Influenced Corrosion Attacks Welds Before Component Is Put Into Service
About 200 feet
of 2-inch, schedule 10, Type 304 stainless steel transfer piping was installed to
replace a bulk handling operation. The product, which could produce nitrous
oxide as a breakdown product, was one that had been routinely handled at the
plant for many years. Within two weeks of when the pipeline began carrying
the product, the line was observed to be leaking. The insulation was
stripped from 20 feet of the horizontal run on either side of the suspected
leaking area and the pipe was visually inspected. When no readily apparent
leak sources could be identified, the welds attaching the 90º elbows to the
straight lengths were dye penetrant inspected. The dye penetrant inspection
found two pinhole sized indications in two circumferential welds.
characterize the failure, the leaking elbow, a suspect elbow about four feet
downstream of the leaking elbow and a flanged end from one straight length
were cut from the line.
The elbow that
contained the leaking welds was longitudinally split to reveal the ID (inner
surface) of the pipe, Figure 1. In the area of the weld perforations there
were reddish orange to black, circular discolorations and/or mounds of
corrosion product on the ID surface of the welds. Away from the
perforations, the circumferential welds were discolored by a “bleeding rust”
deposit. A blue/black band was evident on both sides of these welds, about
¼-inch from the weld bead. This band was typical of oxidation that takes
place when stainless steel is welded without a protective atmosphere. The
weld area in the pipe ID showed a lack of penetration in addition to the
bleeding rust colored deposits.
Figure 1. MIG wire from longitudinal
Figure 2. Cross section of a pit at the
weld interface showing corrosion under the surface.
elbows and the straight pipe section contained longitudinal weld seams. The
straight pipe section contained a longitudinal weld seam with an unusual
“feature.” A length of wire-like metal, which extended out of the weld bead, apparently had remained attached to the weld seam throughout the
construction and operation of the transfer line, Figure 1. The wire was rectangular
in cross-section and appeared to have been a feed wire for an MIG type
were cold formed from straight lengths of pipe and not cast. This type of
construction, if executed with consideration for good craftsmanship
standards, was allowed by the plant pipe code and considered acceptable for
the intended service.
There was a pinhole on the OD (outside surface) of one of the leaking welds. The pinhole was located on the edge of the weld, close to
the pipe base metal. A pinhole on the ID of the same weld was not directly
opposite the OD pinhole. Instead, it was slightly away from the OD pinhole
and on the edge of the weld. The locations of the two pinholes suggested
that the leak path was slanted through the weld.
section of the weld in the area of the leak was prepared for metallographic
examination. There was severe corrosion under the ID surface, Figure 2.
The corrosion was predominantly concentrated in the weld metal. The corrodent dissolved the austenitic matrix of the weld and left behind a
network of ferrite, which became embedded in the corrosion product.
Because of the
relatively short time between start up and the detection of the leak (two
weeks), the plant was concerned that the materials of construction were not
to specification. To confirm that the pipeline complied with ASTM A 312 for
Type 304 stainless steel pipe, a sample of the pipe was analyzed for its
chemical composition, another was subjected to tests to determine its
mechanical properties. The material was found to be in compliance to the
How Did The Pipe Fail?
several issues to sort through for this failure analysis. A review of the
plant pipe code found that the pipeline was not fabricated to the welding
standard. The oxidations around the welds and the lack of penetration
provided sufficient evidence that the ID of the pipe was not protected with
an inert atmosphere during welding.
appeared to have been the only deviation (though a significant one) from the
pipe code. The pipe contained the unusual feature
of weld wire protruding from the ID side of the longitudinal weld. However,
this longitudinal weld was not attacked in any fashion in the areas
available for inspections. In addition, the mechanical and chemical tests
showed that the pipe met the property requirements of the ASTM standard A
132 referenced in the pipe code.
failed as a result of selective pitting attack of two circumferential
welds. There was evidence of corrosion by ferric chloride, which
selectively attacks the austenitic phase of welds in the 300 series
stainless steels. Ferric chloride is an aggressive medium which, under the
correct conditions, can produce the corrosion rates sufficient to have
penetrated the welds within a month or less. However, ferric chloride at
the concentrations sufficient to cause the observed damage, was not part of
the product process stream. To have caused this attack, a concentrator
mechanism was necessary.
For Microbiologically Influenced Corrosion (MIC)
literally hundreds of different microbe types that have been found to be
capable of thriving in what are considered to be extraordinarily hostile
environments – but they have to become established first. The
microbes, some of which can be found in well water, tend to establish
themselves at weld sites, especially at rough surfaces and crevices.
They begin by producing a protective mound, a biomass, that resists common
biocides. Their metabolic activity effectively pumps selected
chemicals from the surrounding fluid to the surface of the metal
below the biomass.
chloride is sometimes a by-product of this activity. Ferric chloride
aggressively attacks stainless steel, particularly when the steel is covered
with a biomass that excludes the oxygen that stainless steel normally uses
to form a protective oxide film. In this case the steel becomes no more
corrosion resistant than carbon steel. Furthermore, considerable amounts of
ferric chloride are released into the surrounding water. This produces the
bleeding rust often associated with MIC, which was present on this piping.
was suspected because the speed of the attack was too high to have been
caused by the plant process, even by the possible formation of nitric acid.
The two weeks that the product was in the line prior to the discovery of the
leak was not a sufficient exposure time. MIC, on the other hand, is known
to be capable of penetrating stainless steel within a period of weeks or
with plant personnel revealed that the line had been hydrotested with well
water nine months prior to process startup. [It has been well documented
that microbes, which have been linked with MIC, exist in this type of water.]
Residual water, at low points in the pipeline, would have provided an
adequate supply of nutrients for the microbes. Since efforts were not made
to completely dry the lines, the lines probably sat with water in the low
points for nine months. This was enough time for the MIC to penetrate the
welds prior to the introduction of the product.
The nature of MIC and the
evidence at hand led us to believe that the pipeline was in good condition
overall – except at the circumferential welds. These welds were
suspect and subsequently examined by radiography. The welds that
showed signs of pitting attack were cut out and repaired. The line was
placed back in service with no further leaks for over two years.
welds could have pitted, the plant had initially planned to replace them all
to prevent further “weld corrosion.” The radiographic inspection determined
which had been attacked and which had not. Only the pitted welds were
replaced and the line was returned to service.
The plant has
since changed hydrotest procedures – they use steam condensate rather than
well water for the testing and they either delay the hydrotest until shortly
before the equipment is placed into service or dry the line thoroughly
immediately after testing.