Abstract
High pressure (HP) and ultra high pressure (UHP) waterjetting for coating removal and surface preparation has several advantages compared to the industry standard abrasive grit blasting process. However, a major drawback of waterjetting is subsequent flash rusting of the steel surface. The color of the flash rust is typically orange, red, or brown of various shades depending on the color of the underlying steel, the nature of the surrounding environment, and the duration of exposure to the environment. Most paint specifications require reworking a flash-rusted surface to reveal bare steel. Additionally, most paint manufacturers will not accept liability if their products are applied over some grades of flash rust. Currently, there is no quantitative or semi-quantitative technique to characterize or categorize the level (or grade) of flash rust. However, descriptive and visual standards developed by SSPC and NACE are available. These standards are routinely used in the waterjetting industry but they are subjective in nature. Attempts have been made in the past or are presently being made by different entities to come up with a more definitive methodology but with limited success.
The present paper discusses the results of a research effort to develop quantitative or semi-quantitative method (or methods) to characterize the level of flash rust formed on carbon steel after HP or UHP waterjetting. The objective was to come up with a simple yet reliable and non-subjective method to identify the grade of flash rust. Several different techniques were evaluated which included: (i) Electrochemical Methods (Linear Polarization Resistance (LPR) tests, interfacial current/resistance measurement, galvanic current measurement and corrosion potential-time response tests), (ii) Colorimetric method, (iii) Modified Tape Test (ISO 8502-3), and (iv) Digital Imaging. Innovative approaches were adopted in using some of the methods. Among all the techniques used, the LPR tests, corrosion potential-time tests, and the colorimetric method showed the most promise. Preliminary guidelines are provided for using these techniques to characterize FR. As far as the authors are aware use of these techniques, particularly electrochemical methods, for characterizing flash rust are not reported in the open literature. More extensive validation is required to develop exact methodologies and protocols in order to formulate guidelines for quantitative characterization of flash rust. Further work is in progress.
