Ron A Sewell, Huntingdon Fusion Techniques Ltd, UK discusses the corrosion problems to be wary of when welding steel pipes.

1. I got 99 problems but
welding ain’t one
Ron A. Sewell, Huntingdon Fusion Techniques Ltd, UK, discusses the corrosion
problems to be wary of when welding steel pipes.
M
anufacturers and users of pipelines in the oil and
gas sectors have long believed that so-called
‘stainless steel’, is indeed stainless. The fact is
that most of these materials not only discolour
but also can corrode when they come into contact with some
endemic media. The black corrosion spots on cutlery after
passing through a dishwasher cycle are an example, but a more
dramatic illustration is salt-water corrosion in pipelines.
Figure 1. Salt water corosion of austenitic stainless steel pipe.
1

2. One area of production receiving little attention is during
pipe and tube fabrication where welding is widely used. Unless
strict welding schedules are not adhered to, however, not only
will discolouration (heat tint) take place but also corrosion
resistance can be significantly reduced.
The mechanism of corrosion
Stainless steels owe their resistance to corrosion to the
formation of a very thin (10-5
mm), transparent surface layer
of chromium oxide. This provides a passive film that acts as
a barrier to penetration by an invasive environment. When
heated to a high temperature in the presence of oxygen, this
film increases in thickness until it becomes visible – the colour
becomes darker with increasing film thickness.
At a critical film thickness, the film becomes unstable and
begins to break down. The fractured zones created offer sites
for localised corrosion, where four principle mechanisms are
involved:
)) Crevice corrosion.
)) Pitting corrosion.
)) Stress corrosion cracking.
)) Microbiologically induced corrosion (MIC).
Crevice corrosion
This is a localised corrosion of a metal surface, which is
attributable to proximity of another metal such as a weld. It is
a locally accelerated type of corrosion and is one of the major
corrosion hazards in stainless steels.
Pitting corrosion
This produces attacks in the form of spots or pits, and takes
place at points where the passive layer might be weakened:
it occurs in stainless steels where oxidation has reduced the
passivity. Once the attack has started, the material can be
completely penetrated within a short time.
SCC
This is characterised by cracks propagating either through or
along grain boundaries. It results from the combined action of
tensile stresses in the material and the presence of a corrosive
medium. It can be induced in some stainless steels by adverse
heat treatments, such as those occurring in weld heat affected
zones.
MIC
This type of corrosion is either promoted or caused by micro-
organisms, typically occurring in industries related to food,
beverage and chemical processing. It is usually referred to by
the acronym ‘MIC’ and is common in welded sections.
To avoid these forms of corrosion, it is essential that the
heat tints are properly removed before the stainless steel
equipment or piping is exposed to aggressive or aqueous
environments. The alternative is to prevent heat tinting during
the welding process by using an inert environment to protect
the surface.
Removal of heat tint
Bright annealing or acid pickling can remove light
discolouration, but heavier deposits may require machining
such as grinding and polishing. Removal clearly requires access
to the area in question, not only for treatment, but also for
debris removal. Even when access is available, none of these
treatments are easy and most can be very expensive.
Whilst it is not too difficult to protect the outside
surface of a weld by using an inert gas as coverage, preventing
oxidation and loss of corrosion resistance on the inside is
often overlooked. The technique of inside protection is known
as ‘weld purging’ and uses inert gas to flush out air and thus
oxygen before and during welding.
Weld purging techniques
Equipment has been developed over the past decade to make
purging much easier. Currently available systems are robust
and suitable for multi-use applications. They can be supplied
to cover the size range between 25 - 2400 mm dia.
Figure 2. Extensive penetration following pitting corrosion in
stainless steel pipe.
Figure 3. Example of PurgElite range of fully integrated
systems covering the 25 - 250 mm pipe range.
2 World pipelines / November 2015
during welding.

3. The most effective devices are those based on connected
inflatable dams. These are programmed to control gas flow
and pressure during inflation and purging, and once placed in
position, require little more input from an operator. The dams
are fabricated using advanced engineering polymers and are
thus suitable for use with oil and gas pipelines and processing
plants where the elimination of contamination is essential.
Purge gas oxygen content can be controlled by using an
oygen monitor. These instruments not only measure oxygen
levels, but will inhibit welding if the level is above that set by
the operator. Recording and analysing software consequently
provides information for quality control puroses.
Conclusion
Even very low oxygen concentrations in weld gases can give
rise to discolouration and corrosion. Controlling oxygen level
in purge gas can be effected simply and efficiently using
contemporary integrated purge systems.
References
1. Eastwood et al. 1993, welding stainless steel to meet hygienic
requirements, document 9, European Hygienic Engineering
Design Group (EHEDG).
2. Microbiologically influenced corrosion of stainless steel
Jörg-Thomas Titz 2nd
symposium on orbital welding in high
purity industries, La Baule, France.
3. Effects of purge gas purity and Chelan passivation on the
corrosion resistance of orbitally welded 316L stainless steel
tubing Pharmaceutical Engineering, Vol 17, No 1 & 2, 1997.
4. Considerations for Orbital Welding of Corrosion Resistant
Materials to the ASME Bioprocessing Equipment Standard
Dr B. K. Henon, Arc Machines, Inc., Stainless Steel America
conference, 2008.
5. Thomas Ammann, purging while welding, BOC Australia,
document 2010.
6. G. Netten , necessity of removal of heat tints on stainless
steel to avoid or minimise corrosion, Vecon Netherlands
Technical Bulletin, July 2005.
7. Effect of the purging gas on properties of 304H GTA welds,
Taban et al. Welding Journal, April 2014, Vol 93.
NOVEMBER 2015 / World pipelines 3
oxygen
purposes
QuickPurge®
Inflatable Pipe
purge systems for
sizes from 8" up to
96" dia
Weld Purge
Monitor® gas
detection and
analysing
instrument