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Chromate Metallic-Ceramic Alternatives
|Metallic-ceramic coatings are used to provide oxidation-resistant and abrasion-resistant coatings for high temperature gas turbine engine components, but they are sometimes used for general corrosion resistance for components that also require abrasion resistance. Metallic-ceramics are a two-coat system, usually comprising a basecoat of Al flakes in a ceramic matrix that traditionally contain hex chrome inhibitors, and a hex-chrome-free topcoat. They are usually applied by spraying. Specification: MIL-C-81751C||
Metallic-Ceramic coatings, especially Sermetel W and its equivalents, contain chromates, and must be sprayed with proper ventilation and emission controls:
- EPA Clean Air Act rules
- OSHA Occupational Exposure to Hexavalent Chromium (Cr6+ PEL is currently 5μgm-3)
- European rules (RoHS, WEEE, ELV, REACH)
- OSD-ATL Memo,
- DFARS Minimizing the Use of Hexavalent Chromium, Federal Register /Vol. 76, No. 87 /Thursday, May 5, p 25569 (2011)
Personnel may be exposed during manufacture, depot overhaul, and operational level touch-up and repair.
- OEM and depot personnel may be exposed to Cr6+ during spray operations.
- Most of the Cr6+ is lost during firing, but some remains in the final coating to provide corrosion resistance.
Alternatives, Applications, Status
A number of non-chrome and the low-chrome metallic ceramic products are now on the market. In most cases the low chrome versions contain some chromate in the formulation, but essentially none in the final fired coating.
|Most Common Alternatives||Examples of Authorizations and Implementations||Specifications|
|Low-Cr metallic-ceramics||Rolls Royce, P&W, Williams International, Honeywell, Turbomeca||Numerous OEM specs|
|Numerous OEM specs|
|Products containing Al flakes in a polymer matrix||Rolls Royce Aircraft Engines||MSRR9253|
Note: Although a number of low and no hex chrome products have been approved and are being used in production, there is very limited data on their performance (see SermeTel Coatings and Processes in ASETSDefense Database).