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Post fabrication tips for stainless steel

9月 19, 2014    
Original text: Oil & Gas E.ssentials, June 2014

Lena Wegrelius, head of the corrosion department at Avesta Research Center, one of three main R&D sites within Outokumpu, shares post fabrication tips for stainless steel.

What types of post-weld treatment are recommended?

"Post-weld cleaning is achieved using either mechanical methods, such as blasting and grinding, or a chemical pickling treatment (or both). The use of a pickling treatment by itself can result in a clean passive surface but this requires the good control of pickling parameters such as acid concentration, temperature, and dwell time. For optimum corrosion resistance, it is recommended that post-weld cleaning be performed using a combination of mechanical cleaning followed by a pickling treatment. This will ensure that all heat tint and surface contaminants are removed."

Is post-weld treatment necessary for duplex grades?

"Heat tint is primarily composed of chromium oxide, which when formed can remove some of the chromium from the underlying alloy and potentially lower the corrosion resistance. To ensure there is no loss of corrosion resistant properties, it is important that lean duplex stainless steels (i.e. LDX 2101® and 2304) receive a proper post-weld treatment. The biggest difference between the two alloy groups is that duplex grades contain higher amounts of chromium, and as a consequence, it is somewhat harder to remove the heat tint from a duplex grade than from an austenitic grade."

What is the difference between passivation and pickling treatments?

"Chemical passivation treatments are used to remove surface contamination, such as free iron, and facilitate the formation of a protective passive film. Stainless steels are typically passivated with either nitric or citric acid solutions. Passivation treatments are recommended after fabrication processes such as rolling, bending, blasting, and machining. However, it is important to note that stainless steel passivates spontaneously in the presence of oxygen and that the passivation treatment’s main function is to clean the surface."

"Pickling is a more aggressive treatment used to remove surface oxides such as weld heat tint or heat treat scale. Pickling is typically done with nitric/hydrofluoric acid solutions that remove oxides as well as some of the underlying stainless steel surface. Pickling treatments restore corrosion resistance and result in a quality passive surface. In this case, there is no need of a subsequent passivation treatment."

Should machined surfaces be passivated?

It is good practice to use a passivation treatment after machining, since machined surfaces tend to be highly deformed and often have a lower corrosion resistance, than a typical cold rolled mill surface.