Advanced environmentally friendly chemical surface treatments for cast magnesium helicopter transmission alloys preservation

Final Report Summary - MAGNOLYA (Advanced environmentally friendly chemical surface treatments for cast magnesium helicopter transmission alloys preservation)

Executive Summary:

The main objective was the development of new eco-friendly chemical conversion technologies as an alternative to CrVI-based coatings, which provide an excellent corrosion protection but present health and environmental hazards. The aim was the protection of cast EV31A alloy used in the fabrication of helicopter transmission components as required by AgustaWestland (the topic manager of the project) and the validation over other Mg alloys, hence fulfilling the demands of the aerospace industry.

The work at TECNALIA was carried out starting from a proprietary process for conversion coating of Mg alloys. Three main steps have been carried out during the optimization process in order to improve the corrosion resistance of the coating:

• Pre-treatment selection: pre-treatment of the substrate is a fundamental step in the preparation route, as it directly affects the performance of the whole system.
• Incorporation of additives to the base conversion treatment in order to improve the corrosion protection (including self-healing capabilities) of the coating.
• Adjustment of operation conditions (pH, dipping time, temperature, etc.) and concentration of additives in order to achieve the best final performance of the coated part.

The optimization process has led to the selection of two different best performing treatments in order to improve the corrosion protection. The laboratory process was scaled-up and a 20 L pilot plant at TECNALIA was used for the production of the plates (Mg alloys EV31A as provided by AW, AZ91 and AM60). After the newly developed treatment, the samples were coated with a primer and a resin (according to the requirements set by AW), both chromium-free.

The specimens were then tested in terms of appearance, composition and morphology, mild environment and salt spray fog corrosion resistance, resin and primer adhesion and evaluation of the chemical resistance of the component. All the requirements for the testing procedures were set and validated by AgustaWestland and are in agreement with the main standards and regulations set for the aeronautic industry.

The samples showed a remarkable corrosion resistance, comparable or superior to the Cr-Mn reference samples.

Project Context and Objectives:

Magnesium alloy castings are used in the fabrication of helicopter gearbox components because the advantages in terms of specific weight / mechanical properties and suitability for the casting process. However, improvements in helicopter performance by the use of magnesium have been offset by corrosion problems. The helicopter industry to regain confidence in magnesium, more corrosion resistant alloys and improved protective finishing systems are needed. Also, these systems must conform to environmental regulations and the use of hazardous substances. Both chemical and electrochemical methods are used for the modification of magnesium surfaces. High voltage micro-arc anodic oxidation is one of the most recently investigated technologies for protecting Mg alloys and its study has been already launched in another Clean Sky project. The idea is that this new technology will hopefully satisfy the coating for most of the casting surfaces but there is need to introduce also a new technology for these areas which are not suitable to be exposed to an electrochemical process, like oil ducts, very close tolerance mating surfaces etc. The review of the state of the art showed that many ways exist to form chromate free chemical conversion layers on the surface of Mg alloys but significant gaps were identified referred to real implementation and commercial significance of these developments and their use in specific applications such as the protection of cast Mg alloys used in helicopters. In this sense, TECNALIA and PROMET consortium partners own proprietary formulations for the surface treatment of non-ferrous alloys with already demonstrated properties for the protection of Mg alloys. In the case of TECNALIA, the starting point will be a prorietary process for conversion coating of Mg alloys. In the case of PROMET several proprietary formulations based respectively on CrIII, cerium and manganese chemistries will constitute the starting point. A selenious acid self-healing process for brush-application will be also tested. This project aims to readjust and optimize these baseline treatments to extend their application to the helicopter transmission EV31A Mg casting alloy as required by the addressed topic, fulfilling all the demanded requirements by the aerospace industry.

The main objective of the project is to develop new eco-friendly chemical conversion technologies for the protection of cast EV31A alloy used in the fabrication of helicopter transmission components as required by the addressed topic, fulfilling all the demanded requirements by the aerospace industry.

The specific objectives are:

• To develop new Cr-VI free conversion treatments for the EV31A (AMS4429) Mg casting alloy with optimum corrosion resistance and primer adhesion properties considering the full sequence of treatment and in accordance with the requirements that apply for the specific topic addressed.
• To select at least the two best performing processes for further characterization and performance testing.
• To write the protocols describing the full sequence process compositions, operation and application procedures for the selected processes.
• To perform the complete testing as required by the addressed topic of the best performing treatments developed and previously selected for further characterization.
• To follow a criss-crossed testing route between the project beneficiaries to perform the designed battery of tests in order to have complementary data for a better assessment of results.
• To check the compliance of the best performing processes developed along the project with the technical requirements given by the topic manager and to provide a reliable comparative ranking of the new treatments in comparison with the reference chromated treatments.
• To identify the potentials and limitations of the new developed protection systems with respect to the existing state-of-the-art.
• To evaluate the economic and ecologic effects of the technical benefits that arise from the new developed processes.
• To coordinate the project in order to achieve the objectives within the agreed time and budget limitations, and with the agreed quality level.
• To agree the dissemination and exploitation activities and draft the associated plans accordingly.

Project Results:

The work at TECNALIA was carried out starting from a proprietary process for conversion coating of Mg alloys. Three main steps have been carried out during the optimization process in order to improve the corrosion resistance of the coating:

• Pre-treatment selection: pre-treatment of the substrate is a fundamental step in the preparation route, as it directly affects the performance of the whole system.
• Incorporation of additives to the base conversion treatment in order to improve the corrosion protection (including self-healing capabilities) of the coating.
• Adjustment of operation conditions (pH, dipping time, temperature, etc.) and concentration of additives in order to achieve the best final performance of the coated part.

The optimization process has led to the selection of two different best performing treatments in the case of TECNALIA. The laboratory process was scaled-up and a 20 L pilot plant at TECNALIA was used for the production of the plates (Mg alloys EV31A as provided by AW, AZ91 and AM60).

In parallel, PROMET has tested potassium permanganate in alkaline or slightly acid media, as well as salts of metals close to chrome in the Periodic Table of elements. Tests have also been performed with lanthanides, such as cerium used industrially as a catalyst in several chemical reactions. Finally Cr(III) has been assessed even though its oxidizing character is much lower than Cr(VI). The treatment selected by PROMET for further testing were an alkaline permanganate based conversion coating and a commercial product, i.e. the Cr(III)-based Surtec 650.

All the specimens were then tested in terms of appearance, composition and morphology, mild environment and salt spray fog corrosion resistance, resin and primer adhesion and evaluation of the chemical resistance of the component. All the requirements for the testing procedures were set and validated by AgustaWestland and are in agreement with the main standards and regulations set for the aeronautic industry. For the purpose of some tests, after the newly developed treatments, some samples were coated with a primer and a resin (according to the requirements set by AW), both chromium-free.

The samples from TECNALIA showed a remarkable corrosion resistance comparable or superior to the Cr-Mn reference samples. On the other side the conversion coating developed by PROMET showed a poorer performance than the reference samples.

Potential Impact:

The conversion coating object of this work is developed as an environmentally friendly alternative to CrVI-based coatings, which provide an excellent corrosion protection but present health and environmental hazards.

Currently, both chemical and electrochemical methods are used for the modification of magnesium surfaces. High voltage micro-arc anodic oxidation is one of the most recently investigated technologies for protecting Mg alloys. However, this technique might not be easy to apply in certain areas of the part to be coated. The suggested treatment is a chemical process and it is suitable to be applied to those areas which are not suitable to be exposed to an electrochemical and/or plasma process, like oil ducts, very close tolerance mating surfaces, etc.

The present conversion coating treatment provides a remarkable corrosion protection for Mg alloys and it is completely free of chromates. It should be emphasized that the resin and primer applied over the coating for further testing are also chromate-free.