SOP: Ferricyanide Paper Test for Porosity of Tin Coating

GIFSQ — Global Institute for Food Safety & Quality

Principle

Pores expose bare steel; the iron ions there react with ferricyanide to form a blue complex — each blue spot marks a pore.

Reagents

a. Ferricyanide solution: prepare a solution containing 1 g of potassium ferricyanide, K₃Fe(CN)₆, and 0.5 g of sodium chloride in 100 ml of water.

b. For electrolytic tinplate only: add to the ferricyanide solution 0.5 g of Tergitol 08, which increases the rate of reaction and renders the paper more limp, securing better contact between the paper and the specimen.

Procedure

Cut samples of tinplate 8 cm × 5.5 cm; clean, degrease using carbon tetrachloride (CCl₄) and dry the samples

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For hot-dipped tinplate: moisten a filter paper strip of the same size as the tinplate sample with the ferricyanide solution and place it in contact with the tinplate for 1 hr, moistening from time to time — dampen the paper only; an over-wet paper lets the blue spots spread and merge until they cannot be counted

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For electrolytic tinplate: place the cleaned test plate on a sheet of glass; dip the filter paper strip in the solution, remove and dry for 5 sec; place the paper on the sheet; remove entrapped air bubbles by pressing gently with the fingers; cover with a sheet of blotting paper and squeeze by running a roller over the surface; remove the blotting paper and moisten the test paper with the ferricyanide solution until the surface is uniformly covered; leave in contact with the specimen for 10 min, moistening from time to time

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Remove the test paper from the specimen, wash in water until free from soluble salts and dry

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Count the blue spots and keep the paper as a permanent record of the porosity of the specimen — express results as number of blue spots per square decimetre (100 sq. cm.) of tinplate

Note

The plain ferricyanide procedure is suitable for hot-dipped tinplate but not for electrolytic tinplate, as the latter gives a uniform blue colour with the test paper — hence the modified procedure with Tergitol 08.

Reference

Macnaughton, D. J., S. G. Clarke & J. C. Prytherch, J. Iron Steel Inst., 12, 159 (1932).