Corrosion is the degradation of a material as a result of its interaction with its surroundings and usually results in converting a refined metal into a more chemically stable oxide with a loss of critical properties.
Stainless steel is a generic term for a family of corrosion resistant alloy steels containing 10.5% or more chromium. All stainless steels have a high resistance to corrosion due to the chromium forming a passive film on the surface of the steel that can protect the material.
Despite its corrosion-resistance, during the welding of stainless steels, regions susceptible to corrosion may develop.
Let’s explain the problem and discuss the processes that A&G Engineering adopt to restore corrosion resistance to stainless steel and ensure long term weld integrity.
Welded joint structure
A welded joint structure has three distinct zones, each with different mechanical and structural characteristics:
- Unaffected base metal
- Heat-affected zone (HAZ) – the portion of the base metal that was not melted during welding but whose microstructure and mechanical properties were altered by the heat.
- Fusion zone – the portions of the material that have been coalesced with a chemical composition that is a mixture of the substrates being welded together and any additional filler material that may have been used.
During welding, as the temperature of stainless steel increases, the speed at which atoms can diffuse through the metal’s structure increases and, at the same time, the speed of any chemical reactions also increases.
Stainless steel sensitisation
When stainless steel is held at a temperature between 425 – 815°C, any carbon within the stainless steel will tend to react with chromium to form chromium carbide. Generally, the carbon within stainless steel will be concentrated within the grain boundaries and when the chromium carbide is formed, the solid solution close to the grain boundaries are then depleted in chromium – the main alloying element that gives stainless steel its anti-corrosion properties. A stainless steel that has been affected in this way is said to be “sensitised”.
Galvanic corrosion
Sensitisation is the loss of alloy integrity and is particularly important in welded metals. If the chromium-depleted zone is exposed to a corrosive environment, it is preferentially attacked by a process called galvanic corrosion – an electrochemical process in which one metal corrodes preferentially, when in electrical contact with a different type of metal, and both metals are immersed in an electrolyte such as water. As the corrosion proceeds, the metal loses strength.
If the chromium content drops below the threshold necessary to maintain a passive film, the region will become susceptible to corrosion and weld decay and is most common in the heat-affected zone.
Minimising sensitisation
One of the common approaches to minimising sensitisation is through material selection such as:
- Using low carbon grades of stainless steels: The lower the carbon content of the “L” grades (e.g., 304L and 316L) means that there is less carbon to bond with the chromium. The low carbon grades have carbon contents of lower than 0.03%.
- Using stabilized grades of stainless steel: When the stainless steel is alloyed with small quantities of titanium or niobium, the carbon preferentially bonds with these elements, ensuring that the chrome content is not lowered significantly. Typical stabilized alloys are type 321, (Ti stabilised) and type 347 (Nb stabilised) stainless steel.
While ‘L’ grade stainless can minimise sensitisation, reduced carbon content results in lower tensile strength and the trade-off for minimising sensitisation is to use thicker material that equates to increased weight and increased material cost.
If using ‘L’ grade stainless steel is not an option, A&G eliminate the risk of weld corrosion using low carbon 316LSi welding wire in conjunction with shielding and backing gas to inhibit the precipitation of chromium carbide ensuring that intergranular corrosion of the weld is minimised.
Passivation
In addition, A&G routinely use a process called post-welding passivation that restores the damaged chromium oxide layer to prevent intergranular corrosion in stainless steel. This can be achieved through chemical treatment or the use of specialised tools to remove surface impurities and accelerate the natural formation of the chromium oxide layer. A&G use a passivation process that complies with ASTM A380 Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems.
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A&G Engineering have been manufacturing stainless steel storage tanks and pressure vessels for 60 years. The unmatched combination of experience, flexibility, expertise, and technology for every A&G project serve to minimise production risk and ensure on-time, on-budget, and as-specified delivery.
If you have a technical query or want to find out how A&G can help with your next project, talk to our expert team by calling us on (02) 6964 3422 or email sales@agengineering.com.au with your inquiry. We look forward to hearing from you.
