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Les facteurs qui différencient les mécanismes de corrosion du revêtement ZnMgAl pour l'acier en environnement naturel de tests accélérés avec du NaCl ont été identifié. Un nouvel électrolyte (RW) qui contient NH4+, HCO3-, Cl- and SO42- a été développé pour des tests accélérés en respectant la vitesse de corrosion relative entre les revêtements à base de Zn et la formation tardive des hydroxydes à double couche (LDH), qui sont connus pour l'exposition naturelle. En considérant le mécanisme de corrosion type dissolution-précipitation, le rôle des ions présents dans l'électrolyte RW sur la cinétique de dissolution et sur l'accumulation de produits insolubles a été étudié. NH4+ et HCO3- ont été démontrés capables d'augmenter la réactivité anodique et la dissolution cathodique de l'Al. La formation des LDH a été retardée par NH4+, HCO3- et SO42-. La réactivité anodique plus élevée du ZnMgAl en comparaison avec le revêtement de zinc pur combinée avec sa meilleure résistance à la corrosion montrent l'importance de la réaction cathodique (réduction de l'oxygène). En comparant les courants dues à la réduction de l'oxygène sur les revêtements frais et sur la surface sous une couche de produits de corrosion, l'effet barrière des sels basiques de zinc (BZS) et LDH a été clairement démontré. Cet effet barrière est disparu sur le revêtement de zinc pur sous polarisation cathodique en raison de la transformation BZS -> ZnO/Zn(OH)2 mais est resté stable sur ZnMgAl dans les mêmes conditions - l'effet d'inhibition du Mg2+ sur le cinétique de cette transformation a été mis en évidence. Un " squelette " résiduel d'Al non-réagi a contribué à la compacité de produits de corrosion. / Factors differing corrosion mechanisms of ZnMgAl coatings on steel in natural environments and in accelerated tests with NaCl solution were identified. A new electrolyte (RW) containing NH4+, HCO3-, Cl- and SO42- was developed, for which the accelerated tests respected the relative corrosion rates of different Zn-based coatings and the delayed formation of layered double hydroxides (LDH) known for field exposure. Considering dissolution-precipitation mechanism of corrosion, the role of ions present in RW electrolyte on the leaching kinetics and on the accumulation of insoluble products was studied. NH4+ and HCO3- were demonstrated to increase the anodic reactivity and cathodic Al dissolution. LDH formation was delayed by NH4+, HCO3- and SO42-. Higher anodic reactivity of ZnMgAl than zinc coating combined with better resistance in corrosion tests indicated the importance of cathodic reaction (oxygen reduction). Comparing oxygen reduction currents on fresh coatings and on the surface under patinas, clear barrier effect of basic zinc salts (BZS) and LDH was demonstrated. The barrier effect was lost under cathodic polarization on Zn coating due to the transformation BZS ->ZnO/Zn(OH)2 but was stable on ZnMgAl in the same conditions – the inhibiting effect of Mg2+ on the kinetics of this transformation was evidenced. A residual “skeleton” of unreacted Al contributed to the increased compactness of the corrosion products.

In the present study, the corrosion behaviour of several magnesia-chrome (MC) and magnesia-alumina spinel (MA) bricks against fayalite type slags was investigated and the role of the spinel phases was highlighted . The experimental results revealed that the corrosion resistance of the MC bricks was superior to the MA bricks against KIVCET slags in static and dynamic conditions . As a result of the interaction between MgO from MC bricks and the slag, a modified forsterite phase (Mg, Fe, Zn, Ca)₂SiO₄ was formed, which destroyed the precipitated complex spinel bonds at the grain boundaries of periclase and magnesia-chromia spinel . Furthermore, both MgO and MgO-MgAl₂O₄ spinel in the MA brick dissolved into the slag, which resulted in modified forsterite phases of (Mg, Fe, Zn, Ca)₂SiO₄ and (Mg, Fe, Zn)(Fe, Al)₂O₄ complex spinels, respectively. In addition, the accretion formation in the KIVCET furnace was investigated through solubility experiments of Cr₂0₃ in the KIVCET slag with various amounts of lead, which revealed that the net contribution of Cr₂0₃ to the spinel formation is the highest in the barren (no Pb) slag, followed by high-lead (11% Pb) and it is the lowest for the low-lead (6% Pb) slag. The amount of spinel solid solution increased consistently with increasing Cr₂0₃ dissolved and the PbO existent in the slag. From examinations of several used bricks from the tuyere area of a Peirce Smith nickel converter, it was found that the corrosion is due to the interaction of the partially oxidized matte penetrating deep into the brick and the magnesia grains forming (Mg, Fe, Ni, Co) xOy spinels . Analyses of brick samples used in the KIVCET Electric Furnace roof identified deep reaching sulphation, which weakened the bonding phase between coarse magnesia grains. In the Bottom Blown Oxygen Converter, a highly aggressive lead and bismuth oxide rich slag penetrated deep into the brick, which destroyed the grain boundaries, causing the refractory to be easily eroded at the refractory-slag interface. Our studies concluded that the spinel phases, either as magnesium chromate, magnesium aluminate or complex spinel [(Mg, Fe)(Cr, Al, Fe)₂O₄], enhanced the corrosion resistance of a basic refractory to fayalite type slags from the non-ferrous smelting and converting furnaces. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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The corrosion behaviour of nickel and monel in aqueous fluoride solutions was studied by potentiostatic polarization techniques and surface examination of the corroded specimens. Nickel does not exhibit the usual active-passive transition for 0 < pH < 4.0 but corrodes rapidly especially at the grain boundaries. In the range 4.0 < pH < 6.5 the nickel-polarization curve contains two active regions. Nickel is passive in contact with a fluoride solution with 6.5 < pH < 12.0. Polarization curves of nickel in fluoride solutions of varied pH's and fluoride ion concentrations in the range 4.0 < pH < 7.0 revealed that the current as a function of potential in the first active region is independent of fluoride ion concentration but dependent on pH. The currents in the first passive and second active regions are a function of pH and fluoride ion concentration. Surface examinations showed that nickel corrodes at the grain boundaries in the second active region. A mechanism has been proposed which accounts for corrosion in the second active region by F⁻ adsorption and passivation by either H₂O or OH⁻ adsorption on the anodically polarized metal surface. A mathematical analysis based on competitive adsorption of these species as a function of electrode potential is shown to be consistent with the experimental data. Monel corrodes at less than half the rate of nickel at the mixed potential in fluoride solutions with 0< pH < 6.5 due to its larger hydrogen overvoltage. Monel exhibits active-passive behaviour similar to nickel but with the passive current up to 6 times as large. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Laboratory tests on steel pipe corrosion subjected to varying velocities of aerated Vancouver city water have been made. The water was circulated through seven vertical sections each containing from eight to ten specimens of standard ¾ inch steel pipe. The water was removed at a constant rate so that the volume of water in the apparatus was renewed from one to two times daily. Exposure times varied from 5 to 25 days with temperatures of 90°F and 130°F. The amount of corrosion was evaluated by determining the weight loss. The solution potential of some designated corroding specimens was measured against a silver-silver chloride reference electrode throughout the time of exposure. No changes in the pH from that of the inlet water occurred and the iron ion content fluctuated corresponding to the fluctuation in inlet water. At 130°F the amount of corrosion, for 10 day tests, was greatest at 1.5 fps velocity, but for tests of greater duration, was greatest at .75 fps velocity. This reversion did not take place at 90°F where the amount of corrosion was greatest at 1.5 fps for all tests. At both temperatures the lowest amount of corrosion occurred at the highest velocity level irrespective of the test duration within the limits investigated. Amounts of corrosion were in all cases higher at 130°F as expected from previous work where maximum temperature effects occurred at about 180°F. The corrosion product consisted mainly of a black fine-crystalline matter (assumed to be magnetite) at the surface of the specimen, with a porous reddish precipitate (assumed to be ferric hydroxide) over the magnetite. The total amount of magnetite increased with increasing temperature for similar conditions of flow and time of exposure. The corrosion mechanism is considered in the light of available oxygen supply, potential change, the effect of velocity, cathodic depolarization and variables involved in the mass transfer of the oxygen. The correlation for the latter with velocity is compared with results from an oxygen-pick up investigation of corrosion. The results of this investigation are qualitatively consistent with previous knowledge of oxygen as a passivator in such systems. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

The effect of crack tip strain rate (CTSR) on environmentally assisted cracking was studied for alloys AA-7075 (Al-Zn-Mg-Cu) and AA-8090 (Al-Li-Cu-Mg) in the artificially aged condition. Fatigue pre-cracked double cantilever beam (DCB) specimen were employed with the crack plane parallel to the rolling plane. The cracking behaviour under monotonic and cyclic loading conditions was investigated in aqueous sodium chloride solutions with and without additions of sodium chromate as a corrosion mhibitor. CTSR values were described in terms of K-rate ∆K/∆t (ie. dK/dt) as a measured average over the loading period of a fatigue cycle. This allowed a comparison with CTSR's of monotonically increasing load or constant load tests. At frequencies ≤1 Hz, the load was applied with a triangular wave form. A high frequency of 30 Hz was obtained by sinusoidal loading. Expressed as K-rate, CTSR values were varied over 7 orders of magnitude from 10⁵MPa√m/s to 10² MPa√m/s. Stress intensities investigated were mainly around region II values with respect to SCC K-log(da/dt) behaviour. At low K-rates, real time crack velocities (da/dt) measured under monotonic slow loading or constant load conditions were comparable to crack velocities obtained with cyclic loading experiments. As the K-rate was increased from low values, typical of constant load experiments, the real time crack velocities decreased. This was caused by plasticity induced crack growth retardation effects and a decrease in crack tip film rupture events during the unloading part of a cycle. The crack propagation rate decreased until minimal crack advance increments per cycle were dictated by mechanical parameters acting on a hydrogen embrittled crack tip region. Under monotonic loading conditions region II crack velocities were not influenced by an increase in K-rate which was explained with a mass transport controlled cracking process. Tests with alloy 7075 at intermediate K-rates and a high R-ratio of 0.78 allowed a crack tunnelling mechanism to operate. This overcame the plasticity induced crack growth retardation and, therefore, cracks propagated at the same rates as during low K-rate tests where no retardation phenomena were encountered. Scanning electron microscope investigations revealed a striated intergranular fracture surface of alloy 7075 if tested at K-rates above the transition value to K-rate independent crack propagation rates. Individual striations could be matched on opposing fracture surfaces and the striation spacing corresponded to the average crack propagation increment per cycle. The striations, therefore, were formed as part of the crack advance during every fatigue cycle. At the lower K-rates no striations were present but micro tear ridges could be found on the intergranular fracture facets indicating that dissolution processes alone did not cause the intergranular crack advance. Alloy 8090 did not reveal significant changes in fractography over the entire K-rate range investigated, except at the highest K-rates where small interlocking steps could be detected on some opposing transgranular fracture surfaces. In general, however, the crack path at all K-rates was mainly intergranular with dimpled fracture facets. Alloy 8090 exhibited a high resistance to SCC with fatigue pre-cracked DCB specimen. Therefore, to obtain crack velocity values with low K-rate monotonic loading tests very long test durations would have been necessary. It is concluded that the transition from intergranular SCC to intergranular CF occurs at a critical K-rate. Below the critical K-rate crack velocities are not increased by cyclic loading. Instead crack growth retardation effects can result in lower real time crack velocities than those typical for constant load tests at comparable stress intensities but much lower K-rates. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate