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Robert J. Stango and Piyush Khullar

Introduction and Background
The development of innovative surface cleaning and surface preparation methods for refurbishing corroded surfaces is essential for safeguarding metallic components that are critical to our infrastructure. To this end, maintenance engineers are constantly searching for cost efficient and effective methods for removing corrosive layers and providing a fresh, receptive surface for newly applied paints and coatings.

Although many different tools and equipment have evolved for surface cleaning and restoration, the grit blasting process has emerged as the most common and widely used method for maintaining an aging, corrosion-prone infrastructure. Success of the grit blasting process can be attributed to its' inherent ability to simultaneously perform the following tasks, which are deemed necessary prior to the reapplication of protective coatings:

• Removal of aggregate foreign substances from the surface
• Exposure of an essentially contamination-free, fresh surface (i.e. exposure of base metal/substrate material)
• Creation of a surface morphology or anchor profile that will be receptive to subsequently applied coatings

However, grit blasting operations are inherently complex, and involve the use of equipment that is large, cumbersome and potentially at odds with both the user and the environment. That is, workers must be encapsulated in a specially enclosed suit that will ensure the flow of clean, breathable air that is free of spent media debris. The use of such equipment is confining and strenuous, and places considerable limitations on the ability of workers to function without taking frequent periods of rest. At the same time, spent media must often be recovered in order to satisfy stringent requirements that are enforced by various environmental protection agencies. Altogether, grit blasting is a costly and prohibitive process, and optional approaches for preparing surfaces are needed that can circumvent these problems, without compromising the quality of cleaned surfaces.

In this article, a newly developed surface preparation process termed bristle blasting is presented that can also satisfy all of the above tasks in a single step. Like grit blasting, the bristle blasting process is an impact/crater-formation based technique that repeatedly strikes the target surface with sufficient kinetic energy to remove contamination and expose a fresh, consistently textured surface. The process utilizes a dynamically tuned rotary wire bristle tool whose tips are both hardened and sharpened. Upon impacting the corroded surface, bristle tips immediately retract, thereby causing a repetition of craters that mimic indentations which are commonly associated with grit blast media. Performance of the bristle blasting tool is examined within the context of cleaning severely corroded API 5L piping, which is commonly used in the petroleum industry. The results obtained for surface cleanliness and texture are shown to be on an equal par when compared to traditional grit blasting processes. Unlike grit blasting, however, this new process uses a light-weight power-driven hand tool that only requires the use of nominal safety equipment such as safety glasses, dust mask, and work gloves.

Description and Use of the Bristle Blasting System
The equipment that is used for bristle blasting is shown in Figure 1, and consists of a hand-held power tool system having a main body, control handle, protective shroud, accelerator bar, dust vacuum, and rotating spindle, which operates at approximately 2,500 rpm. Although the pneumatic version of the bristle blasting system is shown in Figure 1,


Figure 1. Overall view of bristle blasting tool system.

an alternate version of the tool is also available that operates on a standard electric power outlet. At the core of the system is the wire bristle blasting tool, which is attached to the power tool spindle. As shown in Figure 2, the tool is comprised of steel wires whose tips are hardened (Rc = 65) in order to ensure efficient corrosion removal and longevity of service life. The bristles protrude through and are securely held by a polymeric/fiber-reinforced belt that is supported by a flexible plastic ring. Together, the plastic ring
Figure 2. Detailed view of bristle blasting tool components and assembly.

and wire-belt assembly are secured by a die-cast hub which, in turn, is fastened to the power tool spindle. Standard use of the tool is depicted in Figure 3(a), whereby a heavily oxidized layer is being removed from the steel surface. Typically, the rotating tool is placed in direct


Figure 3. Removal of corrosive layer via bristle blasting process (3(a)), and final corrosion-free surface obtained at conclusion of application (3(b)).

contact with the corroded surface, and light forces are applied by the user while moving the tool horizontally (i.e., from left-to-right) along the surface as shown. Continued use of the tool in this manner generates a series of cleaned horizontal rows, which leads to the final, corrosion-free surface shown in Figure 3(b).

Mechanical Principles of Operation
Recent experimental studies on the impact mechanics of rotary bristles have shown that, for a properly designed bristle geometry and synchronous rotational speed, collision of the bristle tip with a target surface is followed by an immediate rebound/retraction of the tip from the impact site (1, 2). That is, the collision results in a single/primary impact crater, similar to the micro-indentation that is characteristic of grit blast processes (3). The duration of this contact event occurs over an extremely short time interval, and a digital high-speed camera is needed to record and optimize the process. The collision sequence is illustrated in Figure 4, whereby 11 (eleven) successive frames have been superimposed in order to capture the complete contact event of a single bristle. One may observe that the incoming bristle approaches the surface (bristle is moving from left-to-right) in frames 1, 2, and 3, and undergoes impact with the metallic surface in frame 4. Subsequently, frame 5 indicates that the bristle tip has rebounded from the surface, and


Figure 4. Successive frames of a single bristle taken from high-speed digital camera depicting the approach (frames 1, 2, and 3), contact/collision (frame 4), subsequent retraction (frame 5), and return to equilibrium position (frames 6-11) of bristle.

has actually retracted toward the rearward direction (i.e., behind the initial impact site). Frames 6 - 11 show successive stages of the bristle motion as further recovery occurs, and the bristle eventually returns to an equilibrium position. The actual impact crater that is formed during contact with a ductile surface (API 5L piping) is shown in Figure 5, and indicates a micro-excavation termed "shoveling", which is similar to those generated by grit blast media (3).

Figure 5. Scanning Electron Microscope (SEM) micrograph of impact crater generated during bristle tip collision with ductile surface (API 5L) material system.

Corrosion Removal Performance and Texture
The corrosion removal performance and surface texture that can be achieved via bristle blasting operations is now examined within the context of removing severe corrosion from API 5L piping, which is commonly used for on-shore and off-shore applications in the petroleum industry. The initial surface of a severely corroded pipe that will be cleaned via the bristle blasting process is shown in Figure 6, whereby a uniform corrosive layer appears on both the internal and external surfaces. Examination


Figure 6. Corroded surface of API 5L pipe prior to cleaning via bristle blasting process.

of this corroded pipe suggests that the standard grade condition SSPC Condition D (100% rust with pits) accurately assesses the degree of surface corrosion.

In Figure 7 (top) the interior surface of a cleaned segment of the pipe is shown after bristle blasting, along with the initial corroded segment place directly below in Figure 7 (bottom) for comparison. Careful examination of Figure 7 (top) indicates that the cleaned


Figure 7. Photograph of cleaned API 5L specimen (top). Condition of initially corroded surface shown for comparative purposes (bottom).

surface has a uniform appearance and is free of corrosion. Detailed characteristics of the bristle blasted surface are shown in SEM micrographs appearing in Figure 8(a) and 8(b). A coarsely textured surface that is free of corrosion and corrosive pits is clearly seen in Figure 8(a) (magnification 20x), and evidence of uniformly dispersed craters formed by


Figure 8. SEM micrographs of the bristle blast treated surface shown in Figure 7 (top). Surface details shown in Figure (8a) at 20x; additional magnification of region indicated by the arrow is shown in Figure (8b) at 100x.

bristle tips is readily apparent. Higher magnification of the surface is shown in Figure 8(b) (100x) whereby individually formed craters can be seen that indicate repetitious impact of the bristle tips with the ductile material. Moreover, the craters closely resemble the morphology of the impact crater shown in Figure 5. Direct measurement of the texture appearing in Figure 7 (top) via standard press-film replica tape indicates that a uniform roughness of Rz - 80 (microns) is obtained.

As one may expect, continued use of the bristle blasting tool can lead to the eventual wear of bristle tips and, therefore, the eventual reduction of both corrosion-removal and texture-generating performance. In order to assess this aspect of tool performance, significant testing has been performed.

Visual Cleanliness
Based upon the surfaces prepared via bristle blasting (see Figure 3(b) and Figure 7 (top)), a direct comparison can be made with visual cleanliness standards that are published by the Steel Structures Painting Council (SSPC), and widely used by the trained workers in the surface preparation community. In each case, the surfaces far exceed the cleanliness that is associated with published norms for various hand and power tools (4). However, the appearance/cleanliness of these surfaces is quite comparable to SSPC "white metal" standards (that is, SP 5 and SP 10) that are typically associated with grit blasting processes (5).

References
1. Wojnar, N., 2006, Design and Application of Rotary Bristle Brush for Peening Applications, M.S. Thesis, Marquette University, Milwaukee, WI 53233.

2. Stango, R. J., and Khullar, P., 2008, Introduction to the Bristle Blasting Process for Simultaneous Corrosion Removal/Anchor Profile, ACA Journal of Corrosion and Materials 33 (5), 26-31.

3. Budinski, K. G., and Chin, H, 1983, Surface Alteration in Abrasive Blasting, Wear of Materials, 311-318.

4. SSPC-VIS 3, Visual Standard for Power- and Hand-Cleaned Steel, Steel Structures Painting Council, Pittsburgh, PA 15213-3724.

5. SSPC-VIS 1, Guide and Reference Photographs for Steel Surfaces Prepared by Dry Abrasive Blast Cleaning, Steel Structures Painting Council, Pittsburgh, PA 15213-3724.

Labels: bristle blasting, corrosion removal, Pipeline surface preparation and cleaning

posted by The Rogtec Team @ 17:13 

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