The Truth about Stainless Steel

The Truth about Stainless Steel

In metallurgy, "stainless steel" is defined as a steel alloy with a minimum content of 13.0% chromium by mass.  Stainless steel does not stain, corrode, or rust AS EASILY as ordinary steel, but it is NOT stain-proof.  There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected to in its lifetime.  Stainless steel is used where both properties of steel and resistance to corrosion are required. 

In the automotive industry, a cheaper, more commonly used alternative to stainless steel is mild steel, or carbon steel.  Carbon steel, also called plain-carbon steel, is steel where the main alloying constituent is carbon.  The American Iron and Steel Institute (AISI) defines carbon steel as:  "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted:  manganese 1.65, silicon 0.60, copper 0.60." 

The term "carbon steel" may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels.  As the carbon content rises, steel has the ability to become harder and stronger through heat treating, but this also makes it less ductile.  Regardless of the heat treatment, a higher carbon content reduces weldability.  In carbon steels, the higher carbon content lowers the melting point.  Eighty-five percent of all steel used in the United States is carbon steel.

Stainless steel differs from carbon steel by the amount of chromium present by percentage of its mass. Carbon steel oxidizes when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion and blocks corrosion from spreading into the metal's internal structure.

High exidation-resistance in air at ambient temperature is normally achieved with additions of a minimum of 13% (by weight) chromium, and up to 26% is used for harsh environments.  The chromium forms a passivation layer of chromium (III) oxide (Cr₂O₃) when exposed to oxygen.  The layer is too thin to be visible, and the metal remains lustrous.  The layer is impervious to water and air, protecting the metal beneath.  Also, this layer quickly reforms when the surface is scratched.  This phenomenon is called passivation and is seen in other metals, such as aluminum and titanium.  Corrosion-resistance can be adversely affected if the component is used in a non-oxygenated environment, a typical example being underwater keel bolts burried in timber.

Manusteelcn Racing exhaust products are manufactured using SUS/T-304 Stainless Steel.  Grade 304 steel is the standard "18/8" stainless steel; it is the most versatile and most widely used stainless steel, available in a wider range of products, forms and finishes than any other.  The "18/8" stands for the chromium and nickel content of the steel.  Grade 304 stainless steel contains 18-20% chromium, which is the main element that gives this grade of steel the corrosion resistance it has.  It has excellent forming and welding characteristics.  With all that said, just because an item is T-304 Stainless Steel, does not mean that the material is 100% stain or corrosion proof.  It is merely more resistant to staining and corrosion.  The weather and environment with higher than normal humidity and precipitation figures allow the steel to be more prone to rust and corrosion, but 304 stainless steel is NOT rust-proof.

Many people question if we truly do use 304 Stainless Steel or not.  For this reason, here is a photo of prototype piping that has not been polished as all of our final production pieces are.  This should clear up any confusion as to the materials we use for our exhaust system products:

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