鉄焼結部品の亜鉛浸透とその耐候性に関する研究

抄録

Diffusion coating was conducted on sintered iron parts with pure zinc powder by packing method. Coatability and atmospheric corrosion resistance of the coated products were discussed. For obtaining the treated layer corresponding to the 3rd Grade of the Specification of Hot-Dipped Zinc Coatings (JIS H-8641), having the coating weight of more than 600g/m², the time of 1-1.5 hat 500°C would be enough for the treatment, which gave satisfactory results in accelerated corrosion test and exposure test.
The coatability test was conducted by dipping the specimen in oil and determining the weight increase by oil impregnation. The results showed that the pores of sintered body were perfectly sealed and corrosion resistance was excellent when the weight increase by oil impregnation was less than 200g/cm². The results of the present experiments were summarized as follows.
(1) When the temperature of treatment was higher than m.p. of zinc, the thickness of the treated layer was increased almost in a straight line with the time, but the deviation was also became larger. However, in the treatment at lower than m.p. of zinc, the relation of the thickness and the time was parabolic, and the deviation was comparatively smaller.
(2) The thickness of the treated layer of sintered iron parts was smaller than that of low carbon steel treated under the same conditions.
(3) The treatment of zinc diffusion coating did not depend upon the effects of furnace atmospheres. For example, no difference of the results was observed between the treatments under hydrogen and the air atmospheres.
(4) The relation between the weight increase and the thickness of treated layer by zinc diffusion coating was not expressed by a straight line. In case of sintered parts, it was supposed to be due to the impregnation of zinc into the pores. In case of the specimen of low carbon steel, the density of treated layer determined by the above curve was 4-5g/cm³ and the treated layer was porous.
(5) Some specimens which showed weight increase of 200g/cm² by coatability test gave approximately 10% of intercommunicating porosity in powder metallurgy, which was proved to be very successfully sealed.
(6) Atmospheric corrosion resistance of sintered iron parts treated with zinc diffusion coating was proved to be excellent by the results of salt spray test and indoor exposure test. For obtaining the treated layer corresponding to the 3rd Grade of the. Specification of Hot-Dipped Zinc Coatings, having the largest coating weight (more than 600g/m² in average), the treatment at 500°C for 1.5h would be enough. The coatability of these specimens was expressed by the weight increase of less than 100g/cm² by oil impregnation.