R.A. Mohammed

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.

The problem of alkaline scale formation from a study on Arabian Gulf water

Abstract An Experimental procedure is developed to study alakaline scale formation from Arabian Gulf water. The brine is refluxed at increasing temperatures under N2 bubbling for five hours. After cooling to room temperature the water is analyzed for HCO−3, CO2−3 in solution and for precipitated CaCO3 and Mg(OH)2. The results indicate the presence of appreciable quantities of soluble CO2−3 exceeding the limits of the solubility product of CaCO3, and reveal the inadequacy of thermodynamic approach for studying scale formation. Solid CaCo3 formation in measurable quantities starts above ca. 65°C, reaches a maximum at 80 °C and declines to a constant value at 90 °C. Mg (OH)2 precipitation starts around 75 °C and increases steadily with rise in temperature. The OH− ions necessary for the precipitation of Mg(OH)2 originate from the thermal decomposition of the CO2−3 moiety of the CaCO3 supersaturating the brine. Experiments with water enriched with additional NaHCO3 result in a marked increase in the deposition of CaCO3. In the meantime the supersaturation with CaCO3 and the formation of Mg(OH)2 decrease to constant values. Factors leading to the formation and stabilization of CaCO3−supersaturated solutions are considered to determine the nature and composition of alkaline scales.

Inhibition of the thermal decomposition of HCO3− A novel approach to the problem of alkaline scale formation in seawater desalination plants

Abstract Alkaline scale formation in seawater desalination plants is triggered by the thermal decomposition of the HCO3−ion. The various techniques used for retarding or eliminating scaling in evaporation units are reviewed in brief. The thermal decomposition of HCO3− is a kinetically controlled, first-order reaction. Like all rate reactions, it is prone to be catalyzed or decelerated by specific agents. Whilst catalysis by Ca2+ ions is well documented, inhibition has never been reported before. Three polymaleate compounds, commonly used as anti-scale agents, were discovered to inhibit HCO3− decomposition. The extent of inhibition depended on the nature and concentration of the additive used. The rate constants of the decomposition reaction were computed at 100°C. The tested compounds were, however, relatively weak inhibitors. Inhibition of the thermal decomposition of HCO3− presents a novel approach to combat scale formation, hitherto not applied before. The present paper is a call for a search for better inhibitors, to be combined with polymers with dispersing activity, to obtain new anti-scale agents with powerful properties.

On the thermal stability of the HCO−3 and the CO2−3 ions in aqueous solutions

Solutions of HCO−3 of concentrations similar to that of the Arabian Gulf water were heated for 5 h between 27.5 and 100°C under reflux and N2 purging. These were then analyzed potentiometrically against standard HCl. Analysis of the titration curves revealed that the conversion of HCO−3 into CO2−3 followed first order kinetics with an activation energy of 17.85 kcal/mol. Partial conversion of CO2−3 into OH− ions occurred when heating was carried out long enough at high temperatures. The implication of these findings on the general problem of alkaline scale formation from seawater is considered.

Contribution to the problem of vapour-side corrosion of copper-nickel tubes in MSF distillers

Abstract The paper describes the vapour-side corrosion (VSC) of 70 Cu30Ni condenser tubes in some MSF distillers. Corrosion is first recognized by above-normal Cu2+ content of the distillate and, at advanced stages, by the increase in electrical conductivity of the product water. Attack takes place at the inlets of tubes in the first high temperature cells and is located mainly at the lower part of the tube bundle. Corrosion starts after a long time of operating the distiller (the induction period). A “corrosion factor” is proposed to characterize tube failure in distillers. It is the number of tubes failing per 1000 operation hours, following the induction period. Corrosion is not due to assault on tube material by CO2 resulting from the thermal decomposition of HCO3− and CO32− ions of seawater. Corrosion involves the primary oxidation of the metal by air leaking through flanges, glands and/or gaskets, followed by dissolution in CO2-loaded vapours condensing on the colder ends of the tubes. Analysis of the distillate reveals the presence of large quantities of O2 in the vapour. A simple experimental set-up to simulate VSC in the laboratory is described. The results obtained therefrom clearly show that: (a) pure water vapour does not produce VSC; (b) CO2-loaded vapours have no measurable effect on VSC; (c) the presence of air together with CO2 is necessary to cause measurable VSC; (d) bombardment of the tubes with water droplets condensing at an upper level greatly accelerates material loss; and (e) dissolved Cu2+ ions catalyze the dissolution of copper-nickel alloy. Mitigation of VSC can be either preventive or curative. Preventive measures involve the monitoring of the O2 content of both brine and distillate, the performance of pressure tests on the distiller, and the change of old gaskets at reasonable intervals. Curative measures involve the plugging and extraction of failed tubes, the application of protective sleeves, and the eventual retubing of the condenser. A change of tube material might be an appealing alternative.

Galvanic corrosion of copper-base alloys in contact with molybdenum-containing stainless steels in Arabian Gulf water

Abstract The condenser tubes of MSF distillers of the Water and Electricity Department of Abu Dhabi are made of either 90 10 or 70 30 CuNi alloys or of Al-Brass and are mounted on 90 10 or 70 30 CuNi tube plates. The tubes suffer pitting-, SCC and erosion corrosion from the water side and general attack from the vapour side. The resulting copper and nickel ions are potential health hazards and harmful pollutants to the environment. In the event of retubing, it is recommended to use more corrosion-resistant, environmentally more friendly tube material. High strength Mo-containing stainless steels qualify both requirements. As these are going to be mounted on the already present copper-base tube plates, galvanic corrosion of the plates is liable to occur. The present study was undertaken to establish the extent of attack that would evolve and its dependence on operation variables. The study was carried out on six Mo-containing stainless steels with Mo contents varying between 0.3 and 6.0 wt% and three copper-base alloys ( 90 10 and 70 30 CuNi alloys and Al-bronze) in sterile Arabian Gulf Water. The study involved the measurement of the open circuit potentials (OCPs) of single metals and the couple potentials (CPs) till constant values. The corrosion current produced upon coupling was followed simultaneously. The effects of varying cathode/anode surface area, of temperature and of stirring on these parameters were examined in detail. The results obtained indicated that the stainless steel cathodes were more polarizable than the copper-alloy anodes. The (CPs) were very near to the (OCPs) of the anodes. The cathode polarizability decreased slightly with increase of cathode/anode are a ratio in the case of couples involving the 90 10 and 70 30 alloys, but was not affected in the case of Al-Bronze. The rates of galvanic attack of the copper alloys increased linearly with cathode/anode area ratios in the range 0.36–6. These rates were the same for couples incorporating the 90 10 and Al-Bronze anodes, and these were higher than those measured with the 70 30 material. In all cases the corrosion rates increased with temperature to reach maximum at 50°C. Above this temperature the rates dropped sharply as result of expulsion of oxygen from solution. Stirring increased the galvanic corrosion rates as result of increased mass transfer of oxygen to the cathodes and of removal of corrosion products from anodes surfaces. Under otherwise the same conditions the corrosion of the tested copper alloys decreased in the order: Al-Bronze 90 10 CuNi alloy> 70 30 CuNi alloy

Physico-chemical studies on BELGARD EV and BELGARD EV 2000 Part III: Analytical procedures for the estimation of active components

Abstract BELGARD EV (BELEV) and BELGARD EV 2000 (BELEV 2) are polymeric formulations used as anti-scale agents in MSF distillers. BELEV results from the self-polymerization of maleic anhydride while BELEV 2 is a terpolymer of the same monomer. The two agents lack analytical procedures for controlling their active constituents. We found that Cd2+, Pb2+ or Hg2+ ions react with the two agents in hexamine buffer of pH 6 to produce gelatinous precipitates of definite composition. The relations between the amounts of cations bound to BELEV (BELEV 2) (m.equiv.-) and the quantities of the agents (mg) are linear, passing through the origin. The results obtained allow the calculation of the equivalent weights of unit cells bound to the cations. For BELEV these amount to 220, 166 and 158 for Cd2+, Pb2+ and Hg2+, respectively. The most probable structure of the formed precipitates are suggested. For BELEV 2 the corresponding figures are 310, 265 and 255, successively. The feasibility of estimating the active constituents of BELEV and BELEV 2 in their preparation is highlighted.

Physico-chemical studies on BELGARD EV and BELGARD EV 2000 solutions. Part II. Potentiometric and conductometric acid-base titrations

Abstract The two anti-scale agents BELGARD EV (BELEV) and BELGARD EV 2000 (BELEV 2) are titrated against standard NaOH solution. Both the potentiometric (pH metric) and the conductometric titration techniques are used. For the sake of comparison the reaction of the monomer (maleic acid, H2M) is also investigated. The two titration techniques reveal that H2M neutralizes along two equal steps. The pK1 and pK2 of the acid are calculated as 2.9 and 6.1, in good agreement with the values quoted in the literature. The potentiometric titration curves of BELEV and BELEV 2 solutions are similar and show a single drawn-out inflexion. Computer analysis of the curves reveal four acid species in case of BELEV and two in case of BELEV 2. The pK values, the percentage composition and the mean average molecular weights of all species are computed. The consumption of NaOH till any one and the same pH value varies linearly with the additives content in solution. This offers a simple method for analysis of BELEV and BELEV 2 preparations. The conductometric titration curves of the two agents indicate the presence of three acid species. Correlation with the pH curves is made in every case. Not all the inflexions in conductometric curves have correspondence in pH curves. The effect of addition of increasing concentrations of H2M to BELEV (BELEV 2) solutions and of increasing BELEV (BELEV 2) concentrations to H2M on the pH-metric curves is examined in detail. The two materials affect the neutralization pattern of one another. The results suggest the masking of the free-acid protons through hydrogen bonding with the polymers.

Uptake of calcium and magnesium from the arabian gulf water on plastic and glass surfaces

Abstract The uptake of Ca2+ and Mg2+ from the Arabian Gulf water of the walls of plastic and glass bottles was followed at room temperature for contact times up to 360 h. Both ions are physically adsorbed in two distinct species, which can be leached out by water and acid elution, successively. The amounts adsorbed are too little to be of practical interest. Both plastic and glass bottles can be safely used to collect and store seawater samples.

Electrochemical simulation of vapour side corrosion of Cu-Ni condenser tubes in multistage flash evaporation distillers

Abstract The corrosion of 70–30 Cu–Ni condenser tubes in the vapour of multistage flash evaporation distillers occurs within a thin water film adhering to the metal surface. This, together with the fact that corrosion involves the alternate formation and dissolution of copper oxides, justifies the application of electrochemical techniques in aqueous solutions for the study of vapour side corrosion. Open circuit potential (OCP) transients were used to establish the effects of vacuum and of O2 , CO2 , and mixtures thereof on the corrosion process. The study was carried out in 4%Na2 SO4 solution at ambient temperatures. The state of vacuum in the distiller was simulated by purging the solution with N2 gas. The OCP of the working electrode tended towards more negative values, revealing the instability of the air formed oxide. This changed slowly to a Cu2O film. Introduction of O2 into the cell resulted in the instantaneous shift of potential towards less negative values. Cuprous oxide was oxidised to hydrated cupric oxide. Reintroduction of N2 gave rise to a clear potential step related to the slow decomposition of Cu(OH)2 to fresh Cu2O. The latter oxide thickened with time. Flushing of the solution with CO2 , following deaeration, also caused the OCP to change to more positive values. The change was, however, a result of the solution turning acid, causing slow attack on Cu2O. This was gleaned from the results of N2 purging, in which a gradual, linear decrease of potential to the Cu2O value occurred. A 1 : 1 mixture of O2 and CO2 caused the OCP of the electrode to move in the positive direction. Both Cu2O and Cu(OH)2 dissolved in the acid solution. Introduction of N2 caused the OCP to shift to the Cu/Cu2O potential. This stage was, however, short lived and the potential moved once more towards less negative values. This behaviour is attributed to the occurrence of the autocatalytic reaction Cu2+ + Cu = 2Cu+. The conclusion finds support in the results of a second cycle, in which the potential fluctuated between those of oxide formation and metal dissolution. Comparison is made between the results of corrosion in aqueous solutions and in the vapour phase. Open circuit potential measurements in solution confirmed and extended the results of weight loss determinations in the vapour phase.