The basic principle of this test is as follows: In atmospheres at nearly ambient temperature with a relative humidity of 75% having a small content of sulphur dioxide, easily visible, rust is formed at discontinuities in tin coatings on the steel surface without the coating itself being effected. The required conditions may be produced inside a closed vessel by adding a measured quantity of sulphuric acid to sodium thiosulphate (10 g/l). The indication given of the distribution of pores and of the presence of uncoated areas and defects outside the character of "normal porosity" is often useful and the method is applicable to articles of small sire and complex shape.
Equipment : A container made of glass or suitable transparent plastics material fitted with an airtight closure of sufficient size to permit the introduction of test specimens. A conventional 10 litre desiccator vessel is suitable but vessels of volume between 2 and 50 litres may be used. Supports and suspensions for test samples must be of inert non-metallic material. A suitable arrangement is to suspend specimens by glass hooks from a simple frame made of plastics material which stands on the base of the test vessel. During tests, the vessel is either placed within a room or other encased space maintained at RT or between 23-25oC or is provided with thermal insulation sufficient to prevent sudden fluctuations of temperature. The test result is not sensitive to temperature within the normal range of ambient temperatures but is affected by sudden fluctuations.
Areas of exposed steel appear as dark rust spots and the coating is assessed according to their number and distribution. A numerical grade may be used to indicate the level of porosity, e.g., Grading as described in IS0 3462 or the samples may be compared with reference material. Actual tested standards may be kept for some weeks and photographs may also be prepared.
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