The quality of lead film coated on steel seems to very depending upon the treatment applied in hot dipping. Various experiments in this respect were carried out and the following result was obtained. (1) Lead coating can be obtained by hot dipping process also on pure iron or cast iron as well as on steel. (2) The thickness of lead coating depends on the surface roughness of specimens. The smoother the specimen is, the less is the thickness of coating, e. g., the amount of lead coating on the specimens polished with sandpaper or buff is 0.0091-0.0121mm thick, and 1.10-1.34gr/dm2. (3) Hydrochloric acid works more effectively for lead coating than nitric or sulphuric acid, and when zinc or tin is added, the amount of lead coating is much increased. (4) Tin chloride or ammonium chloride powder sprayed onto the specimen after hydrochloric acid etching works better for coating than it is used in molten or aqueous solution. (5) It becomes clear that ammonium chloride contributes as flux only to “primary coating” and tin chloride to both primary and especially secondary coating (cf. 4th report). (6) When lead powder is mixed to ammonium chloride by up to 30% or to tin chloride by up to 150%, it not only makes the coated film thicker, but also gives bright and smooth film on the specimen.
We investigated the corrosion resistance of aluminized steel, which had been diffusion heat-treated at 700-1000°C for 1/4-2 hours in atmosphere, in the solutions with pH value of 1-14 range at room temperature and 60°C. At the same time, we studied the effect of diffusion heat-treatment on tensile strength and repeated bending value of the steel. The thickness of diffused alloy layer grows remarkably with substantial increase of diffusion rate when the base metal is heated above Ac3, 900°C. The diffusion layer has almost nothing to do with heating time. The aluminized steels diffusion heat-treated at 700-800°C have lower tensile strength than that of the steel as hot-dip aluminum coated, and it is less than 52kg/mm2. The aluminized steels diffusion heat-treated at the temperature above 900°C become to have higher tensile strength than that of the steel as hot-dip aluminum coated due to the remarkable growth of hard and brittle diffused alloy layer. On the contrary, the repeated bending value of these steels decreases rapidly by diffusion heat-treatment at the temperature above 900°C. The corrosion resistance of aluminized steel in acidic, neutral and alkaline solution with pH value of 1-14 range at room temperature and 60°C can be much more improved by diffusion heat-treatment at high temperature, however, the embrittlement of these steels by the growth of diffused alloy layer should be kept in mind. For example, the diffused alloy layer of aluminized steel heat-treated at 1000°C is only attacked less than 10% in the thickness after 6 weeks immersion in the acidic and alkaline solutions with pH value of all range at room temperature, especially less than 2% in the thickness in the nearly neutral solutions. This diffused alloy layer is only attacked respectively 19% and 23% in the thickness after 24 hours immersion in the solutions with pH 1.25 and 13.6 at 60°C. Even if the diffusion heat-treatment is performed at the temperature of less than 850°C, the diffused alloy layer has remarkably superior corrosion resistance in the acidic and alkaline solutions than metal and alloy layer of steel as hot-dip aluminum coated not only at room temperature but even at 60°C.
Heat resisting tests of aluminized cast iron, in the air, coal gas, and coal gas with 10% by volume H2S, were carrid out at 800°C, 900°C and 1000°C, respectively. The results were as follows: Aluminized cast iron has generally better heat resistance than non-aluminized cast iron. It has poorer resistance in coal gas than in the air, the poorest being in the coal gas with H2S gas.
In the case of hot dip coating, the intermediate alloying layer which is formed between coating and base metal has an effect on the characteristics of the coated metal. When steel is aluminium-coated the intermediate alloying layer of iron-aluminium causes cracking and peeling of the coating because of its hardness and brittleness. The effect of copper addition up to 10 pct. in the aluminium bath has been determined to lighten the intermediate layer. This attempt to lighten the intermediate layer was for the purpose of improving the repeated bending property of the coated metals. The repeated bending test has been done on the dipped metals in aluninium-copper baths of different conditions, and a limit of fracture has been plotted for a copper content of each bath. The copper addition is effective on lightening the intermediate layer, but has little effect on improving the bending property of the coated metals in comparison to the aluminium bath without copper addition.