Nickel continued passivity

When stainless steel is struck the passive film is reduced and an adherent flash of nickel forms on the active metal surface. Deposition is continued in a normal bath. 

Another way to protect a metal uses an impervious metal oxide layer. This process is known as passivation, hi some cases, passivation is a natural process. Aluminum oxidizes readily in air, but the result of oxidation is a thin protective layer of AI2 O3 through which O2 cannot readily penetrate. Aluminum oxide adheres to the surface of unoxidized aluminum, protecting the metal from further reaction with O2. Passivation is not effective for iron, because iron oxide is porous and does not adhere well to the metal. Rust continually flakes off the surface of the metal, exposing fresh iron to the atmosphere. Alloying iron with nickel or chromium, whose oxides adhere well to metal surfaces, can be used to prevent corrosion. For example, stainless steel contains as much as 17% chromium and 10% nickel, whose oxides adhere to the metal surface and prevent corrosion. 

It is possible for the passivation (oxide) layer on the surface of a metal to be continuously removed or not allowed to develop, by erosion from particulate matter or gas bubbles. Not only is the surface eroded but the removal of the protective oxide layer allows corrosion to take place. The problem is accentuated by the presence of an obstruction or debris, on the metal surface that diverts and accelerates the flow near the surface along a defined path. Sato et al [1977] report experimental data on erosion-corrosion resistance of condenser tubes fabricated from various cupro nickel alloys. They suggest that high iron bearing cupro nickels are superior in respect of erosion corrosion by clean sea water. 

Oxygen is the common cathodic reduction species found in water, which is responsible for continued corrosive attack on some engineering materials, such as low carbon steel. However, passive engineering alloys utilize the oxygen to form thin, tenacious, and adherent protective oxide films. Some common alloys with protective films are stainless steels, nickel alloys, copper-base alloys and aluminum alloys. The oxygen concentration at ambient temperatures and atmospheric pressure is approximately 6-8 mg/L. An increase in temperature decreases oxygen solubility, whereas an increase in pressure increases oxygen solubility. 

In the natural atmosphere noble metal coatings, such as chromium, nickel, and their alloys, form passive films on carbon steel surfaces. A defect-free continuous layer of chromium provides excellent protection for a carbon steel substrate. However, deposited layers of chromium and nickel commonly have some defects, such as cracks. The corrosion rate of the carbon... 

The passivity of metals like iron, chromium, nickel, and their alloys is a typical example. Their dissolution rate in the passive state in acidic solutions like 0.5 M sulfuric acid may be seriously reduced by almost six orders of magnitude due to a poreless passivating oxide film continuously covering the metal surface. Any metal dissolution has to pass this layer. The transfer rate for metal cations from this oxide surface to the electrolyte is extremely slow. Therefore, this film is stabilized by its extremely slow dissolution kinetics and not by its thermodynamics. Under these conditions, it is far from its dissolution equilibrium. Apparently, it is the interaction of both the thermodynamic and kinetic factors that decides whether a metal is subject to corrosion or protected against it. Therefore, corrosion is based on thermodynamics and electrode kinetics. A short introduction to both disciplines is given in the following sections. Their application to corrosion reactions is part of the aim of this chapter. For more detailed information, textbooks on physical chemistry are recommended (Atkins, 1999 Wedler, 1997). 

Figure 8 Passivity of nickel. Increase of anodic current with time for nickel prepassivated in pH 7.65 borate buffer solution (23% 0) at 0. 4 V (vs. Hg/Hg2S04) for 5 min and then transferred to a pH 1.0 Na2S04 (100% solution for continued polarization at 0.4 V. Also shown is the percentage of 0 detected in the oxide film by SIMS at various times of exposttre to the pH 1.0 Na2S04 solution. (From Ref 57.)...

The oxidation of the working electrode results in the formation of an oxide layer comparable to passivation in wet electrochemistry. For the oxidation reaction to continue, oxygen or nickel ions will have to diffuse through an oxide layer of increasing thickness. The thickness of this layer can be calculated from the total anodic charge. For the Ni/NiO electrode at 750°C and a scan rate of 20 mV/s, this yields a thickness of about 1 nm of NiO. The reduction reaction will be affected in a similar way, but now a thin Ni layer will impede the transport of oxygen (see Figure 15.15). 

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