Pipe Fabrication Institute

Engineering Standard
ES-50
Internal Oxidation for Piping Welds
Walter J. Sperko, P.E.
Sperko Engineering Services, Inc.
Greensboro, NC
Sperko@asme.org
 Origins of PFI ES-50
Stainless Steel was developed at Brown Firth
Research Laboratories in Sheffield, England by
Harry Brearley. The first heat of stainless steel
was melted on 13 August 1913; it contained
12.8% chromium. Its primary use was in making
rustproof cutlery.
 Origins of PFI ES-50

Sheffield Stainless
Multi-tool from the
1920s
 Origins of PFI ES-50
• Austenitic Stainless Steel was developed by W. H.
Hatfield at Firth-Vickers in 1924 and was marketed
under the trade name "Staybrite 18/8”
• That’s type 304. . .
 Origins of PFI ES-50
What makes stainless steel “rustproof?”
• Chromium oxide film on the surface. It forms naturally
with exposure to the atmosphere.
• The oxide layer “passivates” the underlying steel
making it resistant to further oxidation and chemical
attack in aggressive environments.
• In mild environments like potable water, surface
contaminants can set up corrosion cells that lead to
pitting attack.
The outlet was
Origins of PFI ES-50
made using
plasma to cut
the hole in the
type 316L
header so that
the branch pipe
could be
welded to the
header.
The cutting Origins of PFI ES-50
dross was not
removed.

Service was
potable water.
 Origins of PFI ES-50
After 2
months in
service, leaks
were noticed.

Examination
showed
pitting around
the cutting
dross.
 Origins of PFI ES-50
Plasma
cutting slag
deposited on
the inside of a
316L potable
water line
caused pitting
 Origins of PFI ES-50
Plasma
cutting slag
deposited on
the inside of a
316L potable
water line
caused pitting
that led to
leaks at each
branch
location.
 Origins of PFI ES-50
In mild environments like potable water, surface
contaminants can set up corrosion cells in stainless
steel that can lead to pitting attack and leaks.
• Internal surfaces need to be clean and free of
contamination for optimum performance in
environments where the fluid being handled is
electrically conductive.
• Pitting attack is not a concern when the fluid being
handled is nonconductive.
Root Origins of PFI ES-50
pass in
carbon
steel.

No gas
backing.
Weld
Origins of PFI ES-50
made
using
GTAW
open
root
and no
gas
backing
 Origins of PFI ES-50
• When welding carbon steel piping from the outside
using GTAW with the root surface exposed to the air,
the oxides that form on the weld pool surface are iron,
silicon and manganese oxides. These oxides melt at
around 2100 to 2400°F. Carbon steel melts at
around 2750°F.
• The oxides that form on a carbon steel weld pool that
is exposed to the atmosphere float on the surface as
liquids.
 Origins of PFI ES-50
• When welding stainless steel piping from the outside
using GTAW with the root surface exposed to the air,
the oxides that form on the weld pool surface are iron
and chromium oxides. These oxides melt at around
2100 and 4400°F respectively. Stainless steel melts
at around 2650°F.
• The oxides that form on a stainless steel weld pool
that is exposed to the atmosphere form a semi-solid
slush on the weld pool surface.
Weld
Origins of PFI ES-50
made
using
GTAW
open
root
without
gas
backing
 Origins of PFI ES-50
Weld made
using argon +
5% hydrogen
gas backing, 1
ppm oxygen
 Origins of PFI ES-50
This surface
can be
ground
smooth and
clean and it
will perform
just as well
as the
previous
slide
condition.
 Origins of PFI ES-50
You can’t always get to the root side to clean it up, so
you have to protect the root side weld metal from the air
sufficiently that you can be inspect it for penetration and
freedom from cracking or other flaws.
Some have used the shielding gas from the torch
flowing through the root opening and (somewhat)
protecting the root surface from the oxygen in the air.
Root pass
made on 304L
stainless using
GMAW
waveform-
controlled
welding, open
root, no gas
backing.
This weld is
inspectable.
 Origins of PFI ES-50
The weld surface and adjoining base metal are heavily
oxidized. This may or may not be suitable for an
application. If the application is potable water, this much
oxidation will probably lead to pitting attack.

You can do the same with GTAW but not reliably unless
the process is automated.
 Origins of PFI ES-50
The other approach to protecting the root surface from
oxidation is to replace the air inside the pipe with inert
gas, usually argon. This is commonly referred to as
“purging.”
The techniques for doing that are described well in AWS
D10.11, Guide for Root Pass Welding of Pipe Without
Backing. It suggests that 5 volume changes is sufficient
and provides the following chart.
At 50 CFH, the time
required to purge
NPS 12 pipe is
about 5-1/2 minutes
per foot of pipe
length. That would
be about 3-1/2
hours for a 40 foot
length of pipe.
 Origins of PFI ES-50
Following D10.11 does not guarantee that the weld
surface will be free of oxides.
• Purge gas flowing at high velocity through a gas-
permeable (i.e. rubber) hose or a fitting that is not
tight will aspirate air into the gas stream.
• Moisture in the pipe will cause discoloration. Moisture
can come from dust that has accumulated in the pipe.
• Surface contaminants like cutting fluids and oil from
air-powered grinders will oxidize on the surface.
 Origins of PFI ES-50
If a contractor follows AWS D10.11 to satisfy a
specification requirement that stainless steel piping be
purged, he will have met the specification simply by
following D10.11. Yet the weld surface may be heavily
oxidized and not suitable for the intended application.
This weld was
purged. If all
the specification
requires is that
the pipe be
purged, this
meets that
requirement.
 Origins of PFI ES-50
AWS D18.1 and ASME Bioprocessing Engineering
addressed the issue of “discoloration” of stainless steel
decades ago by publishing pictures showing various
levels of discoloration.
AWS D18.1
 Origins of PFI ES-50
For a piping fabricator or erector installing industrial
piping, these standards presented three issues:
1. The samples showing the levels of discoloration were
made on tube that had a polished ID surface rather
than the pickled surface found in industrial piping.
2. The standards tied the discoloration to the ppm oxygen
level in the purge gas.
3. There was no guidance on selection of an acceptable
level of discoloration for a variety of service conditions
 Development of PFI ES-50
We used ASTM A-270 Type 304L stainless steel tube 3 inch
OD with a wall thickness of 0.065 with a pickled mill finish.
To ensure uniformity of weld appearance, an orbital GTAW
welding head was attached to the pipe and set up so that
the current would melt through and create a liquid surface
as the head proceeded around the pipe. While we started
with separate argon and oxygen supplies, we found that
controlling the oxygen at the PPM level was reliable using
argon and an argon 2% oxygen.
The oxygen concentration low was 10 ppm high was 2%
 Development of PFI ES-50
Rather than provide a “ppm” criteria, ES-50 identifies the
discoloration by number and asks the engineer to select and
specify the discoloration that is acceptable for his
application by number and leave it up to the fabricator or
contractor to figure out the most economical way to meet
the selected discoloration.
1 2 3 4 5 6 7 8 9 10 11
 Development of PFI ES-50
The standard also points out that, to get less discoloration,
one has to purge for a longer time, so purchaser should
expect higher cost of fabrication and installation for less-
discolored surfaces.
 Technical Background of PFI ES-50
What causes the change in colors?
The thickness of the oxide layer. Incident light reflects off
the metal surface. When the oxide is thin, all the light is
reflected so stainless steel looks white. When the oxide film
gets thicker, light is absorbed by the oxide layer except for
yellow; as the oxide thickness increases, the yellow
becomes darker, then it turns successively orange, brown-
red, blue, purple, brown, grey until, ultimately, it forms an
opaque grey/brown film.
 Technical Background of PFI ES-50
How do the oxide layers affect corrosion resistance?
The initial oxides form as iron diffuses to the surface forming
iron oxides of hematite and magnetite. These oxides reflect
yellow light. Chromium remains below the surface, ready to
maintain the chromium oxide barrier that keeps stainless
steel stainless. This is true up to about color number 6.
 Technical Background of PFI ES-50
Eventually chromium diffuses to the surface and begins to
oxidize. The layers turn orange, then brown-red then blue.
This lowers the chromium concentration in the layer beneath
the oxide layer.
As more chromium diffuses to the surface, the oxide turns
purple, then brown then grey, and a chromium-depleted
layer forms beneath the oxide layer.
 Technical Background of PFI ES-50
• As the oxide layer thickens, it cracks, allowing corroding
media to attack the diminished substrate beneath the
oxide layer causing pitting in mild environments.
• Strong acids, on the other hand, will dissolve these oxide
layers uniformly, restoring the stainless steel to its original
corrosion resistance.
• Nonconductive fluids (hydrocarbon fuels and similar), ,
generally do not cause corrosion unless they are
contaminated by sulfides, chlorides and similar.
 Implementing PFI ES-50
Engineers using PFI ES-50 need to evaluate the service
conditions. There are many reasons for selecting stainless
steel:
• Corrosion resistance (e.g. when handling strong acids)
• Corrosion resistance (to keep the product from
contamination)
• Toughness for cryogenic applications
 Implementing PFI ES-50
Engineers need to look at previous use of stainless steel in
their application to see what any precedent is. If some level
of discoloration has been found to be work reliably, select
the appropriate color number and specify that PFI ES-50 be
followed and that color number.
Let the contractor or fabricator figure out how to get there.
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