Lead anodic corrosion

Lea.dAnodes. A principal use for lead—calcium—tin alloys is lead anodes for electrowinning. The lead—calcium anodes form a hard, adherent lead dioxide layer during use, resist corrosion, and gready reduce lead contamination of the cathode. Anodes produced from cast lead—calcium (0.03—0.09 wt %) alloys have a tendency to warp owing to low mechanical strength and casting defects. 

Copper-containing lead alloys undergo less corrosion in sulfuric acid or sulfate solutions than pure lead or other lead alloys. The uniformly dispersed copper particles give rise to local cells in which lead forms the anode and copper forms the cathode. Through this anodic corrosion of the lead, an insoluble film of lead sulfate forms on the surface of the lead, passivating it and preventing further corrosion. The film, if damaged, rapidly reforms. 

Purification actually starts with the precipitation of the hydrous oxides of iron, alumina, siUca, and tin which carry along arsenic, antimony, and, to some extent, germanium. Lead and silver sulfates coprecipitate but lead is reintroduced into the electrolyte by anode corrosion, as is aluminum from the cathodes and copper by bus-bar corrosion. 

A note of caution about roof fasteners. A common mistake is to fix a galvanised or aluminium roof in place with nails or screws of a different metal copper or brass, for instance. The copper acts as cathode, and the zinc or aluminium corrodes away rapidly near to the fastening. A similar sort of goof has been known to occur when copper roofing sheet has been secured with steel nails. As Fig. 24.6 shows, this sort of situation leads to catastrophically rapid corrosion not only because the iron is anodic, but because it is so easy for the electrons generated by the anodic corrosion to get away to the large copper cathode. 

With metals other than Fe, the percent of the ac current leading to corrosion can be considerably different. Cu and Pb behave similarly to Fe [36], whereas A1 [36] and Mg [39] corrode much more severely. This has to be watched with sacrificial anodes of these materials if they are subjected to ac. 

From these two examples, which as will be seen subsequently, present a very oversimplified picture of the actual situation, it is evident that macroheterogeneities can lead to localised attack by forming a large cathode/small anode corrosion cell. For localised attack to proceed, an ample and continuous supply of the electron acceptor (dissolved oxygen in the example, but other species such as the ion and Cu can act in a similar manner) must be present at the cathode surface, and the anodic reaction must not be stifled by the formation of protective films of corrosion products. In general, localised attack is more prevalent in near-neutral solutions in which dissolved oxygen is the cathode reactant thus in a strongly acid solution the millscale would be removed by reductive dissolution see Section 11.2) and attack would become uniform. 

Lead is characterised by a series of anodic corrosion products which give a film or coating that effectively insulates the metal mechanically from the electrolyte (e.g. PbS04, PbClj, PbjO, PbCrO<. PbO, PbO, 2PbC03.Pb<0H)z), of which PbS04 and Pb02 are the most important, since they play a part in batteries and anodes. Lead sulphate is important also in atmospheric passivation and chemical industry applications. 

Pavlov, D. and Rogachev, T. Dependence of the Phase Composition of The Anodic Layer on Oxygen Evolution and Anodic Corrosion of Lead Electrode in Lead Dioxide Potential Region , Electrochim. Acta., 23, 1237 (1978)... 

Electrolyte-sulphuric acid (5% wt.%) plus an inhibitor (0-5kgm ) such as diorthotolyl thiourea, quinoline ethiodide or /3-naphthol quinoline. The temperature should be 75°C, the cathode current density 2000 Am and the time of cathodic polarisation 3 min. The anode should be carbon or lead. If lead anodes are used, lead may deposit on the specimens and cause an error in the weight loss. If the specimen is resistant to nitric acid the lead may be removed by a flash dip in 1 1 nitric acid. Except for this possible source of error, lead is preferred as an anode, as it gives more efficient corrosion product removal. 

Fig. 9. Indirect electrosynthetic process for the oxidation of anthracene to anthraquinone [10]. The charcoal column serves to remove organics which lead to corrosion and poisoning of the lead dioxide (Pb02) anode...

Traditionally, anodic oxygen evolution from acid solution—particularly from aqueous electrolytes containing sulfuric acid—has been performed at lead anodes that are passivated and stabilized against corrosion by a selfforming coating of Pb02. 

In the development of photoelectrochemical (PEC) solar cells, one of the most difficult problems is the corrosion problem. In any solvent, but particularly in solvents with water present, anodic currents flowing from the solid to the solution will usually lead to corrosion. Specifically the corrosion will take the form of anodic oxidation of the semiconductor, with the products remaining as a film, dissolving into the solution, or evolving as a gas. Any such action will degrade the solar cell. 

Apart from simple corrosive attack on a metal surface, Gabel reported a more specific situation leading to corrosion. This occurred when a dry-film-lubricated metal surface was in contact with an unlubricated anodized aluminium surface. Laboratory investigation at 95% relative humidity and 49°C (120°F) confirmed that in this situation the corrosion resistance of the anodized aluminium was reduced from 1100 hours to fewer than 200 hours. It was not clear whether dissimilar metals were involved, but the occurrence suggested that corrosion might be due to the development of an external potential in the presence of an electrolyte. 

The oxidation of aniline in sulfuric acid was studied by Kirk et al. on a packed-bed anode made up of 1 mm spherical lead pellets in contact with a pure lead anode collector plate [27]. The lead pellets were oxidized to lead dioxide for an hour in sulfuric acid (pH = 2) at 300 A/m. No mention was made of the corrosion resistance of these anodes. The anodic destruction of aniline to CO2 was found to occur through the formation of benzoquinone and maleic acid. At a current of 2 A, the initial rate of aniline (2.7 mM) oxidation was very rapid with more than 90% of the initial aniline being oxidized within one hour. After 5 h of operation, 72.5% of the... 

Pure or alloyed lead may be employed as anode in sulfuric acid addition of 1% silver, 0.3% tin, and a little cobalt raises its resistance toward corrosion. Other metals may improve the yield of a given electrode process. Thus, addition of antimony and cadmium to a lead anode [152] is advantageous in the oxidation of o-toluenesulfonamide to o-benzoylsulfonimide (saccharin). The same effect may be obtained, however, by using an uncoated, unalloyed lead anode if Sb203 is added to the anolyte [153]. It seems possible that the dissolution of some antimony from the alloyed lead anode takes place and produces the same effect as the Sb203 in the anolyte. 

The membranes on the market today are more stable than those available in the 1970s. but they would probably not be suitable. One critical problem occurring in all electrolyses in divided cells using organic solvent arises from solvent migration. Even low concentrations of organics in the anolyte can lead to severe problems with regard to anode corrosion. 

The role of O2 is here twofold first it acts as an electron acceptor, secondly it is involved in the anodic corrosion reaction leading to the formation of SO4. This is in agreement with the results obtained with CdS electrodes as discussed in Section 8.1.4. Interestingly, the corrosion rate was considerably reduced after treatment of the particles in a solution of Cd " ions, which led to a blocking of S or HS radical sites at the surface, as already described in Section 9.1.1 [39]. Obviously, the formation of S radicals by holes is hindered. 

Several other attempts have been made by various authors to avoid anodic corrosion at n-type electrodes and surface recombination at p-type electrodes, by modifying the surface or by depositing a metal film on the electrode in order to catalyse a reaction. It has been frequently overlooked that the latter procedure leads to a semiconductor-metal junction (Schottky junction) which by itself is a photovoltaic cell (see Section 2.2) [14, 27]. In the extreme case, then only the metal is contacting the redox solution. We have then a pure solid state photovoltaic system which is contacting the solution via a metal. Accordingly, catalysis at the semiconductor electrode plays a minor role under these circumstances. 

Lead Alloying lead with silver, tin, or cobalt often improves the corrosion resistance of lead anodes. In many cases, the surface of a lead anode is acmaUy lead dioxide [58]. Pb02 electrodes are stable in sulfate media at low pH and the oxygen overpotential is high, but the material has poor mechanical properties and corrodes in HCl. 

Halide ions, according to the adsorption theory of passivity, tend to break down passivity by competing with the passivator for adsorption sites on the metal surface. Should a halide ion find a vacant site and closely approach the surface, hydration and dissolution of metal ions are favored, and the anodic reaction can proceed with low activation energy, in contrast to the high activation energy required when a passivator is adsorbed. The anode reaction, if it persists, is confined to localized areas where the competitive process first succeeds, because surrounding metal immediately becomes cathode of an electrolytic cell, and is protected by flow of current from further anode activity, a process called cathodic protection. This attack at specific sites leads to corrosion pitting typical of metals otherwise passive that are actually corroded by their environment. 

The anodes are made of various materials and the choice is determined by the physical conditions, the electric field pattern, current densities, cost and anode corrosion. Anode current densities vary between 10 amperes per metre squared for silicon iron to more than 1000 amperes per metre squared for platinised and lead alloys. 

Furthermore, even if such a condition is reached and current circulates through the reinforcement, this does not automatically lead to corrosive attack, since the anodic process that takes place at potentials higher than about +600 mV SCE is oxygen evolution, instead of iron dissolution. Nevertheless, attack may occur if the current flows for sufBciently long periods of time [4,5]. Initiation of corrosion can be ascribed to the depletion of the alkahnity in the vicinity of the anodic area promoted by the anodic reaction of oxygen evolution (2H2O —> O2 + 4H + 4e. ... 

With regard to the fact that the anodic metal dissolution leads to corrosive... 

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