Abstract
Authorities agree that the basis of successful coating and lining work is quality surface preparation. For the last fifty years, abrasive blast surface preparation has been considered the best method of achieving quality surface preparation for industrial coating and lining projects.
Until very recently, inspection and evaluation of abrasive blasted surfaces consisted of a macro, visual inspection only, and was time dependent. Standards developed by organizations such as NACE and The Society for Protective Coatings (SSPC)(1) were primarily visual inspection standards, observing and documenting removal of rust, mill scale and other contaminants, and production of a relatively uniform gray steel surface. Inspection was to be done without magnification, at a specified visual distance from the prepared surface, according to a written standard description, or according to companion comparison photos. Time dependence was a key factor, because in many instances the prepared surface degraded rapidly. The abrasive blasted surface would flash rust or "rust back" quickly, losing the required percentage of "white" (actually gray) steel surface.
Measurement of blast profile and of the number of peaks and valleys in a given area were added to better quantify the quality of abrasive blast preparation, but they still did not address potential non-visible contaminants left on a blast prepared surface.
Laboratory analysis and microscopic examination of abrasive blast prepared steel samples showed that abrasive blasting, even to the highest visual standards, left contaminants or sometimes actually deposited them from contaminants in the blasting media.
Field testing of abrasive blasted visually "white metal" steel surfaces on offshore structures in the Gulf of Mexico in the 1970’s consisted of licking the newly blasted steel and tasting salt. In the 1980’s, field tests such as Bresle were developed to confirm the presence of salt on "white metal" abrasive blasted steel surfaces. It was found that despite detergent washing and potable water rinsing of surfaces before abrasive blast preparation, and use of salt-free blasting media, salt tests of visual "white metal" surfaces still showed significant levels of contaminants, and measured levels of salt varied widely on different areas of the same prepared surface.
