Passivity chromium

Chromium passivates very effectively down to very negative potentials even in strongly acidic electrolytes (Fig. 5). The cathodic current density of hydrogen evolution is followed by a small potential range of E = —0.4 to O V of anodic metal dissolution where Cr dissolves as Cr2+. At E > 0 V Cr passivates with a drop of the current density to less than 0.1 pA cm 2. In this potential range Cr3+ is the corrosion product. RRD studies have been applied to determine quantitatively the formation of Cr3+ ions. In principle the dissolution of Cr3+ at a Cr disc may be studied with two concentric analytical rings with their reduction to Cr2+ at the inner ring and its... 

Both dilute and concentrated nitric acid render chromium passive, so does cold, concentrated sulphuric acid and aqua regia. 

The metals used to manufacture cans, ends and closures are either steel (tin plated or chromium passivated) or aluminium. In most cases they are coated on the food contact surface with a resinous or polymeric protective coating to avoid interaction between the foodstuff and the metal. However, there is a well defined sector of the tinplate food packaging market where no protective organic coating is needed or used. 

Chemical analysis of the solutions after anodic dissolution have shown that the oxidation state of chromium in the dissolution products depends on the alloy composition and, correspondingly, on the potential of alloy dissolution. At potentials less positive than the potential of the onset of pure-chromium passivity breakdown, chromium dissolves from the nickel-based alloys as Cr(III). The alloys with chromium contents of not more than 15% dissolve in this manner in NaCl solution. At higher Ea, chromium from the alloy dissolves, for the most part (about 90%), in the form of Cr(VI). This is true for all alloys in Na2SC>4 (or NaNC>3) solution and for the alloys containing more than 25% chromium in NaCl solution. 

D. F. McFarland and 0. E. Harder found that normal nitric acid dissolves 0-35 mgrm. per sq. in. per week. W. Hittorf said that nitric acid makes chromium passive. F. Wohler, E. Jager and G. Kriiss, H. St. C. DeviUe, and C. C. Palit and... 

J. J. Berzelius also noted the resistance the metal offers to attack by aqua regia. A. Granger observed that chromium is attacked by phosphorus at 900°. W. Hittorf found that phosphoric acid makes chromium passive. W. Rohn found that 10 per cent, phosphoric acid dissolves no chromium in 24 hrs. when cold, or in an hour when hot. He also found that 10 per cent, acetic acid dissolves 0-13 grm. per sq. dm. per 24 hrs. in the cold, and 0-03 grm. per sq. dm. per hr. when hot. 

Diban N., Mediavilla R., Urtiaga A., and Ortiz L, Zinc recovery and waste sludge minimization from chromium passivation baths, J. Hazard. Mater. 192, 801, 2011. 

Chromium passivates strongly in acid sulphate media. Hence an inert anode is always employed in chromium plating. It is generally a lead alloy which immediately covers with lead dioxide on positive polarization in the electrolyte. The alloying elements are tin, antimony and silver which are added to the lead to improve its mechanical properties and to reduce the overpotential for oxygen evolution. 

The family of nickel alloys is large. Dillon [132] divides them into two groups those that rely on the resistive properties of nickel itself, and those that include chromium and depend on its forming a passive film. Stainless steels have a similar chromium-passivation effect, but we do not include them here among the nickel alloys. 

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