Passivation of stainless steels

Monel and nickel are the preferred materials of constmction for cylinders and deHvery systems however, copper, brass, steel, and stainless steel can be used at room temperature, providing that these metals are cleaned, dried, and passivated with a fluoride film prior to use. Studies have shown that fluorine passivation of stainless steel and subsequent formation of an iron fluoride layer prior to WF exposure prevents reaction between the WF and the stainless steel surface (23). 

The nature of the reference electrode used depends largely on the accuracy required of the potential measurement. In the case of breakdown of passivity of stainless steels the absolute value of potential is of little interest. The requirement is to detect a change of at least 200 mV as the steel changes from... 

Table 10.33 Critical current density and current density to maintain passivity of stainless steel (Fe-18 to 20Cr-8 to l2Ni) in different electrolytes (after Shock, Riggs and Sudbury )...

Environment Increase redox potential of solution Addition of anodic inhibitors Passivation of stainless steel by additions of O2, HNO3 or other oxidising species to a reducing acid Additions of chromates, nitrates, benzoates, etc. to neutral solutions in contact with Fe inhibitive primers for metals, e.g. red lead, zinc chromate, zinc phosphate... 

COi Compressor Drive E = electric motor, ST = steam turbine e Air is needed to provide oxygen for passivation of stainless steel equipment. 

ASTM A380-99, Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems , in Annual Book of ASTM Standards. 1999, ASTM, Philadelphia, PA. 

I. Olefjord and B.O. Elfstrom, The Composition of the Surface during Passivation of Stainless Steels, Corrosion, Vol 38, 1982, p 46-52... 

BSSA. (2014). Passivation of stainless steels (WWW Document). URL http //www.bssa.org. uk/topics.php article=68. 

Hermas, A.A., Wu, Z.X., Nakayama, M., and Ogura, K. (2006) Passivation of stainless steel by coating with poly(o-phenylenediamine) conductive polymer. J. Electrochem. Soc., 153, B199-B205. 

The contact electric resistance technique has been applied by Ramo et al. [742] to study the corrosion tmd passivation of stainless steels and of chromium plated stainless steels in alkaline pulping liquors. 

Tungsten presents some chemical similarities to molybdenum, and therefore its possible role as an alloying element to improve the corrosion resistance of stainless steels has been investigated. In contrast to molybdenum, there are, however, relatively few data on the effect of tungsten on the passivity of stainless steels (Bui et al., 1983 ... 

The passivity of stainless steel is the result of the presence of a corrosion-resistant oxide film on the surface. In most material environments, it will remain in the passive state and tend to be cathodic to ordinary iron or steel. When chloride concentrations are high, such as in seawater or in reducing... 

To understand the influence of allo5dng elements on the passivity of stainless steels, researchers have combined electrochemical and siufece analysis. Polarization diagrams provide the first indication of the overall influence of alloy additions on the active-passive transition, passivity, and pitting resistance. However, siuface analysis by X-ray photoelectron spectroscopy (XPS) of prepassivated siufaces provides a direct observation of the location and the chemical state of an alloying element. Such... 

Numerous studies have attempted to elucidate the role of Mo in the passivity of stainless steel. It has been proposed from XPS studies that Mo forms a solid solution with CrOOH with the result tiiat Mo is inhibited from dissolving trans-passively [9]. Others have proposed that active sites are rapidly covered with molybdenum oxyhydroxide or molybdate salts, thereby inhibiting localized corrosion [10]. Yet another study proposed that molybdate is formed by oxidation of an Mo dissolution product [11]. The oxyanion is then precipitated preferentially at active sites, where repassivation follows. It has also been proposed that in an oxide lattice dominated by three-valent species of Cr and Fe, ferrous ions will be accompanied by point defects. These defects are conjectured to be canceled by the presence of four- and six-valent Mo species [1]. Hence, the more defect-free film will be less able to be penetrated by aggressive anions. A theoretical study proposed a solute vacancy interaction model in which Mo " is assumed to interact electrostatically with oppositely charged cation vacancies [ 12]. As a consequence, the cation vacancy flux is gradually reduced in the passive film from the solution side to the metal-film interface, thus hindering vacancy condensation at the metal-oxide interface, which the authors postulate acts as a precursor for localized film breakdown [12]. 

Chen G, Clayton CR, Sadowski RA, Gillow JB, Francis AJ (1995) Influence of sulfate-reducing bacteria on the passive film formed on austenitic stainless steel, AISI 304 NACE International, Houston TX Corrosion Chen G (1996) An XPS study of the passivity of stainless steels influenced by sulfate-reducing bacteria (PhD dissertation). State University of New York at Stony Brook, New York, p 279... 

O Laoire C, Timmins B, Kremer L, Holmes JD, Morris MA (2006) Analysis of the acid passivation of stainless steel. Anal Lett 39 2255-2271... 

Molybdenum is an alloying element which is known to increase passivation of stainless steels. Steels of type 316 contain molybdenum as a minor constituent and promote passivation. Sodium molybdate forms a complex passivation film at the iron anode of ferrous-ferric molybdenum oxide. 

The passivity of stainless steels is attributed to either the presence of a corrosion-resistant oxide film or an oxygen-caused polarizing effect. 

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