Preventing Corrosion in Stainless Steel Welding for Industrial Applications

Despite their reputation for durability, stainless steels are vulnerable to corrosion, especially during and after welding, when improper techniques can degrade their protective chromium oxide film.

The study highlights the importance of weld purging—a process that displaces atmospheric gases in the weld zone with an inert gas, typically argon—to prevent oxidation during welding. Various purging methods are evaluated, from simple, homemade solutions to advanced inflatable seals, with the latter being the most effective for ensuring weld quality and preventing corrosion.

Advanced tools like Water Soluble Weld Purge Film™ and PurgEye® Oxygen Monitoring Instruments are recommended for maintaining low oxygen levels, which are crucial for preserving the corrosion resistance of stainless steel welds. The case study also discusses applications beyond pipework, emphasizing the need for inert gas protection in welding flat or contoured components used in various high-stakes industries.

By focusing on preventative measures rather than post-weld cleaning, the study concludes that effective weld purging is essential for achieving high-quality, corrosion-resistant welds, leading to long-term cost savings and enhanced durability in critical industrial applications.

Products Mentioned:

• Water Soluble Weld Purge Film™
• Argweld Weld Trailing Shields®
• Inflatable Weld Purge Systems
• PurgEye® Oxygen Monitoring Instruments

Examples of Corrosion in Stainless Steel

Stainless steels are prized for their corrosion resistance, making them a material of choice in industries where contamination could have severe consequences, such as food, beverage, pharmaceutical, semiconductor, petrochemical, and nuclear sectors. However, the term "stainless" is somewhat misleading, as stainless steel can still suffer from corrosion under certain conditions, particularly during and after welding.

Even everyday items like stainless steel cutlery can develop dark spots after a cycle in the dishwasher, a mild form of corrosion caused by exposure to salts. A more severe example is the corrosion in stainless steel pipework exposed to seawater rich in chlorine-containing salts. Despite their name, most stainless steels are not resistant to corrosion from chlorides and can degrade if not adequately protected during welding.

• Domestic Cutlery: Small dark spots after washing.
• Industrial Pipework: Heavily corroded after exposure to seawater.

These examples illustrate that stainless steel is resistant to many forms of corrosion but is not immune. If proper precautions are not taken, the welding process itself can exacerbate these issues by reducing the steel’s natural corrosion resistance.

The Impact of Welding on Corrosion Resistance

The key to stainless steel’s corrosion resistance is a thin but highly durable chromium oxide film that forms on its surface. This film can self-repair after minor damage, like scratching, but welding can alter the surface layers of the steel, hindering this natural protective mechanism.

When stainless steel is exposed to temperatures above 400°C, often reached during welding, the chromium oxide film can degrade, losing corrosion resistance. This degradation is visible as discolouration, ranging from yellow/gold to blue and eventually black as the temperature increases.

• Discoloration Impact: Indicates loss of corrosion resistance and potential for severe degradation if not addressed.

Post-Weld Operations and Preventative Measures

While post-weld cleaning can remove discolouration and restore some corrosion resistance, this process is expensive, time-consuming, and not consistently fully effective, especially in hard-to-reach areas. This makes preventative measures, such as weld purging, a more efficient and reliable approach.

Weld Purging: This technique involves displacing air (and thus oxygen) in the weld zone with an inert gas, typically argon, to prevent oxidation during welding.

Available Purge Methods

Various purging techniques exist, each suited to different situations:

1. Simple, Homemade Solutions include using materials like screwed-up newspaper, sponge, or cardboard discs. While inexpensive, they are unreliable and can lead to contamination and subsequent weld failures.
2. Expandable Pipe Plugs: These are effective for short stubs or variable pipe fittings but less suited for joints far from access points.
3. Soluble Barriers: These can be glued to the internal diameter of pipes and flushed away after welding, but they have limitations, such as leakage and the risk of contamination due to the water content in paper barriers.
4. Water Soluble Weld Purge Film™: A superior alternative to paper, this film does not contain water and provides a better seal with a strong adhesive, reducing the risk of contamination and ensuring a reliable purge.
5. Flexible Discs Connected by a Tube: These are quick to deploy and easy to remove, but reliability is an issue due to their limited sealing area.
6. Inflatable Seals: These are the most reliable and effective tube and pipe purging methods. They offer controlled inert gas pressure and flow and are designed for easy and rapid deployment, minimising overall welding time.

Monitoring Oxygen Content in the Purge Gas

Even minute amounts of oxygen can cause significant discolouration and loss of corrosion resistance in stainless steel welds. Specialised instruments are necessary to measure and control oxygen levels in the purge gas. Maintaining an oxygen level below 50 ppm (0.005%) for stainless steel is essential. Modern instruments offer features like integrated recording and data logging and can even be programmed to halt welding if oxygen levels rise above a set threshold.

Applications Beyond Pipework

Weld purging and inert gas protection are not limited to pipes and tubes. Flat or contoured components, such as those used in the electrical, electronic, medical, food processing, and pharmaceutical industries, also require careful protection during welding to prevent corrosion. For these applications:

• Argweld Weld Trailing Shields®: These can protect the front of welds from oxidation.
• Flexible Enclosures: These allow components of various sizes to be welded in an utterly inert atmosphere, ensuring complete protection from oxidation.

Conclusion

Preventing corrosion in stainless steel welds is critical for maintaining the integrity and longevity of the welded components. While post-weld cleaning can address some issues, it is often more efficient and cost-effective to prevent discolouration and degradation from occurring in the first place through proper weld purging techniques. Advanced purging systems and monitoring instruments can ensure that oxygen levels are kept low enough to preserve the material’s corrosion resistance, ultimately saving time and money while guaranteeing high-quality welds.