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Blast clean test papers
Article Type: Methods From: Anti-Corrosion Methods and Materials, Volume 55, Issue 2.
A visual examination enables you to judge whether any rust or mill scale remains on the surface of steel after blast cleaning. Unfortunately it is not possible to see whether blast cleaning has removed all traces of the water- soluble iron salts that form at the bottom of rust pits during the corrosion process. If these salts are present, they rapidly induce corrosion and this is evidenced by the formation of rust spots within a few hours.
The test papers enable you to check that all the iron salts have been removed by blast cleaning. The potassium ferricyanide test involves the reaction of soluble ferrous iron salts with potassium ferricyanide to form ferric hexacyano ferrate which is commonly known as prussian blue (Figure 1). The relatively high-tinting strength of prussian blue enables the reaction to be used as a very sensitive test for ferrous iron.
Figure 1 The potassium ferricyanide test involves the reaction of soluble ferrous iron salts with potassium ferricyanide to form prussian blue (ferric hexacyano ferrate)
A convenient way of conducting the test is to use paper impregnated with potassium ferricyanide. A thin film of distilled water is applied an area of the blast cleaned surface and when this has nearly all evaporated, the test paper is held against the surface for about 15s. Any soluble iron salts present are drawn out of the rust pits by capillary action and react with the potassium ferricyanide to form blue spots. The presence of blue spots on the test paper indicates that the surface should be re- blasted. Once the test proves negative, the test area alone should be re-blasted and the entire surface primed within the hour.
For further information, please contact PRA Coatings Technology Centre: Tel.:+44 (0)20 8487 0800