A.M. Shams El Din

Corrosion inhibition by naturally occurringsubstances—I. The effect of Hibiscus subdariffa (karkade) extract on the dissolution of Al and Zn

The thermometric, the weight-loss and the galvanostatic polarization techniques wereused to establish the inhibition of the dissolution of Al and Zn in HCl and NaOH by different concentrations of aqueous extract of Hibiscus subdariffa (Karkade). The extent of corrosion inhibition as measured by the three techniques is comparable. The results indicated that the additive acts by way of adsorption on both cathodic and anodic corrosion areas. Curves representing the variation of the reaction number (R.N.), in thermometric experiments, and the decrease in weight as a function of the concentration of the additive, are invariably sigmoid in nature. When present in enough amounts, the additive decreases the dissolution rate by as much as 85 per cent of the value recorded in its absence. The two main constituents of the aqueous extracts of Hibiscus subdariffa, namely the organic acids and the colouring materials were separated and tested independently for surface activity. Both constituents were effective in retarding the dissolution of the two metals; but the activity of the colouring portion was considerably higher than that of the organic acids.

The behaviour of the copper electrode in alkaline solutions upon alternate anodic and cathodic polarization

Abstract Two main views are found in the literature regarding the type of oxide formed on copper anodes in alkaline solutions. The one considers that copper oxidizes to Cu2O and CuO (or Cu(OH)2) before oxygen evolution. Muller is, however, of the opinion that a still higher oxide, Cu2O3, forms. The discrepancy has been clarified by studying the behaviour of the copper electrode upon repeated anodic oxidation and cathodic reduction. Both views are correct. Copper oxidizes only to Cu2O and Cu(OH)2 in the first anodic half cycle and more to Cu2O3 at advanced cycles. Cu2O3 results from the oxidation of HCuO2− ion. The passivation of copper follows the relation iτ1/2 ⋍ 80(OH−), i being the current and τ the time elapsing until oxygen evolution starts. Cu2O and Cu(OH)2 are reduced at more negative potentials than their formation values. This is related to the semi-conducting properties of these oxides.

On the anodic passivity of tin in alkaline solutions

Abstract The anodic oxidation of tin was studied galvanostatically in NaOH solutions of different concentrations. Primary passivity is attained in all solutions when the metal is covered with a film of Sn(OH) 2 or SnO. Permanent passivity sets in when Sn(OH) 4 forms as a continuous layer on the electrode surface. In concentrated alkali solutions larger quantities of electricity are charged to the electrode to compensate for the chemical dissolution of the hydroxides. As a result of the competition between the anodic formation and chemical dissolution of Sn(OH) 4 , the potential of the tin electrode oscillates between the Sn/Sn(OH) 2 and Sn(OH) 2 /Sn(OH) 4 , values. For any one solution the oscillation phenomenon depends primarily upon the polarizing current. Permanent passivity is governed by the relation ( i - i 0 )τ n = const. where i 0 is the current below which no passivation takes place, τ the time elapsed till oxygen evolution and n is a constant. Interruption of the polarizing current when the electrode is at oxygen-evolution potential gives rise to a pH-independent arrest at potentials more positive than any of the known redox values for tin. This arrest is ascribed to the chemical dissolution of metastannic acid formed at oxygen-evolution potentials. The anodic behaviour of the tin electrode in solutions containing stannite and stannate is similar to that in solutions devoid of these ions. An increase in the tin content of the solution is equivalent to the elevation of the polarizing current; passivity sets in more readily in the concentrated solutions.

Efficiency of organic acids and their anions in retarding the dissolution of aluminium

Abstract The effect of a number of aliphatic, aromatic and amino acids, as well as of their anions, on the dissolution of Al in 2N HC1 and in 1·5N NaOH was studied by the thermometric method. In both media the additives caused a decrease in the maximum reaction temperature and a corresponding reduction in the reaction number (R.N.). Curves representing the variation of the percentage decrease in R.N. with the logarithm of the molar concentration of the additives were constructed. These reveal, in the case of aliphatic acids, that the additives are adsorbed through their carboxylic groups, and that dissolution-inhibition is independent of the chain length. Aromatic acids, on the other hand, appear to lie flat on the surface of the corroding metal. Amino acids in HCI are stronger corrosion inhibitors than the corresponding aliphatic acids, suggesting that adsorption occurs through both the protonated −COOH and −NH 2 groups. In acid solutions all additives act as cathodic inhibitors. The acid anions are weaker corrosion restrainers in NaOH than the corresponding acids in HCl. Anomalies in the R.N.-log C curves of amino acids in alkaline solutions are recorded and are explained on the basis of a change in the mode of adsorption of the additive anions.

On potentiometric acid-base titrations in molten salts: The system K2Cr2O7K2CrO4KOH.☆

An experimental procedure is described for the potentiometric titration of the acid K2Cr2O7 with the base KOH in molten KNO3 at 370°. An oxygen electrode is used as indicator electrode. The titration curves obtained are characterized by a potential drop of ca 700 mV at the equivalent point. The curves are analysed theoretically. Above 400° the dichromate attacks the nitrate substrate, as also does the acid CrO3 at 370°. The titration of molten KOH with K2Cr2O7 does not follow expectation.

Mechanism of corrosion inhibition by sodium molybdate

Abstract Sodium molybdate, Na2MoO4, is a non-toxic, environment-friendly corrosion inhibitor for the protection of soft-water cooling systems. The agent is classified as anodic inhibitor, but its use requires the presence of oxygen or other oxidizing agents in solution. Also, the lack of inherent oxidizing characteristics allows its mixing with a large number of organic compounds, promoting inhibition synergism. Little and contradicting information is known about the mechanism of action of MoO42− as corrosion inhibitor. We have examined in some detail the inhibition of corrosion of mild-steel by Na2MoO4. Work was carried out in distilled and tap waters as well as in NaCl and Na2SO4 solutions of increasing concentrations. The nature of the MoO42−/steel surface interaction was established from the measurement of the variation with time of the open circuit potential (OCP) of steel coupons in the test solutions under a wide variety of conditions. The type of potential/time curve obtained depended primarily on whether or not dissolved oxygen was present in the solution. In air-free distilled water, the OCP of the steel changed towards more negative values and a well defined arrest was recorded. The arrest represented the destruction of the air-formed oxide film on the metal surface. The length of the arrest increased with the MoO42− content in the solution. The additive fortified to some extent the oxide film. The OCP ultimately tended to a constant final steady potential, Est, corresponding to the free corrosion potential under immune conditions. On the other hand, in aerated water the potential tended directly towards positive values, denoting the ready passivation of the steel. In aerated tap water, a different behavior was recorded. Dilute MoO42− solutions promoted corrosion and led to final negative potentials. Concentrated solutions, on the other hand, induced passivation. The active/passive transition followed an S-shaped curve when the final steady-state potentials were plotted as function of the logarithm of additive concentration. The point of inflexion of the curve was the threshold concentration of the inhibitor. The same behavior as in tap water was noted in solutions of NaCl and Na2SO4. Two factors influenced the value of the threshold concentration. The first was the concentration of foreign ions in the solution. The threshold concentration, Cinh, moved towards higher values when the concentration of aggressive anions (e.g., Cl− or SO42−), Cagg, was increased. The two variables were related as log Cinh = a + b log Cagg, where “a” and “b” were constants. The constant “b” was the ratio of electric charges carried by the two counter-acting anions. The double logarithmic relation signified the occurrence of competitive adsorption. A rise in solution temperature also augmented Cinh. The plot of log Cinh as function of the reciprocal of absolute temperature (Arrhenius plot) was linear. The activation energy of passivation was calculated to be 17.33 kcal/mole. The high value suggested chemical bond formation. The factors influencing the stability of the passive state were identified. These were: the time of treatment with the inhibitor, temperature, type and concentration of foreign ion in solution, continuous supply of O2 and eventual reduction in inhibitor concentration. X-ray mapping of the distribution of Mo on the surface suggested the role of MoO42− and O2 to be the healing and repair of defects in the air-formed oxide. Prior to bond formation, the two species were assumed to be absorbed on the surface.

Film Thickening on Nickel in Aqueous Solution in Relation to Anion Type and Concentration

AbstractThe variation of the open circutt potential of the Ni electrode in strongly aerated soluiion of different concentrations of Cl−, Br−, I−, NO−3, SO2−4 and CrO2−4 is followdd till steady-stete values are established. In all solutions the steady potentials, Esb, are approached from negative values, denoting film repair and thickening. Except in relatively concentrated Cl− media, no Ni2+ is detected in solution. Plots of the Ni electrode potential, E, as a function of the logarithm of immersion time, t, are linear, satisfying the general relationship:E = al + b1log t ……(1)where a1 and b1 are constants. Equation (1) is derived theoretically by taking into account the p-conducting properties of NiO, and assuming that the adsorption of the anions on the oxide film generates the electric field necessary to promoee the diffusion of Ni2+ through the oxide. The rates of oxide thickening in the presenee of different concentrations of the various anions are computed and discussed. Expressions are deduced relat...

Corrosion inhibition by molybdate/polymaliate mixtures

Abstract The inhibition of the corrosion of mild steel in continuously aerated potable water by the addition of sodium molybdate, Na2MoO4, was examined by two independent techniques. The first involved the measurement of open circuit potentials of coupons as a function of time till steady-state, constant potentials, Est, were established. The plot of Est as a function of the logarithm of [MoO42−] disclosed consecutive domains of active corrosion, localized (pitting) corrosion and full passivation. The curve did not allow, however, the evaluation of corrosion rates. This information was ascertained from weight loss determination. The rate of corrosion/log [MoO42−] was mirror image of the potential curve. BELGARD EV® (BELEV), a polymeric maleic anhydride preparation used as an anti-scalant, acted in the neutralized state as a moderate adsorption inhibitor affecting anodic sites. Complete surface coverage was ensured in the presence of ca 100 ppm inhibitor in solution. Mixtures of molybdate and BELEV exhibited inhibition synergism. The combined effect of the mixture exceeded the sum produced by the two individual agents. Differences in behaviour was noted between solutions containing low (30 ppm) and high (≥50 ppm) BELEV. The former solutions failed to inhibit corrosion completely whatever the concentration of MoO42− was. With higher BELEV additions inhibition was achieved at definite [MoO42−]/[BELEV] ratios. These were the compositions which raised the metal potential to or above that characterizing a pitting attack in MoO42− solutions only. Visual observation of the metal coupons and test solutions confirmed the above conclusion. Successful mixtures of MoO42− and BELEV could be worked out which ensured complete inhibition and substantial cost savings.

On the pitting corrosion of tin in aqueous solutions

Abstract Four independent techniques were employed to prove that Sn undergoes pitting corrosion. The first was based on the measurement of the variation of the open circuit potential of the Sn electrode in aerated Cl − solutions of various concentrations. Steady-state potentials were attained slowly and erratically from negative values, and were more positive the higher the dilution. Sn electrodes prepassivated in CrO 4 2− solutions responded readily to additions of Cl − ion. The potentials developed were more noble than those measured in presence of the passivator alone, and changed to positive values with the increase of the concentration of the pitting corrosion agent. Attack was under cathodic control. Galvanostatic polarization of the Sri electrode was carried out in 0.005 to 0.1M NaOH, in the presence of various additions of Cl − . Above a certain Cl − content, contingent upon the alkali concentration, the aggressive anion prevented the evolution of Oa on the electrode, and oscillations in the E−t curves were recorded. Competitive adsorption of Cl − and OH − is assumed to occur, which affected both the quantity of electricity, Q p , consumed along the oxide formation steps, and the rate of potential increase, d E /d t , following oxidation. Plots of the two variables as a function of the Cl − ion concentration exhibited a definite break at the value characteristic for the initiation of pitting attack. Potentiodynamic polarization showed that the pitting corrosion potential progressively shifts towards negative values as the concentration of the aggressive agent in solution was increased. The pitting corrosion currents resulting from the addition of Cl − to Sn electrodes prepassivated in CrO 4 2− solutions were measured. The dependence of the maximum currents on solution composition is explained on the basis of competitive adsorption.

Efficiency of alkylamines and alkylammoniumions in retarding the dissolution of Al

Abstract The dissolution of Al in HCI and NaOH solutions of varying concentrations has beenstudied by the thermometric method, and the dependence of the rise in temperature (ΔT) and the reaction number (R.N.) on solution concentration has been established. The results indicate that metal dissolution in each medium follows a different mechanism. The effect of concentration of a number of alkylamines on the dissolution of Al in 3N HCI and in 2N NaOH has been examined; these additives have been shown to cause a lowering of R.N. The alkylamines are more strongly adsorbed than the alkylammonium ions, and interfere with the anodic reaction. The curves relating the percentage decrease in R.N. to the logarithm of inhibitor concentration are invariably sigmoid in shape, and the inflexions on the curves depend on both the medium and amine used. Inhibition increases with the chain length and number of the alkyl groups of the amine. Retardation of dissolution is due to blocking of the surface by the additive. A relation has been established between the cross-sectional areas of the amine molecules and the concentration necessary to cut down dissolution to the same extent.