20-09-2016, 09:54 AM
1455248322-EROSIONANDCORROSIONBEHAVIOUROFTiN.docx (Size: 1.18 MB / Downloads: 5)
ABSTRACT
The erosion and corrosion resistant of TiN & AlCrN-coated austenitic 316L-stainless steel was studied. TiN & AlCrN films with a thickness of 4μm were obtained by using the pulsed-arc plasma-assisted physical vapour deposition technique. The erosion–corrosion experiments were performed in a test machine in which the impingement velocity, impact angle, concentration of solids and pH of the solution were controlled. Solid Particle Erosion test were carried out on the coated specimens at a velocity 30m/s, erodent discharge rate 5g/min, and testing duration of each specimen is 5minutes at angles of 60°, 75° & 90° degree by using ASTM-G76 Air Jet Erosion Tester. The corrosion resistance of the samples was studied using potentiostatic measurement of sea water environment 3.5%NaCl and at pH 3. Polarization curves were simultaneously obtained to relate the electrochemical effects to the erosive wear mechanism. And a best coating is chosen for the austenitic316L steel to increase resistance to corrosion, erosion & extended life.
1. INTRODUCTION
Austenitic stainless steels are used in many components where corrosion resistance is crucial, such as slurry handling in food and chemical industries. Nevertheless, under mechanical action of hard particles, those steels show high plastic deformation and wear. If a corrosive solution carries the particles, the surface damage due to corrosion increases as a consequence of synergistic mechanisms between corrosion and erosion. Austenitic steel exhibits excellent resistance to chloride attack and against complex sulphur compounds employed in the pulp and paper processing industries. The addition of 2% to 3% of molybdenum increases its resistance to pitting corrosion and improves its creep resistance at elevated temperatures. The low carbon content reduces the risk of inter-granular corrosion (Due to carbide precipitation) during welding, reducing the need for post weld annealing. Finally it displays good oxidation resistance at elevated temperatures. Titanium nitride (TiN) coatings are used as an alternative method to improve mechanical resistance in austenitic and corrosion resistance in martensitic stainless steels. As a result of high hardness, melting point and good chemical stability, TiN films can enhance the surface properties under wear and corrosion conditions. Aluminium and its alloys have been widely used as Corrosion Resistant materials due to their, excellent corrosion resistance, besides being largely used in the industries, such as, aeronautic, aerospace and automotive. Aluminium alloy (AlCrN) coatings are recommended for anti corrosion applications. Coating of these alloys to steel generally results in the increase in corrosion resistance.
The dependence between film properties and deposition conditions in PVD coatings has been extensively studied, and several authors have shown that substrate-to-film adherence, thickness and porosity are very important features. Many studies on erosion andcorrosion behavior have shown that TiN & AlCrN coatings can improve the corrosion response of a material if micro structural defects like porosity and pinholes are controlled.
The aim of this work was to study the interaction between erosion and corrosion mechanisms in TiN & AlCrN-coated Austenitic316L stainless steels, submitted to solid patricle impingement and corrosion–erosion conditions. The effects of impact velocity mean impact angle and particle addition to the flow are considered.
2.4 Surface properties tests
The Air Jet Erosion Tester is designed to impinge test samples with controlled flux of erodent. The solid particle erosion (SPE) test system involves repeated impacts of the erodent particles driven by pressurized air striking the surface of the test sample. The amount of mass loss of the tested sample divided by the mass of erodent can present the wear rate, or the volume loss can be also evaluated to rank the erosion resistance of full range of materials. The erodent particles (typically blocky Al2O3 particles) are usually used and fed using the air pressure and driven through a tube leading to the heating chamber connected to two heaters just before the nozzle exit. The result of the test is reported as the loss of weight of the sample. It may be converted to volume loss for comparative ranking of materials. Solid Particle Erosion test were carried out on the coated specimens of TiN & AlCrN at a velocity 30m/s, Erodent discharge rate 5g/min, Operating frequency 20Hz and testing duration of each specimen is 5minutes at angles of 60°, 75° & 90° degree by using ASTM-G76 Air Jet Erosion Tester.
PITTING CORROSION
Potentiostatic polarization test were carried out at sea water environment (3.5%NaCl) and at pH 3 using potentiostat with the start potential as -250mV , Reverse potential as 250mV and Sweep rate as 100mV/min.
3.2.1 Pitting Corrosion for AlCrN Coated Specimen Under 3.5% NaCl Solution In pH Value = 3
The corrosion protection performance of the AlCrN coating was studied by linear sweep voltammetry. The below fig presents the potentiostatic polarization curve of the AlCrN coated 316L stainless steel substrate recorded after 5mins. This coated sample shows an anodic behavior with limited current density in the range of +0.55V over its current corrosion, before the potential exceeds the breakdown potential. The behavior of the coating as reported.