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Craig Woolhouse, Elcometer Limited
Edge Lane, Manchester, M43 6BU, United Kingdom
Contamination of blast cleaned steel surfaces prior to the application of a protective coating leads to premature coating failure and a high risk of corrosion of the steel. It has become common practice to include a salt contamination test prior to the application of the first coat to ensure that the required cleanliness has been achieved. One simple and quick method for carrying out this test is the Saturated Filter Paper Method for extracting the salts from the surface with a conductivity meter to assess the resulting sample to determine the concentration of the soluble salts. This method is described in SSPC Guide 15. Testing using this method has shown that soluble salts, particularly sodium chloride crystals, do not spread evenly over a surface and therefore the area tested and the number of tests in a given area are significant in determining if the blasted surface is in fact clean in respect of soluble salts. This paper will discuss the advantages inherent in the saturated paper method compared to other patch based extraction methods and describe how the average and local peak soluble salt contamination over a significant area can be determined. Reference will also be made to the possibility of developing a test surface doped with a known concentration of sodium chloride so that the accuracy of testing can be verified.
