Stainless steels passivity

Anodizing employs electrochemical means to develop a surface oxide film on the workpiece, enhancing its corrosion resistance. Passivation is a process by which protective films are formed through immersion in an acid solution. In stainless steel passivation, embedded ion particles are dissolved and a thin oxide coat is formed by immersion in nitric acid, sometimes containing sodium dichromate. 

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1—4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active-passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6... 

The reason for the protective action is known to lie in the fact that the nitric acid, even in a very small concentration, enforces passivation of the stainless steel. Passivation is based on the formation of a thin layer of chromium (III) oxide, which brings the attack to a virtual halt ... 

Fig- 7.65 Schematic EPR (electrochemical potentiokinetic reactivation) curves for three amounts of sensitization of an austenitic stainless steel. Passive film formed at (1). Downscans pass through maximum attack at (2). Environment 1 N H2S04 + 0.01 M KSCN at 30 °C. Curve (3) is observed if passive film continues to form on downscan. Source Ref 93... 

K. OsozawaandN. Okato, Effect of Alloying Elements, Especially Nitrogen, on Initiation of Pitting in Stainless Steel, Passivity and Its Breakdown in Iron and Iron-Base Alloys, R.W. Staehle and H. Okada, Ed., National Association of Corrosion Engineers, 1976, p 135-139... 

Type 316,317 stainless steel (passive) Type 304 stainless steel (passive) Type 410 stain less steel (passive) Nickel (passive)... 

Mo Stainless Steel (Passive) 18-8 Stainless Steel (Passive) Chromium Steel >11% Cr (Passive) Inconel (Passive)... 

Wang and Li (2001) consider that yttrium also helps to increase the resistance of the AISI304 stainless steel passive film to mechanical failure. In this case, it was demonstrated that the electron work function of the passive film on yttrium-containing steel was higher than that on yttrium-free steel, implying that yttrium stabihzed the passive film, making it stronger and more chemically stable. 

A series of reported studies provide some insights into the nature and the role of bound water in the passivity of austenitic stainless steels [4-6]. The studies primarily focused on the passivity of t)rpe 304 stainless steel, passivated in deaerated 0.5 M H2SO4. Radiotracer studies were conducted with tritiated water in order to determine the quantity of water bound into the lattice following the formation of the passive film. In addition, the rate of desorption of bound water from the film was determined by dioxane solvent extraction. Dynamic rupture and self-repair of the passive film were seen to be critically influenced by the nature of the bound water. Two classes of bound water were determine from XPS, radiotracer, Coulombic titration, pitting incubation, and noise analysis [4-6]. The two classes of bound water were of the following types (a) M-H2O and M-OH (aquo and ola-tion groups) and (b) M-0 or M-OOH with 0X0 and olation bridges. 

---