This article presents an overview of the developments in stainless steels made since the 1990s. Some of the new applications that involve the use of stainless steel are also introduced. A brief introduction to the various classes of stainless steels, their precipitate phases and the status quo of their production around the globe is given first. The advances in a variety of subject areas that have been made recently will then be presented. These recent advances include (1) new findings on the various precipitate phases (the new J phase, new orientation relationships, new phase diagram for the Fe–Cr system, etc.); (2) new suggestions for the prevention/mitigation of the different problems and new methods for their detection/measurement and (3) new techniques for surface/bulk property enhancement (such as laser shot peening, grain boundary engineering and grain refinement). Recent developments in topics like phase prediction, stacking fault energy, superplasticity, metadynamic recrystallisation and the calculation of mechanical properties are introduced, too. In the end of this article, several new applications that involve the use of stainless steels are presented. Some of these are the use of austenitic stainless steels for signature authentication (magnetic recording), the utilisation of the cryogenic magnetic transition of the sigma phase for hot spot detection (the Sigmaplugs), the new Pt-enhanced radiopaque stainless steel (PERSS) coronary stents and stainless steel stents that may be used for magnetic drug targeting. Besides recent developments in conventional stainless steels, those in the high-nitrogen, low-Ni (or Ni-free) varieties are also introduced. These recent developments include new methods for attaining very high nitrogen contents, new guidelines for alloy design, the merits/demerits associated with high nitrogen contents, etc.

Recent developments in stainless steels

The history of the Point Defect Model for the passive state: A brief review of film growth aspects

The formation kinetics and electric properties of anodic barrier oxides on Al, Hf, Nb, Ta, Ti, and Zr depend on the migration of ions, controlled by an electric field strength of up to 107 V cm−1. The high field model, based on ideas of Guntherschulze, Betz, Verwey, Mott, and Cabrera, explains only parts of the experiments. Contradictory models and investigations of the last 60 years are compared with new results, which enable a more detailed understanding of many aspects of the high field model, e.g. the shape of experimental transients, the time and field dependent concentration of mobile ions, the nature of the mobile species and the interaction of anion and cation transport, and the location of the rate determining step.

Thin anodic oxide layers on aluminium and other valve metals: high field regime

Kinetics of the reaction of oxygen with clean nickel single crystal surfaces: I. Ni(100) surface

Kinetics of the reaction of oxygen with clean nickel single crystal surfaces. I - Ni/100/ surface. II - Ni/111/ surface

Low-temperature oxidation is a reaction, occurring at or below room temperature, between a solid and a gas. It usually involves the combination of oxygen with metals, and it has the greatest commercial impact in the presence of moisture, as in corrosion. Cabrera and Mott put forward a theory of low-temperature oxidation, based on the assumption that cation migration occurs under the influence of a potential built up across the growing oxide film. Recent experimental results require that this theory be expanded to explain recent observations such as anion migration during oxide growth and the transition from the initial chemisorbed monolayer to a bulk, threedimensional oxide. The additional ideas put forward in the present paper may be summarized as follows. Low-temperature oxidation is controlled by the nature of the oxide; whether it is a network former or a modifier. A period of fast, linear oxidation is followed by a slow logarithmic reaction whose rate, in turn, can increase if the oxide film crystallizes to form grain boundaries. The initial fast oxidation is a continuation of the chemisorption process. Place exchange (anions and cations interchanging positions) occurs when the energy due to the image force of an oxygen ion is greater than the bond energy holding the ion in place. A stable film forms when this bond energy is greater than the image force energy. The oxygen ions formed on the oxide surface then set up a potential across the film. This potential provides the driving force for continued reaction. Oxide growth during this later stage is a slow, logarithmic process. A barrier to ion transport exists at the gas-oxide interface in the case of anion migration and at the metal-oxide interface in the case of cation migration. In both cases, the field built up across the oxide lowers the barrier sufficiently so that ion migration can occur. Network modifiers allow cation migration. The reaction rate is sensitive to crystallographic orientation of the metal, but not to oxygen pressure. A constant voltage is maintained across the film, so that the Cabrera-Mott theory explains the logarithmic kinetics. Network-forming oxides allow onion migration. The number of anions, and hence, the rate of reaction, is sensitive to oxygen pressure, but not crystallographic orientation of the metal substrate. Since the potential is a result of the mobile anions, the film tends to grow under constant field. The logarithmic kinetics then must be explained by an increasing activation energy for ion transport, as proposed by Eley and Wilkinson. The logarithmic growth rate can be increased by the presence of water vapor if the water introduces “dangling” bonds into an oxide network structure. Crystallization of the oxide film also increases its rate of growth and results in the formation of oxide islands.

Low-temperature oxidation

The role of oxide perfection is a major factor in controlling the rate of oxidation and corrosion of metals. Studies of the reactivity of tantalum, silicon, and amorphous metals are particularly revealing in this regard. Two states of oxide perfection are found: single crystal and vitreous. The former is seldom encountered in practice because it requires epitaxial growth of a single crystal oxide on the metal. However, bond flexibility in vitreous oxides allows them to adapt to various substrates, even polycrystalline ones. Degradation of both single crystal and vitreous oxide results in the formation of polycrystalline oxide. Here, grain boundaries provide paths for easy ion movement and resultant fast oxide growth. Stabilization of vitreous oxide involves bond strength, multiple configurations with equivalent energies, substrate effects, and purity. The control of these factors is the technological challenge to achieving more perfect, and hence more protective, oxides.

F. P. Fehlner

Papers

Low-temperature oxidation

The Role of Noncrystalline Films in the Oxidation and Corrosion of Metals

Reply to M. J. Pryor's discussion of “low-temperature oxidation”

Critique of “on some aspects of low-temperature and anodic oxidation of metals and semiconductors”