Developing Green and Sustainable Electroplating for the Aerospace Industry

Interview conducted by Jake WilkinsonMay 6 2016

Professor Karl Ryder, of the University of Leicester, is a leading academic in the field of electrochemistry. As part of his research he has been developing novel environmentally-friendly methods of electroplating the components used in the aerospace and automotive industry, which currently use hazardous and toxic solvents. AZoM spoke to Karl about the novel solvents his research group are developing for electroplating and electropolishing, and the integral role optical profiling plays in his research.

Please could you introduce our readers to the research you carry out at the University of Leicester?

I’m part of a research group at the University of Leicester that carries out electrochemical and materials based research. Our research is primarily on surface coatings produced using techniques such as metal plating, but we research other metal electro-finishing methods like electro-dissolution and surface etching.

The main focus of our research is to look at conventional industrial processes of coating surfaces and to try and make them more sustainable and less toxic. These coatings are frequently used for wear-protection, to prevent corrosion or purely for aesthetics.

At the moment, chromium plating is commonly done using chromium six. Chromium six is one of the most carcinogenic materials known. There's a huge desire to find electroplating methods that don't use chromium six. My research group is looking at chromium, nickel and cobalt in particular.

As part of our research we need to understand the new coatings, we need to understand the structure, thickness, morphology, shape, the way that the deposits grow, and so on which requires surface characterization methods.

We are also developing methodologies for more complex casting methods such as single crystal super alloy castings which is important for the automotive and aerospace industries.

Components used in the aerospace industry are electro-finished before they are assembled. Image Credits: shutterstock.com/ID1974

What is single crystal super alloy casting, and why is it important to industries such as aerospace?

Single crystal casting is an advanced casting technique which has been developed over the last fifteen years.

It’s a method used to cast a coating for components which will be highly stressed, such as the turbines in a modern aerospace engine. These blades have to convert the majority of the jet’s combustive force, produced by the burning fuel, into rotation. These components are required to function at very high temperatures, typically between 1300 and 1700 °C, and, because they rotate so rapidly, incredibly high mechanical stresses.

To achieve this they essentially have to be flaw free. They're fabricated in such a way that there are no grain boundaries in the coating, which could be a focus point for mechanical stress resulting in cracks. If these blades were to crack bits of the blade would be thrown off at 10,000 RPM which, when close to a fuel tank in flight, is extremely dangerous.

The purpose of single crystal methodology is to cast these very highly stressed components as a single crystal, so that there are no grain boundaries meaning there is no possibility of cracking or failure.

The casting of the components must be completely flaw free to ensure they do not fail at extreme operating temperatures. Image Credits: shutterstock.com/Jaromir Chalabala

How do manufacturers and researchers ensure that the crystallization is uniform across an entire surface?

They use a combination of inspection techniques. When a blade or component comes out of the casting furnace, it has to be heat treated, which homogenizes and sets the crystal structure.

Once the sample has been heat treated it is chemically etched using chemicals such as iron chloride. This etching can give a visual indication of any possible defects in the crystal structure. Shaded areas are indicative of grain boundaries, if any shaded areas are observed then the casting is scrapped completely.

We then electro-polish the surface. Electropolishing is one way of removing, in a very controlled way, small amounts of a surface. Electropolishing allows us to visualize grain boundary structures which shouldn't be there.

If the grain boundary is localized at the surface, and only 10 µm or so deep, then electropolishing can be used to remove the grain boundary to give a flawless component. However, if the grain boundary runs through the entire casting then it cannot be removed and we have to begin the casting process.

A sample of a polished Ni based superalloy that has been electrolytically etched. The etch profile is visible and the etch depth can be measured using a Zeta-20 Optical Profiler.

What solvents and metals are conventionally used for electropolishing processes?

Common metals that are electropolished include stainless steels, nickel based alloys and occasionally cobalt based alloys. Currently, chemicals such as sulfuric acid, phosphoric acid, mixtures of both and organic based sulfonic acids are used, all of which are aggressive. On top of this toxic additives are often added such as hydrogen fluoride and chromic acid.

My team work towards developing solvent and electrolyte systems which can be used for electro-polishing yet are nowhere near as aggressive or toxic. We work on electrolyte systems which are metal specific. As part of this research we have been working with Rolls Royce’s aerospace division to develop a new method of etching super alloys.

Another one of the electrolytes we have developed is a mixture of choline chloride, a salt found in food additives, and ethylene glycol, which is antifreeze. If these two components are combined in a specific way they can be used to electropolish stainless steel to the same standard as the toxic phosphoric acid which is currently used. This was a huge step forward for us.

This combination of components is an example of a deep eutectic solvent (DES) which are electrolytic liquids which show behaviour similar to molten salts. They have a range of interesting properties that makes them useful solvents for electrochemists. These include a low vapour pressure, so unlike organic solvents they do not evaporate at room temperature, and a high affinity for metal salts meaning they will dissolve most materials.

The ionic nature of DES systems means electrochemical processes will occur at lower energies, making the reactions required in processes such as electropolishing easier to achieve.  

We have also worked on other electrolyte systems which are also metal specific. We have been working with Rolls Royce’s aerospace division to develop a new method of etching super alloys.

Super alloys are often casted as a single crystal using the HIP (hot Isotactic Press) method, where a powder is compressed in a mould at extreme temperatures and pressures to create the desired component. This is then electrochemically etched and electro-polished to remove any grain boundaries. The success of this can be determined by optical profiling.

A sample of Ni alloy produced using a hot isostatic press (HIP) imaged using a Zeta-20 Optical Profiler. The Ni alloy has been electrolytically etched using a deep eutectic solvent. The image shows the step edge between the native (unetched) surface and the dissolved (etched) surface.

What are the characteristics of ions that are soluble in deep eutectic solvents, and which of these are suitable for use in electropolishing?

There are two parts to this question. The characteristics of ions that are soluble in deep eutectic solvents compromise a whole range of metal salts. Deep eutectic solvents have an extremely high ionic concentration. Almost any salt that you add becomes a metal halide salt in solution which allows the dissolution of a wide range of ions. It's that that really means that you can dissolve a wide range of things

To answer the second part of your question on which of these are suitable for electropolishing, this is dependent on the substrate. For example, stainless steel is an alloy that's often electropolished using phosphoric and sulfuric acid. As stainless steel is an alloy of iron and nickel it’s important that the two different metals are removed in the same ratio as their concentrations in the alloy itself. If only the nickel you would be left with an iron surface which would rust rapidly.

One of the strengths of deep eutectics is that we know from our own measurements that when you electropolish alloys they tend not to de-alloy. They tend to come out in the same proportions as they are present in the solid. That means we can electropolish a whole range of different types of substrates, both pure metals and mixtures of metals, like stainless steels and super alloys, with little complications.

How do you determine if an electropolished surface is uniform after you've electropolished it?

When we have a substrate that we want to electro-polish we determine it’s initial surface roughness which we measure using a Zeta Optical Profiler and also using an Atomic Force Microscope (AFM). We then electropolish the surface, as this happens the surface of the substrate becomes more reflective and optically brighter and this allows us to make subjective, qualitative judgements on the surface’s uniformity.

Following this we would optically profile the surface again to determine the difference in surface roughness following the electropolishing. This is the important stage as it gives us an objective and quantitative way of determining how effective the electropolishing has been.

The ability to optically profile our samples has made our research faster and more accurate. Before we used the Zeta Optical Profiler we were limited to using only an AFM, which whilst highly accurate has some big disadvantages. Firstly, we could only image surface sections of 100 x 100 µm so we had to assume this section was representative of the entire surface as the substrate. To try and counter any error from this assumption we would have to take multiple measurements over the entire surface which was extremely time consuming.

With the Zeta Optical Profiler we can rapidly take multiple quantitative measurements over the entire surface of the substrates at a high accuracy which saves us time and gives us more confidence in our results. We can also capture high quality 2D images with an infinite depth of focus; meaning features at the top of the surface are in focus and the features at the bottom of the surface are in focus. The Zeta Optical Profiler does this with incredible versatility, it's very easy to change the magnification.

We looked at the range of different technologies available and the Zeta Optical Profiler was offering the best technology for its price.

Sectional views of a Co-based superalloy bolt, used as a fixing component in a F1 engine imaged using a Zeta-20 Optical Profiler.  Pitch and roughness information are visible. Optical Profiling gives a 2D projection with an excellent depth of field.

Do you use the Zeta Optical Profiler for any other aspects of your research?

Yes we do. My team is also involved in the development of other sustainable technologies. Part of this research involves making alternatives to construction materials, such as MDF (Medium Density Fibre Boards) and leather.

As part of this research they use the Zeta Optical Profiler extensively to determine the surface characteristics of those types of materials.

Some of my colleagues in the Chemistry Department are also using the Zeta Optical Profiler in their microfluidics research. They are measuring the characteristics of microfluidic channels which are buried within the substrate. This would only be possible using an instrument such as the Zeta Optical Profiler which is capable of profiling features such as a subsurface, and transparent, microfluidic channel.

The Zeta-20 Optical Profiler.

In the future, can you see your electropolishing methods being integrated into large scale manufacturing processes?

Yes absolutely. In fact, it’s already happening right now.

To help speed up this process we have a facility at the University of Leicester called the Ionic Liquids Demonstrator, which is a scale-up unit. Here we develop processes which allow us to move from the litre scale, which we have conducted our research at, to a 100 litre scale for use in industry.

At the Ionic Liquids Demonstrator we are working on several scale-ups involving chromium, zinc alloys and, more recently, aluminum; which is the first of it’s kind in the UK.

The scale up-process is one of the most difficult things to do. A lot of things occur that cannot be easily anticipated and the process can be hard to control. Careful consideration needs to be made on controlling the mass flow (for example by pumping or stirring), how to maintain large amounts of suspended non-liquids and also how to dissipate heat in a safe and controlled way.

All of this large-scale research is carried out at an electroplating facility in Gloucester which is used for commercial aerospace electroplating for our partner, Airbus. We don’t want to wait for the future, we’ve made it happen now.  

Where can our readers find out more about your research and the Zeta Optical Profiler?

You can hear more about our research through my teams website or by reading the papers supplied below. More information on the Zeta Optical Profiler can be found here.  

• The Electrodeposition of Silver Composites using Deep Eutectic Solvents, Andrew P. Abbott, Khalid El Ttaib, Gero Frisch, Karl S. Ryder and David Weston, Phys. Chem. Chem. Phys., 2012, 14, 2443.
• The Effect of Additives on Zinc Electrodeposition from Deep Eutectic Solvents, Andrew P. Abbott, John C. Barron, Gero Frisch, Karl S. Ryder and A. Fernando Silva, Electrochimica Acta, 2011, 56, 5272.
• Electroplating using Ionic Liquids, Andrew P. Abbott, Gero Frisch and Karl S. Ryder, Annual Review of Materials Research,2013, 43, 1.

About Prof. Karl Ryder

Karl Ryder is an electrochemist with research interests in the electrochemical processing in novel and environmentally sustainable electrolytes. This encompasses electroplating, electrodissolution, materials finishing  as well as metal recovery, recycling and energy storage technologies.

Prof. Ryder works closely with a range of strategic industries including aerospace, automotive and electronics. He is lead academic for the Materials Centre at University of Leicester and is a recipient of a Royal Society Industry Fellowship to work with Rolls-Royce Aerospace on applications of electrochemical processing of super-alloy components for turbine engines.  

Prof. Ryder is also technical director of Scionix Ltd.  Scionix is a University spin-out company set up to formulate, manufacture and commercialize Ionic liquid electrolyte technology.

His electrochemical research spans the fields of ionic liquids (including deep eutectic solvents), electrodepostion and dissolution, electropolymeristaion, electrochemical energy storage and industrial electroplating.