In this study, high-strength aluminum alloy AA7055 deposits are prepared using a recently developed cold spray process that employs in-situ microforging. The in-situ hammering effect is achieved by mixing large shot-peening particles into the spray powder and is shown to enhance interparticle bonding along with the plastic deformation of deposited particles. The underlying mechanisms are discussed based on the characterization of interface microstructure and the distribution of oxide film at the interparticle interface.

The corrosion behavior of thermally sprayed Al-Zn alloy coatings immersed in NaCl aqueous solution was determined by using electrochemical impedance spectroscopy. Ohmic resistance and capacitance of the corrosion product film formed on the sprayed coating surface, and the corrosion resistance and electric double layer capacitance at the interface between the sprayed coating and the solution were evaluated. As immersion time in NaCl solution increased, the corrosion rate of the Al-2mass%Zn coating increased, but the corrosion rate for Al-6mass%Zn and Al-15mass%Zn coatings decreased due to the influence of the corrosion product.

In this investigation, aluminum-copper, aluminum-zinc, and zinc coatings were flame sprayed onto a sand-blasted mild steel substrate. The coatings were immersed in seawater and were examined on a monthly basis using EDS and XRD analysis to assess corrosion and marine fouling behaviors. The spraying and test procedures are described and the results are presented and discussed. Zinc and aluminum-zinc coatings with high zinc content proved to have good anti-corrosion and anti-fouling properties, but the aluminum-copper coatings did not.

MoB/CoCr with excellent durability against molten metal has been developed to be applied to aluminum die casting parts in automobile and to hot continuous dipping rolls in Zn and Al- Zn plating lines for steel industries. Our previous study has revealed that the MoB/CoCr coating prepared by high velocity oxy-fuel (HVOF) spraying has much higher resistance against attack of the molten Al-45wt%Zn alloys compared to conventional WC/12wt%Co coating from evaluation by laboratory scaled molten metal immersion test. However, performance with no damage is only limited when applying onto the substrate with smaller thermal expansion similar to the MoB/CoCr coatings. This paper describes the solution to prepare the MoB/CoCr coating without damage onto austenitic stainless steel of AISI316L with larger thermal expansion, whose material is commonly utilized for the hot continuous dipping rolls. It has been found that use of undercoat is effective to reduce the negative influence of large difference in thermal expansion between the MoB/CoCr coating and substrate of AISI316L. Optimized thickness of topcoat and undercoat and its combination showed intrinsic performance of low reactive MoB/CoCr without any mechanical damage such as crack and/or peeling.

In the continuous hot-dip galvanization process, the corrosion and chemical stability of the sink roll in the galvanizing bath are important issues that affect quality and productivity. In order to protect the sink roll, carbide cermet and/or ceramic coatings were deposited on the surface of the sink roll. The WC-, Cr 3 C 2 - cermet coatings were deposited by high velocity oxygen fuel (HVOF) spray, respectively. The coating samples were immersed in molten Zn-alloy containing 50wt.% aluminum at 833K for 24 hr and 144 hr, respectively. The inter-diffusion and inter-reaction of Zn, Al and elements in the coating and corrosion behavior of these coatings were investigated by XRD, SEM and EPMA, etc. The corrosion mechanisms of the carbide cermet coatings and ceramic coatings in molten High Al-Zn-alloy were proposed.

In order to develop protective coatings for sink rolls in continuous hot-dip galvanizing, a sprayed MoB/CoCr cermet coating was formed on a 316L stainless steel by the HVOF spraying process and its durability in the molten 55%Al-Zn- 1.5%Si bath (923 k) has been investigated by SEM and EDS. The immersion test revealed that the MoB/CoCr coating has much higher durability (640 hours) in 55%Al-Zn-1.5%Si bath than the conventional sprayed coatings (120 hours), such as WC-Co, WC/Co/Cr and ceramics. It was found that the failure of MoB/CoCr coating is mainly caused by the mismatch of coefficient of thermal expansion (CTE) between the top coating and the substrate. The failure procedure is that first crack is generated because of heat stress, then the crack proceeds and causes scaled delamination, at the same time molten Al-Zn will enter into cracks and/or Al-Zn reaches the undercoat and/or substrate, finally molten Al-Zn dissolves the substrate.

MoB/CoCr, a novel thermal spray material, with high durability in molten Al and/or Al-Zn alloys has been developed to utilize for die casting parts of Al alloy, and for hot continuous dipping roll in Zn and Al-Zn plating lines. The durability of the MoB/CoCr coatings prepared by high velocity oxy-fuel (HVOF) spraying has been investigated using a molten metal immersion tester. The immersion tests revealed that the MoB/CoCr coating has much higher durability without dissolution in the Al alloys than conventional spray coatings, such as WC/12%Co and WC/10%Co/4%Cr, as well as various other surface modification methods. The MoB/CoCr coating also has showed higher durability in the molten Al-Zn alloy when the coefficient of thermal expansion (CTE) of the substrate is similar to that of the MoB/CoCr. The lifetime of the coating on stainless 316L substrate, widely used as the hot continuous dipping roll, was insufficient because of the generation of cracks due to a large difference in the CTE between the MoB/CoCr coating and substrate. Basic anneal examinations have revealed that optimization of coating structure by changing undercoat material and its thickness is effective to improve the lifetime.

This report presents results of a new technology that looks promising to apply high quality corrosion resistant coatings in limited special areas. This technology could minimize or eliminate maintenance activities experienced today with existing corrosion preservation management systems. Four passivation coatings (aluminum, zinc, 85Zn- 15Al, and 55Al-45Zn) were deposited and evaluated using the Cold Spray process. The composite powders were not chemically alloyed but mixed by weight percent (powder alloyed) to prepare the composite mixture. Each coating was deposited with both nitrogen and helium gas. Deposition efficiency measurements were made for each powder/gas combination. Coatings were applied to DH-36 steel specimens and analyzed for quality of the bond line between the coating and substrate, porosity, bond strength, weight percent of metal constituents resident in the composite coatings, and integrity of the coating after 1000 hours of exposure to accelerated salt spray testing. Cold sprayed results are compared with specimens sprayed with aluminum using the flame sprayed process. Cost comparisons are made for depositing the various powders using both Cold Spray and flame spray processes.

The sealing effect of various sealants applied to sprayed coatings in sodium chloride aqueous solution was investigated with the galvanostatic technique in which the same quantity of electricity is fed to each test material coated with sealant film cut crossly. The amount of dissolved sprayed coating sealed with sealant can be obtained as the difference between the dissolved material volume at the crosscut part and that in the solution. From the result it was found that the higher the sealing effect of sealant the more aluminum is dissolved from the sprayed coating at the crosscut part, which is considered to mean in case of the sealant poor in sealability, sprayed coating is dissolved even where it is coated with sealant and electrolytic current is dispersed. This proves that the galvanostatic technique is effective in evaluating the sealing effect of sealants.

Zinc and aluminium coatings have been used widely to protect steel structures from corrosion in aggressive and hostile conditions. The more recent development of zinc 15wt% aluminium alloy in a wire form has demonstrated that arc-spray coatings can be produced with a resistance to red rust which is superior to that of the single metals. Competitive 'pseudo' alloy or composite coatings produced by co-spraying wires of zinc and aluminium have been shown to achieve resistance to salt spray conditions similar to this conventional alloy. Work described in this paper confirms these findings and goes on to demonstrate an additional advantage of co-spraying an aluminium -5wt% magnesium instead of aluminium with the zinc. The importance of providing a fine dispersion of the two phases in the 'pseudo' alloy is highlighted and an alternative method of providing a similar dispersion by using a 'cored' wire (e.g. Al-5wt%Mg wire in a zinc sheath) approach has been demonstrated. The importance of 'self sealing' in these coatings after the initial loss of zinc is discussed which is related to the coating microstructure and their electrochemical behaviour in chloride solutions.

Thermal spraying has been used to protect many steel structures from aqueous corrosion using Zinc and Aluminium, and to some extent their alloy coatings to provide galvanic protection. The lifetimes of the coatings can approach 50 years even when exposed in severe marine environments. Zinc coatings work by continuously sacrificing themselves and slowly dissipating over time. Aluminium coatings passivate more readily and form a barrier layer, the passivity makes them less able to protect damaged areas and to self heal. A new ternary coating system involving Aluminium, Zinc and Magnesium has been shown to be capable of providing both a passive barrier layer as well as being able to give galvanically active protection. Salt spray tests have shown that the resistance to red rust of these new coatings increases by 300% over similar thicknesses of the separate metal coatings. Processing by arcspray is straightforward and both adhesion and deposition efficiency are better than where Zinc is sprayed alone.