Applying poor welding practice to marine pipes can severely impact on their ability to withstand corrosion, even when ‘stainless alloys’ are involved, warns metallurgist and consultant Dr. Michael Fletcher.
Welds carried out on most metals without adequate inert gas coverage oxidise.
The effect is even noticeable with many stainless steels. To some, it is an inconvenient feature that can be removed after welding, but this may be difficult and costly, especially if access is restricted.
Unfortunately, any oxidation can result directly in a reduction in corrosion resistance and, in some cases, loss of mechanical strength. This is significant in marine pipe applications where stainless steels along with titanium and nickel alloys are employed principally for their corrosion resistance and mechanical properties.
It will come as a surprise to many that oxygen contents as low as 50ppm (0.005%) in the welding gas can produce discolouration or ‘heat tint’.
The mechanism of corrosion
Stainless steels owe their resistance to corrosion to the formation of a very thin (a few microns) 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.
Four principle mechanisms are involved:
1. Crevice corrosion
Localised corrosion of a metal surface 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 (see Figure 1).
2. 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 (see Figure 2).
Fig 1. Crevice corrosion adjacent to stainless steel pipe weld
Fig 2. Extensive penetration following pitting corrosion in stainless steel pipe
3. Stress corrosion cracking
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.
4. Microbiologically induced corrosion (MIC)
Corrosion promoted or caused by microorganisms, typically 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 heat tints are properly removed before the 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.
Light discolouration can be removed by bright annealing or acid pickling 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.
The alternative to potentially time-consuming, difficult and expensive cleaning of heat tint is to avoid it during the welding process. This can be undertaken by protecting the joint from oxidation by using an inert gas such as argon. The upper side of the joint is protected by the inert gas used in the torch. The underside, known as the underbead, needs separate treatment. This technique is referred to as weld purging.
Seals are inserted on either side of the weld root and inert gas is admitted to displace air in the space between them. Many options for sealing are available but the only totally reliable and sufficiently versatile systems are those based on inflatable seals.
Considerable design effort has been applied by the designers and manufacturers to these solutions over the past decade or so. Currently available systems address the problems of controlled inert gas pressure and flow, the need for easy and rapid deployment and removal to limit overall welding time, thermal resistance and leak-tight access for oxygen monitoring equipment. They also provide a large pipe contact area and therefore excellent and reliable sealing.
Coupled with these advantages comes flexibility to allow access and removal through pipe bends, abrasion resistance and the use of materials that meet food, semiconductor and nuclear compliance standards.
The Argweld systems are examples of fully integrated inflatable purge equipment and can accommodate tube and pipe diameters from 25-2,400mm.
Fig 3. Schematic representation of fully integrated, inflatable weld purge system for use in pipe and tube fabrication. These devices are available for diameters between 25mm and 2,400mm
Oxygen level measurements
Clearly a knowledge of the oxygen level in the purge gas is essential, recognising that a level below 50ppm may be necessary. This can be accommodated with an oxygen monitor and sensitive versions of these, referred to as purge monitors, have been developed specifically for the welding industry. Commercially available weld purge monitors can combine monitoring down to 10ppm with software for data recording, analysis and quality control.
During the last few years many industries, including the marine sector, have revised their weld acceptance standards upwards in striving to manufacture products with better mechanical properties and much improved corrosion resistance. The welding accessory manufacturers have responded by developing equipment capable of meeting these standards but increased use of this equipment is vital in the pursuit of quality. Industry sectors with demanding standards for weld quality need to be aware of the corrosion hazards arising from poor welding practice. Simply assuming that so-called ‘stainless alloys’ are totally resistant to corrosion can be risky. SBI
- Microbiologically influenced corrosion of stainless steel”, Jörg-Thomas Titz, 2nd symposium on orbital welding in high purity industries, La Baule, France
- “Purging while welding”, Thomas Ammann, BOC Australia Document, 2010
- “Effects of purge gas purity and Chelant passivation on the corrosion resistance of orbitally welded 316L stainless steel tubing”, Pharmaceutical Engineering Vol 17 Nos 1 & 2, 1997
- “Purge welding stainless steel for cleanability and corrosion resistance”, foodprocessing.com.au. May 2010
- “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
- “Necessity of removal of heat tints on stainless steel to avoid or minimise corrosion”, G. Netten, Vecon Netherlands, Technical Bulletin, July 2005
- “Effect of the purging gas on properties of 304H GTA welds”, Taban et al, Welding Journal Vol 93, April 2014
- Huntingdon Fusion Techniques Ltd, Burry Port, UK www.huntingdonfusion.com
Author: Michael Fletcher
Dr M J Fletcher is a qualified metallurgist with extensive experience in welding and non-destructive testing.
He works as an independent consultant, providing support to a wide range of manufacturing industry on a global basis.
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