Yu. I. Kuznetsov

Corrosion inhibitors in conversion coatings. II

This review covers main approaches to development of inhibited conversion formulations for chemical oxidation of aluminum alloys. A mechanism explaining the formation of conversion coatings on aluminum alloys from chromate conversion baths is proposed; the protective properties of the resulting coatings are characterized. It is demonstrated that various modified methods for the preparation of chromate conversion coatings on aluminum enhance their protective properties, however, without solving ecological problems. Among alternative corrosion inhibitors used in chemical oxidation of aluminum alloys, molybdate-containing formulations are worth noting and have been much investigated. The most promising approach is to develop corrosion inhibitors for use during both the formation of a conversion coating and its subsequent filling with compounds chemisorbable at the modified oxide.

On the Inhibition of the Carbon Dioxide Corrosion of Steel by Carboxylic Acids

The inhibitive effects of aliphatic carboxylic acids on steel corrosion in the liquid and vapor phases of carbonate media were studied. It was shown that the acids become more effective with an increase in their hydrophobicity and can inhibit both the cathodic and anodic processes on steel because of their high adsorbabilities. Lauric acid was found to be the most effective inhibitor of carbon dioxide corrosion among the carboxylic acids studied. Although its molecule includes a relatively long alkyl fragment (C11H23), this acid is rather volatile and can be used as VCI. Caprylic acid in a concentration of 3.7 mmol/l inhibits steel dissolution in the temperature range from 30 to 100°C and increases the apparent energy of corrosion activation.

Corrosion Inhibition of Steel in Lithium Bromide Brines

The inhibiting effect of phosphomolybdenic acid (PMA) on the corrosion of steel (St3) in 65% lithium bromide solution has been studied in a temperature range from 80 to 230°C and at pH 5.0–8.5. Corrosion and electrochemical studies showed that the efficiency of PMA as a corrosion inhibitor increases with an increase in temperature and pH of the LiBr brine. The ambiguous behavior of PMA in solutions at low temperatures and pH was analyzed. A blend of PMA with a low-temperature inhibitor (IFK) was suggested. It was shown that these inhibitors manifest synergism and the composition has high inhibiting properties at elevated temperatures. The effect of PMA on the effective activation energy of the corrosion process was analyzed.

Inhibition of copper dissolution in water solutions of triazoles

The influence of the chemical structure of certain triazoles on the inhibition of copper dissolution in water solutions at pH 7.40 is studied by electrochemical and ellispometric means. It is established that adsorption of the studied triazoles on copper at potential E = 0.0 V relative to a normal hydrogen electrode is polymolecular, the first layer is described by the Frumkin equation with values of the free adsorption of energy values (−ΔGa0) = 50.5–70.1 kJ/mol. In addition, possible orientations of triazole molecules with respect to a surface of oxidized copper are revealed.

SiO2-based nanocontainers of a novel type: If the template micelles are functional, why remove them

A new method is proposed for creating containers based on mesoporous silica nanoparticles for diverse amphiphilic compounds. The method is based on the use of the micelles of required functional compound (i.e., the compound that is subsequently loaded into the containers) as a template when synthesizing nanoparticles. The prospects and advantages of this approach are exemplified by the synthesis of SiO2 particles using the micelles of cetyltrimethylammonium bromide (which can be used as an inhibitor of hydrogen sulfide corrosion) as a template.

New possibilities of metal corrosion inhibition by organic heterocyclic compounds

It is shown that many heterocyclic organic compounds, e.g., various azoles, are capable of adsorption on metals with formation of strong bonds with the surface. The approach based on the linear free energy relationship (LFER) principle offers a description of the efficiency of corrosion inhibition by these compounds and provides an understanding of the nature of chemical bonds formed by an organic inhibitor with a metal that needs to be protected. Knowledge of the specifics of interaction between azoles and metal surfaces can be helpful in choosing a promising corrosion inhibitor for drastic conditions, for example, in chemical mechanical planarization slurries where simultaneous mechanical polishing and electrochemical dissolution of the surface usually occur. Corrosion inhibition of steel at high temperatures (≥ 150°C) in mineral acid solutions is yet another example of a new successful use of azoles. It is demonstrated that new possibilities for improving the anticorrosive protection of metals are provided not only by using mixtures of some azoles with carboxylate-type organic corrosion inhibitors but also by constructing bilayer nanocoatings from aqueous solutions. For example, strong adsorption of small amounts of rather an exotic compound, dimegin (disodium salt of deuteroporphyrin), not only promotes the passivation of iron in neutral aqueous solutions but also impedes the local depassivation of the metal. Furthermore, it offers wide possibilities for further improvement of adsorption of other heterocyclic chemical compounds due to preliminary modification of the metal surface even by a small dimegin concentration.

Features of formation of magnetite coatings on low-carbon steel in hot nitrate solutions

The process of anticorrosion magnetite coating (MC) formation on low-carbon steel is studied in alkali-free nitrate converting media at the temperatures of 70–98°C reduced as compared to those used (130–145°C) in standard technologies of steel bluing: formation of such coatings in alkaline nitrate solutions. Alongside with the conventional corrosion-electrochemical methods of analysis of the formed MCs, the regularities of the MC surface reliefs were studied using the method of atomic force microscopy combined with the technique of flicker-noise spectroscopy (FNS) for processing digitized images and obtaining the parameters of the MC surface structure in different nanometer ranges. It was shown that it is necessary to introduce additives of metal nitrates with a low cation radius into the ammonium nitrate converting solution to obtain MCs with a high corrosion stability at the first stage of MC formation and the final stage must consist in the further “passivation” of MCs: MC treatment by aqueous solutions based on nontoxic carboxylates. According to the FNS analysis of the surface structure of the formed MCs, a significant decrease of the FNS “point” factor, an indicator of MC corrosion instability, occurred during the final treatment. On this basis, one could characterize quantitatively the results of accelerated corrosion tests: no steel corrosion occurred on the thus formed coatings for 42 days under standard severe conditions: 100% relative humidity and daily “showering”. The performed study reveals fundamental possibilities for solving the problems of standardization of the anticorrosion coating surface based on the analysis of their surface profile in the nanometer range.

Current State of the Theory of Metal Corrosion Inhibition

The general regularities of the action of metal corrosion inhibitors are analyzed. It is noted that the creation of inhibitors is a big scientific problem, requiring that achievements of many fields of physical chemistry be used. The limitations involved in application of adsorption inhibitors, whose theory of action has been developed most thoroughly, are considered. The importance and theoretical aspects of the development of composite materials and inhibiting hydrophilic monomers, that undergo polymerization following adsorption on metal surface, are discussed. The search for ways to manage the polymerization of inhibitors in thin adsorption layers can become a new efficient direction of corrosion inhibition. Special cases of inhibition, which cannot be rationalized in terms of the adsorption theory and which require that complexation be taken into account, are discussed. As the theory of corrosion inhibition develops, the scope of inhibitors expands. Besides the combination of cathodic protection with corrosion inhibitors, the latter are used more and more widely in conversion coatings and in water-borne paints. The strict technological and environmental requirements that are applied to inhibitors in this case do not prevent the progress of this scientific direction and do not diminish its prospects.

Adsorption of organic anions on iron in aqueous solutions: An ellipsometric study

Adsorption of anions of higher aromatic amino acids on the surface of iron from aqueous solutions is studied by an ellipsometric method concurrently with electrochemical measurements. It is demonstrated that ellipsometry is an effective method for studying in situ regularities governing the adsorption of anions of organic acids on the surface of metallic electrodes. Variations of ellipsometric parameters with the adsorbate concentration make it possible to obtain adsorption isotherms and determine adsorption characteristics.

On the zinc and copper dissolution in phosphate solutions

The copper and zinc dissolution in phosphate solutions is studied over wide pH range (from 4.5 to 11.7). The cathodic reaction rates are shown to depend on the pH of the solutions; they are minimum at a pH of 9.5 for zinc and 11.7 for copper. Phosphate directly participates in the cathodic process on zinc because of the reduction of phosphate to phosphite anions. Thermodynamic calculations, corroborated by electrochemical studies and analyses of the electrode surfaces by different methods, showed that zinc passivates itself by the deposition of either ZnO, Zn(OH)2, Zn(H2PO4)2, and ZnHPO4, in the solutions of low acidity, or ZnO, Zn(OH)2, ZnHPO4, and Zn3(PO4)2 at a pH of 9.5 to 11.7. On copper, the passive films are formed of CuH2PO4 and CuHPO4 in weakly acid phosphate solutions or of CuO, Cu(OH)2, CuHPO4, and Cu3(PO4)2 at a pH of 9.5 to 11.7.