日本金属学会誌

J. Japan Inst. Metals, Vol. 1, No. 6 (1937),
pp. 219-225

On the Protective Coatings of Magnesium Alloys Formed by Hot Acidic Solutions of Some Metallic Phosphates

Hikozo Endo1, Tsuneyuki Saito1

1東北帝大金屬材料研究所

Abstract:

A protective coating of some basic phosphates on a surface of magnesium alloys was prepared by the following process:-magnesium alloyed with manganese, zinc or iron was immersed in a mixed solution of a weak phosphoric acid and manganese, zinc or iron phosphate respectively and heated at 98_??_400°. When a coating of a basic phosphate was formed on the surface of specimen, the latter was washed in boiling water. The coating of these phosphates was more effective for magnesium alloys against corrosion of them by sodium chloride solution than that of magnesium phosphate formed by the process found by one of the writers 8 years ago, because the metallic phosphate, for example, manganese phosphate is firmly coated on manganese or on manganese oxide; this manganese remained on the surface of the specimen owing to the dissolution of magnesium or redeposited on the surface of the specimen. The specimens having the coating of metallic phosphate on their surface was then pressed and further coated with coal-tar by heating them in an autoclave. In this work many corrosion-resisting magnesium alloys were tested, and it was found that the coating of manganese phosphate was best, followed by zinc phosphate, iron phosphate in most cases being not so effective, except the case of PF(1) described below with the specimen in which the result was comparable as above two cases. The coating of manganese phosphate on the surface of alloys which contain manganese such as A, B AZG, L in Table 1 was more effective than other coatings, arid it was also confirmed that the manganese phosphate coating was fairly good if manganese content in alloy is more than 0.5%. Alloys containing aluminium 6_??_7% and a small quantity of other elements together with manganese were indifferent to form the coating, while in the case of alloys in which manganese content was below 0.2% and that of zinc 2_??_3%, the effect of the coatings formed by manganese and zinc phosphates were almost the same, for example, in the case of specimen C, manganese and aluminium contents were only 0.2% and 3.5% respectively and zinc was comparatively large; the coating of zinc phosphate was a little better or almost the same as manganese phosphate. In most cases the coating of iron phosphate was not so effective except the case of _??_ in which the baking of coal-tar in autoclave was carried out at 240° for 5 hours with a pressure of 70kg/cm2. An improvement of baking with coal-tar in third process was made in the following manner:-in order to absorb sufficient coal-tar in the phosphate coating, first of all, the coated specimen was placed in vacuum vessel and the air absorbed was expelled and then placed it in burning tar in autoclave or a solvent of coal-tar was next made to absorb in the coating and put in coal-tar with high pressure in autoclave. The result was found to be more effective.


(Received 1937/06/29 Published 1937/10/20)

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