Steel parts used in aerospace, unlike their aluminium counterparts, are subject to a variety of demands and therefore the coating specified for a steel part must be compatible with the base metal as well as the intended use.
Prepare the Steel Before Any Coating
Grit blasting is commonly used to prepare steel surfaces for coating. As steel is a high strength material (greater than 1,000 MPa) it is also important to consider a post blast treatment such as a stress-relief bake prior to applying the coating to prevent hydrogen embrittlement. This treatment is particularly important for components such as landing gear and actuator components.
Hard Chrome for Wear and Dimensional Rebuild
Hard Chrome for wear on hydraulic rods and other bearing surfaces. Typical build up values are from 25 microns up to 500 microns. Typical hardness values are in the region of 900 HV. Note: hexavalent chromium based processes are tightly controlled for health and safety reasons. Check requirements for any new application.
Electroless Nickel for Corrosion and Geometry
Within steel components that contain blind holes or bores, threaded holes or surfaces within components that are to be renovated, retouched, or resurfaced, Electroless Nickel coating can be provided within these areas with total and even coverage, without the normal throwing power limitations of normal electrolytic processes. There is more on Aerospace Surface Treatments at https://www.poeton.co.uk/about/industries/aerospace-defence/.
Zinc Nickel for Fasteners and Galvanic Compatibility
Zinc Nickel coatings are commonly used as aerospace fasteners. Zinc Nickel coatings with a 8 to 14% Nickel content provide over 1,000 hours of salt spray testing and have excellent electrochemical properties for corrosion resistance and galvanic corrosion when joining steel parts to carbon-fibre/ aluminium parts.
Thermal Spray for Dimensional Restoration
Worn or out-of-tolerance parts can often be restored to original dimensions by application of a thermal spray coating. Typically, a tungsten carbide or chrome carbide coating is used for these types of repairs, with typical thicknesses of 100-500 microns. This process is well suited for repairing of shafts, parts of housings and bearing journals.
Surface treatment decisions for aerospace steel components must consider the steel, the expected service load and the exposure. If any of these are incorrectly specified then the surface treatment that looks to be the correct ‘off the shelf’ option when specified on steel, will fail in service.
