SOP: Determination of Tin Crystal Size by Acid Etching
GIFSQ — Global Institute for Food Safety & Quality
Principle
How a tin coating’s grain size sits relates to how it corrodes in some media — grain coarser than No. 9 (ASTM) is judged satisfactory. Grain is seen either under polarized light on the untouched surface or after a simple acid etch; the etching bath below brings the crystal pattern out clearly.
Reagents
a. Etching solution: dissolve 100 g of ferric chloride (FeCl₃) crystals and 0.5 to 1 g of sodium sulphide (Na₂S) in 1 litre of water.
Procedure
Immerse the test plate for 5 to 15 sec in the etching solution
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Wash in water and observe the crystal size
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Assess the grain size: for non-ferrous metals generally, grain is judged at 75× against standard charts; for tinplate the grains are compared by eye, unaided, with the reference charts. This is comparative practice within the tinplate industry rather than a standard in the strict sense.
References
- Hoare, W. E., E. S. Hedges & B. T. K. Barry, The Technology of Tinplate, p. 379.
- Barry, B. T. K. (personal communication).