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What Is Stainless Steel and Why Is it Stainless?

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What Is Stainless Steel and Why Is it Stainless?
Issue Time:2016-08-01

In 1913, English metallurgist Harry Brearly, working on a project toimprove rifle barrels, accidentally discovered that adding chromiumto lowcarbon steel gives it stain resistance. In addition to iron, carbon,and chromium, modern stainless steel may also contain other elements,such as nickel, niobium, molybdenum, and titanium. Nickel, molybdenum, niobium,and chromium enhance thecorrosion resistance of stainless steel. It is theaddition of a minimum of 12% chromium to the steel that makes it resist rust,or stain 'less' than other types of steel. The chromium in the steel combineswith oxygen in the atmosphere to form a thin, invisible layer ofchrome-containing oxide, called the passive film. The sizes of chromiumatoms and their oxides are similar, so they pack neatly together on thesurface of the metal, forming a stable layer only a few atoms thick. If themetal is cut or scratched and the passive film is disrupted, more oxide will quicklyform and recover the exposed surface, protecting it from oxidativecorrosion.

(Iron, on the other hand, rusts quickly because atomiciron is much smaller than its oxide, so the oxide forms a loose ratherthan tightly-packed layer and flakes away.) The passive film requires oxygen toself-repair, so stainless steels have poor corrosion resistance in low-oxygenand poor circulation environments. In seawater, chlorides from the salt willattack and destroy the passive film more quickly than it can be repaired in alow oxygen environment.

Types of StainlessSteel

The three main types of stainless steels are austenitic,ferritic, and martensitic. These three types of steels are identified by theirmicrostructure or predominant crystal phase.

Austenitic:

Austenitic steels have austenite as their primary phase (face centeredcubic crystal). These are alloys containing chromium and nickel (sometimesmanganese and nitrogen), structured around the Type 302 composition of iron,18% chromium, and 8% nickel. Austenitic steels are not hardenable byheat treatment. The most familiar stainless steel is probably Type304, sometimes called T304 or simply 304. Type 304 surgical stainlesssteel is an austenitic steel containing 18-20% chromium and 8-10% nickel.

Ferritic:

Ferritic steels have ferrite (body centered cubic crystal) as theirmain phase. These steels contain iron and chromium, based on the Type 430composition of 17% chromium. Ferritic steel is less ductile thanaustenitic steel and is not hardenable by heat treatment.

Martensitic:

The characteristic orthorhombic martensite microstructure was firstobserved by German microscopist Adolf Martens around 1890. Martensitic steelsare low carbon steels built around the Type 410 composition of iron, 12%chromium, and 0.12% carbon. They may be tempered and hardened. Martensite givessteel great hardness, but it also reduces its toughness and makes it brittle,so few steels are fully hardened.

There are also other grades of stainless steels, such as precipitation-hardened,duplex, and cast stainless steels. Stainless steel can be produced in a varietyof finishes and textures and can be tinted over a broad spectrum of colors.

Passivation

There is some dispute over whether the corrosion resistance of stainlesssteel can be enhanced by the process of passivation. Essentially, passivationis the removal of free iron from the surface of the steel. This is performed byimmersing the steel in an oxidant, such as nitric acid or citricacid solution. Since the top layer of iron is removed, passivation diminishessurface discoloration. While passivation does not affect the thickness oreffectiveness of the passive layer, it is useful in producing a clean surfacefor a further treatment, such as plating or painting. On the other hand, if theoxidant is incompletely removed from the steel, as sometimes happens in pieceswith tight joints or corners, then crevice corrosion may result. Most researchindicates that diminishing surface particle corrosion does not reduce susceptibilityto pitting corrosion.

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