Single crystals and bicrystals have been used to study pitting corrosion and intergranular corrosion of Al and Al-Cu alloys in 0.5M NaCl solution. The critical pitting potential and pit density were examined as a function of a number of factors. These included crystallographic orientation; the bulk solution chemistry including CI- concentration, NO₃- addition and pH; the effect of Cu alloying; and the effects of homogenizing and aging on the alloy crystals. The susceptibility for pitting corrosion was found to depend on crystallographic orientation in pure Al with {111} showing maximum pitting and {011} and {001} exhibiting progressively less pitting. This crystallographic effect was not observed in the Al-3 wt% Cu alloy. The addition of Cu to pure Al was found to raise the Epit and produce a higher pit density on the surface. The increase of CI⁻ concentration was found to enhance pitting corrosion, producing a higher pit density and lowering the Epit. Addition of NO₃- to the solution decreases pitting corrosion, reduces the pit density and substantially shifts the Epit to a more noble potential. A model of pitting corrosion is proposed, based on a local kinetic balance between the repassivation process and the dissolution process at the bare metal surface at the base of a preexisting oxide flaw on the crystal surface. The model successfully accounts for the observed effects of the Cu alloy addition, and the solution composition variations on pitting corrosion. In the alloy bicrystals, it was observed that pitting corrosion in the grain boundary region was dependent on the composition and thermal history of the crystal. In most of the homogenized Al-Cu bicrystals, the presence of the grain boundary did not influence the pitting corrosion. In a 0.1 wt% Cu alloy with a tilt boundary of 28° about the <001> direction preferential pitting along the grain boundary was observed. The preferential pitting is attributed to nonequilibrium depletion of Cu at the high angle tilt boundary. Preferential attack is also observed at grain boundaries in as-grown and in aged bicrystals. This is attributed to Cu segregation in the crystals and the lower value of Epit associated with the Cu depleted regions. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The thermal fouling data of Watkinson were recomputed on the assumption that increases in pressure drop were caused entirely by roughness of the fouling deposit and not at all by blockage. The results indicate that roughness played some role in the sand-water runs, but no role in the gas oil runs. The anomalous increases in overall heat transfer coefficient with degree of fouling in the first two sand-water runs were thus explained by the effect of roughness on liquid film heat transfer coefficient. The original loop was modified, the principal change being the replacement of the manual temperature recording system by a digital Data Logging System. Fluid and wall temperatures, as well as differential pressure, could thus be automatically recorded as a function of time. Better control of independent operating variables such as fluid velocity, inlet temperature and heat flux could therefore be achieved, and the effects of temporary fluctuations in these operating conditions could be observed and separated from fouling effects. Thermal fouling runs were made in the modified heat transfer loop on samples of Bayer Process spent liquor from the ALCAN aluminum refinery in Arvida, P.Q. The only discernible trends in the results were increases in rate and degree of fouling with increasing heat flux to a maximum value, followed by a sharp decrease, at liquor velocities less than 5 ft/sec, and the absence of thermally detectable fouling at higher velocities. The unreproducibi1ity of the results is believed to be attributable, at least in part, to changes in chemical composition of the liquor throughout the experiments. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

No description available.

Aluminum powder metallurgy (PM) parts have found applications in automotive, aerospace and transportation. Sintered aluminum parts have been developed and compete with traditionally fabricated ingot metallurgy (IM) products for specific applications. To extend the range of application of (PM) alloys which offer the advantage of net and near net shape production, processing parameters and corrosion behaviour of the aluminum alloys need to be improved. In this research, processing parameters and corrosion behaviour of a commercial Al-Mg-Si aluminum alloy (Alumix 321) were investigated. This alloy is the PM equivalent of wrought AA6061. Four sintering temperatures (610 °C, 620 °C, 630 °C, 640 °C) and two pressing pressures (200 MPa, 400 MPa) were used and the optimum pressing and sintering procedure was selected. In addition to different processing routes of aluminum powder metallurgy alloys, a series of electrochemical experiments on both (IM) and (PM) aluminum alloy was performed with the aim of correlating corrosion behaviour with production techniques. As a modification step, post sintering treatments and surface alteration techniques were applied. Hot rolling, hot swaging, repressing, resin impregnation and shot peening were performed and their effect on corrosion behaviour was investigated; their effect on density, hardness, and microstructure was also studied. Hardness after hot swaging and hot rolling increases and near full density was achieved (? 99%), while for resin impregnation and shot peening surface nature and roughness were affected, respectively. Electrochemical techniques such as open circuit potential (OCP), Tafel extrapolation (TE), cyclic polarization (CP) and stair step polarization (SP) were performed on the ingot, wrought, and post sintered alloys immersed in a 3.5 wt% NaCl solution. Electrochemical experiments show that corrosion current decreases as a result of post sintering treatments. The electrochemical experiments also show different corrosion mechanisms that were later confirmed by the metallographic analysis. The corrosion product and corroded surfaces of the alloys were characterized by optical microscopy, scanning microscopy (SEM), energy dispersive spectroscopy (EDS), wavelength dispersive spectroscopy (WDS), and X-ray diffraction (XRD). Results show that pitting is the main corrosion mechanism of the wrought alloy. However, powder metallurgy alloys show pitting, crevice, and intergranular corrosion. / Effect of processing parameters on corrosion behaviour of Alumix 321 PM alloy

All-aluminum microchannel heat exchangers are designed to significantly reduce refrigerant charge requirements, weight, reduced brazed joints, and decreased potential for leakage by increasing reliability. Al 3003 alloy is corrosion resistant and can be formed, welded, and brazed but the issue with all-aluminum heat exchangers is localized corrosion (pitting) in corrosive environments. Currently, there is no universally accepted corrosion test that all coil manufacturers use to characterize their products. Electrochemical testing method of cyclic polarization was employed in this investigation and relevant parameters including electrolyte corrosive agent and its concentration, electrolyte pH, and applied potential scan rate was varied to find an optimal set of parameters. Results of cyclic polarization of Al 3003 in electrolytes containing various concentrations of NaCl were compared with those of the tests in Sea Water Acidified Accelerated Test (SWAAT) electrolyte and it is shown the SWAAT electrolyte (4.2% sea salt acidified to pH of 2.9) is by far stronger (in terms of corrosivity) than typical 3.5% NaCl solution used in most corrosion testing. Corrosion rates (g/m2yr) of Al 3003 measured in this investigation were comparable to those provided by ISO 9223 standard corresponding to C1 through CX categories. Duration of cyclic polarization test is much shorter than that of SWAAT and results obtained in this test is more reproducible compared to those of SWAAT. Scanning electron microscopy micrographs show typical pit depths of about 50 μm.

No description available.

Aluminum has high resistance to attack in many environments but corrodes rapidly in most inorganic acids. The purpose of this investigation was to attempt to determine the rates and mechanism of the surface reactions for the corrosion of aluminum in hydrochloric acid vapors as a function of the vapor concentration, vapor composition, and temperature. Corrosion tests were made by suspending aluminum foil samples, five centimeters by two centimeters, by a glass thread. The samples were located in the vapor over hydrochloric acid solutions. The glass thread with the sample was attached to the center of a glass rod. One end of the rod was fixed; the other was suspended from one end of the arm of an analytical balance, permitting periodic weighing of the corroding samples without removal from the flask. The tests were conducted above hydrochloric acid solutions whose concentration varied from 2 to 32 weight percent. Ten temperature levels, from zero to 48°C, were employed so that the vapor pressures of the various acid concentrations overlapped. Weighings were made at 10 minute intervals for 200 minutes. The corrosion products were analyzed by standard x-ray diffraction techniques. The rate of weight gain was found to be a linear function with a respect to time and exponential with respect to hydrochloric acid concentration. The rate passed through a maximum at a hydrogen chloride partial pressure of 1.77 mm Hg. The decrease in rate above the maximum is due to the formation of a protective aluminum trichloride six hydrate (Al Cl₃ • 6 H₂O) film. Temperature increased the rate of weight gain at all hydrogen chloride partial pressure. Based on the analysis of the corrosion products the following mechanism is proposed for the corrosion of aluminum. For hydrogen chloride partial pressures below 1.77 mm Hg, the hydrogen chloride acts at the electrolyte for the following electrochemical reaction. 2 Al + (3 ÷ X)H₂O = Al₂O₃ • X H₂O ÷ 3 Hg For hydrogen chloride partial pressures above 1.77 mm Hg., the hydrogen chloride enters into the ration as follows 2 Al + 6 H Cl + 12 H₂O = 12 Al Cl₃ • 6H₂O + 3 Hg The energy of activation is approximately constant over the entire experimental range indicating that the same mechanism is controlling both proposed corrosion reactions. The controlling reactions would be the primary electrode reactions which are identical for both regions. / Master of Science

In this thesis, corrosion data were subjected to statistical analysis for the purpose of obtaining meaningful estimates of the specific rate of reaction and the energy of activation and obtaining a response equation for prediction of weight gain. A two-stage analysis was used in the estimation of specific rate and activation energy. First, an estimate of the reaction rate for each test run was obtained from linear regression analyses of weight gain on time. A weighted linear regression was then performed with reciprocals of estimated variances of rates as weights; the estimated order of reaction was obtained. It was shown that the mechanisms of the two possible reactions, i.e., formation of aluminum oxide or AlC13x6H20, are different. At the lower HCl partial pressures (Al2O₃ the product}, the specific rates and the activation energy were estimated in two ways: (1) the method of weighted linear regression when stoichiometric first order is assumed; (2} the method 0f weighted covariance analysis when empirical order is assumed. The data in the A1c13x6H20 portion were inconclusive. The prediction equation was obtained by an analysis of covariance; the model was that of a two-way classification with time as a covariate. Main effects ( temperature and pressure) were estimated. Finally, a response equation, tor prediction of the weight of corrosion product (A12O₃ ), was found. This equation is valid for HCl partial pressures o,QO) to 2.90 mm Mg; temperatures 17 °C to 44 °C, time of exposure $0 to 130 minutes. / Master of Science

M. Tech. Chemical Engineering. / Discusses the effect of ferrous gluconate (FG) and zinc gluconate (ZG) as novel corrosion inhibitors on the corrosion rate of mild steel and aluminium alloy in 3.5% NaCl and 0.5 M H2SO4 media was investigated by electrochemical and weight loss techniques. The effect of inhibitor concentration was investigated. The concentration of these inhibitor ranges from 0.5 to 2.0% g/v at a temperature of 28OC. The synergetic effect of these inhibitors was also studied. High resolution scanning electron microscopy equipped with energy dispersive spectroscopy (HR-SEM/EDS) and Raman spectroscopy was used to characterize the surface morphology of the metals before and after corrosion. Experimental results revealed that ferrous gluconate and zinc gluconate in 3.5% NaCl and 0.5 M H2SO4 solution decreased the corrosion rate at the different concentrations studied. Maximum inhibition efficiency of 100% was achieved for mild steel at 0.5% g/v concentration of FG, 0.5% g/v concentration of ZG and 1.5% g/v synergetic of FG + ZG in 3.5% NaCl solution. Similarly, 100% inhibition efficiency was obtained for aluminium alloy at different media studied (3.5% NaCl and 0.5 M H2SO4). The experimental results obtained from potentiodynamic polarization method showed that the presence of FG and ZG in 3.5% NaCl and 0.5 M H2SO4 solutions decreases the corrosion current densities (icorr) and corrosion rates (CR), and increases the polarization resistance (Rp). It was observed that the inhibitor efficiency depends on the corrosive media, concentration of the inhibitor and the substrate material. The adsorption characteristics of FG and ZG were best described by the Langmuir and Freundlich isotherms. Good correlation exists between the results obtained from both polarization and weight loss methods.

M. Tech. Engineering Metallurgy. / Aims to improve the mechanical and chemical properties of mild steel, by developing highly corrosion resistant surface coatings of zinc-aluminum using the electro-deposition techniques. Properties that are targeted are specifically hardness, wear and corrosion resistances.