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Unidirectional surface roughness of varying magnitudes were created on both nickel and mild steel by grinding on SiC papers with grit sizes from G60 (roughest) to G1200 (smoothest) and the corrosion resistance in 0.5M H2SO4 solution was determined using a potentiodynamic polarization technique. A different trend of corrosion rate versus roughness was seen for the active-passive metal (nickel) and non-active-passive metal (mild steel). For nickel there was an increase in corrosion rate with increasing roughness, whereas for mild steel the corrosion rate decreased with increasing surface roughness. Furthermore, through a detailed examination of the surface before and after corrosion using techniques including profilometry, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), it was established that different corrosion mechanisms were operative for nickel and mild steel. For both metals, the smaller grit sizes produced a rougher surface with wider and deeper grooves. In the case of nickel, the higher roughness provided a greater contact area between the corrosive medium and metal and there was trapping of the corrosive ions in the deep grooves. Both of these factors would lead to an increase in corrosion rate. Also, for the smoother nickel surfaces, it is easier to form a stable passive film. For mild steel, which does not form a passive film, corrosion rates are generally much higher than for nickel. For the rougher surfaces with the deeper grooves, the corrosion product, FeSO4, can fill the grooves thereby acting as a barrier to further ingress of the corrosive ions to the un-corroded metal. WIT Transactions on Engineering Sciences, Vol 90, www.witpress.com, ISSN 1743-3533 (on-line) © 2015 WIT Press doi:10.2495/MC150321 Materials Characterisation VII 355

Due to their high chemical reactivity, relatively low melting point and low hardness, magnesium and magnesium alloys have relatively poor corrosion and wear resistance. Since both corrosion and wear are surface phenomena, a number of surface engineering techniques have been used to improve corrosion and wear performance. Whilst some surface hardening/strengthening methods have led to improvements in wear properties, they have not, in themselves, significantly improved the corrosion performance. Plasma electrolytic oxidation (PEO) has the potential to produce hard, compact oxide coatings that are well adhered to the magnesium alloy substrate. Such coatings can provide both improved wear and corrosion resistance. In this paper we describe how by changing the PEO processing parameters (substrate alloy; electrolyte; current or voltage; processing time) we can change the nature of the PEO oxide coatings (thickness; microstructure; porosity; phase content; composition) which, in turn, effects the corrosion and wear performance. All PEO-coatings have a three-layer structure with a porous outer layer, and intermediate dense layer and a thin inner dense layer. From a corrosion aspect, the performance of coatings is determined by the time taken for corrosion to initiate since this is much shorter than time taken for the coating to degrade. For PEO-coated Mg-alloys, this initiation time is primarily determined by the thickness, porosity and phase content of the inner dense layer at the coating/substrate interface. With respect to tribological properties, the coefficient of friction (COF) in dry sliding wear increases with increasing surface roughness of the PEO coatings. The wear rate is primarily determined by the thickness and hardness of the intermediate dense layer. Surface and Contact Mechanics including Tribology XII 163 WIT Transactions on Engineering Sciences, Vol 91, www.witpress.com, ISSN 1743-3533 (on-line) © 2015 WIT Press doi:10.2495/SECM150151 Coatings containing less porosity and higher spinel-phase (MgAl2O4, Mg2SiO4) content are harder and more wear-resistant.

Magnesium and its alloys are attractive to the automotive industry for their inherent light-weight which leads to highly fuel-efficient design. However, due to a low melting temperature (650°C), magnesium has relatively poor elevated temperature mechanical properties, e.g., creep. This has, therefore, restricted its use in applications such as engine components. Magnesium is also a highly reactive metal and has inherently poor corrosion and wear resistance. Improved corrosion and wear performance can be obtained through alloying and microstructural engineering. However, for enhanced corrosion and tribological properties, the use of surface engineering techniques involving coatings is mandatory. Plasma Electrolytic Oxidation (PEO), also known as “Micro-Arc Oxidation (MAO)”, has been used to successfully produce oxide layers on magnesium alloys with excellent tribological and corrosion resistant properties. By controlling the PEO process parameters, uniform, relatively pore-free and well adhered coatings can be produced which can provide adequate corrosion protection. The coating requirements for good tribological properties are somewhat different than for good corrosion performance. However, good tribological performance combined with good corrosion performance can be obtained through control of the PEO processing parameters.

Induction hardening is a key manufacturing process in the production of ductile iron automotive camshafts. Its desirable features include low-cost, a wearresistant surface, and ease of manufacturing. Due to the asymmetrical geometry of the camshaft, induction hardening generates non-uniform heating/cooling which leads to a variability in both the microstructure of the surface hardened case, principally the amount of retained austenite (RA), and the level of residual stress (RS). Both RA and RS can affect the contact fatigue performance of the camshaft. In the present study, automotive camshafts containing six lobes per camshaft were induction hardened to different schedules and the RA and RS (surface) variations were determined both between different lobes on the same camshaft and between camshafts that were induction hardened to different schedules. RA and RS measurements were made using x-ray diffraction methods by two independent, certified testing laboratories. Because of their low cost and ease of testing, we have also used optical metallographic methods to determine RA. A correlation was obtained between the RA values obtained by x-ray diffraction with those obtained by optical metallography. This data for ductile iron expands the database that was available for steels to higher carboncontent ferrous alloys. Finally, a correlation is made between RA content and RS level in order to define a robust process window.

Zr oxide coatings were deposited using plasma electrolytic oxidation (PEO) on Zr-2.5wt%Nb (Zr-2.5Nb) alloy, which is currently used for pressure tubes in the CANDU nuclear reactor. The effects of two PEO processing factors, electrolyte concentration and current density, on the microstructure and properties of the coatings were systematically investigated. The coating morphology and chemical composition were determined using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Potentiodynamic polarization corrosion testing was used to examine the corrosion resistance of the coatings in a dilute aqueous, LiOH solution. Pin-on-disc wear tests were performed under dry and lubricated sliding conditions. SEM was also used to characterize the wear traces on the coated and uncoated surfaces. A 30-day autoclave experiment was carried out to study the corrosion performance of the Zr-2.5Nb substrate, PEO coatings and a commercial autoclaved black oxide coating. Based on the results of this research, recommendations are made as to processing parameters for the production of oxide coatings with improved wear and corrosion resistance.

Ferritic nitrocarburizing is being proposed as an alternative to gas carbonitriding to improve the surface characteristics of SAE 1010 plain carbon steel automotive components without producing unacceptable part distortion. Navy C-rings are specially designed specimens for the evaluation of the distortion (size and shape) that result from any heat treatment process. In this study, gas ferritic nitrocarburizing and nitrogen cooling was used to heat treat Navy C-ring specimens of varying thicknesses (2.8 mm–19.05 mm) at temperatures ranging from 510°C to 595°C. For each combination of part thickness and heat treatment temperature, the following parameters were evaluated: size and shape distortion; XRD and OM (optical microscopy) to characterize the nature of the nitride layers formed at the surface; X-ray determination of the residual stresses in the nitride surface layers. The various combinations of nitrocarburizing temperature and time resulted in an expansion of the OD (Outside Diameter) dimension and a small deterioration in flatness. The ID (Inside Diameter) changed from a small expansion (+0.02%) for the thickest specimens (19.05 mm) to a small contraction (-0.02%) for the thinnest specimens (2.8 mm). The gap tends to close up as the thickness decreases from the thickest to the thinnest specimens. Microstructural differences were found in the nitrided layers formed at the different heat treatment temperatures. The residual stresses in the outer e-nitride layers (Fe3N) were typically tensile. On the basis of these results, preliminary recommendations are made as to heat treatment parameters to lower distortion.

The low specific gravity of magnesium has made Mg-alloys attractive for automotive and aerospace applications. Conventional Mg casting alloys have been based on the Mg-Al system with additions of Zn, Mn or Si, e.g. AZ91 alloy (Mg-9.0Al-1.0Zn, wt.%). Such alloys, which have good castability, mechanical properties and corrosion resistance, are widely used in the automotive industry. However, due to the rapid degradation of the mechanical properties at elevated temperatures, especially the creep resistance, application of these alloys has been limited to specific components that operate at temperatures below 150 C. New creep-resistant Mg casting alloys are required for application as transmission cases (temperatures up to ~175 C), engine blocks (~250 C) and pistons (~300 C). The development of such high performance creep-resistant alloys requires the development of microstructures in the cast alloy that both prevent grain boundary sliding and restrict the motion of lattice dislocations within the primary α-Mg grains. The development of Mg-Al-Ca casting alloys is described where Ca-containing eutectic phases are precipitated along the grain boundaries and where dislocation motion within the grains is restricted due to solid solution hardening and the presence of dispersed nanoscale eutectic phases