Stainless steels
are iron-chromium alloys with a minimum of 10.5% chromium. When the steel
contains at least that much chromium, a thin, transparent, protective passive
film forms on the surface. The passive film forms spontaneously as a result
of the reaction between the chromium in the steel and the oxygen in the air.
The passive film is a barrier between the steel and the environment and prevents
corrosion attack of the underlying steel as long as the passive film remains
intact. This protective passive film is quite thin, on the order of 10 to
20 atoms thick, and its actual composition depends on the alloying elements
in the stainless steel and the environment to which it is exposed. For example,
the passive film on a piece of Type 304 stainless is not quite the same when
it is exposed to air and when it is exposed to potable water.
Role of Alloying Elements
Chromium is the essential ingredient in stainless steels. As the chromium
content is increased above 10.5%, the passive film becomes stronger and is
able to resist more aggressive environments, particularly those containing
chlorides. High chromium also helps the passive film to heal itself more rapidly
if it is disrupted, for example, by scratching of the surface.
At elevated temperatures the chromium reacts with the oxygen in the air to
form a thick, visible oxide layer. The color and thickness of the chromium
oxide will depend on the temperature and time of exposure. The chromium oxide
layer is visible and is thick enough to be scraped off and measured. This
is quite unlike the thin, transparent, protective, passive film.
Various alloying elements are added to stainless steels to improve corrosion
performance in specific environments, or to modify or improve the mechanical,
or physical properties of the stainless steel. There are several hundred different
stainless steels, all formulated to provide a specific combination of corrosion
resistance, weldability and mechanical properties.
Nickel is the most common alloying element in stainless steels. It
changes the crystal structure of the steel from ferritic to austenitic. The
austenitic structure has improved ductility, formability and weldability.
Nickel also improves corrosion resistance in reducing environments such as
sulfuric acid. The most common stainless steel is Type 304 which has about
18% Cr and 9% Ni.
Molybdenum is added to stainless steels to improve their resistance
to pitting and crevice corrosion in chloride-containing environments. Type
316 is the most common Mo stainless steel. Its nominal composition is 17%Cr-10%Ni-2%Mo.
Since about 1970, nitrogen has been an important alloying addition
to stainless steels. The high performance austenitic stainless steels and
the second-generation duplex stainless steels all contain a deliberate addition
of 0.10 to 0.50% nitrogen. For these stainless steels, nitrogen improves pitting
and crevice corrosion resistance, makes them stronger, and retards the formation
of sigma phase during welding. In duplex stainless steels, nitrogen promotes
the re-formation of austenite at higher temperatures and helps maintain an
acceptable austenite-ferrite phase balance in the as-welded condition. Precipitation
of intermetallic phases such as sigma reduce the toughness and corrosion resistance
of stainless steels.
Additions of copper to a stainless steel increase the corrosion resistance
in reducing environments such as sulfuric acid. Alloy 20 and 904L are examples
of stainless steels with deliberate copper additions.
Other alloying additions are used to enhance specific properties. For example,
sulfur is added to the free machining stainless steel, Type 303, for
improved chip breaking during turning operations. Aluminum and silicon
additions improve the oxidation resistance of other stainless steels.
Chemical composition limits and mechanical property requirements for some
of the more common wrought stainless steels are shown in Tables 1 and 2. The
stainless steels are grouped according to their family or metallurgical type:
austenitic, duplex, ferritic, martensitic, and precipitation hardening. The
grades listed here are generally available in one or more product forms but
some have been included for historical purposes.
The grades are generally listed in order of their UNS number. For some there
are subcategories that reflect the common industrial practice. For example,
the duplex stainless steels are separated into first-generation and second-generation
duplex. The second-generation duplex grades contain a deliberate alloying
addition of nitrogen to improve the corrosion resistance and toughness of
these materials, to permit their use in the as-welded condition.
Are some Ni-base alloys stainless
steels?
About ten years ago, ASTM adopted the Euronorm definition of “steels”.
As a result, a number of stainless steels which had been given ASTM B (non-ferrous)
specification listings began to be added to the ASTM A (steel) specifications.
These grades were originally given N08xxx UNS numbers because they were in
the B specifications but were true stainless steels not nickel-base alloys.
They have retained their original “N” UNS number as they have been
added to the ASTM A specifications, so users can recognize that these are
the same grades that have been in use for many years. Stainless steels such
as 904L (N08904) are now in all of the stainless steel product specifications
but for the next several years are expected to be “grandfathered”
by continued listing in the B specifications, as well.
The chemical composition limits reflect those shown for the major alloying
elements in ASTM, AMS, and AWS specifications. The values shown are maximums
unless ranges or minimums are listed.
Forms of Stainless Steel Corrosion
When stainless steels are selected properly, fabricated correctly and maintained
adequately, they will perform without attack indefinitely. This is the case
in the vast majority of applications. However, if the environment becomes
overly aggressive, for example due to a process change or a process upset,
the passive film may be overwhelmed, usually on a very localized basis, and
no longer be a protective barrier for the stainless steel underneath. Corrosion
will then occur. For stainless steels, there are several forms of possible
attack including: pitting, crevice corrosion, stress corrosion
cracking, galvanic corrosion, and intergranular corrosion.
