The Southern African Stainless Steel Development Association (sassda) has developed a new lifecycle costing (LCC) programme to enable the costs of using stainless steel in projects to be compared to alternatives based on realistic estimates of the total costs of products or structures across their full service lives.
Stainless steels have traditionally been specified in applications where the primary requirement is for corrosion resistance. However, since their invention over 100 years ago, stainless steels have also been recognised for other attributes such as durability, versatility, quality, sustain- ability, hygiene and aesthetic appeal.
It is this combination of properties that has seen stainless steels become the material of choice in a wide variety of uses: from the utensils and kitchenware used to prepare food; in a range of applications in the transport industry; as process equipment in the food and beverage industry; for the manufacture of pharmaceutical products; in the medical field; and through to very demanding applications in the chemical processing and power generation industries.
Stainless steels contain at least 11% chromium and this forms a chromium-rich passive layer on the surface of the steel. It is this passive layer that confers corrosion resistance on stainless steel. However, the key to stainless steel is that the passive layer is self-healing. Unlike coated carbon steel, which will rust or corrode if the coating is scratched or damaged, stainless steel has the ability to regenerate and heal the passive layer spontaneously. It is this corrosion resistance and the passive layer properties that make stainless steels such an ideal choice in so many applications. Higher alloyed stainless steel can resist very aggressive chloride, acidic or alkaline solutions, while the lower alloyed stainless steels can resist atmospheric corrosion. The heat resisting grades of stainless steels can resist oxidation up to temperatures as high as 1 200 °C. Thus knowledge of the application and the corrosive environment allows the selection of the most appropriate and cost-effective grade.
There are three main groups of stainless steels that are classified according to their microstructure as austenitic (comprising about 72% of all stainless steels), ferritic (about 25%), duplex (about 2%) and the balance being other (e.g. martensitic) or unclassified grades.
Apart from their corrosion resistant properties, there are many other reasons to specify stainless steel. For example, the strength of stainless steel allows thinner sections to be used than with other materials. Some grades of stainless steels, such as the duplex stainless steels, have strength levels double that of standard austenitic or ferritic grades.
If ductility and formability are critical, such as in deep-drawing applications (e.g. pots and pans and sinks), then austenitic stainless steels have outstanding properties. Austenitic stainless steels also remain tough at very low temperatures, even down to that of liquid nitrogen. On the other hand, the martensitic stainless steels are extremely hard and thus are ideal for knives as they can retain a sharp edge.
Stainless steels are often selected for their visual appeal. They are available in industrial finishes, where aesthetics are not important, but they are also used in mirror finishes, highly polished finishes, brush and scratch finishes, depending on the visual effect sought.
Stainless steels are recognised as the most hygienic surface in the food and beverage industry. The stainless steel will not contaminate the product and the smooth surface ensures that bacteria can be easily removed. This excellent cleanability has seen stainless steel become the preferred choice in a wide range of industries from pharmaceuticals and hospitals to kitchens and breweries.
Stainless steels are 100% recyclable with- out any loss in quality no matter how many times the process is repeated. When products reach the end of their useful lives, over 80% of the stainless steel is collected and recycled.
Stainless steels are durable and have low maintenance costs due to their corrosion resistance. There is no coating or painting requirement and normal maintenance would simply be occasional cleaning.
The LCC advantage
Stainless steel may not always be the cheapest candidate material for an application when considering upfront costs. However, its durability and ease of maintenance compensate for the sometimes higher initial purchasing costs and it is often the least expensive choice in a lifecycle cost comparison.