Recently, SpaceX, the aerospace design and manufacturing corporation headed by the Howard Hughes-esque billionaire Elon Musk, announced that they will build their prototype, Mars rocket entirely out of polished stainless steel. Polished stainless steel will not only provide the sleek, aesthetic look for the SpaceX rocket, but the molecular structure of the polished stainless steel will ensure the rocket’s durability at cryogenic temperatures such as those temperatures reached in space. The SpaceX rocket is an extreme example of polished stainless steel’s durability. The reality is that polished stainless steel has a wide variety of applications. Architects and design professionals may favor polished stainless steel for its aesthetic qualities but one of polished stainless steel’s prime qualities is its durability.
A Brief Look into the Metallurgy behind Polished Stainless Steel
To understand why polished stainless steel is used in such a wide variety of applications, it is important to look into the metallurgy of polished stainless steel. First, we have to look at stainless steel itself: Stainless steel is an alloy containing iron, carbon, and at a minimum 11.5% chromium by mass. In other words, stainless steel is an amalgamation of different elements. All steel contains iron and carbon, but the introduction of chromium into the elemental compound creates stainless steel. Chromium’s elemental properties are responsible for the inherent corrosion resistance of the alloy. Chromium prevents rust or deterioration of the alloy when exposed to water and air. For architectural applications where the material is exposed to the environment, stainless steel is the ideal metal. The iron-chromium oxide compound forms a tight adherent film on the surface which resists attack by the atmosphere and a wide variety of industrial gases and chemicals. The bond formed between iron and chromium is why SpaceX chose polished stainless steel for their Mars rocket. Interestingly, the iron-chromium bond gives stainless steel its regenerative qualities or, in other words, the ability to heal itself.
How Polishing Further Strengthens Stainless Steel
Again, polished stainless steel is not only used for its aesthetic qualities. The polishing process does accentuate the natural reflection of stainless steel, but it also improves the alloy’s durability. Polishing is the mechanical process of removing surface flaws and creating a uniform finish or pattern on a stainless-steel product. Polishing is accomplished by passing a product under a series of abrasive cloth belts resulting in the abraiding or grinding of the surface. As the abrasive belts meets the surface, the friction caused by the coarse belt hitting the product’s surface removes the initial surface of the product (including any surface flaws).
The polishing process is similar to the process sanding wood. Generally, polishing will not change the tolerance of a product. Polishing involves consistently removing layers of stainless steel. The removal of layers is so fine that the product’s dimensions do not change. Polishing will reduce the coefficient of friction thereby making the surface smoother. Polishing passivates the material by allowing the stainless steel to heal itself by creating a new oxide film when the product’s chemical properties interact with atmospheric elements. The polished surface is more aesthetically pleasing and passivation further accentuates the regenerative properties of the polished stainless steel.
Although polished stainless steel is heavier than other alloys, its durability is unparalleled. SpaceX and other industries have and will continue to rely on this important alloy.
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