Due to its poor corrosion resistance, carbon steel is often replaced by stainless steel to build storage tanks for use in water treatment units, food, chemical and para chemical industries. For years, 316L has been widely used. Recently, duplex (austenitic-ferritic) stainless steels have been selected due to improved corrosion resistance and higher mechanical properties.
Corrosion resistance properties of duplex and austenitic stainless steels of the 300 series are presented and compared in relation to the problems, which are generally experienced in storage tanks.
In addition, a user-friendly software package has been developed to estimate and compare investment and life cycle costs for tanks made of different materials. This software takes into account API (USA) and CODRES (French) calculation codes. Some cost simulations, as well as actual construction costs, are presented and discussed.
Duplex stainless steels appear to be very cost competitive, since investment costs are generally close to that of coated carbon steel and lower than that of 316L. Life cycle costs of duplex stainless steel tanks are always far lower due to their superior corrosion resistance. This allows maintenance operations to be minimized or eliminated.
FOREWORD
Due to its availability, low cost, and ease of fabrication, carbon steel has been used for decades to build storage tanks, even in undeveloped countries.
Unfortunately, this material is readily oxidized in weak corrosive environments and even in the atmosphere. Its life is often limited to a short time unless it is protected or its initial thickness is significantly increased. The poor corrosion resistance of carbon steel is due to the fact that iron is able to form a stable and protecting oxide film on its surface only when the pH is between 9.5 and 12 and the temperature is near ambient. Such conditions are rarely encountered in practice. In addition, depending on the actual pH value, oxygen in the atmosphere may seriously affect the material. In fact, as soon as the pH is lower than about 10, oxygen induces a passivation of aerated zones while dearated areas become active (corrosion sites) due to a strong galvanic coupling effect. This process often occurs in storage tanks at the interface between the liquid and the vapor phase. In addition, carbon steel may be corroded by soils, often requiring the tank bottoms to be cathodically protected.
For these reasons, stainless steel tanks in the food industry, water treatment units and the para chemical industry often replace carbon steel tanks.
