Effects of Waterjet Cleaning on Surface Preparation
Dr. Frenzel* presented a summarizing paper on the effects of waterjetting on the surface and surface preparation. Here are the highlights of her paper.
In waterjet cleaning, the jet has two effects on the surface: direct impact that is controlled by the velocity of the jet and shear that is controlled by the volume of the jet.
Direct impact causes erosion to break down the cohesion of the coating to be removed. Shear stress overcomes the adhesion forces. A higher velocity produced with a smaller orifice tends to erode the surface while a higher volume from a larger orifice tends to shear or hydraulically life the coating.
Compared to grit blast, waterjetting gives comparable adhesion values to the treated surface even though it does not create detectable profile and produces very smooth comparable surfaces. Experimental results indicated that waterjetting increased the wettability of the surface, which leads to a better contact between the coating and the bare steel and thus a better adhesion. Waterjetting can induce surface plastic deformation similar to shot peening and thus enhance the fatigue strength of treated parts by 20-30%. This indicates that waterjetting changes the surface energy, which may be also the explanation for the enhanced wettability.
Immediately after waterjetting, a light golden color was observed on the treated steel surface. One explanation is that ultrasonic cavitation from the droplets of the jet causes instantaneous chemical reaction and forms a thin layer of tightly adherent oxides or hydroxides, which resists new corrosion.
Waterjetting creates a micro-profile on the treated surface instead of a familiar “peak-to-valley” profile. The peak-to-peak spacing on these micro-profiles is about 20-50 microns and the peak-to-valley depth is 10-75 micron. On the contrary, the profile created by grit blast with 60 mesh crushed steel grits has a peak-to-peak spacing of 230 microns.
Waterjetting not only cleans the surface, but also attacks the pores of the microstructure and creates pits with undercuts. These pits with undercuts have excellent mechanical/adhesive qualities for coating. Experimental results showed that a higher stress (6,000 psi) was required to strip off thermal spray coating from a waterjetted surface, compared to 3,000 psi from a grit-blasted surface.
Waterjetting improves adhesion also by expanding the surface area and thus increasing number of potentially reactive sites, which allow molecular associations between the substrate and the paint. This was supported by a test that directly compared a coating over grit blasted and WJ surfaces.
* Frenzel, L. (2007) What effect does waterjet cleaning have on the surface and surface preparation? Proceedings of the 2007 American WJTA Conference and Expo, August 19-21, Houston, Texas, Paper 1-A.
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