Chang Sheng Liu

CORROSION BEHAVIOR OF A ZIRCONIUM-TITANIUM BASED PHOSPHONIC ACID CONVERSION COATING ON AA6061 ALUMINIUM ALLOY

The conversion coating was formed by dipping AA6061 in a fluorotitanate/zirconate acid and amino trimethylene phosphonic acid (ATMP) solution at room temperature. The formation process and the anti-corrosion performance of the conversion coating were investigated using electrochemical test and salt spray test (SST), respectively. The electrochemical test shows that the Zr/Ti and ATMP coating improves the corrosion resistance of AA6061 as good as the chromate (VI) coating. But the results of SST show that the corrosion resistance of Zr/Ti and ATMP coating is not as good as the chromate (VI) coating. The corrosion area is less than 2% after 72 h.

CORROSION RESISTANCE OF HOT DIP GALVANIZED STEEL PRETREATED WITH BIS-FUNCTIONAL SILANES MODIFIED WITH NANOALUMINA

The corrosion behavior of hot dip galvanized steel pretreated with bis-[triethoxy-silylpropyl] tetrasulfide (BTESPT) modified with alumina particles was studied. The corrosion resistance of the passiving films was evaluated by Tafel polarization curve and electrochemical impedance spectroscopy. The films formed on the galvanized steel substrate were characterized by Fourier transform infrared spectroscopy and energy dispersive X-ray spectrometry. The surface morphology of the treated hot dip galvanized steel samples was observed by Field Emission Scanning Electron Microscope. The results show that the pretreatments on the basis of silane films modified with nanoalumina particles have reduced both anodic and cathodic current densities, and increased total impedance in the measured frequency, consequently, improving corrosion protection for hot dip galvanized steel during immersion in NaCl solutions compared to chromate films and silane films.

AMINO-TRIS-（METHYLENEPHOSPHONIC ACID） LAYERS ADSORPTION ON AA6061 ALUMINUM ALLOY

The amino-tri-(methylenephosphonic acid) layers were adsorbed on the surface of AA6061 aluminum alloy for improving the lacquer adhesion and corrosion inhibition as a substitute for chromate coatings. The surface structure and characteristic of the amino-tri-(methylenephosphonic acid) layers on AA6061 aluminum alloy were investigated by means of XPS and ATR-FTIR analysis. The analyzed results showed that the amino-tri-(methylenephosphonic acid) adsorption layers adsorb on the surface of aluminium alloy via acid-base interaction in a bi-dentate conformation. After the amino-tri-(methylenephosphonic acid) layers were coated with epoxy resin, the layers showed good adhesive strength and favorable corrosion resistance in contrast to chromate coatings.

A Comparative Study of Zirconium-Based Coating on Cold Rolled Steel

An experimental zirconium-based conversion coating on cold rolled steel was investigated in the present work. The following tests on differently pretreated panels, such as bonderite®NT-1, oxsilan®9810/1 and lz-4356, were conducted to evaluate corrosion performance include adhesion test, T-bent test, neutral salt spray test and warm salt water dipping test and electrochemical studies were also performed. The whole results showed that the experimental zirconium-based conversion coating had well-pleasing corrosion protection properties and adhesion properties, and similar lever performance as compared to the other industrial coatings. The experimental zirconium-based conversion coating was very hopeful about industrial application.

Process Optimization and Properties of Laser Cladding High Vanadium High Speed Steel Coatings on Nodular Cast Iron

As withstanding very high loads, thermal cycling leading to thermal fatigue, and severe environmental in the steel industry, rolls with long service life are specially required. High speed steel with high vanadium content is a newly-developed wear-resistance material that has been studied and used in some countries for making steel rolls. As a surface treatment technology, laser cladding can fabricate coating to improve the wear resistance of substrate. In this paper, the substrates for laser cladding were nodular cast iron rolls, Nd: YAG solid pulsed laser was used to explore the feasibility of preparation high vanadium high speed steel (HVHSS) coatings. The Nd: YAG laser cladding results that the coated layers combined metallurgically with the substrate with a lot of microcracks. The average microhardness up to 650 HV is more than 2 times as high as that of the substrate. After laser remelting, a fully dense and crack free HVHHS coating with an excellent metallurgical bonding was deposited. The presence of VC in the coating mainly improves the microhardness of coating up to about 650 HV.

Laser Surface Alloying of NiCr-Cr3C2 and C-B-W-Cr Powders on Nodular Cast Iron Rolls

The laser surface alloying(LSA) technique was firstly used to form wear resistant layers on nodular cast iron rolls with two different powders, A: NiCr-Cr3C2 and B: C-B-W-Cr. The microstructure, element distribution and phases of the layers were investigated. Results indicate that the alloying layers had pores and cracks, as well as a metallurgical bonding with the substrates. For layers with powder A, as the thickness of the pre-layers decreased, the numbers of pores and cracks of the LSA layers decreased but the thicknesses of them were close. Phases were barely influenced by the thickness of the pre-layer, while the content of them were affected. For layers with powder B, as the laser specific energy increased, the numbers of pores and cracks in the LSA layers decreased and the thicknesses of them increased. The microstructure in the layers with different powders was different.

Preparation of the Carbon Fiber/Cu Alloy Matrix Self-Lubricating Composite Materials

Using 663-tin bronze, Ni, W, nanoAl2O3, MoS2, graphite, CaF2, and Ni coated graphite as the matrix alloy powder, in which copper-coated carbon fiber of 5%, 7%, 9%, 11% and 13% in volume fraction were added as the reinforcing phase, a novel type carbon fiber/copper-matrix self-lubricating composite materials was prepared by means of powder metallurgy. The results indicate that the mechanical properties of the composite materials are improved after adding copper-coated carbon fibers. The composite materials reach optimal overall mechanical performance under testing when the volume fraction of the added copper-coated carbon fibers is 11%.: with a hardness of 57.8 HV and a compressive strength of 222 MPa. The addition of carbon fiber also improved the friction and wear properties of the composite materials. Increasing the volume fraction of fiber, was found to increase the wear resistance and improve self-lubricating performance. A volume fraction of 11% gave a friction coefficient of 0.09 and abrasion loss of 4mg.

Study on Microstructure of Laser In Situ Formation of TiBX and TiC Titanium Composite Coatings

Two kinds of mixed powders:Ti-6Al-4V/B/C and Ti-6Al-4V/B4C which are pre-pasted or synchronized fed on Ti-6Al-4V substrates separately were scanned by a 500W pulsed YAG laser to induce in situ formation of titanium composite coatings contained TiBx and TiC ceramic reinforced phases. The influences of laser processing parameters including Pulse Frequency (PF), Pulse Width (PW), Laser Power (P) and Scanning Speed (V) together with the powder proportions on the microstructure and properties of the coatings were investigated. Microstructures, phase components of the coating were analyzed by OM, SEM, TEM and XRD respectively. Experimental results show that two and more kinds of ceramic reinforcements were in situ formatted in the matrix of Ti-6Al-4V. TiB and TiC ceramics were formed evenly with the morphology of needle, tiny dendrites and disperse particles in the prepasted single path specimens. For the powder feed laser cladding layers, the ceramic reinforcements were TiB (needlelike), TiB2 (hexagonal prism or rodlike), a small amount of TiC (disperse particles) and non fully reacted B4C. The microhardness increased with the increase of the amount of B4C and B+C additions. When the added B and C contents are the same, the microhardness of the coating with B4C addition is higher than that of the coating with B+C addition. The average micro-hardness of a powder prepasted (with 20 wt.% B4C addition) multi-path laser cladding layer formed under the optimized processing parameters is up to 800HV, which is more than 2 times of that of the substrate (340Hv), and the wear weight loss of the layer decreased nearly 3 times that of the substrate.

Corrosion Protection and Composition of Galvanized Steel Substrates by Bis-[3-(triethoxysilyl)-Propyl]Tetrasulfide and Gamma-Aminopropyltriethoxysilane

Compound silane films were prepared on galvanized steel by immersing method using mixture of bis-[3-(triethoxysilyl)propyl]tetrasulfide and γ-aminopropyltriethoxysilane. The corrosion resistance of galvanized steel substrates was compared after treatment with bis-[3-(triethoxysilyl)propyl]tetrasulfide, γ-aminopropyltriethoxysilane and their mixture in 3.5%NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy. The results indicated that corrosion current density of the mixed silane films was reduced evidently and self-corrosion pressure of that was shifted to left markedly and total impedance value of that increased about two orders of magnitude. The analytical characterisation was performed by Fourier transform infrared spectroscopy and atomic force microscopy. The results showed that bis-[3-(triethoxysilyl)propyl]tetrasulfide and γ-aminopropyltriethoxysilane were all connected with the former of films.

Microstructure and Cavitation-Erosion Mechanism of Laser Cladding NiCrSiB Coating on CrNiMo Stainless Steel

A Ni-based alloy NiCrSiB laser cladding layer was made on the surface of CrNiMo stainless steel specimens using NiCrSiB powder. Cavitation erosion behavior of the laser cladding layer and the CrNiMo stainless steel was investigated by ultrasonic vibration cavitation erosion equipment The results shown that laser cladding layer has dense microstructure and has metallurgical combination with CrNiMo stainless steel substrate. Because of impact effect of cavities collapse, micro-cracks caused material broke off from samples. NiCrSiB laser cladding layer have better micro-crack propagate resistance property than CrNiMo stainless steel.Laser cladding layer has better cavitation resistance properties because of strengthening effect by precipitated phases such as M23[CB]6、CrB、CrSi、Fe2B、B4C and work hardening effect during cavitation erosion process.