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Economics enter into every business decision. But, the important criteria should not be the initial cost of a material, but its life-cycle cost or cost effectiveness. It usually is much more cost effective to specify a material that will provide an extended life, particularly in areas that are difficult to repair or in components that would cause major shut-downs in case of failure. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook.
The following two extreme alternatives describe the consideration given to economic factors when selecting materials for specific service:
A low initial cost system largely based on carbon steel and cast iron which will require considerable maintenance over the life of the plant. Such a system is a reasonable choice in areas where labor costs are low and material is readily available
A system based mainly on alloy materials which, if correctly designed and fabricated, will require minimum maintenance and will function reliably. Rising labor costs in most industries, together with the need for high reliability in capital intensive plant has produced a trend to this type of system.
In practice many systems are a mixture of these extreme options resulting in the high initial costs of one and the high maintenance costs of the other.
Corrosion-resistant alloys are used where corrosive conditions prohibit the use of carbon steels and protective coatings provide insufficient protection or are economically not feasible. Examples of these alloys include stainless steels, nickel-base alloys, and titanium alloys. According to U.S. Census Bureau statistics, a total of 2.5 million metric tons of raw stainless steel was sold in the United States in 1997. With an estimated cost of $2.20 per kg ($1 per lb) for raw stainless steel, a total annual (1997) production cost of $5.5 billion was estimated. It is assumed that all production is for U.S. domestic consumption. The total consumption of stainless steel also includes imports, which account for more than 25 percent of the U.S. market. Thus, the total consumption of stainless steel can be estimated at $7.3 billion.
Where environments become particularly severe, nickel-base and titanium alloys are used. Nickel-base alloys are used extensively in the oil production and refinery and chemical process industries, where conditions are aggressive. Furthermore, there is an increased use of these alloys in other industries, where high temperature and/or corrosive conditions exist. With the average price for nickel-base alloys at $13 per kg in 1998, the total sales value in the United States was estimated at $285 million.
The primary use of titanium alloys is in the aerospace and military industry, where the high strength-to-weight ratio and resistance to high temperatures are properties of interest. However, titanium and its alloys are also corrosion-resistant in many environments, and have therefore found application in oil production and refinery, chemical process, and pulp and paper industries. In 1998, it was estimated that 65 percent of the titanium mill products were used for aerospace and 35 percent for non-aerospace applications. The total annual consumption cost for titanium and titanium alloys for corrosion control applications is estimated at $150 million.
The total 1998 consumption cost of the corrosion-resistant metals and alloys is estimated at $7.7 billion. (reference)
