Corrosion nickel hydroxides

Nickel hydroxides have been used as the active material in the positive electrodes of several alkaline batteries for over century [1], These materials continue to attract much attention because of the commercial importance of nickel-cadmium and nickel-metal hydride batteries. In addition to being the cathode active material in nickel-metal hydride batteries, Ni(OH)2 is an important corrosion product of the anode during cycling. There are several reviews of work in the field [2-10],... 

In normal battery operation several electrochemical reactions occur on the nickel hydroxide electrode. These are the redox reactions of the active material, oxygen evolution, and in the case of nickel-hydrogen and nickel-metal hydride batteries, hydrogen oxidation. In addition there are parasitic reactions such as the corrosion of nickel current collector materials and the oxidation of organic materials from separators. The initial reaction in the corrosion process is the conversion of Ni to Ni(OH)2. 

However, certain corrosion inhibitors, such as y-alumina, calcium carbonate, zinc oxide, sodium oxalate, etc., show insufficient corrosion inhibition. For some corrosion inhibitors, the mechanical strength of the PAS resin is deteriorated. More satisfactory corrosion inhibitors are nickel compounds, such as nickel carbonate, nickel hydroxide, and nickel cit-rate. The anticorrosive effect is already satisfactory when the corrosion inhibitor is added in amounts of 0.1 %. 

The equilibrium potential of the nickel-hydroxide electrode is slightly above that of water decomposition. In this respect the situation resembles that of the lead-dioxide electrode, but the much lower value of this potential allows the use of nickel as conducting element, since corrosion of this metal can be neglected, at least under normal conditions. For this reason, corrosion is not shown in Fig. 1.31. (Only in foam electrodes with an extremely large surface area of the substrate, nickel corrosion may slightly disturb the current balance in sealed cells.)... 

Design factors which the NiMH manufacturer must consider for good shelf life are the oxidation and corrosion resistance of the metal hydride alloy, the amount of precharge on the metal hydride electrode, the nickel hydroxide active material formula, and the quality of the cobalt conductive network in the positive electrode. 

The atmospheric corrosion of nickel is similar to that of zinc. Nickel exists solely as Ni + and instantaneously forms nickel hydroxide and, subsequently, NiS04 6H2O, has been observed [61b]. After prolonged exposure, an amorphous basic nickel sulfate is formed, frequently mixed with small amounts of carbonate, with less protective abUity. This phase can crystallize and form another basic nickel sulfate, with higher stabUity and protective abUity [61a]. No evidence of other anions in the corrosion products has been found so far. The corrosion rates of nickel are comparable to those of copper [70]. 

Table 3.40 Bimetallic corrosion between nickel-iron alloys in sodium hydroxide ...

The corrosion rate of nickel in sodium hydroxide is adversely affected by heat transfer by small amounts of oxidisable alkaline sulphur-containing salts, e.g. Na2SOj, NajS Oj, Na S and, at high temperatures, by alkaline oxidising agents, viz. NaClOj and NajOj. In the former circumstance Alloy 600 is more resistant than nickel, but not in the latter. When Alloy 600 is used for service in caustic alkalis, it should be stress relieved after fabrication to minimise the possibility of stress-corrosion cracking. 

Low-carbon and chromium-nickel steels, certain copper, nickel and aluminium alloys (which are all widely used in marine and offshore engineering) are liable to exhibit stress-corrosion cracking whilst in service in specific environments, where combinations of perhaps relatively modest stress levels in material exposed to environments which are wet, damp or humid, and in the presence of certain gases or ions such as oxygen, chlorides, nitrates, hydroxides, chromates, nitrates, sulphides, sulphates, etc. 

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