Protective film theory

Several theories have appeared in the Hterature regarding the mechanism of protection by -PDA antiozonants. The scavenger theory states that the antiozonant diffuses to the surface and preferentially reacts with ozone, with the result that the mbber is not attacked until the antiozonant is exhausted (25,28,29). The protective film theory is similar, except that the ozone—antiozonant reaction products form a film on the surface that prevents attack (28). The relinking theory states that the antiozonant prevents scission of the ozonized mbber or recombines severed double bonds (14). A fourth theory states that the antiozonant reacts with the ozonized mbber or carbonyl oxide (3) in Pig. 1) to give a low molecular weight, inert self-healing film on the surface (3). 

In spite of many objections, ozone scavenger and protective film theories cannot be neglected because of serious experimental evidence. They should be considered as an important part of the overall antiozonant mechanism. Their role prevails in rubber solution or in very thin rubber films. It is very probable that a part of an antiozonant is wastefully depleted just because of direct ozonation. 

Involvement of the Ozonized Rubber Moieties in Antiozonant Mechanism. The rubber chain relinking theory (30) is consistent in part with the self-healing film formation theory (37) a reaction between an antiozonant or some of its transformation products and ozonized elastomer is considered. Either scission of ozonized rubber is prevented in this way or severed parts of the rubber chain are recombined (i.e., relinked). A "self-healing" film resistant to ozonation is formed on the rubber surface. Such a film formed by the contribution of nonvolatile and flexible fragments of the rubber matrix should be more persistent than any film suggested in the protective film theory. 

Surface spectroscopy also supports preferential attack of ozone on the antiozonant. Although all antiozonants must react rapidly with ozone, not all highly reactive materials are antiozonants. Something else in addition to the scavenging effect is required. The protective film theory contends that ozonized products, to... 

Objections to the protective film theory are less clear cut. Murray (1972) has stated that if the film were to be composed only of antiozonant or ozonized antiozonant, then some rather unrealistic rates of diffusion would have to be assumed in order to form such a film in the time period observed . On the other hand the results of Andries et al (1975), who studied the surface of ozonized natural rubber compounds by attenuated total reflectance spectroscopy obtained results consistent with this theory (and for that matter the scavenger theory) but not with the re-linking and diversionary theories. 

The development of acidity within an occluded cell is by no means a new concept, and it was used by Hoar s as early as 1947 in his Acid Theory of Pitting to explain the pitting of passive metals in solutions containing Cl ions. According to Hoar the Cl ions migrate to the anodic sites and the metal ions at these sites hydrolyse with the formation of HCl, a strong acid that inhibits the formation of a protective film of oxide or hydroxide. Edeleanu and Evans followed the pH changes when aluminium was made anodic in Cl solutions and found that the pH decreased from 8-8 to 5-3. 

Theory of protective film formation supposes creation of a rubber surface film from PD oxidation and/or ozonation products. 

The precise mechanism resppnsible for the passivity conferred on metals by anodic inhibitors, such as chromate, is not known. While some early workers thought that a protective salt film (e.g., chromate) was formed, this view is not generally applicable, since passivity can occur in a system where the salt film would be freely soluble (e.g., iron in nitric acid). It is, however, generally accepted that passivity is associated with the formation of a protective film, and current views ascribe the action of anodic inhibitors either to adsorption at anodic sites or to continuous repair of the protective film. The former view has received attention in recent publications by Cartledge ), while the latter is favored by Evans (2). However, work on aluminum has suggested that true passivity is associated with the crystal structure of the film, which in turn determines its stability. This principle has recently been introduced by one of the authors (3) and is developed below into a general theory of passivity. 

The Oxlde-Fllm Theory. The oxlde-f11m theory describes the state of Improved corrosion resistance through the formation of a protective film on the metal substrate this consists of the reaction products of the metal with its environment. Such a film Is a new phase, even If It Is as thin as a single monolayer.(21) Electron diffraction (21) and elllpsometric(33) studies give the experimental evidence for the theory. In this case the physicochemical properties of the metal relative to a corrosive medium depend to a large degree on the properties of the protective film. The properties of the film, however, are not uniquely determined. 

The relinking and self-healing film theories require chemical interaction between the antiozonant, or ozonized antiozonant, and the rubber or ozonized rubber. The evidence for these interactions is sparse in the literature. The products of the ozone-antiozonant reaction are soluble in acetone. Thus if only the scavenger and protective film mechanisms are operative, no nitrogen from the antiozonant should be left in the rubber after ozonation and subsequent acetone extraction. Nitrogen analyses of extracted rubber showed, however, that some of the nitrogen was unextractable this nitrogen was presumably attached chemically to the rubber network. 

The structure of the passive film on alloys, as with passive films in general, has been described both by the oxide-film theory and by the adsorption theory. It has been suggested that protective oxide films form above the critical alloy composition for passivity, but nonprotective oxide films form below the critical composition. The preferential oxidation of passive constituents (e.g., chromium) may form protective oxides (e.g., Cr203) above a specific alloy content, but not below. No quantitative predictions have been offered based on this point of view, and the fact that the passive film on stainless steels can be reduced cathodically, but not stoichiometric Cr203 itself, remains unexplained. 

Faraday, to whom Schonbein s first communication was sent as a letter, made further experiments. In his second paper he gives a history of earlier work previously unknown to him and Schonbein. As a result of experiments Faraday concluded that the peculiar condition of iron is due to the protection of the iron by a coat of oxide. .. so thin as not to be sensible , or, alternatively that the superficial particles of the metal are in such relation to the oxygen of the electrolyte as to be equivalent to an oxidation.. . having thus their affinity for oxygen satisfied , which would cover several theories proposed later. The oxide-film theory was experimentally confirmed by U. R. Evans. ... 

Both the protective-film-rupture theory and the high-index-plane theory may be valid for one and the same system, depending on conditions. This has been demonstrated by the behavior of an iron wire strained in an aerated nitrate solution.When straining was done in the active region of potentials, the observed increase of anodic current was of the magnitude expected on the basis of the high-index-plane theory (within a power of 10). However, in the passive region, a 1500-2000-fold increase was observed which can be explained only in terms of the film-rupture mechanism. 

There are two theories regarding the formation of these films. The first theory states that the film formed is a metal oxide or other reaction compoimd. This is knovm as the oxide film theory. The second theory states that oxygen is adsorbed on the surface, forming a chemisorbed film. However, all chemisorbed films react over a period of time with the imderlying metal to form metal oxides. Oxide films are formed at room temperature. Metal oxides can be classified as network formers, intermediates, or modifiers. This division can be related to thin oxide films on metals. The metals that fall into network-forming or intermediate classes tend to grow protective oxides that support anion or mixed anion/cation movement. The network formers are noncrystalline, whereas the intermediates tend to be microcystalline at low temperatures. 

In contrast to cathodic protection, anodic protection is relatively new. Edeleanu first demonstrated the feasibihty of anodic protection in 1954 and tested it on small-scale stainless steel boilers used for sulfimc acid solutions. This was probably the first industrial apphcation, although other experimental work had been carried out elsewhere. This technique was developed using electrode kinetics principles and is somewhat difficult to describe without introducing advanced concepts of electrochemical theory. Simply, anodic protection is based on the formation of a protective film on metals by externally applied anodic currents. Anodic protection possesses unique advantages. For example, the applied current is usually equal to the corrosion rate of the protected system. Thus, anodic protection not only protects but also offers a direct means for monitoring the corrosion rate of a system. As an... 

Many chemists and engineers believe that, in order to have a good protective film, one must have a third component in addition to the polar heads attached to the surface and the coat of hydrocarbon tails. The third component is a film of oil attracted to the hydrocarbon tails. This theory gains support from the fact that some of the semipolar film formers are not effective in 100% aqueous systems, where there is no oil available to form a film over the hydrocarbon tails. ... 

The relinking (14) and self-healing film (3) theories require chemical interaction between the antiozonant and ozonized mbber. The evidence for these interactions is meager (35,36). Overall, there seems to be no clear evidence in the Hterature for PDA derivatives becoming attached to mbber chains as a result of ozone attack. Much fundamental work in this area remains to be done, however. It seems clear at this point that any antiozonant—mbber interaction must be much less important than the scavenging effect of the antiozonant. In summary, the scavenger model is beheved to be the principal mechanism of antiozonant action. Ozone—antiozonant reaction products form a surface film that provides additional protection (37). 

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