Anticorrosion Mechanism

Inhibited polymer films are distinguished from barrier-type films by their electrochemical activity. This property arises from the ability of these films to liberate Cl from the polymer matrix and transfer it to the mated metal surface. The Cl strongly affects the kinetics of electrochemical reactions leading to corrosion damage of metals. The inhibiting mechanism of corrosion can be treated in this case as follows. 

With this aim, the kinetics of the variation of the polarizing resistance (Rp) of a steel electrode has been studied [118] in the vicinity of its stationary potential when the electrode is in contact with an inhibited polymer film in aqua solutions of electrolytes (HCl and Na2S04). The films were produced by hose extrusion of PE-based compositions with PI (mineral oil) and oil-soluble Cl. 

It is common knowledge that there exists an inversely proportional relation between the polarizing resistance of the electrode system and corrosion rate, which significantly increases Rp upon contact with the electrolyte [119]. 

Comparing curves of Rp — tr (Fig. 2.44) [118], we conclude that, when there are no adhesive contacts of the inhibited film material and the electrode is absent, Rp and the kinetics of its variations will be defined by the penetrability and deformation of the film material along with the adsorption and hydrophobic properties of the Cl. 

The initial values of the polarizing resistance of the electrode in contact with modified PE films are 1.5-2 times higher than those of unmodified ones and are in agreement with the hydrophobic properties of the Cl. Hydrophoby rises through the series MO, SIM, Ukramin, AKOR. The kinetic curves of Rp —tr show two characteristic sections. The slope in the first second (initial) is dependent upon the PI and Cl content in the film material. The inclination of the second section remains constant for all samples. Consequently [118], Rp attenuation on the first section is the result of the contact area of the electrolyte and metal surface enlarging with time as the adsorbed Cl molecules are removed from it. The variation velocity of Rp in the second section is 

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Flake zinc pigments have a barrier effect and also act by a cathodic anticorrosive mechanism. Compared with zinc dust coatings, flake zinc pigments are formulated with lower pigment volume concentrations [5.178]. 

IRRAS can be extremely useful for studying in situ the corrosion and anticorrosion mechanisms [295]. For example, in order to understand high-temperature corrosion processes on AISI type 304 stainless steel, Guillamet et al. [284] measured the spectra by IRRAS of a steel plate exposed for 1 min to air at high temperatures. Comparison with the vlo bands of a series of oxides indicated that the main product is a-Fc203 (not Fc304, as suggested earlier for corrosion of... 

Various anticorrosion mechanisms have been described, the most common being ennobhng, dislocation of the oxygen reduction site, and the release of dopant anions. 

Although the Langelier index is probably the most frequently quoted measure of a water s corrosivity, it is at best a not very reliable guide. All that the index can do, and all that its author claimed for it is to provide an indication of a water s thermodynamic tendency to precipitate calcium carbonate. It cannot indicate if sufficient material will be deposited to completely cover all exposed metal surfaces consequently a very soft water can have a strongly positive index but still be corrosive. Similarly the index cannot take into account if the precipitate will be in the appropriate physical form, i.e. a semi-amorphous egg-shell like deposit that spreads uniformly over all the exposed surfaces rather than forming isolated crystals at a limited number of nucleation sites. The egg-shell type of deposit has been shown to be associated with the presence of organic material which affects the growth mechanism of the calcium carbonate crystals . Where a substantial and stable deposit is produced on a metal surface, this is an effective anticorrosion barrier and forms the basis of a chemical treatment to protect water pipes . However, the conditions required for such a process are not likely to arise with any natural waters. 

The mechanism of the action of zinc phosphate is shown in Figure 69. Zinc phosphate dihydrate pigment is hydrated to the tetrahydrate in an alkyd resin binder [5.84], The tetrahydrate is then hydrolyzed to form zinc hydroxide and secondary phosphate ions which form a protective film of basic iron(III) phosphate on the iron surface [5.80]. The anticorrosive action of zinc phosphate depends on its particle size distribution. Micronization improves the anticorrosive properties [5.85]-[5.87], The effect of corrosion-promoting ions on the anticorrosive properties of zinc phosphate is described in [5.88], [5.89],... 

The electrochemical action of red lead results from the fact that lead has valencies of 2 and 4 in lead orthoplumbate Pb(IV) compounds are reduced to Pb(II) in the cathodic region [5.147]. The chemical anticorrosive effect is a result of lead soaps that are formed when fatty acids in the binder react with the red lead. The lead soaps permeate the paint film as lamellae, and give good mechanical strength, water resistance, and adhesion to the steel surface. Furthermore, the corrosion-promoting chloride and sulfate ions are precipitated by lead(II) ions [5.148]. 

Maybe, the proposed mechanism of the action of zinc phosphate is more related to theoretical considerations than to fully proven knowledge. However, practical experience has shown that zinc phosphate is an active anticorrosive pigment, but the protective effect of lead and chromate pigments can only be achieved in certain systems [5.70]. 

Owing to their anticorrosive properties TPs (e.g., 205) serve as rust inhibitors in lubricants for magnetic recording materials (95JAP(K)07/65352) or in chemical-mechanical polishing systems (02JAP(K) 134442). 

The differences in the usage of rubber compounds for various applications. For example, compounds used for anticorrosive tank linings are quite different from compounds tailored for other dynamic working conditions and mechanical applications [1],... 

Matthews, 2000). A phosphate salt solution and an aqueous suspension of HAp powder were used as an electrolyte for PEO and EPD, respectively. The corrosion resistance was assessed by potentiodynamic polarisation tests in a buffered physiological solution (see also Kwok et al., 2009). The results of this study indicated that a hybrid combination of PEO and EPD produces an osseoconductive phase-pure ITAp layer and an anticorrosive titania interlayer that promise to provide good mechanical and biochemical stability in the highly corrosive environment of the human body. Other studies involving titania bond coats include Albayrak and Altintas, 2010 Paluszkiewicz, Dlugon andKwiatek, 2012 Rath etal., 2012 and Jain etal. (2013). 

The occurrence of A-nitrodiethanolamine at various levels in all sorts of aqueous fluids in the mechanical industry results primarily from the reaction between alkanolamines and nitrite ions. The alkanolamines, such as di- or tri-ethanolamines (or their derivatives) are used as anticorrosives, lubricants or emulsifiers. The nitrite ions are frequently present in trace amounts or they are intentionally added to inhibit corrosion (metalworking fluids). 

One of the reasons for local corrosion at the metal-polymer interface is sorption of electrolytes by polymers and permeability of the polymer barrier towards electrolytes. Sorption of electrolytes (acid solutions, bases and salts) leads to essential variation in the service characteristics of the protecting polymer coatings and anticorrosion packaging films under mechanical loads. These variations under mechanical loads, especially in seals and friction joints, are much deeper and can affect mechanisms of contact interactions. 

This non-equilibrium chemical and physical (mechanical) state of inhibited plastics is caused by incorporated Cl and their carriers (PI) in concentrations exceeding the thermodynamic compatibility threshold with the polymer. The thermodynamic non-equilibrium is a useful property of inhibited plastics since it expands their anticorrosion functional features. The greater the deviation from the non-equilibrium state of such systems, the quicker the relaxation processes in them accompanied by the release of excess Cl into the ambient medium and to the object being protected. The structure and properties of the components of inhibited plastics Cl depend on their composition and can be carried to the mated part by either diffusion in the gaseous (thanks to Cl volatility) or liquid phase via its exudation, its mixture with a PI, or washing of water-soluble Cl and diffusion through the material moisture. These processes bring about mechanical relaxation of the polymer matrix. The point at which the inhibited anticorrosion material moves into a state of thermodynamic equilibrium is usually correlated with its protective ability. 

Corrosion protection of metal ware by plastics containing contact inhibitors can be exercised according to the schema in Fig. 1.15, b. An anticorrosion plastic element 3 contains an inhibitor in the polymer matrix micropores. Element 3 contacts the protected metal surface 1. Corrosive medium 2 penetrates into the contact gaps and wets the article surface. Corrosion inhibitor 4 either spontaneously or under some effect, e.g. of mechanical forces, releases from the micropores of the polymer matrix into the contact gaps and adsorbs on the metal article surface. In case of mutual displacement of the contacting article 1 and element 3, spreading of the adsorption layer over the article surface is accelerated. 

Polyolefine films, mostly manufactured from polyethylene (PE), are most extensively used in anticorrosion techniques [8,12-20]. They are characterized by elasticity, low water and vapor permeability, mechanical strength, frost-resistance, inertness, perfect heat-sealing properties and non-toxicity. 

The structure and properties of inhibited films may alter within a wide range in response to their composition and production techniques. Their anticorrosion effect depends largely on the mechanism of Cl transportation... 

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