This work, within EPSRC LATEST2 (Light Alloys Towards Environmentally Sustainable Transport 2) Programme Grant, is focused on the development of environmentally-friendly corrosion inhibitors, nanocontainers loaded with inhibitors and epoxy coatings for corrosion protection of the aerospace alloy AA 2024-T3. More specifically, the electrochemical techniques of image assisted electrochemical noise, electrochemical impedance spectroscopy, split-cell technique and potentiodynamic polarization were used for the qualitative and quantitative evaluation and characterization of environmentally-friendly corrosion inhibitors on AA 2024-T3. Scanning electron microscopy observations complemented the electrochemical measurements. It was found that the organic inhibitor 2-mercaptobenzothiazole provides excellent corrosion inhibition properties to AA 2024-T3 in 3.5% sodium chloride solution. Among the tested nitrates, cerium nitrate provides the best inhibition performance. The concentration of the nitrate salt is critical in determining the corrosion protection. An excessive concentration of nitrate ions results in the dissolution of copper-containing oxides, increasing the corrosion rate. Different types of core-shell structured nanocontainers were synthesised with the methods of distillation precipitation polymerization, emulsion polymerization and sol-gel. The nanocontainers were characterized by scanning electron and transmission electron microscopy observations. The corrosion inhibitor 2-mercaptobenzothiazole was encapsulated into the nanocontainers. The encapsulation of 2-mercaptobenzothiazole was evaluated with energy dispersive X-ray analysis mapping micrographs from transmission electron microscopy measurements. Epoxy coatings with nanocontainers loaded with 2-mercaptobenzothiazole were applied on AA 2024-T3 for protection against corrosion. The corrosion protection properties of the coatings were evaluated with electrochemical impedance spectroscopy. The results indicated that epoxy coatings provide excellent barrier properties to AA 2024-T3 in the demanding environment of 3.5% sodium chloride solution with low frequency impedance values more than 1 GOhm cm2 for over 4000 hours of testing. Coatings containing nanocontainers loaded with 2-mercaptobenzothiazole tested with an artificial scribe revealed protection of the AA 2024-T3 substrate in the scribed area, decrease of the anodic delamination in the early hours and decrease number of cathodic dark areas after long immersion time. Overall, epoxy coatings with encapsulated inhibitors can be considered as a promising system for potential replacement of hexavalent chromium treatments on aerospace alloy AA 2024-T3.

Dans cette thèse, la dynamique d'évaporation de gouttes sessiles de solutions salines sur des surfaces planes inertes et métalliques a été étudiée et le phénomène de corrosion pour les surfaces ferriques caractérisé. En premier lieu, nous nous sommes intéressés à la dynamique d'évaporation de gouttes sessiles salées sur des surfaces inertes pour une large gamme de concentrations en sel, d'humidité relatives, de tailles de goutte et d'angles de contact. Notre étude révèle les domaines de validité du modèle classique d'évaporation, processus contrôlé par la diffusion de la vapeur dans l'air et met en évidence l'impact de flux (de Marangoni) induits par des gradients de concentration (tension de surface) en sel sur la dynamique d'évaporation et les dépôts salins obtenus après évaporation de la goutte. De plus, nous nous sommes consacrés à l'évolution spatio-temporelle de gouttes sessiles de solutions salines sur des surfaces métalliques. Contrairement au modèle simplifié de la goutte d'Evans, nous avons montré que le processus de corrosion s'étend aux abords de la ligne de contact, avec la formation d'un film périphérique. La ligne triple est déstabilisée par des gradients de tension de surface induits par des variations de composition ionique au cours du processus de corrosion et la migration des cations vers la périphérie de la goutte. Enfin nous avons étudié le phénomène de corrosion du métal induit par l'évaporation de gouttes sessiles salées. Le processus de corrosion, en particulier la localisation des réactions anodiques et cathodiques sur la surface métallique en contact avec la goutte est corrélée à la distribution spatiale du sel au sein de la goutte s'évaporant. / In this thesis we investigate the evaporation behaviour of sessile droplets of aqueous saline solutions on planar inert and metallic surfaces and characterise the corrosion phenomenon for iron surfaces. First we study the evaporation behaviour of sessile salty droplets on inert surfaces for a wide range of salt concentrations, relative humidities, droplet sizes and contact angles. Our study reveals the range of validity of the well-accepted diffusion-controlled evaporation model and highlights the impact of salt concentration (surface tension) gradients driven Marangoni flows on the evaporation behaviour and the subsequent salt deposit patterns. Furthermore we study the spatial-temporal evolution of sessile droplets from saline solutions on metallic surfaces. In contrast to the simple, generally accepted Evans droplet model, we show that the corrosion spreads ahead of the macroscopic contact line with a peripheral film. The three-phase contact line is destabilized by surface tension gradients induced by ionic composition change during the course of the corrosion process and migrations of cations towards the droplet perimeter. Finally we investigate the corrosion behaviour under drying salty sessile droplets on metallic surfaces. The corrosion process, in particular the location of anodic and cathodic activities over the footprint droplet area is correlated to the spatial distribution of the salt inside the drying droplet.

La cartographie de potentiel est couramment utilisée afin de détecter les zones de corrosion à risque dans les ouvrages en béton armé. Cette méthode utilise une électrode de référence positionnée à la surface du béton afin de mesurer la différence de potentiel entre un point à la surface du béton et une connexion au réseau d'armatures. Toutefois, il existe deux inconvénients majeurs à cette technique : la connexion au treillis d'armatures qui nécessite d'y avoir accès et la vérification de sa continuité électrique dans la zone de mesure. Dans le but de ne plus être soumis à ces inconvénients, une nouvelle méthode est proposée. Au lieu de se connecter au treillis, une seconde électrode de référence est utilisée, elle aussi positionnée à la surface du béton étudié. Cette configuration de mesure ne donne plus des potentiels électriques, mais des gradients de potentiel entre les deux électrodes de mesure. Par la mise en place de simulations numériques et d'essais expérimentaux, cette configuration de mesure est étudiée. En ce qui concerne le travail expérimental, deux dalles de béton armé de 3x3x0,15m ont été coulées afin d'être proche des conditions rencontrées sur site. Ensuite que des zones corrodées, dont la taille est contrôlée, sont créées de manière accélérée à l'aide d'un dispositif de migration de chlorures. L'épaisseur d'enrobage de la dalle ayant une influence sur les mesures, il a été choisi de couler deux dalles. La première dalle comporte un treillis dont l'épaisseur d'enrobage est constante alors que la seconde dalle possède une épaisseur d'enrobage variable. En ce qui concerne la modélisation, une étude paramétrique utilisant la méthode des éléments finis est réalisée. Cette modélisation permet l'étude de l'influence de nombreux paramètres tels que la résistivité, la taille de la zone corrodée, l'épaisseur d'enrobage, le procédé de mesure, etc. C'est l'analyse des courants d'échange, mais aussi des cartographies de potentiels et de gradients de potentiel qui permettent la corrélation entre les résultats expérimentaux et de simulation. C'est alors qu'il est possible de conclure sur la faisabilité de cette méthode et de ses avantages par comparaison avec la mesure classique. / Half-cell potential mapping is commonly used to detect corrosion risks in reinforced concrete structures. This method uses a reference electrode positioned on the surface of concrete for measuring potential difference by using a voltmeter connected to the reinforcement. However, there are two major drawbacks in the implementation of this method: the necessity to make an electrical connection to the reinforcement and the electrical continuity of this reinforcement. In order to overcome these disadvantages, a new method is proposed. Instead of using electrical connection to rebar, a second reference electrode is used, also positioned on the surface. These two electrodes configuration gives the electrical potential gradient on the concrete surface. By performing both experimental work and numerical modelling this method configuration is tested. For experimental work, two reinforced concrete slabs (3x3x0.15 meter sized) were cast to be close to actual structural conditions. The corroded areas are created with an accelerated method, using the migration of chloride ions. Corrosion size and localisation are controlled during the experiment. The thickness of the concrete cover has an influence on the measures, for this reason in the first slab reinforcement depth is fixed, while it is variable in the second slab. Concerning modelling, a parametric study using a finite element model is performed. This model allows the study of the influence of several parameters such as resistivity, corroded area size, concrete cover and also the measurement process. It is the analysis of the corrosion current, as well as potential mapping that allows the correlation between the experimental and modelling results. Discussion of both results concludes on the feasibility of this method and confirms its benefits compared to the usual half-cell potential mapping.

Corrosion is a serious industrial concern. According to a cost of corrosion study released in 2002, the direct cost of corrosion is approximately $276 billion dollars in the United States approximately 3.1% of their Gross Domestic Product. Key influences on the severity of corrosion include: metal and electrolyte composition, temperature, turbulent flow, and location of attack. In this work, mechanistic models of localized and flow influenced corrosion were constructed and these influences on corrosion were simulated.<p>A rigourous description of mass transport is paramount for accurate corrosion modelling. A new moderately dilute mass transport model was developed. A customized hybrid differencing scheme was used to discretize the model. The scheme calculated an appropriate upwind parameter based upon the Peclet number. Charge density effects were modelled using an algebraic charge density correction. Activity coefficients were calculated using Pitzers equations. This transport model was computationally efficient and yielded accurate simulation results relative to experimental data. Use of the hybrid differencing scheme with the mass transport equation resulted in simulation results which were up to 87% more accurate (relative to experimental data) than other conventional differencing schemes. In addition, when the charge density correction was used during the solution of the electromigration-diffusion equation, rather than solving the charge density term separately, a sixfold increase in the simulation time to real time was seen (for equal time steps in both simulation strategies). Furthermore, the charge density correction is algebraic, and thus, can be applied at larger time steps that would cause the solution of the charge density term to not converge.<p>The validated mass transport model was then applied to simulate crevice corrosion initiation of passive alloys. The cathodic reactions assumed to occur were crevice-external oxygen reduction and crevice-internal hydrogen ion reduction. Dissolution of each metal in the alloy occurred at anodic sites. The predicted transient and spatial pH profile for type 304 stainless steel was in good agreement with the independent experimental data of others. Furthermore, the pH predictions of the new model for 304 stainless steel more closely matched experimental results than previous models.<p>The mass transport model was also applied to model flow influenced CO2 corrosion. The CO2 corrosion model accounted for iron dissolution, H+, H2CO3, and water reduction, and FeCO3 film formation. The model accurately predicted experimental transient corrosion rate data.<p>Finally, a comprehensive model of crevice corrosion under the influence of flow was developed. The mass transport model was modified to account for convection. Electrode potential and current density in solution was calculated using a rigourous electrode-coupling algorithm. It was predicted that as the crevice gap to depth ratio increased, the extent of fluid penetration also increased, thereby causing crevice washout. However, for crevices with small crevice gaps, external flow increased the cathodic limiting current while fluid penetration did not occur, thereby increasing the propensity for crevice corrosion.

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The major concern regarding the use of hot-dip galvanized (HDG) steel as reinforcement in concrete has been the high rate of corrosion experienced by the zinc during the first hours in the fresh, wet, and highly alkaline concrete. The present work was aimed at clarifying three issues associated with these concerns. The first involves the metallurgical phases at the surface of the zinc coating. The concentration of zinc at the surface is a function of processing procedures, surface treatment, and exposure to weathering. Differences in the coating surface composition influence the corrosion behaviour of HDG steel reinforcing bars when they are embedded in concrete. The second issue involves the increasing use of supplementary cementing materials in concrete, which change the chemistry of the concrete pore fluid, and also influence the corrosion. The third issue is that the initial corrosion is accompanied by hydrogen evolution, which could increase the pore volume of adjacent cement and thereby, decrease the bond stress between the concrete and the steel. In order to limit the hydrogen evolution, a chromate layer is applied after galvanizing. The results of the project have demonstrated that, during zinc corrosion in ordinary Portland cement (OPC) concrete, calcium hydroxyzincate formed on untreated HDG steel provided sufficient protection against corrosion. Therefore, it is concluded that treating HDG rebar with dilute chromic acid is unnecessary as a method of passivating zinc. A layer of zinc oxide and zinc carbonate formed through weathering on HDG bars slightly increased the initial corrosion rate and passivation time compared with the non-weathered rebars. HDG steel with an alloyed coating, i.e. consisting of Fe-Zn intermetallic phases, required a longer time to passivate than those with a pure zinc surface layer. The lower zinc content of the surface limited the rate of CHZ formation and, hence, delayed passivation. However, regardless of the surface condition, the coating depth loss after two days of embedment in OPC concrete was insignificant. In concretes containing 8% cement replacement with silica fume, or 25% cement replacement with slag, the initial corrosion rates were higher than those in OPC due to higher pH and lower calcium contents of the concrete pore solution. The higher corrosion rates lead to initial depth losses which are considered significant. However, in these concretes, chromate treatment was also shown to be unnecessary. Through porosity assessment of cements adjacent to HDG bars, it was found that hydrogen evolution accompanying zinc corrosion did not have an impact on the pore volume of cements: any additional pores created by hydrogen gas are filled by the zinc corrosion products. The negligible difference in pore volume between cements adjacent to chromated and non-chromated bars further confirmed that chromated treatment was unnecessary.

Concrete is strong in compression; however, it is quite fragile in tension. To overcome this flaw, concrete is frequently reinforced with bars typically made of low grade, low carbon steel. The environment inside of concrete is favorable for steel; unfortunately when passive steel is exposed to chlorides, active corrosion can initiate, resulting in damage to the structure. One source of chloride contamination is through anti-icing agents which are used to inhibit the formation of ice on roadways, ensuring safe driving conditions. This represents a serious concern from both the cost associated with rehabilitation (Canadian infrastructure deficit in 2003 was $125 billion [1]) and as a safety concern to the public. In Canada, 5 million tonnes of road salts are used each year [2], of which Ontario uses 500 to 600 thousand tonnes [3]. As a result, the Ministry of Transportation Ontario (MTO) has requested a study of four frequently used anti-icing agents: 25.5% NaCl, 31.5% MgCl2, 37.9% CaCl2 and 32.6% multi Cl- (12% NaCl, 4% MgCl2 and 16% CaCl2). The objective of the study is two-fold, the first is comparing the effects of the solutions on steel embedded in concrete (high pH environment) and the second is to compare the effects of the anti-icing agents to a variety of construction steels in atmospheric conditions (neutral pH). Macro-cell and micro-cell corrosion in concrete were tested using both modified ASTM G109 prisms and concrete beams with 6 embedded black steel bars. Unfortunately, these tests proved inconclusive; all of the steel remained passive. This was a result of casting a high quality concrete in laboratory conditions which ultimately lead to minimal diffusion of the anti-icing solutions. Therefore, it is recommended that for short term corrosion testing (<2 years), poor quality concrete or cement paste should be used. Micro-cell testing in synthetic concrete pore solution contaminated with the anti-icing solutions was conducted in order to obtain results in the period of the M.A.Sc. program and to directly observe the corrosion. The initial concentration of Cl- in each solution was 0.00% Cl-; this was incrementally increased by 0.005% Cl-/week. Potentiostatic linear polarization to resistance measurements and pH measurements were used to monitor the corrosion on a weekly basis. The results of this test showed that MgCl2 has the most detrimental effects due to the drop in pH (from 13.5 to 9.1) caused by Mg replacing Ca in Ca(OH)2 to form the less soluble Mg(OH)2. The transition from passive to active corrosion initiated at 0.7, 0.4-0.9, 0.6 and 0.6% Cl- for NaCl, MgCl2, CaCl2 and multi Cl-, respectively. The active corrosion current densities were 11mA/m2 for NaCl, CaCl2 and multi Cl-, whereas MgCl2 had active corrosion rates of ~100 mA/m2. One bar exposed to CaCl2 showed corrosion rates as high as 600 mA/m2. This was a result of crevice corrosion between the shrink fitting and the rebar. Once the expansive corrosion products broke through the shrink fitting and ample supply of oxygen became available, allowing the corrosion rates to spike dramatically. The following steels were tested directly in the diluted solution in a cyclic corrosion chamber: stainless steels: 304L, 316LM, 2101, 2205, 2304, XM28; corrosion resistant steel reinforcing bars (rebar): galvanized rebar, guard rail (galvanized plate steel) and MMFX; carbon steels: black steel rebar, box girder, drain, weathering steel. The reinforcing bars were virgin steels whereas the remaining steels were components from the field. The testing regime followed SAE J2334 using the anti-icing solutions diluted to 3% by wt. Cl- as the immersion liquid. Unfortunately, the mutli Cl- solution was not tested due to time constraints. The mass change per unit area was measured every five cycles. All stainless steels exposed to all anti-icing solutions exhibited similar changes in mass per unit area, less than 10 g/m2. All plain carbon steels including weathering steel exhibited mass changes per unit area of more than 1000 g/m2 with some variability between the various anti-icing solutions and steel types, although the black steel rebar typically outperformed the other carbon steels. The corrosion products of MMFX were non-adherent, resulting in inconclusive results. The galvanized layer on the guard rail, which had been exposed to the environment in service, proved to be more protective than the fresh zinc coating on the galvanized rebar. When exposed to the MgCl2 solution, the mass change of both new and used galvanized steels was comparable to that found in the stainless steels. When exposed to NaCl solutions, the galvanized guard rail also exhibited this trend, whereas the new galvanic coating did not, suggesting that with exposure to the atmosphere a galvanic coating will protect the steel against NaCl. In all cases galvanized steel exposed to CaCl2 solutions exhibited mass changes per unit area of less than 100 g/m2 this is considered moderate, as this value is one order of magnitude higher than the stainless steels and one order of magnitude lower than the carbon steels exposed to the same test. It is recommended that galvanic coatings be utilized in areas heavily exposed to anti-icing solutions. The weathering steel offers no advantages over carbon steels when directly exposed to anti-icing solutions. Furthermore, in areas with high amounts of exposed galvanized steel, CaCl2 should be avoided. Between the four solutions tested, NaCl solutions are recommended as the anti-icing agents that, overall, causes the least amount of damage to both the reinforcing steel in concrete and to exposed metallic components. NaCl is followed by multi Cl- and CaCl2. Even though MgCl2 causes less damage when directly exposed to carbon steels and galvanized steels than CaCl2, it is much easier to repair external components than internal components. Therefore, MgCl2 is not recommended.

The major concern regarding the use of hot-dip galvanized (HDG) steel as reinforcement in concrete has been the high rate of corrosion experienced by the zinc during the first hours in the fresh, wet, and highly alkaline concrete. The present work was aimed at clarifying three issues associated with these concerns. The first involves the metallurgical phases at the surface of the zinc coating. The concentration of zinc at the surface is a function of processing procedures, surface treatment, and exposure to weathering. Differences in the coating surface composition influence the corrosion behaviour of HDG steel reinforcing bars when they are embedded in concrete. The second issue involves the increasing use of supplementary cementing materials in concrete, which change the chemistry of the concrete pore fluid, and also influence the corrosion. The third issue is that the initial corrosion is accompanied by hydrogen evolution, which could increase the pore volume of adjacent cement and thereby, decrease the bond stress between the concrete and the steel. In order to limit the hydrogen evolution, a chromate layer is applied after galvanizing. The results of the project have demonstrated that, during zinc corrosion in ordinary Portland cement (OPC) concrete, calcium hydroxyzincate formed on untreated HDG steel provided sufficient protection against corrosion. Therefore, it is concluded that treating HDG rebar with dilute chromic acid is unnecessary as a method of passivating zinc. A layer of zinc oxide and zinc carbonate formed through weathering on HDG bars slightly increased the initial corrosion rate and passivation time compared with the non-weathered rebars. HDG steel with an alloyed coating, i.e. consisting of Fe-Zn intermetallic phases, required a longer time to passivate than those with a pure zinc surface layer. The lower zinc content of the surface limited the rate of CHZ formation and, hence, delayed passivation. However, regardless of the surface condition, the coating depth loss after two days of embedment in OPC concrete was insignificant. In concretes containing 8% cement replacement with silica fume, or 25% cement replacement with slag, the initial corrosion rates were higher than those in OPC due to higher pH and lower calcium contents of the concrete pore solution. The higher corrosion rates lead to initial depth losses which are considered significant. However, in these concretes, chromate treatment was also shown to be unnecessary. Through porosity assessment of cements adjacent to HDG bars, it was found that hydrogen evolution accompanying zinc corrosion did not have an impact on the pore volume of cements: any additional pores created by hydrogen gas are filled by the zinc corrosion products. The negligible difference in pore volume between cements adjacent to chromated and non-chromated bars further confirmed that chromated treatment was unnecessary.