Protecting oxidizable metals

R ently, the use of oxidized PANI has been r ognized as an inter ting means of protecting oxidizable metals. Debeny (I) was the first to show that a PANI film coated on stainless steel by electrochemical oxidation of aniline acted as a protective primer. This protection has been explained by a redox effect of the doped PANI, which maintains the metallic oxide film in its passive state. These result were confirmed by MacDiarmid et al. (2) and later, Wessling et al. (3) showed that chromating could be replaced by the direct deposition of a PANI dispersion on the metal. 

Galvanize. To protect an easily oxidizable metal (such as iron) with a less oxidizable metal (such as tin or zinc) by electric means Ref Hackh s Diet (1944), 366-R... 

Anode, sacrificial — a rather easily oxidizable metal, e.g., zinc, magnesium, aluminum, electrically connected with a metal construction to be protected from corrosion. Due to the formation of a -> galvanic cell the sacrificial anode is oxidized instead of the metal to be protected. Sacrificial anodes are the oldest and simplest means for electrochemical corrosion protection. 

Illustration of this effect in the case of a two-step process for aniline electropolymerization on mild steel and zinc from an aqueous electrolyte was reported by Lacroix et In acidic medium (the usual conditions for the electrodeposition of PANI) the direct electropolymerization of aniline on zinc or steel fails, because large amounts of metal dissolve before the aniline polymerizes. This can be avoided if the surface is first treated by depositing a thin polypyrrole film in neutral salicylate medium. This PPy layer behaves as a thin noble-metal layer and can be used for the electrodeposition of a PANI film of controllable thickness in an acidic medium. Using this pretreatment, no or very little metal dissolution occurs in this second step. The films exhibit very stable electroactivity in acidic electrolytes, similar to that of PANI deposited on platinum, which indicates that the underlying oxidizable metal is fully protected (Figure 16.1). 

Several strategies have been used to improve adhesion of ECP upon oxidizable metal. Bilayer coatings, composed of two types of ECP deposited electrochemically, are among these strategies and several groups have reported that they provide better protection by enhancing the adherence of the first layer to the metal. Many examples have already been described in Section 16.2 of this chapter. However, the interface between the metal and the first layer remains ill defined with no covalent bonds between the metal and the polymer. 

Recently, this difficulty was overcome when the starting diazonium was the aniline dimer (4-aminodiphenylamine or ADPA) [157]. In that case the grafted layer is electro-active and aniline oxidation remains possible in aqueous sulfuric acid and leads to a strongly adherent PANI film on the modified surface. The first experiments were performed on carbon and extension to an oxidizable metal has not yet been reported. However, as it has been proven that the covalent grafting of diazonium salts was also successful with iron and other oxidizable metals [178-182], we think that the same procedure might be applied in the case of iron. A strong increase of the adhesion should be expected, and therefore an improvement of the protection properties. 

It was also reported that sodium salicylate produced strongly adherent PPy film on various oxidizable metals in aqueous media by one stop electrosynthesis [16]. In the process, sodium salicylate with the metal ion produces a thin protective layer, which slows dissolution of the metal without hindering pyrrole monomer oxidation. 

It should be also emphasized that the recent applications involving bioelectrochemistry increase rapidly and steadily, while those for corrosion protection, although still in the infancy, seem promised to a brilliant future, especially when the ECPs are associated to other materials like sol-gel coatings. This latter field should probably open on extremely promising applications, especially in the field of the protection of very oxidizable metals like aluminum. 

Such a panel of solar cells is encapsulated by pol5rmeric encap-sulants to form a solar cell assembly. This assembly may be further sandwiched between two protective outer layers to form a weather resistant module. The protective outer layers are formed from sufficiently transparent material to allow photons to reach the solar cells. In modules, where PVB is used as the encapsulant material, it has been foimd that the PVB tends to discolor over time, when in contact with an oxidizable metal component. 

Thiols on oxidizable metals have been proven to act as good molecular corrosion inhibitors [97-99,104], provided the interface between the thiol group and the metal is well controlled. On nickel, this molecular barrier can resist several weeks to protect against corrosion [90]. However, mostly this happens by the formation of a thick and compact inhibitor film, which is not discussed further here. 

The electroless nickel/electroless palladium/immersion gold (ENEPIG) finish is simply an ENIG finish into which an intermediate layer of palladium (0.1 to 0.5 pm thick) is deposited between the electroless nickel and immersion gold coatings. Palladium is deposited from an autocatalytic plating bath. Palladium is a noble metal applied to protect oxidizable Ni. Palladium (m.p. 1552°C) is not fusible but rather dissolves in the molten solder in a manner similar to gold, and... 

It should exhibit a spontaneous self-healing property for self-repair in case failure occurs due to cracking or spallation of the layer. So the coating should act as a reservoir for the highly oxidizable metallic constituent(s) for early development of a protective scale. 

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