New standard a disappointment; Time for a change of policy?

An extravagantly titled standard1 has been published covering a method for determining chloride ion on steel surfaces. This is not the first time that a standard of this type (and general title) has come into the public domain; our archives contain also the text of the 1992 edition of a British standard2 which was adopted by the ISO in 2000.

It has long been known that the chloride ion is a powerful influence on the performance of paint systems on steel. There is a need to quantify how much chloride ion is present at the interface between steel and paint, and the need for a standard such as this one is indisputable. In the 14 years since the original document was published, science has moved on considerably. It was therefore with a keen sense of anticipation that this new document was received.

Unfortunately, the modifications to the previous document which have been made in the new edition fall far short of fundamental. The technology on which the 1992 edition was based has apparently not changed so fundamentally that another edition should have been necessary, and searches must be made elsewhere for the rationale behind the new document. Perhaps the number of the document gives some clue; the 1992 document is a British standard, the 2000 version of this text is an international standard, and the new document is a European standard. It could be argued that maintaining standards in an up-to-date form is essential for the correct interpretation of science and technology; it is equally important to consider whether the information so imparted really justifies the re-publication of 18 pages of text at a price per page of about €5.

The one important fact which emerges from the revised text is the caution that the method (involving a volumetric determination of chloride ion by titration against mercury(II) nitrate using bromophenol blue/diphenylcarbazone mixed indicator in acid solution) is not applicable to surfaces treated with chromate, nitrite or amine. The inclusion of amine is new, and arises from the use of this class of inhibitor in wet-blast cleaning. It is stated that amine inhibitors can form a hydroxide boundary layer (not water-soluble) over the substrate and prevent the swabbing water contacting the underlying salt and removing it. However, the standard does not preclude the use of the method in the presence of other inhibitors which may operate through similar mechanisms. It might have been rather more helpful if the standard had something to say about other potential wet-blast corrosion inhibitors.

The only other change in technique occurs where the chloride-containing contaminant is removed from the test surface by swabbing with water. The 1992 edition required only that the swabbing process was continued for at least five minutes. The new standard increases the time to 'at least ten minutes'. However, the effect of this change is rather spoiled by noting that 'a more exact way of sampling is described in ISO 8502-6'.3 It is difficult to understand how, if a more exact method is current (and has been for the past five or so years), it wasn't called up in the text of what is supposed to be a referee method of analysis.

The remaining changes are cosmetic at best. 'Water' is no longer defined in the 'Normative references' section, but figures identically in the 'Reagents and Materials' section. The effect of this change is to renumber all the 'Reagents and Materials' section. 'Chalk' and 'absorbent cotton pads (cotton wool)' are no longer 'Apparatus and Materials' but have become 'Reagents and Materials', thus effecting a change in the numbering of the 'Apparatus' section. The preparation of the potassium chloride standard solution is part of the 'Reagents and Materials' section instead of being part of the 'Procedure' section, and the method outlined is exactly what young chemists of the 50s were taught not to do. "Chemistry is not playing shopkeepers," said their wise teachers, a maxim that saved much expensive laboratory time, and which apparently no longer applies. The required procedure appears now to be to weigh out a specified quantity of reagents correct to 1mg and dilute it to a litre of water - in the old days we used to weigh out a known quantity (correct to 0.1mg) of reagent, dilute it to a known volume, and calculate the normality (or should that be molality?) of the solution with a slide rule.

Similar remarks could apply to the section covering the preparation of the standard mercury(II) nitrate solution. This section is doubly confusing because not only is it not part of the 'Procedure' section, but it calls up a reagent (the mixed indicator solution) which the standard hasn't yet defined.

Conclusion

Had this standard been a first edition, many of the criticisms of the text could have been excused - after all, standards are written for the benefit of 'those skilled in the art', and most of us with pretentions to skill knew what the standard meant. However, it falls short of acceptable that infelicities of language and layout remain unaddressed 14 years after the original publication.

It would be helpful if those who earn their corn through the sale of standards could reconsider their policies in respect of standard reissues.

References

1. BS EN ISO 8502-2:2005 Preparation of steel substrates before application of paints and related products - tests for the assessment of surface cleanliness. Part 2 Laboratory determination of chloride on cleaned surfaces [Return ]

2. BS 7079 Part B2:1992 (ISO 8502-2:2000) Preparation of steel substrates before application of paints and related products - Methods for the assessment of surface cleanliness. Part B2 Method for the determination of chlorides on cleaned surfaces [Return ]

3. ISO 8502-6:2000 Preparation of steel substrates before application of paints and related products - tests for the assessment of surface cleanliness. Part 6: Extraction of soluble contaminants for analysis - the Bresle method[Return ]

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