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Chromate-Free Coating Systems for Aerospace and Defence Applications (Free-Coat) Phase 1 – 11 December 2018

Chromate-Free Coating Systems for Aerospace and Defence Applications (Free-Coat) Phase 1 – 11 December 2018

Overview of the project

PRA World Limited (‘PRAW’) are undertaking a research project to establish the current state-of-the-art with respect to the performance of hexavalent chromate-free (‘Cr VI-free’) coating systems.  The topic of chrome-free protection of aluminium on aircraft and military land vehicles has been of major interest to manufacturing companies, organisations and the UK government involved in the aerospace and defence sector for several decades. In the absence of detailed concrete results, it is important to establish the current state-of-play for chromate-free systems, which need to meet the stringent and demanding requirements for use on aircraft (external & internal) bodies and components. The project will evaluate the potential to replace these hexavalent chromates and protective systems (coatings + pre-treatments) with alternative ‘chromate-free’ technologies. The conclusions to this project will also form a basis for future studies and further research and development into chromate-free systems.

Pera International have provided £50,000 of research funding to enable PRAW to undertake this research project.


Objectives: The project is intended to produce generic research results (that is, results capable of being utilised by a number of companies within a class of trade) as follows:

  1. Research the state of the art in respect to chromate-free complete protective systems for aerospace and defence applications, key focus is on Cr VI-free coating systems for the internal and external of aircraft bodies.
  2. Provide an overview of anonymised results that may be shared with a wider audience (subject to agreement), so that appropriate decisions may be made with respect to the future continued use of Cr-VI substances such as Strontium Chromate
  3. Enhance the understanding and communication lines with all stakeholders, specifically by informing interested parties regarding the progress that has been made, and the remaining challenges related to the development of chromate-free systems that meet the requirements.


Following a review of the state of the art of hexavalent chromate-free coating systems for corrosion protection for aerospace and defence applications (, PRAW is moving into the next phase of comparative testing of the most promising coating systems.

The evaluation will be based on complete Cr(VI)-free protective systems nominated by up to four leading suppliers.  It is proposed that each of the participants will prepare a total of 48 standard aluminium alloy (2024-T3) panels as follows:

  1. Cleaning and deox of all 48 panels
  • Solvent cleaning – procedure to be agreed, followed by Deox using Deoxidine 624
  1. Pre-treatment
    • 24 panels (1a) Silane-based Sol-gel conversion coating – Socogel B0202
    • 24 panels (1a) Participants selected pre-treatment
  2. Single coat primer
    • Participant’s primer applied at 15-25 microns dry film thickness to 24 panels (1/2a) and 24 panels (1/2b) to one side only
  3. Two-coat primer + topcoat
    • Participant’s matt topcoat to be applied at total (primer + topcoat) 50-60 microns dry film thickness to 12 panels (1/2a/3) and 12 panels (1/2b/3) to one side only

In addition, 24 control panels (Alodine standard Cr-based pre-treated 2024-T3 panels) will be prepared:

  1. 12 x Single coat primer (BS 2X 33) applied to 2024-T3
  2. 12 x Two-coat matt white system BS 2X 33 + BS X 34) applied to 2024-T3

Test panels which will be subjected to comparative performance testing (including chemical resistance, adhesion, corrosion resistance) with the standard chromium-based benchmark samples.  Panels will be tested by PRAW:

  • Adhesion BS EN ISO 2409 (dry)
  • Flexibility BS EN ISO 1519
  • Filiform corrosion BS EN 3665 (1000 hrs)
  • Hydraulic fluid resistance ISO 1518
  • Neutral salt spray BS EN ISO 9227 (3000 hrs)

Four market leading manufacturers of coating systems for corrosion protection for the aerospace and defence industries were identified to represent progress achieved to-date in the development of chromate-free systems.  Progress has suffered delays, not least as a result of the ongoing impact upon companies of the COVID-19 pandemic.

Custom test panels (102 x 152 x 1mm (0.040 x 4 x 6”), bare aluminium alloy 2024-T3) have been purchased from Q-Labs and have been forwarded to the participants for coating.  Testing of the coated panels is expected to commence late 2021.


For further information please contact Ben Naden  01664 501428