The corrosion behaviour of three coating types (Al-Ni, 70 wt. % Al-Ni / 30 wt. % Al2O3 and 30 wt. % Al-Ni / 70 wt. % Al2O3) applied by plasma spray technique on carbon steel specimens has been studied in 3.5% sodium chloride solution. The coated specimens were tested under corrosion-only and wear-corrosion conditions and the extent of the corrosion attack was assessed by electrochemical techniques and observations by optical (OM) and scanning electron (SEM) microscopes. The presence of the intermetallic alloys AlNi3 and AlNi (in traces) was detected in the coatings by X-Ray Diffraction (XRD) analysis. Therefore, tests were also carried out on massive AlNi3 and AlNi specimens (prepared by fusion of stoichiometric amounts of the elements) and the results were compared to those obtained from the coatings themselves, sprayed on glass and from the coated steel specimens. It was found that the coatings suffered from active corrosion while the intermetallic alloys were noble passive materials, likely because the spraying process induced the solidification of a high fraction of random amorphous Al-Ni solid solution (undetectable by XRD) in the coatings, while only a limited amount of ordered intermetallic alloys was present. Short circuiting tests and SEM-Energy Dispersion X-Ray (SEM-EDX) analysis showed that the substrate was galvanically protected by the coatings themselves, particularly when they contained a high Al-Ni phase fraction.
CORROSION PERFORMANCES OF Al-Ni AND Al-Ni COMPOSITE COATINGS ON CARBON STEEL IN A SALINE ENVIRONMENT
MONTICELLI, Cecilia;ZUCCHI, Fabrizio;FRIGNANI, Alessandro
2003
Abstract
The corrosion behaviour of three coating types (Al-Ni, 70 wt. % Al-Ni / 30 wt. % Al2O3 and 30 wt. % Al-Ni / 70 wt. % Al2O3) applied by plasma spray technique on carbon steel specimens has been studied in 3.5% sodium chloride solution. The coated specimens were tested under corrosion-only and wear-corrosion conditions and the extent of the corrosion attack was assessed by electrochemical techniques and observations by optical (OM) and scanning electron (SEM) microscopes. The presence of the intermetallic alloys AlNi3 and AlNi (in traces) was detected in the coatings by X-Ray Diffraction (XRD) analysis. Therefore, tests were also carried out on massive AlNi3 and AlNi specimens (prepared by fusion of stoichiometric amounts of the elements) and the results were compared to those obtained from the coatings themselves, sprayed on glass and from the coated steel specimens. It was found that the coatings suffered from active corrosion while the intermetallic alloys were noble passive materials, likely because the spraying process induced the solidification of a high fraction of random amorphous Al-Ni solid solution (undetectable by XRD) in the coatings, while only a limited amount of ordered intermetallic alloys was present. Short circuiting tests and SEM-Energy Dispersion X-Ray (SEM-EDX) analysis showed that the substrate was galvanically protected by the coatings themselves, particularly when they contained a high Al-Ni phase fraction.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
