The Importance of Corrosion Resistance in Stainless Steel Applications
Stainless steels are prized across many industries for their corrosion resistance. This quality is essential in dairy, food processing, pharmaceuticals, and semiconductor manufacturing sectors, where contamination can have severe consequences. However, if not managed correctly, the welding process can compromise this vital property.
Understanding the Mechanism of Corrosion
The corrosion resistance of stainless steels and other chromium-containing alloys is due to forming a very thin, transparent surface layer of chromium oxide. This passive film acts as a protective barrier against environmental factors. However, when stainless steel is heated to high temperatures during welding, this protective film thickens and may become visible as discolouration. The film can become unstable at a critical thickness, leading to localised corrosion. Four principal corrosion mechanisms are particularly relevant:
Weld Decay in Stainless Steel
During welding, stainless steel can suffer from a phenomenon known as "weld decay." This occurs when chromium in the steel forms chromium carbides along the grain boundaries, depleting the surrounding areas of chromium. If the chromium content falls below 12%, the steel loses its corrosion-resistant properties, leading to an intergranular attack. This localized galvanic corrosion can severely weaken the material at the weld site.
Impact on Mechanical Strength
The loss of chromium during welding not only reduces corrosion resistance but also affects the material's mechanical properties. For example, in chromium/molybdenum/vanadium alloys, which are valued for their high-temperature creep resistance, any chromium reduction can degrade these essential properties. This is particularly critical in industries where high strength and durability are required.
The Role of Welding in Corrosion Reduction
Welding processes can cause oxidation, visible as discolouration, particularly in stainless steel. While this discolouration might seem like a minor inconvenience, it can significantly reduce corrosion resistance and mechanical strength. This is especially important in applications like dairy, food, pharmaceuticals, and semiconductors, where stainless steels are used precisely for their corrosion resistance.
To mitigate this, inert gases such as argon or helium are used during welding to protect against oxidation. The gas shield from a GTAW torch can protect the upper surface of the joint, but internal surfaces, such as the inside of pipes and tubes, require special attention. Effective internal protection, known as weld purging, has been developed over the past 25 years to address this need.
Pipe and Tube Purging Systems
Effective weld root protection systems are crucial for maintaining the integrity of stainless-steel welds. These systems work by sealing the inside of a pipe on either side of the weld zone and displacing air with an inert gas. The seals must be reliable, leak-tight, and easy to insert and remove. The quality of the inert gas and the flow characteristics are also critical to maintaining a high level of protection.
Primitive purging systems, using materials like paper, wood, or polystyrene, often fail to provide adequate protection and could even catch fire during welding. Modern systems, such as those using inflatable dams, are far more effective and are now available for pipe and tube diameters from 25 mm to 2400 mm.
Measuring Residual Oxygen
An effective weld purge process must be supported by reliable oxygen detection equipment. Modern weld purge monitors are designed to measure oxygen levels accurately, down to as low as 10 ppm. This level of sensitivity is crucial for welding reactive and refractory alloys, where even small amounts of oxygen can compromise the quality of the weld.
For example, the PurgEye® 600 instrument by Huntingdon Fusion Techniques can accurately measure oxygen levels from 1000 to 10 ppm, providing the precision needed for high-quality welding operations.
Conclusion
Maintaining corrosion resistance in stainless steel welds is crucial, especially in industries where pipework cleanliness is vital for product quality. The use of advanced purging equipment and monitoring instruments is essential to prevent the loss of chromium during welding. By ensuring effective protection and precise control over the welding environment, the integrity of stainless-steel structures can be preserved, safeguarding their resistance to corrosion and mechanical strength.
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