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INVESTIGATION OF THE CORROSION OXYDATION AND THERMAL STABILI TY OF NDFEB TYPE MAGNETS AND DEVELOPMENT OF IMPROVED MAGNET MATERIAL THROUGH ALLOYING SUBSTITUTIONS AND COATINGS

Objective

mAGNETS WILL BE PREPARED BY TWO DISTINCT PROCESSES OF POWDER METALLURGY SINTERING AND RAPID SOLIDIFICATION- BONDING. tHE PROJECT CAN BE CONSIDERED IN THREE PARTS: - sTUDY OF THE PROTECTIVE COATINGS AND ALLOY MODIFICATIONS, WHICH INCLUDES COATING MAGNETS WITH A THIN PRotective coating; coating the powder or ribbon with a protective layer; alloy modifications of the magnet and modified ribbon processing. - Corrosion behaviour studies and structural investigation of the magnets and coatings, which includes atmospheric and electrochemical tests and surface analysis studies. - Magnetic characterization and further structural studies. The technical advantages of solving the corrosion-oxidation problems for these types of magnet will lead to the wider application of magnets in devices and motors.
The project can be considered in 3 parts:
study of the protective coatings and process modifications, which includes coating magnets with a thin protective coating, coating the powder or ribbon with a protective layer and modified ribbon processing;
corrosion behaviour studies and structural investigation of the magnets and coatings, which includes atmospheric and electrochemical tests and surface analysis studies;
magnetic characterisation and further structural studies.
Zinc and zinc alloy electrodeposited coatings have been used, with a suitable cycle, to protect sintered magnets. Good results were obtained after salt spray and industrial atmosphere corrosion tests. An electrochemical method for corrosion behaviour assessment of magnets of this type, as such passivated or coated, is proposed. It is based on cathodic treatments and transient voltage response to double amperostatic square pulses. Encouraging results have been obtained for very thin inorganic coatings on MQ type powder which, when polymer bonded, led to a marked improvement in corrosion resistance compared with commercial MQI based magnets. Further development and evaluation are in progress.

A general mechanism for the corrosion behaviour of neodymium iron boride magnets has been established, related to the magnet heterogeneity with the presence of different phases. An electrochemical method for assessing the corrosion resistance of magnets, with and without coatings, has been developed. It is based on the study of the transient voltage at the magnet surface after a second cathodic current pulse in a suitable aggressive solution and its dependence on the amount of cathodic current circulated. Suitable pretreatments have been tried and interesting results have been obtained with passivation pretreatments giving neodymium phophorus at grain boundaries. Coatings on sintered and plastic magnets have been tried by several methods. Good results are obtained with zinc and cobalt layers on sintered magnets and a sol gel glass on powders for plastic magnets. Improved active chemical depostion (ACD) nickel phosphorus coatings have been realized with an alkaline brass flash plating to start ACD deposition.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Ingegneria C. Olivetti and C. SpA
Address
Via G. Jervis 77
10015 Ivrea
Italy

Participants (1)

Politecnico di Milano
Italy
Address
Via Mancinelli 7
20131 Milano