[Haichuan Chemical Technology Documentation] Abrasive blasting surface treatment – Does the pipeline anti-corrosion coating industry truly understand this technology?

Pipeline Project Case 3 – During the installation of a three-layer polypropylene (3LPP) pipeline project in the Middle East from 2015 to 2016, visual inspections revealed coating delamination between the FBE primer and the steel substrate at the pipe ends (delamination at the pipe ends). During the problem assessment phase, induction coils are used to heat the pipeline above the softening point of the polypropylene (PP) adhesive, thereby facilitating the removal of the coating. In some areas, the FBE coating also peeled off simultaneously when the PP layer was removed, and surface corrosion marks could be seen on the exposed pipe base. The specifications for pipeline coating work on this project follow Shell’s standard DEP31403031-Gen, as well as the relevant standards set by the International Organization for Standardization (ISO) 21809-1 for external coatings on three-layer polyolefin (3LPO) pipelines. In the three aforementioned cases, the sintered epoxy powders (FBE) used were all products from internationally renowned brands, with decades of experience in practical applications in pipeline coating. Painted steel pipes are produced by internationally recognized top-tier steel manufacturers. The Process Qualification Test (PQT) for pipeline coating construction, as well as the production quality inspections, all passed successfully, meeting the requirements of the project’s technical specifications and relevant international standards. The test results were also approved by the experienced on-site personnel of the owner and third-party inspectors. However, the fundamental reason for coating peeling in these three projects is exactly the same: although the surface profile height required by the project’s painting specifications was achieved, the sharpness of the surface profile was insufficient, and the painting specifications did not provide a clear definition for this requirement (this will be discussed further in this article). This raises a question: Are the typical surface profile heights and abrasive blast cleaning requirements specified in current pipeline coating standards and codes adequately defined to ensure adhesion between the pipeline coating and the steel substrate? Table 1 provides an overview of the requirements regarding abrasives used for coating main pipelines, as specified in current ISO, API, NACE, and Shell DEP standards, as well as the requirements for surface profile. API 5L9 specifies that, in the qualification testing of FBE materials, steel grit (G40, with a Rockwell hardness of HRC 55 or higher) must be used to prepare laboratory test plates; however, it allows the coating contractor and the purchaser to agree to use steel balls and/or steel grit in actual coating applications. NACE SP0394, ISO 21809-1, and ISO 21809-2 also stipulate that the steel grit used in production must be compatible with the qualification testing of FBE materials, but they do not clarify whether only steel grit is allowed to be used in production or if steel shot can also be utilized. In addition, ISO 21809 or Shell DEP31403031 specify special abrasive requirements for high-strength steels (such as X80, X100, X120) or stainless steel pipes. None of the aforementioned standards/specifications impose mandatory requirements regarding whether recyclable abrasives may be used, nor do they specify how to ensure that the abrasive blasting equipment is in good condition and that the abrasive mixture is clean and suitable for use in pipeline projects. These requirements rely more on the pipeline coating contractors/operators, whose experience, skills, capabilities, and quality assurance standards vary widely. Shell DEP is the only specification that requires conductivity testing of the abrasive medium.

Regarding the requirements for surface cleanliness levels, almost all current pipeline coating standards/specifications require that the cleanliness level should be at least Sa 2½ as defined in ISO 8501-1 (very thorough sandblasting cleaning) to Sa 3 (sandblasting until a seemingly clean steel substrate is obtained), or correspond to preparation levels ranging from NACE No.2/SSPC-SP 10 (near-white metal grade) to NACE No.1/SSPC-SP 5 (white metal grade). To meet the specified surface cleanliness levels and profile requirements, the abrasives used for centrifugal sandblasting cleaning in pipeline painting plants must be clean, and their type, particle size, shape, and hardness must all be appropriate. A balanced and controllable ratio of particle size and shape is equally crucial. It is also necessary to determine the process scheme: whether to use a single shot blasting machine with an appropriate mixture of steel pellets/steel grit, or to employ two shot blasting machines, with the first one using only steel pellets and the second one using only steel grit. When steel balls and steel grit are used together, operators must exercise strict control and add the abrasives precisely in order to maintain a stable ratio between the two. Steel shot is in the form of spherical particles; due to its geometric shape, it has the highest mass per unit volume, which allows for the conversion of that mass into higher kinetic energy per unit weight. However, the surface treated with it becomes smoother (suitable for removing oxide scale, but not suitable for creating a rough anchor pattern). Steel grit has a stronger cutting effect than steel balls, enabling the creation of a rougher surface and sharper contours. It is worth noting that the pipeline coating industry no longer uses the term \"sharp-edged anchoring profile\" to describe or define the desired surface profile. The latest revisions of the aforementioned international standards seem to specify only the contour height between the peak and trough, but do not mention the sharpness of the contour edges. Standard NACE No.2/SSPC-SP 10 states in Appendix A: “If surface profile control (minimum/maximum values) is considered essential for the performance of the coating, it shall be specified in the coating procurement documents (project specifications).” The typical surface profile height achievable through industrial abrasive blasting is shown in Table 6 of SSPC-SP COM. “However, Appendix A contains non-mandatory explanatory notes for this standard; it does not constitute a requirement and is provided solely as a reference for good construction practices. A well-defined sandblasted profile is key to ensuring good adhesion of the FBE coating to steel pipes. Figure 2 shows the surfaces of two pipes that have been treated with abrasive blasting. The contour height values measured using the Testex replication tape method in accordance with ISO 8503-5 are identical, but the microstructural features differ significantly, with notable differences in the sharpness of the edges (for example, the number of contour peaks can be counted using a surface profilometer, or the contour roughness can be assessed using ISO roughness comparison plates). Obviously, if the pipeline project specifications follow only the height requirements outlined in any of the aforementioned international standards/specifications, both types of sandblasted surfaces will be able to pass the project process qualification tests and production quality inspections.

The industry has long been misled by misleading terms related to surface profile. Since abrasive blasting is one of the key steps in surface preparation before pipeline painting, this raises a question: are the existing painting standards and specifications sufficient to help us establish appropriate requirements for surface profile, thereby ensuring adhesion between the coating and the steel substrate and meeting the design life and performance requirements during construction and operation of the pipelines? Regrettably, the answer to the question of whether it is possible to resolve the coating delamination failure issues encountered recently in those three international pipeline projects located in North America, South Asia, and the Middle East is: no. Part of the reason for this problem is that the industry has been misled by misleading terms related to surface profiles. The determination of the surface profile depends on its definition: ISO 8503‑1 defines it as the height of the main peak relative to the main trough ； ASTM D7127 describes it as the positive and negative vertical deviations measured from the average line near the centerline of the profile being tested. ASTM D4417 does not currently define any profiles; it only describes three different measurement methods: A—profile comparison plate, B—height micrometer, C—transfer tape. To some extent, defining a surface profile as height or vertical deviation is misleading: it implies a simplified description of “surface profile characteristics” – according to the definition of “profile” in the Cambridge English Dictionary, the profile of a surface is merely its linear height of roughness. The industry has been misled by these misleading definitions/terms, thus ignoring the fact that a surface treated by sandblasting is a three-dimensional \"surface,\" rather than having linear height or length. A specific range of the vertical height of the surface profile is not the true determinant of the surface topography. As Chris Bates pointed out in 2004, for a high-quality surface profile, the key factor is \"surface area\" and its maximization, in order to achieve full contact between the coating and the substrate. Its purpose is to increase the “surface area,” thereby enhancing the apparent adhesion per unit area. Two-dimensional measurement values (such as Ra, Ry, Rz, etc., which represent the linear height of the surface profile or the number of peaks per unit length) can provide some reference, but they cannot reflect the actual condition of the surface. The profile formed by surface preparation using abrasive blasting relies on its anchoring effect on the coating not merely on an increased linear peak-to-valley height, but rather on an increased total surface area that can combine with the coating after it has been wetted. For coatings attached to a surface by mechanical, chemical, thermal, or composite bonding, the greater the (micro) surface area, the more bonding sites there are, and thus the better the adhesion of the coating to the substrate. To maximize the surface area, a well-defined substrate surface should be more similar to a file-like surface rather than a smooth one. Regardless of the height of the linear contour, arc-shaped or disc-shaped (non-angled) surface contours are not acceptable. Adhesion can also be defined as the process in which two objects in contact with each other and bonded together require an external force or thermal motion to break their bond. The strength of adhesion depends not only on the energy expended during contact formation but also on the interactions present at the contact interface. For the contact between a liquid coating and a solid substrate, this process is called wetting. Surface wettability can be defined as the tendency of a liquid to spread on a solid surface. When the surface energy of a liquid is lower than that of a solid, the liquid can wet the solid. Under the influence of gravity, a liquid applied to a solid surface will spread spontaneously until the cohesive force of the liquid, gravity, and capillary force (surface tension) reach equilibrium, resulting in a stable state. When the contact angle is greater than zero, the force balance or equilibrium state at the solid-liquid interface can be expressed by Young’s equation (see Figure 3).