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Passivation is a common treatment to maximize the corrosion resistance of stainless steel. Nitric acid is generally used and involves several ecological problems, citric acid could be a promising and environmentally friendly alternative to nitric acid. In this work citric acid has been extracted from lemon waste using and eco- friendly procedure. The stainless steel samples have been treated in both nitric and citric acid (commercial and extracted) and corrosion test have been performed. The results show how citric acid can be used as substitute of nitric acid in passivation treatment.

I materiali compositi a matrice metallica (MMC) sono materiali avanzati con elevate proprietà fisiche emeccaniche. In questo lavoro vengono studiati e messi a confronto il comportamento a corrosione di duetipologie di MMC con matrice in alluminio e rinforzo particellare in allumina. I dati ottenuti con le proveelettrochimiche, eseguite immergendo i provini in soluzione 3.5% NaCl, mostrano come la corrosione negliMMC sia influenzata sia dalla presenza delle particelle di allumina ma soprattutto dai composti intermetallicipresenti nella matrice che risultano essere i siti preferenziali di innesco della corrosione per pitting.

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).

In questo articolo vengono illustrati i risultati di prove di resistenza alla corrosione eseguite su campioni di alluminio AA 6082 T6 rivestiti con un film di Al2O3 depositato mediante letto fluido. Sono state preparate 4 serie di provini con diversi tempi di deposizione (tra 30 e 240 minuti). Le prove di corrosione sono state eseguite mediante Spettroscopia di Impedenza Elettrochimica (EIS), Rumore Elettrochimico (ENA) e Impedenza Elettrochimica Localizzata (LEIS). I risultati hanno evidenziato una migliore resistenza alla corrosione dei campioni trattati fino a 120 minuti, mentre prestazioni più scadenti sono state registrate per i campioni sottoposti a tempi di deposizione superiori.

The aim of this paper is the study of the influence of addition of Si, Ni and Co on the microstructure and mechanical feature of Is-ct gold alloys, in order to reduce production rejects in the investment cast process. For this reason, a typical alloy containing Ag, Cu and Zn usually used for jewelry manufacturing, has been used to prepare four master alloys with variable concentrations of Si, Co and Ni; 18-ct gold alloys were also prepared. It has been demonstrated that the increasing of Si concentration induces embrittlement in gold alloys. XPS analysis on fractured samples revealed the presence of SiO2 at the grain boundaries of samples containing 0.25 wt.% of Si. The addition of Ni and Co permitted to obtain gold alloys with good mechanical properties and surface finishing. It has been also demonstrated that Ni and Co form insoluble intermetallic compounds with Si that decrease SiO2 content at the grain boundaries thus preventing embrittlement. XRD analysis permitted to identify the compounds as Ni3Si and Co2Si. (C) 2001 Elsevier Science B.V. All rights reserved.