| 摘要 |
PROBLEM TO BE SOLVED: To provide a rust preventing method capable of efficiently executing treatment in the flow of a production process and capable of maintaining sound coating at least for one or two months even if a cast iron pipe subjected to zinc thermal spraying is stored outdoors. SOLUTION: Zinc thermal spraying is applied on the cast surface of a cast iron pipe and is thereafter coated with an aq. soln. of water glass whose pH is controlled to 9 to 11. When water is added to water glass, sodium hydroxide is formed as the following formula of mNa2O.nSiO2+(m+x)H2O→2mNaOH+ nSiO2.xH2O. Based on the principal that, when being brought into contact with an aq. soln. contg. NaOH, thin, dense and solid passive coating Zn(OH)2 is formed on the surface of zinc, the aq. soln. of water glass is pooled into the recessed parts in the surface of zinc thermal sprayed coating 1, passive coating 2 is formed with this part as the center, and, moreover, a colloidal silica 3 applied so as to smoothen the ruggedness in the surface of the passive coating 2 is formed, which protects the passive coating itself from the outside.
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