Material science

1. The effects of alloying elements
The different alloying elements have specific effects on the properties of a stainless steel. It is the
combined effect of all the alloying elements, heat treatment, and, to some extent, impurities that
determine the property profile of a certain steel grade. It should be noted that the effect of the
alloying elements differs to some extent between the different types of stainless steel.
Chromium (Cr)
This is the most important alloying element and it gives stainless steels their basic corrosion
resistance. All stainless steels have a Cr content of at least 10.5% and the corrosion resistance
increases the higher chromium content. Chromium also increases the resistance to oxidation at
high temperatures and promotes a ferritic microstructure.
Nickel (Ni)
The main reason for adding nickel is to promote an austenitic microstructure. Nickel generally
increases ductility and toughness. It also reduces the corrosion rate in the active state and is
therefore advantageous in acidic environments. In precipitation hardening steels nickel is also
used to form the intermetallic compounds that are used to increase strength. In martensitic
grades adding nickel, combined with reducing carbon content, improves weldability.
Molybdenum (Mo)
Molybdenum significantly increases the resistance to both uniform and localized corrosion. It
slightly increases mechanical strength and strongly promotes a ferritic microstructure. However,
molybdenum also enhances the risk for the formation of secondary phases in ferritic, duplex, and
austenitic steels. In martensitic steels it increases the hardness at higher tempering temperatures
due to its effect on carbide precipitation.
Copper (Cu)
Copper enhances corrosion resistance to certain acids and promotes an austenitic
microstructure. It can also be added to decrease work hardening in grades designed for
improved machinability. It may also be added to improve formability.
Manganese (Mn)
Manganese is generally used to improve hot ductility. Its effect on the ferrite/austenite balance
varies with temperature: at low temperature manganese is an austenite stabilizer, but at high
temperatures it will stabilize ferrite. Manganese increases the solubility of nitrogen and is used to
obtain high nitrogen contents in duplex and austenitic stainless steels. Manganese, as an
austenite former, can also replace some of the nickel in stainless steel.
Silicon (Si)
Silicon increases resistance to oxidation, both at high temperatures and in strongly oxidizing
solutions at lower temperatures. It promotes a ferritic microstructure and increases strength.
Carbon (C)
Carbon is a strong austenite former that also significantly increases mechanical strength. In
ferritic grades carbon strongly reduces both toughness and corrosion resistance. In martensitic
grades carbon increases hardness and strength, but decrease toughness.

2. Nitrogen (N)
Nitrogen is a very strong austenite former that also significantly increases mechanical strength. It
also increases resistance to localized corrosion, especially in combination with molybdenum. In
ferritic stainless steels nitrogen strongly reduces toughness and corrosion resistance. In
martensitic grades nitrogen increases both hardness and strength but reduces toughness.
Titanium (Ti)
Titanium is a strong ferrite and carbide former, lowering the effective carbon content and
promoting a ferritic structure in two ways. In austenitic steels with increased carbon content it is
added to increase the resistance to intergranular corrosion (stabilized grades), but it also
increases mechanical properties at high temperatures. In ferritic grades titanium is added to
improve toughness, formability, and corrosion resistance. In martensitic steels titanium lowers
the martensite hardness by combining with carbon and increases tempering resistance. In
precipitation hardening steels, titanium is used to form the intermetallic compounds that are used
to increase strength.
Niobium (Nb)
Niobium is a strong ferrite and carbide former. Like titanium, it promotes a ferritic structure. In
austenitic steels it is added to improve the resistance to intergranular corrosion (stabilized
grades), but it also enhances mechanical properties at high temperatures. In ferritic grades
niobium and/or titanium is sometimes added to improve toughness and to minimize the risk for
intergranular corrosion. In martensitic steels niobium lowers hardness and increases tempering
resistance. In the US it is designated Columbium (Cb).
Aluminum (Al)
If added in substantial amounts aluminum improves oxidation resistance and is used in certain
heat-resistant grades for this purpose. In precipitation hardening steels, aluminum is used to form
the intermetallic compounds that increase the strength in the aged condition.
Cobalt (Co)
Cobalt is used in martensitic steels, where it increases hardness and tempering resistance,
especially at higher temperatures.
Vanadium (V)
Vanadium forms carbides and nitrides at lower temperatures, promotes ferrite in the
microstructure, and increases toughness. It increases the hardness of martensitic steels due to
its effect on the type of carbide present. It also increases tempering resistance. It is only used in
stainless steels that can be hardened.
Tungsten (W)
Tungsten is present as an impurity in most stainless steels, although it is added to some special
grades, for example the superduplex grade 4501, to improve pitting corrosion resistance.
Sulfur (S)
Sulfur is added to certain stainless steels to increase their machinability. At the levels present in
these grades, sulfur slightly reduces corrosion resistance, ductility, weldability, and formability. At
Outokumpu the trademark PRODEC (PRODuction EConomy) is used for some grades with
balanced sulfur levels for improved machinability. Lower levels of sulfur can be added to

3. decrease work hardening for improved formability. Slightly increased sulfur content also
improves the weldability of steel.
Cerium (Ce)
Cerium is one of the rare earth metals (REM) and is added in small amounts to certain heat-
resistant grades to increase resistance to oxidation at high temperatures.
How corrosion occurs
The corrosion resistance of stainless steel is attributed to the thin passive film that forms
spontaneously on its surface in oxidizing environments if the steel has a minimum chromium
content of approximately 10.5%.
As the film adheres strongly to the metal substrate and protects it from contact with the
surrounding environment, the electrochemical reactions that cause corrosion are effectively
stopped. If locally destroyed, for example by scratching, the film can 'heal' by spontaneously
repassivating in an oxidizing environment.
All types of corrosion affecting stainless steel are related to permanent damage of the passive
film, through either complete or local breakdown. Factors such as the chemical environment, pH,
temperature, surface finish, product design, fabrication method, contamination, and maintenance
procedures can all affect the corrosion behavior of steel and the type of corrosion that may occur.
More information on corrosion types, corrosion testing, and corrosion in specific environments
and applications can be found in the Outokumpu Corrosion Handbook, which also includes a
large collection of corrosion data for stainless steels. Contact your local sales office to get your
copy.
Corrosion can be divided into two categories: wet corrosion and high temperature corrosion.
Wet corrosion
Wet corrosion refers to corrosion in liquids or moist environments, and includes atmospheric
corrosion. It is an electrochemical process that involves an anode and a cathode, connected by
an electrolyte. The metal oxidizes (corrodes) at the anode, forming rust or some other corrosion
product. At the cathode, areduction reaction takes place – typically the reduction of oxygen or
hydrogen evolution. Preventing corrosion involves stopping these reactions taking place.
Typically, stainless steel does not corrode in the same manner as carbon or low-alloy steel,
which rust due to constantly changing anodes and cathodes on the whole surface. In order for
this process to occur on stainless steel, the passive film needs to be completely broken down in
environments such as non-oxidizing acids like hydrochloric acid. More commonly, the passive
film is attacked at certain points, causing various types of localized corrosion.
Wet corrosion forms
There are several different forms of wet corrosion, including:
• Pitting corrosion
• Crevice corrosion

4. • Uniform corrosion
• Environmentally assisted cracking
• Stress corrosion cracking
• Sulfide stress cracking
• Hydrogen induced stress cracking
• Corrosion fatigue
• Atmospheric corrosion
• Intergranular corrosion
Common causes of pitting and crevice corrosion
Pitting and crevice corrosion have very similar causal factors. Stainless steels are particularly
susceptible to pitting and crevice corrosion in media containing halide ions such as chlorides.
Therefore, high-risk environments for pitting and crevice corrosion include seawater and process
solutions containing high chloride concentrations.
Other factors that increase the probability for pitting and crevice corrosion are increased
temperature, low pH, and the addition of oxidative chemicals, for example by chlorination. Both
pitting and crevice corrosion can have serious consequences and therefore must be avoided.
Pitting corrosion
This type of corrosion is highly localized, with discrete pits on the free surface of stainless steels.
If the passive layer is damaged or locally weak, pitting corrosion can initiate, and the small area
that is unprotected by the passive film becomes the anode. As this anodic area is very small
compared to the large cathode area of the undamaged passive film, the corrosion rate is high
and a pit is formed.
Crevice corrosion
As its name implies, this type of corrosion occurs in crevices and confined spaces. Crevices can
be caused by component design or joints such as flanges and threaded connections, but also
under deposits formed on the surface during maintenance.
As the oxygen content is limited inside a tight crevice, the passive layer is weakened and, just as
with pitting, dissolved metal ions in the crevice lower the pH and allow chloride ions to migrate
into the crevice. Eventually the passive layer breaks down and the aggressive environment
facilitates the corrosion attack. Compared to pitting, crevice corrosion results in larger but
shallower attacks.
Resistance to pitting and crevice corrosion
It is well known that increasing the chromium content and adding molybdenum and nitrogen as
alloying elements increases stainless steels' resistance to pitting and crevice corrosion.
The Pitting Resistance Equivalent (PRE), often also given as PREN to indicate the influence of
nitrogen in the steel, can be used in order to rank and compare the resistance of different
stainless steels in terms of their resistance to pitting corrosion. It takes into account the effect of
the most important alloying elements. One frequently used equation for stainless steels is PRE =
% Cr + 3.3 × %Mo + 16 × %N.

5. It is important to remember that the calculated PRE only gives an indication of the resistance of
stainless steels and gives no information on their behavior in real environments. Therefore, it
should only be used for roughly comparing the pitting corrosion resistance of different grades.
Avoiding pitting and crevice corrosion
There are a number of measures that can be taken in order to avoid pitting and crevice
corrosion. These include:
• Selecting a highly alloyed stainless steel grade
• Lowering the chloride content of the corrosive environment
• Increasing the pH
• Decrease the content of oxygen and other oxidizing species from the environment, or
eliminate them altogether
• Use a design that avoids the need for tight crevices and which discourages stagnant
conditions and the formation of deposits
• Employ good fabrication practices that produce smooth and clean surfaces, and ensure
that weld oxides are removed
Uniform corrosion
Uniform corrosion occurs when the passive layer is destroyed on the whole, or a large part, of
the steel surface. This means the anodic and cathodic reactions occur on the same surface at
constantly changing locations, much like corrosion on carbon steel. The result is more or less
uniform removal of metal from the unprotected surface. Uniform corrosion can occur on stainless
steels in acids or hot alkaline solutions.
In an environment with constant temperature and chemical composition, uniform corrosion
occurs at rather a constant rate. As a result, in contrast to pitting and crevice corrosion, the
corrosion rate can be measured. This rate is often expressed as loss of thickness over time, for
example mm/year. Stainless steel is normally considered to be resistant to uniform corrosion in a
specific environment if the corrosion rate does not exceed 0.1 mm/year.
Resistance to uniform corrosion generally increases with increasing levels of chromium, nickel,
and molybdenum. However, in strongly oxidizing environments molybdenum has proved to be
detrimental to corrosion resistance.
Uniform corrosion is considered to be easier to predict than localized corrosion. While every
effort should be made to avoid pitting and crevice corrosion completely, some degree of metal
loss due to uniform contamination can often be tolerated. The exception is in applications where
contamination is unacceptable, such as for hygiene reasons in food-handling equipment.
Environmentally assisted cracking
This phenomenon is caused by the combined action of mechanical stress and a corrosive
environment. Once initiated, crack propagation can be very rapid and result in critical failure.
Environmentally assisted cracking can be caused by a number of environmental species
[CR2] including chlorides, hydrogen, and hydroxides. In order for cracking to occur the
mechanical tensile stresses must exceed a critical level. These need not necessarily be applied
stresses, but can also be residual stresses from manufacturing operations such as forming and
welding.

6. Stress corrosion cracking
Like pitting and crevice corrosion, stress corrosion cracking (SCC) most frequently occurs in
chloride- containing environments. Elevated temperatures (> 60 °C for chloride environments
and > 100 °C for alkaline environments) are normally required for SCC to occur in stainless steel.
Nevertheless, there are cases where cracking can occur at temperatures as low as 30 °C, for
example in swimming pool environments.
A common cause of SCC is evaporation on hot stainless steel surfaces. Liquids with low chloride
content that would normally be considered harmless can cause chloride concentrations high
enough to cause SCC. One example of where this can occur is underneath thermal insulation on
piping.
Standard austenitic grades, such as 4307 and 4404, are generally sensitive to chloride-induced
stress corrosion cracking. High nickel and molybdenum content increases the resistance of
austenitic stainless steels, which is why the high alloyed austenitic grades 904L, 254 SMO®, and
654 SMO® show excellent resistance to chloride-induced SCC. Stainless steels with a duplex
microstructure generally have high resistance to SCC, as have ferritic grades.
Sulfide stress cracking
Sulfide stress cracking (SSC), a form of hydrogen-induced cracking, is the cracking of a material
under the combined action of mechanical tensile stress and corrosion in the presence of water
and hydrogen sulfide (H2S). It is of particular importance in the oil and gas industry, as natural
gas and crude oil can contain considerable amounts of hydrogen sulfide (often referred to
as sour service).
To assess the corrosivity of process fluids containing hydrogen sulfide, the partial pressure of
hydrogen sulfide has to be considered, together with pH, temperature, chloride, carbon dioxide,
and oxygen content.
Susceptibility to hydrogen embrittlement is most severe at or below ambient temperature,
whereas chloride-induced SCC is most severe at high temperatures. Consequently, the
combined risk of cracking due to hydrogen sulfide and chlorides tends to be most severe for
austenitic, and especially duplex, stainless steel grades in the 80–100 °C range.
Hydrogen-induced stress cracking
Another hydrogen embrittlement failure mode that can be a concern in the oil and gas industry is
hydrogen-induced stress cracking (HISC), where hydrogen is introduced when the material is
undercathodic protection in seawater. The hydrogen is a result of the increased cathodic
reaction, hydrogen ion reduction, on the stainless steel surface.
Even high-alloyed stainless steels can be subjected to full cathodic protection in offshore
applications, as these steels are typically connected to carbon steel and other low-alloyed steels
already under protection. Ferritic, martensitic, and duplex stainless steels are generally more
susceptible than the austenitic grades to hydrogen embrittlement.
Corrosion fatigue

7. A material that is subjected to a cyclic load can fail due to fatigue at loads far below the ultimate
tensile strength. If the material is simultaneously exposed to a corrosive environment, failure may
occur at even lower load levels – and even more quickly.
Failure resulting from a combination of cyclic load and a corrosive environment is known as
corrosion fatigue. In many cases there is no pronounced fatigue limit, as observed in air, but a
gradual lowering of the fatigue strength with an increasing number of load cycles.
Corrosion fatigue cracks are usually less branched than stress corrosion cracks, although both
forms of corrosion cause brittle failures. Corrosion fatigue can occur at ambient temperature and
in environments that could be considered harmless with regard to other forms of corrosion.
As with stress corrosion cracking, residual stresses from manufacturing processes can adversely
affect resistance to corrosion fatigue. Increasing the mechanical strength of stainless steels also
increases their resistance to corrosion fatigue so duplex stainless steels are often superior to
conventional austenitic grades.
Atmospheric corrosion
Atmospheric corrosion is a collective term to describe the corrosion on metal surfaces in the
atmosphere. The atmosphere may be indoor or outdoor, and many different corrosion forms may
be involved.
Stainless steel that is exposed to an aggressive atmospheric environment is primarily affected by
staining, sometimes referred to as tea staining. However, not all discoloration is necessarily the
result of corrosion. It can also be discoloration from dirt or extraneous rust caused, for example,
by iron particles on the surface. However, if the chloride level is high enough, stainless steel can,
over time, also be attacked by localized corrosion such as pitting and crevice corrosion.
According to the ISO 9223 standard, the corrosivity of the environment is classified C1 to CX,
where C1 is the least corrosive and CX the most aggressive. The corrosivity classes are a good
tool for selecting materials that suffer uniform corrosion under atmospheric conditions, such as
carbon steel or zinc.
However, with their passive layer, stainless steels exhibit a totally different corrosion mechanism.
This means it is not easy to apply the corrosivity classes in ISO 9223 to stainless steels, and they
are therefore not the best tool for selecting stainless steels for atmospheric conditions. The
higher the corrosion class, the higher alloyed stainless steel needs to be used, with the range
going from ferritic grades up to superaustenitic and superduplex grades.
How the stainless steel is exposed to the atmosphere is also of great importance. In areas with
rainfall, sheltered conditions prevent rinsing, and corrosivity is increased. In dry areas with little or
no rainfall, sheltering will protect steel from aggressive pollutants and thus decrease corrosivity.
Surface condition and surface roughness can affect the performance of stainless steels. On a
coarse surface, dirt, particles, and corrosive chemicals are easily retained, increasing the
susceptibility to atmospheric corrosion. A smooth surface will facilitate rinse-off and is therefore
less susceptible. Rinse-off is also facilitated if a ground or polished surface is vertically
orientated. The lower the alloying levels of the stainless steel, the greater the impact of the
surface finish on the resistance to atmospheric corrosion.
Intergranular corrosion

8. This type of corrosion was previously a potential risk for stainless steel because of its high
carbon content (0.05–0.15%). Modern steelmaking methods, and especially the use
of AOD (Argon Oxygen Decarburization), have enabled lower carbon content, meaning that
intergranular corrosion is rarely a problem today.
Nevertheless, it can occur if stainless steels are exposed to temperatures in the range 550–850
°C. Chromium carbides with very high chromium content can then be precipitated along the grain
boundaries, causing the material nearby to be depleted of chromium and thus become less
corrosion resistant. A stainless steel which has been heat-treated in a way that produces such
grain boundary precipitates and adjacent chromium-depleted zones is said to be sensitized.
Sensitization can be a result of welding or of hot forming at an inappropriate temperature.
Sensitization might also increase sensitivity to other forms of corrosion such as pitting, crevice
corrosion, and stress corrosion cracking. Measures against intergranular corrosion by preventing
carbide precipitation include:
• Using low-carbon stainless steel (< 0.05%)
• Using steel that is stabilized with e.g. titanium or niobium (which bind the carbon as
titanium or niobium carbides and prevent chromium carbides from forming)
• Ensuring the shortest possible holding time in the 550–850 °C temperature range
• Solution annealing at 1,000–1,200 °C, at which temperature chromium carbides are
dissolved, followed by rapid cooling in water or air
Galvanic corrosion
Galvanic corrosion can take place if two dissimilar metals are electrically connected and exposed
to a corrosive environment. Galvanic corrosion, is usually not a problem for stainless steels but
can affect other metals in contact with them.
In their passive state, stainless steels are nobler than the majority of other metallic construction
materials in most environments. Galvanic coupling to metals such as carbon steel, galvanized
steel, copper, and brass can therefore increase the corrosion rate of these metals. Galvanic
corrosion between different stainless steel grades is generally not a problem providing that each
grade remains passive in the environment in question.
If the surface of the less noble metal is small relative to the nobler metal, the corrosion rate can
become very high. This is the case if carbon steel bolts are used to fasten stainless steel sheets,
which can lead to severe galvanic corrosion on the bolts. Similarly, defects in coating or paint on
a less noble material can result in a small anodic area and lead to high corrosion rates. It is
therefore preferable to coat or paint the nobler metal in a galvanic couple in order to reduce the
risk of galvanic corrosion.
Problems with galvanic corrosion can often be avoided by proper design and through electrically
insulating dissimilar metals.
High temperature corrosion
In addition to the electrochemically based wet corrosion, stainless steels can suffer high
temperature corrosion and oxidation. This can occur when a metal is exposed to a hot
atmosphere containing oxygen, sulfur, halogens, or other compounds able to react with the
material.

9. As with wet corrosion, stainless steel used for high-temperature applications must rely on the
formation of a protective oxide layer at the surface. The environment must be oxidizing in order
to form the protective layer, which consists of oxides of one or several of the alloying elements.
An environment is often termed oxidizing or reducing as meaning "with respect to iron", since a
so-called reducing atmosphere can oxidize elements such as aluminum and silicon, and often
even chromium.
Some common cases of high-temperature corrosion are described here. More complex cases
such as corrosion in exhaust gases, molten salts, and chloride/fluoride atmospheres are
described in the Outokumpu Corrosion Handbook, which you can request from your local sales
office.
Oxidation
When stainless steels are exposed to an oxidizing environment at elevated temperatures, an
oxide layer is formed on the surface, acting as a barrier between the metal and the gas.
Chromium increases the oxidation resistance of stainless steels by the formation of a chromia
(Cr2O3) scale on the surface.
When the chromium content is increased from 0 to 27%, the maximum service temperature
increases from around 500 °C to 1,150 °C. At temperatures above 1,000 °C, aluminum oxides
are more protective than chromium oxides. The amount of aluminum required for the formation of
a protective layer will, however, make the alloy rather brittle and hence fabrication will be difficult
and costly.
The sensitivity to temperature variations can be reduced by the addition of small amounts of so-
called reactive elements such as yttrium, hafnium, and rare earth metals (REM) such as cerium
and lanthanum. Small REM additions will lead to the formation of a tougher and more adherent
oxide layer, improving cyclic oxidation resistance, erosion-corrosion resistance, and
oxide spallation resistance. These are important properties when the component is subjected to
temperature changes or mechanical deformation.
Although oxides as a rule are beneficial, there are a few elements that tend to form liquid or
gaseous oxides, leading to so-called catastrophic oxidation. Catastrophic oxidation generally
occurs in the 640–950 °C temperature range and for this reason molybdenum, which forms low-
melting-point oxides and oxide-oxide eutectics, should be avoided in material for service at
temperatures above 750 °C.
Sulfidation
Different sulfur compounds are often present as contaminants in flue gases and some process
gases. Chemically, sulfidation is similar to oxidation. However, sulfides have a lower melting
point than the corresponding oxide, so there is risk of forming molten corrosion products.
Nickel in particular can form nickel-sulfur compounds with a low melting point, resulting in a rapid
deterioration of the alloy. In addition, sulfide scales are generally much less protective than the
corresponding oxide scales, leading to a faster corrosion rate. In order to avoid nickel-sulfur
compounds, nickel-free materials, such as ferritic high temperature grades, should be selected in
reducing sulfur environments.
Under conditions where it is difficult to form a protective oxide layer (reducing environment), the
corrosion resistance is considerably lower and is directly dependent on the bulk chemical
composition of the alloys. In these conditions, steels with high chromium content and little or no
nickel are superior.