Skin deep... success on the surface

TWI Bulletin, March - April 2004

Creating a hard ceramic coating on aluminium alloys....by plasma electrolytic oxidation

Fig.1. Plasma discharge around a component immersed in electrolyte
(Courtesy Keronite Ltd)

A uniform thickness coating can be developed over areas with complicated geometry and restricted access surfaces, such as inner surfaces and holes. The process is capable of coating alloys of aluminium, magnesium and titanium. A typical coating thickness can range from 25-100µm depending upon the substrate alloy and process conditions.

The coating hardness depending upon a substrate alloy can be as high as 1800HV. The electrolyte is environmentally safe and is a low concentration alkaline solution of a proprietary composition. A bath with a capacity of 200 litres and a power rating of 80kW can coat a substrate up to 0.25m ² surface area at a rate of 1µm.min ^-1. Substrate preparation often requires only degreasing of the component and the process is operated between 20-50°C. The coating can be further impregnated with various materials such as polymer, ceramic or metallic substances to improve functional properties. By applying an appropriate sealant, the coating can be made resistant to many chemicals, acids and alkalis.

The PEO process can offer an alternative to a number of widely used processes including hard anodising, hard chrome plating and thermal spraying. While many successful applications have been demonstrated for these more established processes, there is a strong need to identify new coating processes that can meet the requirements of recent environmental legislation and the continual drive for better coating performance. In particular, the PEO process is an environmentally clean method, using a solution that requires no significant treatment prior to disposal. A PEO coated aluminium surface may provide a lightweight alternative to a steel surface coated with hard chrome or by thermal spraying.

Potential applications of the PEO coatings include pistons, cylinder blocks and liners, sliding bearings for the automotive industry; valves and pumps for the oil and gas and chemical industries; anilox rolls and thread guides for the printing and textile industries. Figure 2 shows some components coated using the Keronite PEO process.

Fig.2. Components coated using Keronite PEO process

Experimental approach

The substrate alloy was aluminium AA6082-T6 of nominal composition Bal Al, 0.69Si, 0.52Mn, 0.67Mg and 0.26Fe. The specimens were ground on 600 grit SiC paper prior to coating. The nominal coating thickness was 50µm. Substrate specimens of 40mm diameter and 6mm thickness were PEO coated by Keronite. Coating cross sections were prepared ready for microscopic examination using standard metallographic techniques to a 1µm diamond finish. A scanning electron microscope (SEM) was used for microstructural characterisation ( Fig.3). Porosity in the coating was measured from the polished cross-section using the In2ViewCat-Pro image analyser connected to an optical microscope. Coating hardness was measured on a polished cross-section using the Vickers diamond pyramid micro-indenter with an indentation load of 50g. Coating composition was determined using the X-ray diffraction (XRD) technique.

The corrosion performance of the coated and uncoated alloys was assessed by exposing the specimens to a salt spray environment following the guidelines described in the ASTM standard B117-97 up to 2000 hours. In addition, electrochemical corrosion behaviour was studied using an accelerated potentiodynamic testing (anodic polarisation) in an N ₂-purged 3.5% NaCl solution of pH 8. The solution temperature was maintained at 25±1°C.

Results

The appearance of the as-coated surface is shown in Fig.3. The SEM image in Fig.3 shows the coating to comprise grain-like structures. These grains are circular in shape and contain some micro-pores and micro-cracks at the grain boundaries. A cross section through the as-prepared coating is shown in Fig.4.

Fig.3. SEM image of PEO coating surface on AA6082 aluminium alloy

Fig.4. SEM image of a cross-section of PEO coating and Vickers micro-indentation hardness values of different layers

The coating has a relatively uniform thickness of about 40-50µm with a distinct two-layer structure. The upper 15-20µm of the coating can be seen to consist of a layer of fused larger grains (about 30-50µm in length)with some larger pores and vertical cracks. The lower 30-35µm layer consists of interlocking grains of much smaller size (about 8-10µm). A porosity level of about 2% was measured from the lower coating layer.

The XRD analysis confirmed the coating consisted of crystalline phases α, γ and δ-Al ₂O ₃. Average Vickers micro-indentation hardness values measured for the upper and lower layers of the coating and on the substrate are shown in Fig.4. The hardness of the lower coating layer was about 1800HV, which is about 10 times that of the substrate.

The surface of the specimens exposed to the salt spray environment was examined after 24, 192, 336, 720, 1000 and 2000 hours. The uncoated alloy displayed a slight white deposit indicating a corrosion attack of the exposed surface after just 24 hours, but this was at a low level up to 2000 hours. The surface of the coated alloy did not display any change and was free from visible signs of corrosion attack after 2000 hours of salt spray exposure. Surface appearances of the uncoated and the PEO coated specimens after 2000-hour exposure to the salt spray environment are shown in Fig.5. The surface of the uncoated alloy was rated nine (in accordance with ASTM D1654-92) at 336 hours and remained such until 2000 hours of exposure. The coated alloy was rated 10 (highest rating) at 2000 hours of salt spray exposure.

Fig 5a - Surface of uncoated AA6082 aluminium alloy after 2000 hours of salt spray exposure

Fig 5b PEO coated AA6082 aluminium alloy after 2000 hours of salt spray exposure

Forward scans of the anodic polarisation curves of the uncoated and the PEO coated alloy AA6082 are shown in Fig.6. The uncoated aluminium alloy displayed an immediate rapid increase of the current density upon slightly increasing the potential from its rest potential 'Ecorr'. This immediate rise of the anodic current density indicates a rapid corrosion occurring at the surface of the uncoated alloy while immersed in a salt solution. Such electrochemical behaviour is often shown by the surface of an actively corroding material which does not impart corrosion protection.

Fig.6. Anodic polarisation plots of the uncoated and PEO coated AA6082 aluminium alloy in de-aerated 3.5% NaCl solution

The coated alloy displayed a very slow increase of the current density, maintaining less than 20µA.cm ^-2 over a wide range of potential scans (about 400mV SCE positive from its Ecorr). This indicated that the coating was providing a good level of corrosion protection to the underlying aluminium substrate. A slow increase of the current density indicated that the coating surface was not completely passive.

Discussion

Examination of the coating cross section demonstrated that a uniform coating of about 50µm thickness could be achieved on AA6082 aluminium alloy with the PEO process. An interesting feature of the PEO coating is that it consisted of multiple layers, a top layer of about 33% and the harder lower layer of about 66% of the total thickness. Coating growth of about 66% inward and 33% outward from the surface is expected. In essence, initial dimension of the substrate can be retained by removing the top layer. The lower coating appeared to have fine micro-pores and was formed of interlocking grains as would be expected for the coatings produced using the PEO process. A significant increase to the substrate surface hardness was imparted by the formation of crystalline phases mainly comprising α, γ and δ aluminium oxide (Al ₂O ₃) and hardness values up to 1800HV were measured on the lower coating layer. A typical hard anodised coating on an aluminium alloy would consist of mainly amorphous aluminium oxide that often has a much lower coating hardness. The extreme hardness provided by the PEO coating layer to the aluminium substrate is of practical importance where very hard surfaces and extremely good corrosion resistance are required for severe wear and corrosion applications. The uncoated alloy displayed a good corrosion resistance in the salt spray environment for 2000 hours.

However, poor electrochemical behaviour displayed by the anodic polarisation plot suggests a susceptibility of the uncoated alloy to rapid corrosion in a fully immersed condition. In this regard, the PEO coating may have worked as an effective barrier by preventing easy penetration of the corrosive solution, albeit not a complete barrier. It is expected that by effectively sealing the fine coating porosity with a suitable sealer, the corrosion resistance of the PEO coating may be further improved.

Conclusions

The following conclusions can be drawn from the work undertaken:

• The PEO coating on AA6082 aluminium alloy consists of multiple layers. The main coating layer has coating porosity less than 2% and is formed of interlocking grains.
  
  
• The PEO coating on AA6082 aluminium alloy consists of crystalline phases of α, γ and δ-Al ₂O ₃. These ceramic phases impart very high coating hardness up to 1800HV.
  
  
• The PEO coating can provide at least 2000 hours of salt spray endurance to the AA6082 aluminium alloy without any visible corrosion attack.
  
  
• Electrochemical study suggests that the PEO coating can provide a good level of corrosion protection to the underlying AA6082 aluminium alloy in an immersed saline aqueous environment.