Corrosion-resistant black conversion coatings for magnesium

Magnesium’s properties of strength, flexibility, and low weight make it an exciting choice for a range of components in the aerospace industry. In the constant innovation of lightweight designs, it enables weight and fuel efficiency. In the frontier of space, magnesium exhibits additional properties that can further improve designs and extend the lifespan of space recruitment.

However, magnesium has its own susceptibilities that can make its use in the extreme conditions of space a challenge, especially in the context of component lifespan. Fortunately, there are advanced coating solutions that not only seek to extend the lifespan of these components in space, but provide low reflectivity and thermal control features for optical space equipment in particular.

The features of magnesium

Magnesium is garnering an increasing level of interest from aerospace engineers thanks to its impressive strength-to-weight ratio. It is the lightest structural metal on earth, with alloys having a density of 1.7-1.8g/cm3, yet exhibits impressive strength due to its hexagonal and close packed crystal structure. Moreover, it is 100% recyclable and the eighth most abundant metal in the earth’s crust, resulting in sustainable components that allow engineers to reduce the weight of designs to enhance propellent efficiency and maximise launch thrust.

In the context of space applications in particular, magnesium offers interesting design possibilities via its additional practical features. An example of this is magnesium alloys having the ability to mitigate deep space radiation exposure up to 30% less than other structural metals such as aluminium. This can directly extend the lifespan of electrical/electronic components that face degradation and failure as a result of radiation. 

The importance of optical components for space equipment

One particular component of space equipment that is sensitive to the deep space environment yet crucial to mission success are optical devices. Deep space explorers, satellites and observation equipment all rely on the accuracy of sensors to perform their role. This can range from analysing the distance between stars to determine their location, or taking high quality images of points of interest in space.

Light from various sources in space can negatively impact sensors; this is seen in the saturation of images and light signal detection failure in the LuoJia-1 satellite as a result of solar stray light. Moreover, these components are also susceptible to extreme temperatures, and require effective thermal control to operate optimally. As a structural metal, magnesium is a strong contender for optical components due to the previously mentioned features, but also brings with it a list of potential challenges.

The pitfalls of magnesium in space

• Corrosion: Magnesium’s outer oxide layer fails to protect components in acidic or even neutral environments, leaving the metal’s surface vulnerable to attack. 
• Wear: Magnesium has low hardness and high chemical reactivity, making it unsuitable for long-term usage in heavy wear applications. 
• Thermal: Magnesium has a relatively low thermal conductivity (68 W/m-K) and melting point, resulting in a deterioration of structural and mechanical performance at 200°C with most alloys. 

Fortunately, many of these factors can be diminished via metal coating technology. Hard anodizing has been a traditional solution that uses an electrolyte bath to produce an outer oxide layer for components that protects from corrosion and enhances its surface hardness. However, hard anodising can result in through-thickness cracks in complex geometry components that reduces fatigue strength, and lacks a significant enhancement of the required thermal characteristics.

Plasma Electrolytic Oxidation and bespoke aerospace solutions

Plasma Electrolytic Oxidation (PEO) is an advanced metal surface coating solution that provides bespoke surface features for light metal components such as magnesium. It utilises an electrolyte bath similar to hard anodising, but uses greater and controlled voltages to produce plasma discharges on the metal’s substrate. This creates a ceramic outer layer that is not only extremely resistant to corrosion, heat and wear, but produces an environmentally safe byproduct that can be easily disposed of.

A key feature of PEO is the porosity of the outer ceramic layer. These pores allow for the absorption of various reagents that are introduced in the electrolyte solution, enhancing surface characteristics that suit specific applications. For aerospace applications, Keronite has developed a new space-grade coating to maximise the optical performance of coated components; Keronite Black-Mag.

Black magnesium coating

Black-Mag is a space-grade coating that maximises optical performance via light absorption/low reflectivity, exhibiting a luminosity indices of 25-28 L*. By introducing amorphous magnesium oxides and other metal oxides into PEO’s bath solution, the resultant outer ceramic coating has remarkable uniformity and a dark black finish. In addition to low reflectivity, black magnesium coatings provide excellent thermal, corrosive and wear resistance; allowing engineers to extend the lifespan of components in space equipment where maintenance is mostly impossible. 

The low reflectivity and thermal resistance of black magnesium coatings enables the use of the light metal for space optical components. Satellite barrels and sun sensors can absorb light to enable the efficient operation of optics whilst bringing lightweighting and life-extending capabilities to designs. Additionally, the aesthetic and protective qualities of black magnesium coating has respective applications for aviation and automotive components that would benefit from the black finish.

Utilise bespoke PEO solutions to extend the lifespan of your components

Black magnesium coating is an exciting new electrolyte technology developed by Keronite to enable the use of the light metal in optical space applications. Better yet, its range of surface properties can also be used in a wide array of additional applications within the atmosphere as well.

Our team of advanced engineers develop a range of electrolyte solutions to suit the specific surface characteristics of different applications. The result is the extended lifespan of lightweight components that would otherwise need to be heavier metals to survive harsh environments like outer space.

To find out more about the details surrounding black magnesium and our other electrolyte solutions, download our datasheet. Or, to find out more about how PEO can provide the surface characteristics you need for your application, contact us today.