Corrosion resistant alumina coatings by Fluidized Bed process [Rivestimenti anti-corrosivi di allumina depositati mediante letto fluido]

Alumina coatings have been deposited on AA 6082 T6 aluminium alloy (table 1) using a novel technique based on a Fluidized Bed (FB). Custom made FB system consisted of a fluidization column, 1.2 m in height with a square cross section of 400 mm made from stainless steel, a section for air flux homogenization and a porous plate distributor. The air feed was provided by a blower. A flow meter, a hygrometer and a thermocouple set were also used to monitor the process and to assure optimal environmental conditions (Q = 650 m3/h, T = 20°C, RH = 40%). Aluminum substrates were mounted on a rotating shaft (Rotating speed = 1 Hz). Four samples have been prepared with processing times ranging from thirty minutes to four hours (table 2) in order to understand the effect of process time on the tribological and corrosion resistant properties. Processing times were chosen on the basis of mass growth preliminary tests. The corrosion behaviour of the Al2O3 coatings was investigated in NaCl solution (3% and 0.5%). by means of Open Circuit Potential (O.C.P), Electrochemical Impedance Spectroscopy (EIS), Localized Electrochemical Impedance Spectroscopy (LEIS) and Electrochemical Noise Analysis (ENA). Blank corrosion tests were performed on uncoated substrate. FB process allowed to coat aluminium alloy substrates with a tough and well-adherent Al2O3 coating as evidenced by microindentation and scratch tests. Figure 1 shows OCP trend for samples 2 and 4. A 20 mV decreasing was detected after 6 days of immersion and it may be presumably connected to the onset of a corrosion attack. Very similar trends were detected for all samples. EIS spectra, evaluated by the use of equivalent circuits, evidenced an increasing of charge transfer resistance (Rct) with immersion time, as shown in figure 2 for sample 2. Figure 3 reports a comparison between the Rct trend of sample 2 and 4 that showed the best and the worst behaviour of all samples. Rct values for the blank test are also reported. The slight increase in Rct observed at the end of tests, is given by the occlusion of the pores by corrosion products. LEIS is a new electrochemical technique that, using a 5 electrodes configuration and a motorized probe, permits to acquire impedance maps of coating surface, able to reveal local degradation events. Figures 4 reports impedance maps of sample 3 at two different immersion times. Maps evidenced the presence of a defect on coating (Fig. 4a) and followed by the onset of a pit (Fig. 4b). The analysis of noise acquisition confirmed the findings of EIS and LEIS. Figures 5 report the potential standard deviation (a) trend of samples 1 and 4. Sample 1 showed strong fluctuations representative of a transient stage at the beginning of test. The further reduction of fluctuation amplitude indicates a good behavior of the sample. Sample 4 confirmed the poor corrosion resistance features. ENA permitted to discriminate three stages (Fig. 5b). The first stage takes place in the first 10 days of test and is due to the permeation of solution through pore coating. During the second stage, between 10th and 15th day the onset of a pitting attack occurred. The third stage, from 15th day until the end of test a pit propagation took place. Current Power Spectral Density (PSD) obtained by the Maximum Entropy Method (MEM) confirmed the occurrence of a pit attack (Fig. 6 e 7). FB allowed the preparation of tough and adherent Al2O3 coatings. EIS and LEIM demonstrated the best corrosion resistant performances were obtained with process time of 60 min (sample 2).