SOP: Pickle Lag Test for Corrosion Resistance of Tinplate
GIFSQ — Global Institute for Food Safety & Quality
Principle
A piece of steel base or detinned plate goes into 6 N hydrochloric acid at 90°C, and the time until iron starts dissolving — or hydrogen starts evolving — at a steady rate is clocked. That opening stretch of sluggish attack is the induction (pickle lag) period. Short lag means good shelf-life; holding it to 10 seconds is a real benefit.
Procedure
In the case of tinplate specimens, remove the tin by dipping in concentrated HCl containing antimony trioxide as in Clarke’s method for measuring coating weight (if base plate is used, degrease)
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Immerse for 30 sec at 90°C in 10° NaOH (carried as printed) to which sufficient hydrogen peroxide is added periodically to maintain oxygen evolution — wear gloves; avoid finger marking of the test samples
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Test by one of the methods below
Apparatus and methods
Autographic pickling rate tester (obtainable from Thwing-Albert Instrument Co.): the instrument is activated by the hydrogen pressure developed from an 8 × 65 mm specimen dropped into a vessel of 125 ml capacity containing 95 ml of 6 N HCl maintained at 90°C; the instrument draws the rate-of-pickling curve automatically. Change the acid solution after every 10th test; use the same source of acid in comparative tests; control acid concentration and temperature rigidly; prevent contamination of acid by contact with rubber.
Alternative methods:
- Weigh a number of specimens, immerse for various time intervals in 6 N HCl at 90°C and reweigh (this method is not accurate and may not give a definite indication).
- Immerse a single specimen in 6 N HCl at 90°C and measure the hydrogen evolved by noting at intervals the pressure at constant volume or volume at constant pressure.
Interpretation of results
Plot hydrogen evolution against time. The curve opens with a short stretch — usually under 20 seconds — where iron dissolves or hydrogen evolves slowly and gathering pace, then climbs into a straight, steady-rate run. Project that straight run back to the time axis; the gap in seconds between the intercept and the origin is the Pickle Lag Value. The results may instead be expressed as the time elapsing before the steady state is reached.
Source note: the book prints “10° NaOH” — carried as printed; the meaning of the degree symbol is not defined in the source.
References
- Willey, A. R., J. L. Krickl & R. R. Hartwell, Corrosion, 12, 433 (1956).
- Hoare, W. E. & S. C. Britton, Tinplate Testing, p. 44.