The differences between carbon steel and stainless steel are generally well understood:
Corrosion resistance: the chromium content of stainless steel makes the product more corrosion resistant than carbon steel. The chromium attaches to oxygen to create a corrosion resistant surface layer of chromium oxide. Carbon steel does not have the chromium content to create a protective layer so oxygen bonds with the iron to create iron oxide or rust.
Mechanical properties: While there are many types and grades of stainless steel and carbon steel, in general terms, stainless steels with their high nickel content are more ductile than carbon steels, and the alloying elements in many stainless steels provide superior tensile strength. Stainless steel is also notable for its superior resistance to temperature-induced cracking.
Appearance: Stainless steel is generally preferred when aesthetics is a factor as the product can retain its attractive appearance without the need for a clear coat or painting. If scratched, the corrosion resistant layer on stainless steel will reform, while the scratched carbon steel surface would need to be retreated.
Cost: Due to the addition of alloying elements such as chromium, nickel, manganese, stainless steels are generally a higher upfront cost outlay than carbon steels which are composed of more affordable iron and carbon elements.
For project managers, the key question is how these factors translate into lifetime cost of ownership for a project? By choosing carbon steel, are you really saving money?
Material costs
A&G completed a worked cost comparison example for a 1,500,000L API650 storage tank, found here.
While stainless steel is more expensive per tonne than carbon steel, its higher strength means less is required for a typical tank fabrication project. In the A&G example, the higher strength of stainless steel means that a lower material thickness product can be used, resulting in a total weight for the stainless steel product that is almost half that of the carbon steel equivalent.
Fabrication costs
Although fabrication practices are similar, welding durations on carbon steel tanks are higher, and consumables cost for stainless steel tanks are slightly higher. Fabricating carbon steel tanks requires preheating and post heating for welds. Carbon steel tanks also need post welding treatment and radiography testing inspection. Depending on design parameters, these processes are not required for stainless steel tanks.
Initial surface treatment
There are additional costs associated with grit blasting and initial treatment of external (and, possibly internal) surfaces of carbon steel tanks. Stainless steel tanks do not incur these costs.
Ongoing maintenance costs
Painting, maintenance, and rework costs for carbon steel varies depending on type of product stored and facility location. Tanks storing aggressive materials, or near coastal environments, may cost more in maintenance.
Depending on the individual application and process, maintenance costs can vary greatly. Based on a tank surface area of 810 square metres, A&G estimate 10-12 years of maintenance will add $279,000 to lifetime cost of ownership and a 30-year lifespan will add over $1m to lifetime costs.
Another significant consideration is the ‘Out of Service Costs’ (when the tank is not being used) for carbon steel tanks during regular maintenance works.
With properly selected types and grades, stainless steel is resistant to tank contents, the external environment and doesn’t need painting.
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It is a myth that stainless steel tanks cost more than carbon steel. As shown above, stainless steel tanks cost your business significantly less and provide many other benefits, from the initial capital outlay phase and over the life of the vessel.
A&G Engineering is Australia’s leading stainless steel tank manufacturer, providing different capacity storage and processing tanks for the petrochemical, winery, brewery, dairy, food and beverage and other industries. Call us on (02) 6964 3422 or sales@agengineering.com.au to find out more about whether a stainless steel tank is the right solution for your needs.
