Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 115-118). Also available on the World Wide Web.

Thesis (Professional Degree)--University of Missouri, School of Mines and Metallurgy, 1935. / The entire thesis text is included in file. Typescript. Title from title screen of thesis/dissertation PDF file (viewed June 15, 2010)

In response to more stringent global CO2 emissions, automotive manufacturers have increased the use of advanced high strength steels (AHSS). Ultra-high strength steels are often used within the body-in-white (BIW) for safety critical parts and structural reinforcements, such as roof rails and side impact beams. Currently, the most commonly used press hardened steel (PHS) grade for these applications is 22MnB5, with a typical composition of 0.22C 1.2Mn 0.25Si 0.005B (wt%). Automotive OEMs have expressed a desire to use Zn-based coatings as they are compatible with the current painting system and have the potential to provide robust cathodic protection. The steel blanks generally undergo direct hot press forming (DHPF) to achieve the necessary martensitic microstructure and target mechanical properties, but this presents challenges for Zn-coated 22MnB5. The adoption of Zn-based coatings within the automotive industry has been inhibited by the prospect of liquid metal embrittlement (LME) resulting from DHPF, as well as the desire to provide robust cathodic protection. Previous literature has reported that a zinc ferrite (α-Fe(Zn)) coating with a global Zn content of at least 30 wt% will provide cathodic protection to the underlying substrate. The main goal of this work was to determine the microstructural evolution and electrochemical properties of galvanized (GI70 – 70 g/m2/side) 22MnB5 substrates as a function of the annealing time at a typical austenization temperature of 900°C. It was found that the Zn-based coatings annealed at 700°C consisted to a mixture of small volume fraction of α-Fe(Zn) and Г-Fe3Zn10. After heating to 900°C, the coating comprised varying volume fractions of α-Fe(Zn) and Zn(Fe) liquid, which transformed to Г-Fe3Zn10 after solidification. The relative fraction of Г Fe3Zn10 was found to decrease with increasing annealing time until the coating completely transformed to α-Fe(Zn) after annealing at 900°C for 240 s. GDOES results found that, when the sample was annealed at 900°C for 240 s, the global Zn content of the coating was less than 30 wt%. Coatings comprising varying fractions of Г-Fe3Zn10 were subjected to uniaxial tensile tests to determine how the coating microstructure affected the mechanical properties in comparison to the uncoated substrate material. It was found that the uncoated substrate material met the mechanical property requirements of σ(UTS)min ≥ 1500 MPa regardless of annealing time. However, σ(UTS) was found to decrease with increasing annealing times for the GI70 coated samples until the target mechanical properties were not met when the sample was annealed at 900°C for 180 s. This was attributed to increased coating thicknesses leading to a decrease in the martensitic cross-sectional area to support the load. Furthermore, the coatings were subjected to a variety of electrochemical characterization techniques, including potentiodynamic and galvanostatic polarization scans, potentiostatic scans, and electrochemical noise tests. Potentiodynamic polarization scans indicated a higher driving force for cathodic protection when the coating contained some fraction of Г-Fe3Zn10. Furthermore, a limiting current density for these samples was observed, demonstrating that Г-Fe3Zn10 corrodes at a slower rate in comparison to α Fe(Zn). Galvanostatic polarization measurements indicated that, when the fraction of Г Fe3Zn10 within the coating was below 15 vol%, the protective properties of the phase were not exhibited. XRD and TEM analysis revealed the formation of three corrosion products on the surface: simonkolleite, hydrozincite, and akaganeite. It was found that, when samples contained greater than 15 vol% Г-Fe3Zn10 in the coating, the predominant corrosion products were a combination of simonkolleite and hydrozincite. When the Г Fe3Zn10 content was below this value, the dominant corrosion product was found to be akaganeite. Furthermore, substrate attack was observed on a sample annealed at 900°C for 420 s when the coating layer was intact, indicating that the α-Fe(Zn) only containing coating obtained at this time does not provide cathodic protection. Based upon the current results, it was determined that a minimum volume fraction of 15 vol% Г-Fe3Zn10 must be present within the coating layer to obtain robust cathodic protection. Furthermore, it was determined that the processing window to develop cathodically protective Zn based coatings while mitigating LME is extremely narrow. This is a result of the fact that it is necessary for at least 15 vol% Г-Fe3Zn10 to be present within the coating microstructure at room temperature, which is liquid at the forming temperatures of 900°C. From the current findings, it was found that it is unlikely that a cathodically protective Zn-based coating can be obtained for DHPF steel parts using 22MnB5 as a substrate material. This is due to the high forming temperature resulting in liquefication of the coating and the rapid cooling rates necessary to achieve the target mechanical properties of σ(UTS)min ≥ 1500 MPa. Thus, it is recommended that the current substrate material be altered such that the part may be formed below the peritectic temperature of 782°C. / Thesis / Master of Applied Science (MASc)

In recent years, the automotive industry has turned to press hardened steels (PHS) to improve passenger safety while enabling vehicle weight reduction. To form the complex shapes required for this purpose, they are often direct hot press formed. It is possible to provide corrosion resistance to these parts by galvanizing the PHS sheets prior to direct hot press forming (DHPF). However, the austenitization of the galvanized steel causes the Zn-based coating to transform into two intermetallic phases. These are iron-rich α-Fe(Zn) and zinc-rich Г-Fe3Zn10. The Г-Fe3Zn10 is liquid during traditional DHPF, and the applied stress can result in liquid metal embrittlement (LME). Recently, two new grades of PHS have been developed, which allow for DHPF at 600-700°C, below the Fe-Zn peritectic temperature at 782°C, thus avoiding LME. These prototype PHS grades are designated 2%Mn (0.2C-2Mn-0.25Si-0.005B (wt%)) and 2.5%Mn (0.2C-2.5Mn-0.25Si-0.005B (wt%)). The objective of this work is to determine the effect of DHPF on the ability of a Zn-based coating to provide robust cathodic protection to the two prototype PHS. Galvanized panels of both the 2%Mn and 2.5%Mn steel were DHPF with a U-shape die at 700°C. The surface and cross-section of the coating were examined to determine the effects of DHPF on the coating surface. Die friction during DHPF resulted in die wiping on the wall of the part, leading to removal of surface Г-Fe3Zn10. In cross-section, coating cracks were present at the wall and corner of the U-shape part due to the deformation during DHPF. Potentiodynamic polarization scans were used to determine the corrosion potential of the coating, and this was used to calculate the driving force for cathodic protection using the difference in corrosion potential between the coating phases and the substrate. It was found that only Γ-Fe3Zn10 provided robust cathodic protection to both steel substrates, and the driving force for cathodic protection was lower for the coated DHPF 2.5%Mn steel. Galvanostatic scans were used to evaluate dissolution kinetics of coating phases. Robust cathodic protection was provided by the galvanized coating for austenitization times of 30 - 120 s for the 2%Mn substrate and 30 - 60 s for the 2.5%Mn substrate. The duration that robust cathodic protection was provided was shortest at the wall of the U-shape part. This result was attributed to die wiping caused by DHPF, where the surface is smoothed by die friction. When there is less Г-Fe3Zn10 in the coating, such as at longer austenization times, surface Г-Fe3Zn10 was removed and an increased amount of α-Fe(Zn) is exposed, which does not provide robust cathodic protection. In addition, coating cracks form along α-Fe(Zn) grain boundaries after austenitization for 180 s on all examined regions of the U-shape part, allowing a greater surface area of the coating exposed to electrolyte, further increasing dissolution of the coating. / Thesis / Master of Applied Science (MASc)

Despite the high demand for press-hardenable steel (PHS) with coatings that provide sacrificial cathodic protection, Zn-based coatings have experienced limited use due to the significant challenges associated with avoiding liquid metal embrittlement (LME) while maintaining robust cathodic protection when using conventional PHS materials and processing techniques. The present research addresses these challenges by reducing the conventional direct hot press forming (DHPF) temperature to between 600–700 °C, such that forming and quenching occurs well below the Zn(Fe)liq → Г-Fe3Zn10 peritectic temperature of 782 °C, thereby removing the conditions necessary for LME to occur while allowing for formation of the cathodically-protective Г-Fe3Zn10 phase. The objective of this work was to define a process window for two galvanized prototype PHS alloys with compositions of 0.20C-2.01Mn-0.26Si-0.005B and 0.19C-2.5Mn-0.26Si-0.005B (wt%) that would result in fully martensitic microstructures, tensile strengths (TS) ≥ 1500 MPa, and robust cathodic protection, defined as attaining ≥ 15 vol% Г-Fe3Zn10 in the coating microstructure, while avoiding LME. Accomplishing this task involved characterizing both grades as a function of austenization time, stamping temperature, and strain imposed by the forming process in order to define process windows that resulted in parts that met the aforementioned property requirements. It was found that the approach of increasing the Mn content relative to conventional PHS grades was successful in improving the hardenability sufficiently to enable the formation of fully martensitic microstructures despite the lower effective cooling rates associated with the reduced DPHF temperatures. Microstructural imaging and tensile testing demonstrated that, for both prototype PHS grades, a process window exists for the production of parts that satisfy the targets of the formation of fully martensitic microstructures and TS ≥ 1500 MPa while exhibiting uniform elongation of about 0.05 followed by significant post-uniform elongation. The effect of DHPF temperature and strain imposed by the forming process on mechanical properties was found to be negligible. Tensile tests and fractography revealed that reducing the DHPF temperature to between 600–700 °C was successful in preventing LME, thereby allowing samples to fracture in a ductile manner. Micro-cracking in the coating of the DHPF part was observed; however, these cracks were arrested at the coating-substrate interface. For all tested conditions, the coating met the target of ≥ 15 vol% Г-Fe3Zn10, implying that robust cathodic protection is expected. Based on the results of these experiments, it was concluded that DHPF process windows that meet all of the property targets include austenization times and DHPF temperatures of 120–180 s and 600–700 °C, respectively, for the 2Mn grade, and 60–180 s and 600–700 °C, respectively, for the 2.5Mn grade. / Thesis / Master of Science (MSc)

In my diploma work i focus on the production hydrogen by electrolysis of water using by electrolyzer. Especially, i concentrate on preparation electrodes for laboratory electrolyzer. I work on principles of increasing electrodes area, increasing functional surface of electrodes, by Nickel – Zinc coatings. By electroplating or chemical plating we spread microlayer of Nickel-Zinc onto electrodes made of steel plate. Then, in alkali - hydrate oxide, we will corrode Zinc components off the alloy and this is how we create pore surface of electrode. From macroscopic view, electrode has same geometrics propositions, however from microscopics view, the electrode has bigger surface area. The hydrogen production increases with unchanget sizes of electrolyzer.