The corrosion resistance of stainless steels can be significantly affected by the welding operation. We therefore would like a way to predict the corrosion resistance, and measure it, if possible. In particular, the pitting resistance of the stainless steels are important. The Pitting Resistance Equivalent Number (PREN) is a way of calculating the pitting resistance of different alloys and welds. The ASTM G48 corrosion test is a more practical way of assessing the pitting resistance of higher alloyed stainless steels.
the pitting resistance, we will also look at other factors that can affect the
corrosion resistance of stainless steels when we weld them.
In relation to the corrosion resistance of stainless steels, it
has been noted that Molybdenum tends to increase the pitting resistance. This is why type 316 stainless steel has a better pitting
resistance than type 304 stainless steel. Type 316 contains around 2.5%
Molybdenum whereas type 304 does not contain Molybdenum.
Molybdenum, (Mo) there are other alloying elements that also tend to have this
effect of improving the corrosion resistance of stainless steels by increasing the pitting resistance. The pitting
resistance equivalent number is a way of taking the percentages of the alloying
elements that improve pitting resistance on stainless steels, and expressing it
in a number that can be compared to other alloys for estimating the pitting corrosion resistance of stainless steels.
pitting resistance equivalent number (PREN) is calculated using the following
= %Cr + 3.3 x %Mo + 16 x %N
this equation, it is clear that Nitrogen (N) has a very large impact in
increasing the pitting corrosion resistance of stainless steels. (It is also a very
strong Austenite former, but that is another story for another day!) The
problem is that Nitrogen is difficult to get into a solid solution in high
percentages, and when we melt the material during welding, the Nitrogen will
tend to escape out of the molten metal. For this reason, welding filler metals
will tend to include other alloying elements to compensate for the reduced
Nitrogen content of welds.
let us compare the differences in PREN between typical 304L, 316L and type 2205 Duplex stainless
the pitting resistance equivalent numbers given above, it becomes clear that
316L is more resistant to pitting corrosion than 304L, but that the most pitting
resistant of the three materials being compared is the type 2205 Duplex
Stainless Steel. (DSS)
note that often client specifications, or material specifications may require a
minimum PREN for materials in certain services such as sea water service.
PREN discussed above is based purely on the chemical composition of the
material. There are however other factors that affect the pitting corrosion resistance of stainless steels,
such as the condition of the surface oxide layer on the stainless steel, or the
exact form that the alloying elements take within the material, or the presence
of localized areas of alloy segregation or loss. For this reason, some codes
and client specifications require a practical test to be done on materials, and
especially weld test pieces, to prove that they have the desired pitting
resistance. Such a test is described in testing standard ASTM G48.
G48 describes 6 different “methods”, which are geared to different types of
alloys and also the type of information required. From a welding perspective,
“method A” is the most widely used test, so we will discuss this method in a
little more detail. Method A is simply called the “Ferric chloride pitting
ASTM G48, Method A, a coupon of the material that is to be tested is placed in
a specific ferric chloride solution and heated to a predetermined temperature
and held for a predetermined time at that temperature. Typical temperatures are
25°C for duplex stainless steels (DSS) and 40°C for super duplex stainless
steels. (SDSS) A typical time for the test is 24 hours, although it can be done
an example, the widely used subsea pipeline code DNV-OS-F101 requires 25 Chrome
duplex stainless steels (SDSS) to be tested at 40°C for 24 hours.
acceptance criteria vary, but generally it is a pass / fail type test based on:
note that some standards or specifications only require the absence of pitting,
without specifying a maximum mass loss.
note that ASTM G48 corrosion testing is generally not a standard requirement of
welding codes such as ASME IX. It is usually done to meet the requirements of a
specific material specification or client standard.
is important to note that there are many factors that can potentially affect
the outcome of the ASTM G48 test. Factors such as the heat input during welding
and the shielding gas practices are directly relevant to the welding procedure.
Unfortunately, factors such as how the welded coupon is cleaned after welding,
and how the laboratory prepares the test samples, can also have significant
impacts to the results. In this regard I recommend that the cleaning practices
should be such as to reduce the surface roughness and ensure a sound,
continuous oxide layer on the surface. A typical example is that an overzealous
use of wire brushes will result in significant scratches on the material
surface, which will typically increase the mass loss found during the test. It
is best to use chemical cleaning techniques to remove any unwanted oxides from
the surface and leave a sound oxide layer on the affected areas.
ASTM G48 standard is really geared toward the testing of base metals, not
welds. As such it recommends that samples are prepared by “polishing” with 120
grit abrasive paper. This is not really practical for welds that have uneven
and curved surfaces, hence the typical use of alternative surface preparation
methods such as stainless steel wire brushes and chemical cleaning. Such
alternative surface treatments are allowed by the ASTM G48 standard.
the temperature on a stainless steel increases, the speed at which atoms can
diffuse through the metal’s structure increases. At the same time, the speed of
any chemical bonding / reactions also increase. These effects can lead to
problems with the corrosion resistance of stainless steels when we weld them. One of the problems is sensitization.
sensitization, any carbon within the stainless steel will tend to react with
the Chromium to form chrome carbides. As a general rule, the carbon will be
most concentrated within the grain boundaries of the stainless steel. When the
carbon then forms the chrome carbides, a lot of chrome is removed from the
solid solution just next to the grain boundaries. These grain boundary areas
are then depleted of the main alloying element that gives stainless steels
their anti-corrosion properties. A stainless steel that has been affected in
this way is said to be “sensitized”.
a sensitized stainless steel is exposed to a corrosive environment, there
will be a preferential corrosive attack along the grain boundaries. This type
of attack can occur very rapidly, and can result in catastrophic failure of the
affected component or structure. In short, sensitization is very bad for the corrosion resistance of stainless steels.
temperature at which this sensitization occurs in stainless steels is generally
accepted to be between 500°C and 800°C, although for some alloys it can be as low
as 420°C and as high as 850°C. Below the sensitizing temperature range the
diffusion of the atoms in the material, along with their reactivity is not
enough to result in the sensitization to occur. Above the sensitization temperature, the
chrome carbides tend to dissolve back into the base metal, reversing the negative effect on the corrosion resistance of stainless steels.
therefore becomes clear that when welding a stainless steel, there will always
be a band along the weld edges that will be exposed to the sensitizing
temperature range. The longer the material is exposed to the sensitizing
temperature, the greater the amount of sensitization. When a welded component
displays reduced corrosion resistance of stainless steels along the sides of the weld, due to this sensitization, it
is termed “weld decay”.
earlier days this reduced corrosion resistance of stainless steels due to sensitization was a significant problem, but it has
been largely eliminated by a number of alloy refinements, such as:
The corrosion resistance of stainless steels that have been sensitized, can be returned by taking the temperature of the entire component high
enough to return the chromium into solution and then cooling rapidly. Typically
this will be a temperature in excess of 1000°C. Unfortunately, this operation
is often not practical. Once large structures have experienced sensitization,
not much can be done other than replacing the affected material.
are standard tests to find out if the corrosion resistance of stainless steels have been reduced due to sensitization. Sometimes
these are required by client standards when higher carbon grades of stainless
steel needs to be able to withstand corrosive environments. One standard to
which such testing is typically done is ASTM A262.
way in which welding can affect the corrosion resistance of stainless steels is
by introducing conditions that can lead to stress corrosion cracking. Stress
corrosion cracking occurs in many different materials, in the presence of
different corrosive media, but it is especially prevalent in austenitic
stainless steels in the presence of chlorides.
occurs when the following three conditions are met simultaneously:
the classic SCC of austenitic stainless steels, the factors come together as
to this very prevalent SCC mechanism in the presence of chlorides, this
situation has been given its own name. It is called Chloride Stress Corrosion Cracking (CSCC) of austenitic
stainless steels, and is one of the main factors to consider when evaluating the corrosion resistance of stainless steels for any application.
the component is small enough, then the SCC corrosion resistance of stainless steels can be restored by relieving the welding related stresses. This can be achieved by subjecting the component to a quench anneal
heat treatment following welding. For many components this treatment is however not practical. This means that it is almost impossible to
prevent CSCC of welded austenitic stainless steels under the operating conditions
mentioned. Under those conditions, other more resistant alloys need to be used.
Typically duplex stainless steels, ferritic stainless steels, super austenitic
stainless steels or nickel or copper based alloys.
corrosion resistance of stainless steels is reliant on a surface layer of a
very adherent chrome oxide. This chrome oxide layer forms all by itself in the
presence of oxygen. When the surface layer is damaged, it generally tends to
re-form rather rapidly in the presence of oxygen, restoring the corrosion
resistance. The big secret is that there needs to be oxygen present to re-form
the chrome oxide layer. Under conditions where the material is starved of
oxygen, rapid localized corrosion is possible.
oxygen starved conditions are typically found locally on the surface of stainless steels under the following
is therefore very clear that any operations that result in “heat tinting” of
the material, such as welding, can severely reduce the corrosion resistance. In
addition, any operation that can introduce iron contamination into the surface
oxide layer can also reduce the corrosion resistance. This reduction of the corrosion resistance of stainless steels can easily happen if
tools contaminated with iron is used on the stainless steels.
examples of this are:
best approach is to try to eliminate any source of surface contamination of the stainless
steel. Where some contamination or heat tinting has occurred, or is inevitable
(e.g. you do not have the necessary stainless steel tooling) then performing a
pickling and passivation treatment on the stainless steel’s surface can restore
the corrosion resistance of stainless steels.