Welding Knowledge - Part 4

Welding can reduce the resistance to corrosion

Most industries using stainless steels do so because of their resistance to corrosion. Industry sectors such as dairy, food and pharmaceutical manufacturers and the semi-conductor producers are major users since the end products must be contamination free and the presence of any corrosion products can have serious consequences.

The Mechanism of Corrosion

Stainless steels and other alloys containing chromium 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.

Four principle mechanisms are involved;

1. Crevice corrosion
2. Pitting corrosion
3. Stress corrosion cracking
4. Microbiologically induced corrosion (MIC)

Crevice Corrosion

Localised corrosion of a metal surface attributable to the 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.

Fig 2. Extensive penetration following pitting corrosion in stainless steel pipe

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.

Fig 3 Stress corrosion cracking in welded joint

Microbiologically induced corrosion

Corrosion promoted or caused by micro-organisms, typically in industries related to food and beverage processing. It is usually referred to by the acronym ’MIC’ and is common in welded sections [refs 1-2].

Fig 4. Extreme example of MIC in stainless steel pipe.

Weld Decay of Stainless Steel

Reduction in the protective chromium content can lead to a phenomenon known colloquially as ‘weld decay’. During welding of stainless steels, local sensitized zones (i.e., regions susceptible to corrosion) often develop. Sensitization is due to the formation of chromium carbide along grain boundaries, resulting in depletion of chromium in the region adjacent to the grain boundary.

This chromium depletion produces very localised galvanic cells.

If this depletion reduces the chromium content below the necessary 12 % that is required to maintain a protective passive film, the region will become sensitised to corrosion, resulting in intergranular attack.

Fig 5. Damage caused by weld decay in the heat affected zone.

Reduction in Mechanical Strength

Another consequence of chromium loss during welding is the effect on mechanical properties. In the chromium/molybdenum/vanadium materials for example, developed for their high temperature creep resistance, enhanced hardenability, wear resistance, impact resistance and machinability, any reduction in chromium content can affect these properties

Welded joints are common. Well made, they offer a smooth transition from one section to another, high strength and are cosmetically attractive. However, the welding process itself can lead to significant loss of corrosion resistance in the joint area and a reduction in mechanical properties unless precautions are taken to prevent oxidation.

The Welding Process

Welds carried out on most metals with inadequate inert gas coverage oxidise. The effect is even noticeable with many stainless steels. To some, the discolouration due to oxidation is an inconvenient feature that can be removed after welding, but this may be difficult and, in any case, 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 dairy, food, pharmaceutical and semiconductor pipe applications where stainless steels are employed principally for their resistance to corrosion.

The inert gas used routinely during fusion welding to protect against oxidation needs careful consideration. It will come as a surprise to many that oxygen contents as low as 50 ppm (0.005%) in this protective gas is rarely totally effective.

Fig 6a. The result of unprotected underbead in welded austenitic stainless steel.										Fig 6b. To ensure no discolouration occurs the oxygen content needs to be reduced to 20 ppm (0.002%).

Effective protection is thus essential and this is achieved by surrounding the joint with an inert gas such as argon or helium. The gas shield associated with a GTAW torch will protect the upper surface of the joint, but the inside of pipes and tubes needs special attention. To meet the need for total internal protection, called weld purging, dedicated equipment has evolved over the past 25 years.

Pipe and Tube purging

Systems for weld root protection are based on sealing the inside of a pipe on either side of the weld zone then displacing air with an inert gas. The seals must be reliable and leak tight, effective and easy to insert and remove. The inert gas must be of a quality commensurate with the need to protect the molten metal. Gas flow should be laminar to maintain a high level of protection and pressure controlled to offer adequate coverage but without expelling molten metal from the joint.

Early, and with hindsight, primitive systems involved the use of paper, card, wood and polystyrene discs. Often these provided at best poor sealing and on occasions burst into flames – satisfactory removal after welding presented challenges. Ensuring that all oxygen had been removed during purging was left entirely to the skill and experience of the operator.

There were regular incidences where protection proved to be inadequate and the joint had to be re-made with consequent expense and loss of time. It comes as a surprise that these practices are still used, even by some prominent fabrication companies across the world.

The most effective solutions are based on the use of inflatable dams and fully integrated systems are now available covering pipe and tube diameters between 25 and 2400 mm.

Fig 7. Pipe and tube purging concept.

If this depletion reduces the chromium content below the necessary 12 % that is required to maintain a protective passive film, the region will become sensitised to corrosion, resulting in intergranular attack.

Fig 8. Inflatable PurgElite® system from the Argweld ® range for tubes between 25 and 610 mm diameter. Diameters up to 2235 mm can be accommodated using QuickPurge® systems.

Residual oxygen measurement instruments

Any effective weld purge process needs to be supported by suitable oxygen detecting equipment. Weld purge monitors have now been developed to meet the need for reliable, robust and sensitive measurements. For reactive and refractory alloy welding these must be capable of accurately measuring oxygen levels down to 10 ppm.

As an example, the PurgEye® 600 instrument manufactured by Huntingdon Fusion Techniques reads down to 10 ppm with extreme accuracy and has a display range from 1,000 to 10 ppm.

Fig 9. The Argweld® PurgEye® 600 monitor, has a USB connection and data logging capability allowing the operator ease of data transfer without the need for a computer connection.

The entire Argweld product range is supported by an extensive library of publications including Technical Notes, White Papers, Conference Proceedings and peer-reviewed International Articles. These are available on-line by application to Huntingdon Fusion Techniques Ltd [ref 9].

Conclusion

Resistance to corrosion is clearly a significant issue in applications where pipework cleanliness is crucial in ensuring product quality. The use of globally well-proven accessories such as purging equipment and monitoring instruments is vital if loss of chromium during welding is to be prevented.

References

1. Accelerated corrosion of 2304 duplex stainless steel. Huabing Li et al. Scientific Reports volume 6, Article number: 20190 (2016)
2. 6th International Symposium on Applied Microbiology and Molecular Biology in Oil Systems. June 2017. San Diego, California
3. Microbiologically influenced corrosion of stainless steel, 2nd symposium on orbital welding in high purity industries, La Baule, France
4. 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
5. Considerations for Orbital Welding of Corrosion Resistant Materials to the ASME Bioprocessing Equipment Standard. Stainless Steel America conference 2008
6. Heat Tint Poses Corrosion Hazard in Stainless Steel. Welding Journal December 2014
7. ASM International. Corrosion in Weldments. 2006
8. Effect of microalloying elements on austenite grain growth in Nb–Ti and Nb–V steels. Karmakar A et al. Mater Sci Technol. 2014,30
9. www.huntingdonfusion.com

 By Michael Fletcher, PhD. Metallurgy

Leeds University
Delta Consultants  

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