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Stainless steel thrives aggressive environments

9月 22, 2014    
 

New innovations combat corrosion in extreme conditions. Outokumpu has conducted long-term field research on corrosion.

Problem

Corrosion resistance of stainless steel depends on a combination of alloy and service environment. Projects in subtropical, tropical and coastal areas often suffer from corrosion even when normally adequate grades such as 316L are used.

The reasons for significantly faster corrosion in regions such as the Middle East include high temperatures, airborne chlorides from the sea, and lack of rainfall that cleans the steel surface.  

"The conditions in Europe and the Middle East are drastically different, but often the same materials are specified," says Tobias Larsson, Business Development Manager, Outokumpu. "Companies based in Europe or the US often just copy the specifications that have been proven to work well in their respective regions."

Solution

Outokumpu has conducted long-term field research on steel corrosion. Results from the company's new test site in Dubai point to interesting possibilities for duplex stainless steels in high-corrosion environments.

"If a common austenitic grade such as 316 is found to have insufficient corrosion resistance, an option is to move up to a grade such as 904L. However, for most customers, this is not cost-effective, so we took a look at duplex grade 2205, comparable in price to 316. Outokumpu has recently started to produce new high-chromium stainless steel 316plus, which endures in the aggressive environment thanks to its high chromium content and the price for this steel grade is lower than 316, so it would also be an option," says Tobias Larsson. "We found that with good cleaning, 2205 provides excellent results even in extreme conditions."

For minimal maintenance, a further option is superduplex grade 2507 or austenitic 254 SMO. However, the increase in cost from 2205 to 2507 is around 50%. For the ultimate balance of performance and price, the answer may lie in a novel use of a proven technique: electropolishing.

"Electropolishing is an electrochemical process that removes material from a metallic workpiece ion by ion. Smoothing the surface reduces corrosion-inducing dust and dirt deposits," says Larsson. "An interesting theory of electropolishing stainless steel is that the alloy components are removed at different rates. This results in a surface layer rich in chromium, improving the passivation of the surface."

Results

At Outokumpu's test site in Dubai, sandblasted and electropolished 2205 duplex steel appeared pristine even after nine months of extreme exposure. By contrast, normal austenitic 316L steel showed heavy staining after only three months.   

These discoveries are already being put to use. Outokumpu supplied 170 tons of structural sections in 2205 duplex steel for the new headquarters of the Qatar Foundation, under construction in Doha. The project required 17 different hollow sections and 35 different L-profiles.