The Corrosion of Duplex Stainless Steels

Duplex stainless steels contain more than 19% chromium and have more than 30% of both ferrite and austenite. They were first invented in the early part of the 20th century, but not until the 1970s and 80s, with the introduction of argon oxygen decarburization (AOD) melting and the recognition of the benefits of nitrogen additions, that duplex stainless steels became attractive for widespread industrial use. Duplex stainless steels have found applications in most industries, including oil and gas, marine, desalination, power, chemical and process, pulp and paper, and mineral processing.

Modern duplex stainless steels are approximately 50/50 austenite and ferrite, and they combine the high strength of ferrite with the ductility of austenite. The addition of nitrogen enables better equi-partioning of elements between the two phases, higher strength in the austenite, better corrosion resistance, and improved weldability, to name but a few benefits.

Since the 1980s, a range of duplex stainless steels has been developed that have higher alloy content, and therefore greater corrosion resistance and strength. The commonly used duplex stainless steels have good corrosion resistance in a wide range of environments and excellent resistance to stress corrosion cracking (SCC). Their high strength and hardness also gives them excellent resistance to both erosion corrosion and cavitation. There are also cast versions of many of the duplex compositions and these have seen extensive use for pumps, valves, and other equipment.

Duplex stainless steels have been adopted by many industries to varying degrees. They are the workhorse corrosion resistant alloy (CRA) of the oil and gas industry, and are also widely used in the chemical and process industries for their SCC and corrosion resistance. Superduplex stainless steel has become the main alloy for piping, pumps, and valves in seawater reverse osmosis (SWRO) desalination plants, because of its resistance to crevice corrosion in seawater. This book covers a variety of corrosion types that may affect duplex stainless steels in service, presenting the available data and, in some cases, how to avoid problems. The final chapter covers the use of standards and specifications to procure good quality material for specific applications. It also covers the requirements over and above standard specifications necessary to obtain good quality material.

In this book, Roger Francis will review various duplex alloy compositions, mechanical properties, and design stresses for vessels and pipes to various codes. He will also cover the basics of welding duplex alloys, both to themselves and to other alloys, and their corrosion resistance. Of most importance, the book will look at a variety of types of corrosion that may affect duplex stainless steels in service, presenting the available data and, in some cases, how to avoid problems.