Choosing stainless steel products can
be difficult and confusing. We have developed this guide to
assist and educate you in choosing the best stainless steel
products to fit your needs.
16 Gauge VS 18 Gauge: What's
16 gauge stainless steel is a higher quality
grade than 18 gauge. This is because 16 gauge is thicker and
heavier making it stronger and more durable. 16 gauge is ideal
for every day heavy duty use making it a worthwhile investment.
304 VS 430: What's the difference?
Type 304 stainless steel (also known as 18/8
stainless steel) is an austenitic stainless steel. It has
higher nickel content than its ferritic 430 counterpart. Type
304 is non-magnetic, more resistant to corrosion than 430,
and is more durable. Type 304 is immune to foodstuffs, sterilizing
solutions, most organic chemicals and dyestuffs, and a wide
variety of inorganic chemicals. Because 304 can withstand
the corrosive action of various acids found in fruits, meats,
milk, and vegetables, it is used for sinks, tabletops, stoves,
refrigerators, and steam tables. It is also used in numerous
other utensils such as cooking appliances, pots, pans, and
Type 430 has good resistance to a wide variety of corrosives
including nitric acid and some organic acids. It attains its
maximum corrosion resistance when in the highly polished or
buffed condition. In general, its resistance to pitting and
crevice corrosion resistance is close to that of Type 304.
Stress corrosion cracking resistance of Grade 430 is very
high, as it is for all ferritic grades.
Types of stainless steel
There are different types of stainless steels:
when nickel, for instance is added the austenite structure
of iron is stabilized. This crystal structure makes such steels
non-magnetic and less brittle at low temperatures. For higher
hardness and strength, carbon is added. When subjected to
adequate heat treatment these steels are used as razor blades,
cutlery, tools etc.
Significant quantities of manganese have been used in many
stainless steel compositions. Manganese preserves an austenitic
structure in the steel as does nickel, but at a lower cost.
Stainless steels are also classified by their crystalline
* Austenitic stainless steels comprise
over 70% of total stainless steel production. They contain
a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient
nickel and/or manganese to retain an austenitic structure
at all temperatures from the cryogenic region to the melting
point of the alloy. A typical composition is 18% chromium
and 10% nickel, commonly known as 18/10 stainless is often
used in flatware. Similarly 18/0 and 18/8 is also available.
* Super austenitic stainless steels, such
as alloy AL-6XN and 254SMO, exhibit great resistance to chloride
pitting and crevice corrosion due to high Molybdenum contents
(>6%) and nitrogen additions and the higher nickel content
ensures better resistance to stress-corrosion cracking over
the 300 series. The higher alloy content of "Super austenitic"
steels means they are fearsomely expensive and similar performance
can usually be achieved using duplex steels at much lower
* Ferritic stainless steels are highly
corrosion resistant, but far less durable than austenitic
grades and cannot be hardened by heat treatment. They contain
between 10.5% and 27% chromium and very little nickel, if
any. Most compositions include molybdenum; some, aluminum
or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo,
29Cr-4Mo, and 29Cr-4Mo-2Ni.
* Martensitic stainless steels are not
as corrosion resistant as the other two classes, but are extremely
strong and tough as well as highly machineable, and can be
hardened by heat treatment. Martensitic stainless steel contains
chromium (12-14%), molybdenum (0.2-1%), no nickel, and about
0.1-1% carbon (giving it more hardness but making the material
a bit more brittle). It is quenched and magnetic. It is also
known as "series-00" steel.
* Duplex stainless steels have a mixed
microstructure of austenite and ferrite, the aim being to
produce a 50:50 mix although in commercial alloys the mix
may be 60:40. Duplex steel have improved strength over austenitic
stainless steels and also improved resistance to localized
corrosion particularly pitting, crevice corrosion and stress
corrosion cracking. They are characterized by high chromium
and lower nickel contents than austenitic stainless steels.
Stainless Steel Grades
The AISI defines the following grades
- 200 Series
• Austenitic iron-chromium-nickel-manganese
- 300 Series
• Austenitic iron-chromium-nickel
• Type 301: Highly ductile,
for formed products. Also hardens rapidly during mechanical
• Type 303: Free machining version
of 304 via addition of sulfur
• Type 304: The most common;
the classic 18/8 stainless steel.
• Type 316: The next most common;
for food and surgical stainless steel uses; Alloy addition
of molybdenum prevents specific forms of corrosion. Also known
as "marine grade" stainless steel due to its increased
ability to resist saltwater corrosion compared to type 304.
SS316 is often used for building nuclear reprocessing plants.
- 400 Series
• Ferritic and martensitic alloys
• Type 408: Heat-resistant;
poor corrosion resistance; 11% chromium, 8% nickel.
• Type 409: Cheapest type; used
for automobile exhausts; ferritic (iron/chromium only).
• Type 410: Martensitic (high-strength
• Type 420: "Cutlery Grade"
martensitic; similar to the Brearley's original "rustless
steel". Also known as "surgical steel".
• Type 430: Decorative, e.g.
for automotive trim; ferritic.
• Type 440: A higher grade of
cutlery steel, with more carbon in it, which allows for much
better edge retention when the steel is heat treated properly.
- 600 Series
• Martensitic precipitation
• Type 630: Most common PH stainless,
better known as 17-4; 17% chromium, 4% nickel
Stainless steel finishes
Standard mill finishes can be applied to flat rolled
stainless steel directly by the rollers and by mechanical
abrasives. Steel is first rolled to size and thickness and
then annealed to change the properties of the final material.
Any oxidation that forms on the surface (scale) is removed
by pickling, and the passivation layer is created on the surface.
A final finish can then be applied to achieve the desired
• No. 0 - Hot Rolled Annealed,
• No. 1 - Hot rolled, annealed
• No, 2D - cold rolled, annealed,
pickled and passivated
• No, 2B - same as above with
additional pass through polished rollers
• No, 2BA - Bright Anealed (BA)
same as above with highly polished rollers
• No. 3 - coarse abrasive finish
• No. 4 - fine abrasive finish*
• No. 6 - matt finish
• No. 7 - reflective finish
• No. 8 - mirror finish
*DLF Products are #4 finished
Stainless Steel Properties
Stainless steels have higher resistance to
oxidation (rust) and corrosion in many natural and man made
environments, however, it is important to select the correct
type and grade of stainless steel for the particular application.
High oxidation resistance in air at ambient temperature is
normally achieved with additions of more than 12% (by weight)
chromium. The chromium forms a passivation layer of chromium
(III) oxide (Cr2O3) when exposed to oxygen. The layer is too
thin to be visible, meaning the metal stays shiny. It is,
however, impervious to water and air, protecting the metal
beneath. Also, when the surface is scratched this layer quickly
reforms. This phenomenon is called passivation by materials
scientists, and is seen in other metals, such as aluminum.
When stainless steel parts such as nuts and bolts are forced
together, the oxide layer can be scraped off causing the parts
to weld together. When disassembled, the welded material may
be torn and pitted, an effect that is known as galling.
Stainless steel's resistance to corrosion and staining, low
maintenance, relative inexpense, and familiar luster make
it an ideal base material for a host of commercial applications.
There are over 150 grades of stainless steel, of which fifteen
are most common. The alloy is milled into sheets, plates,
bars, wire, and tubing to be used in cookware, cutlery, hardware,
surgical instruments, major appliances, industrial equipment,
and building material in skyscrapers and large buildings.
Stainless steel is 100% recyclable. In fact, over 50% of
new stainless steel is made from re-melted scrap metal, rendering
it a somewhat eco-friendly material.
Even a high-quality alloy can corrode under certain
conditions. Because these modes of corrosion are more exotic
and their immediate results are less visible than rust, they
often escape notice and cause problems among those who are
not familiar with them.
Passivation relies upon the tough layer of oxide described
above. When deprived of oxygen (or when another species such
as chloride competes as an ion), stainless steel lacks the
ability to re-form a passivating film. In the worst case,
almost all of the surface will be protected, but tiny local
fluctuations will degrade the oxide film in a few critical
points. Corrosion at these points will be greatly amplified,
and can cause corrosion pits of several types, depending upon
conditions. While the corrosion pits only nucleate under fairly
extreme circumstances, they can continue to grow even when
conditions return to normal, since the interior of a pit is
naturally deprived of oxygen. In extreme cases, the sharp
tips of extremely long and narrow pits can cause stress concentration
to the point that otherwise tough alloys can shatter, or a
thin film pierced by an invisibly small hole can hide a thumb
sized pit from view. These problems are especially dangerous
because they are difficult to detect before a part or structure
fails. Pitting remains among the most common and damaging
forms of corrosion in stainless alloys, but it can be prevented
by ensuring that the material is exposed to oxygen (for example,
by eliminating crevices) and protected from chloride wherever
Pitting corrosion can occur when stainless steel is subjected
to high concentration of chloride ions (for example, sea water)
and moderately high temperatures.
Weld decay and knife line attack
Due to the elevated temperatures of welding or during
improper heat treatment, chromium carbides can form in the
grain boundaries of stainless steel. This chemical reaction
robs the alloy of chromium in the zone near the grain boundary,
making those areas much less resistant to corrosion. This
creates a galvanic couple with the well-protected alloy nearby,
which leads to weld decay (corrosion of the grain boundaries
near welds) in highly corrosive environments. Special alloys,
either with low carbon content or with added carbon "getters"
such as titanium and niobium (in types 321 and 347, respectively),
can prevent this effect, but the latter require special heat
treatment after welding to prevent the similar phenomenon
of knife line attack. As its name implies, this is limited
to a small zone, often only a few micrometers across, which
causes it to proceed more rapidly. This zone is very near
the weld, making it even less noticeable. Modern steel making
technologies largely avoid these problems by controlling the
carbon content of stainless steels to <0.3% and historically
such grades were referred to as "L" grades such
as 316L; in practice most stainless steels are now produced
at these low carbon contents.
Stainless steel can actually rust quite rapidly if
it fails to form its protective oxide layer. This tends to
happen when the stainless has had carbon steel forced into
its surface, as by being dragged over carbon steel during
installation, brushing with carbon steel, grinding with a
contaminated wheel, or temporary welds to carbon steel.
This is a largely historical problem related to the
high carbon contents of steels from the past, for modern steels
it is very rarely an issue.
Some compositions of stainless steel are prone to inter-granular
corrosion when exposed to certain environments. When heated
to around 700 °C, chromium carbide forms at the inter-granular
boundaries, depleting the grain edges of chromium impairing
their corrosion resistance. Steel in such condition is called
sensitized. Steels with carbon content 0.06% undergo sensitization
in about 2 minutes, while steels with carbon content under
0.02% are not sensitive to it.
It is possible to reclaim sensitized steel by heating it
to above 1000 °C and holding at this temperature for a
given period of time dependent on the mass of the piece, followed
by quenching it in water. This process dissolves the carbide
particles, and then keeps them in solution.
It is also possible to stabilize the steel to avoid this
effect and make it welding-friendly. Addition of titanium,
niobium and/or tantalum serves this purpose; titanium carbide,
niobium carbide and tantalum carbide form preferentially to
chromium carbide, protecting the grains from chromium depletion.
Use of extra-low carbon steels is another method and modern
steel production usually ensures a carbon content of <0.03%
at which level inter-granular corrosion is not a problem.
Light-gauge steel also does not tend to display this behavior,
as the cooling after welding is too fast to cause effective
In the presence of reducing acids or exposition to reducing
atmosphere, the passivity layer protecting steel from corrosion
can break down. This wear can also depend on the mechanical
construction of the parts, e.g. under gaskets, in sharp corners,
or in incomplete welds. Such crevices may promote corrosion,
if their size allows penetration of the corroding agent but
not its free movement. The mechanism of crevice corrosion
is similar to pitting corrosion, though it happens at lower
Stress corrosion cracking
Stress corrosion cracking is a rapid and severe form of stainless
steel corrosion. It forms when the material is subjected to
tensile stress and some kinds of corrosive environments, especially
chloride-rich environments (sea water) at higher temperatures.
The stresses can be a result of the service loads, or can
be caused by the type of assembly or residual stresses from
fabrication (e.g. cold working); the residual stresses can
be relieved by annealing. This limits the usefulness of stainless
steel for containing water with higher than little parts per
million of chlorides at temperatures above 50 °C.
Stress corrosion cracking applies only to austenitic stainless
steels and depends on the nickel content.
Sulphide stress cracking
Sulphide stress cracking is an important failure
mode in the oil industry, where the steel comes into contact
with liquids or gases with considerable hydrogen sulfide content,
e.g. sour gas. It is influenced by the tensile stress and
is worsened in the presence of chloride ions. Very high levels
of hydrogen sulfide apparently inhibit the corrosion. Rising
temperature increases the influence of chloride ions, but
decreases the effect of sulfide, due to its increased mobility
through the lattice; the most critical temperature range for
sulphide stress cracking is between 60-100 °C.
Galvanic corrosion occurs when a galvanic cell is
formed between two dissimilar metals. The resulting electrochemical
potential then leads to formation of an electric current that
leads to electrolytic dissolving of the less noble material.
This effect can be prevented by electrical insulation of the
materials, eg. by using rubber or plastic sleeves or washers,
keeping the parts dry so there is no electrolyte to form the
cell, or keeping the size of the less-noble material significantly
larger than the more noble ones (eg. stainless-steel bolts
in an aluminum block won't cause corrosion, but aluminum rivets
on stainless steel sheet would rapidly corrode.
Contact corrosion is a combination of galvanic corrosion
and crevice corrosion, occurring where small particles of
suitable foreign material are embedded to the stainless steel.
Carbon steel is a very common contaminant here, coming from
nearby grinding of carbon steel or use of tools contaminated
with carbon steel particles. The particle forms a galvanic
cell, and quickly corrodes away, but may leave a pit in the
stainless steel from which pitting corrosion may rapidly progress.
Some workshops therefore have separate areas and separate
sets of tools for handling carbon steel and stainless steel,
and care has to be exercised to prevent direct contact between
stainless steel parts and carbon steel storage racks.
Particles of carbon steel can be removed from a contaminated
part by passivity with dilute nitric acid, or by pickling
with a mixture of hydrofluoric acid and nitric acid.