Surface Treatments to Protect Conventional and Rheo-High Pressure Die Cast Al-Si Alloys from Corrosion

Rheocasting process integrated with the high pressure die casting method (Rheo-HPDC) has established itself as a new promising technology to produce high-quality components. However, different types of microstructural segregation induced by the semi-solid process influence the properties of the final component. The semi-solid microstructural features and the new compositions require detailed corrosion studies and verification. The first part of this thesis deals with microstructural and corrosion studies of the conventional and Rheo-HPDC Al-Si alloys. In this part, corrosion properties of two Al–Si alloys containing 2.5 and 4.5 wt % silicon cast by Rheo-HPDC method were examined in the diluted Harrison solution using polarization and electrochemical impedance spectroscopy (EIS) techniques on as-cast and ground surfaces. The microstructural studies revealed that samples taken from different positions (with respect to the feeding gate) contain different fractions of solid and liquid parts of the initial slurry. It was shown that the Rheo-HPDC Al-Si alloys are prone to the localized form of corrosion inside the eutectic region at the interface of aluminum with silicon phase and intermetallic particles. Electrochemical behavior of as-cast, ground surface, and bulk material was shown to be different due to the presence of a segregated skin layer and the surface quality. Corrosion properties of the two Al-Si alloys cast by the conventional and Rheo-HPDC process were also evaluated and compared in 0.01, 0.05, 0.1 and 0.6 M NaCl solutions. The conventional HPDC and semi-solid alloys presented similar EIS responses. However, the semi-solid samples with a lower fraction of the eutectic phase showed slightly higher impedance values in the more diluted sodium chloride solutions. Corrosion morphological features, including localized corrosion, trenching and co-operative corrosion rings were comparable for both types of alloys. However, in the anodic polarization test, the semi-solid alloys presented a higher resistance to pitting corrosion. To protect aluminum alloys from corrosion, chromium-based conversion coating has been successfully used for decades, due to its extensive protection. However, rising concerns and new restrictions on the environmental hazards of Cr (VI) compounds have led to intensive efforts to develop alternative coatings. The second and third parts of this thesis address the effort to investigate two alternatives in this field. Cerium-based conversion coatings were deposited on the conventional and Rheo-HPDC Al-Si alloys by immersion in cerium nitrate aqueous solutions. Different parameters were studied to optimize the conversion coating, and NaCl or H2O2 were also added to the solution to modify or accelerate the deposition process. The results revealed that applying cerium-based conversion coating on Al-Si alloys, is possible and a selective deposition is obtained due to the presence of iron-rich intermetallic particles inside the eutectic region. Under the accelerated conditions, the deposition mechanism includes dissolution of the aluminum matrix, selective dissolution of aluminum from the noble intermetallic particles, oxidation of iron from these particles, and the deposition of cerium hydroxide/oxide layer. The results revealed that the improvement in corrosion resistance in the presence of selectively deposited cerium-based conversion coating is more significant compared to the homogenous coating obtained from the conversion solution containing H2O2. The aluminum alloy with a higher amount of silicon showed more active surface during the conversion process which reduces the required concentration of Ce(NO3)3 but also makes it difficult to work with more aggressive solutions. In the third part of this thesis, the possible protective effect of polypyrrole coating on pure aluminum and Rheo-HPDC Al-Si alloys was investigated. Different electropolymerization solutions containing the Py monomer, SDS, DHBDS (Tiron), C6H8O7 and NaNO3 were used. The presence of nitrate anions led to the passivation of the aluminum electrode (both pure and alloy) during the electropolymerization and to the deposition of a thicker/more conductive coating. These facts resulted in longer and more efficient corrosion protection in NaCl solutions. This polypyrrole coating was able to keep the alloys’ surface potential noble for at least 168 hours. Which can be attributed to the anodic protection provided by the reduction of the polymer. It was shown that the presence of silicon phase or intermetallic particles has a positive effect on the electropolymerization of polypyrrole film. Therefore, the coatings deposited on the alloys possess higher thicknesses compared to those deposited on the pure aluminum. In the presence of chloride ions, all coatings suffered from the formation of blisters as a result of severe (localized) galvanic interaction of polypyrrole with aluminum. This may question the application of polypyrrole coating in concentrated NaCl solutions. However, it is shown that the protection efficiency can be improved by altering the solution chemistry which affects the polymer/metal interface and the conductivity and the barrier properties of the coating. Therefore, the application of polypyrrole in corrosion protection is not totally ruled out but needs specific considerations

A Survey over the Dentists’ and Endodntists’ Approaches towards the Management of Endodontic Emergencies in Mashhad, Iran

Introduction: Pain or swelling caused by various stages of inflammation/infection of the pulp/periradicular area is called endodontic emergencies. Determining the most effective method of emergency treatment is a challenging issue in endodontics. The goal of this study was to determine and compare the level of knowledge in general dentists and Endodontists about endodontic emergency treatment plan in Mashhad, Iran in 2012-2013. Methods and Materials: In this cross-sectional descriptive study, 152 questionnaires were distributed among 120 general dentists and 32 Endodontists of Mashhad. The questionnaire contained two separate parts. The first part included demographic information and in the second part different treatment protocols were suggested for 12 various conditions of pulp/periapical emergencies, and the participants were asked to choose the correct option(s). To determine the relationship between qualitative variables, the chi-square analysis was used. The level of significance was set at 0.05. Results: There were significant differences between treatment plans presented by general dentists and Endodontists about endodontic emergencies, especially in cases of necrotic pulp and subsequent swelling. Conclusion: Level of knowledge of dentists about the indications of incision and drainage, intra-canal medicament, root filing beyond the apical foramen and antibiotic prescription was not enough. These findings highlight the importance of refreshing courses for general dentists to improve their competency in the management of endodontic emergencies.Keywords: Emergency; Endodontics; Endodontists; Flare Up; General Dentists; Level of Knowledg

Identifying the Conditions under Which Online Reviews Translate into Product Sales: A Sentiment Analysis Approach

Number of available online consumer reviews has increased dramatically. This research-in-progress study draws on Cue-Summation and Communication Persuasion theories to define and validate the online review positiveness as a multidimensional formative index and conduct a sentiment analysis, using secondary data from Amazon.com, to identify the conditions under which online reviews translate into product sales. Particularly, this study focus on the role of product involvement, review time, product price, and review score inconsistency on the impact of online review positiveness on product sales. Potential contributions to theory and practice are also discussed

The effect of task type and task sequence on the learning of EAP vocabulary in medical students

Background and aims: Vocabulary plays the main role in the foreign language learning and without vocabulary knowledge is not established any communication. The current study was aimed to investigate the effect of input tasks, input plus output tasks, and output plus input task on vocabulary learning of advanced medical students. Methods: The participants of this pre-experimental study were 45 advanced students majoring different fields of medicine, selected after taking a TOEFL proficiency test. To collect the relevant data the students were randomly divided into three groups. After taking VKS test for choosing the vocabulary which the students don’t know, a pretest was administered. In the following three weeks the students received different types of treatment based on their group to learn the meaning of the intended vocabulary. A post-test was administered two days after the treatments were completed. Results: The analysis of the results showed that all three methods of treatment had an effect on the learning of students. However, one way ANOVA showed that the two groups receiving both input and output tasks with different sequences had a more significant improvement in their vocabulary learning. Conclusion: The results showed that the two mixed methods proved to be the most effectiveness to improve the learning of EAP vocabulary learning in students. The result of this study is useful for material developers and EFL teachers, especially those teaching English for academic purposes