable,’ he said.