Pipes and other associated drilling equipment for the gas and oil industry need to be able to operate under challenging conditions, including abrasive wear, the flow of corrosive materials and demanding environments. Protection against these factors is provided by properly applied coatings, both external and internal, and these help to maintain optimal flow while extending the life of the equipment and the pipe.
Surface preparation has a vital role in ensuring the efficiency and effectiveness of the coating. Surface preparation aims to facilitate the adhesion of a coating on the equipment. The level of adhesion determines whether the coating can offer effective protection by becoming part of the substrate, or whether it is just a thin film lying on the surface. If the surface is not well prepared, the coating cannot adhere properly, and this often results in time-consuming and costly equipment failure.
Grit Blasting Overview
A common method of surface preparation for coatings that require both a high degree of surface cleanliness and an anchor pattern is grit blasting. Preparing the surface with grit blasting increases the adhesion strength of the coating as well as the roughness of the surfaces. It is also the only method that can remove intact rust and mill scale completely and produce an even roughness with a controlled anchor pattern.
Most grit blasting users aim for a ‘working mix’ of grit sizes. This means that they want a mixture of differently sized grit, rather than one single grit size. Since different things do different things to the profile, working the surface with different grit sizes maximizes blasting efficiencies. The working mix used is dependent on the unique profile that the user wants to create on the surface. One user may want to create a rougher profile, while another may require a smoother profile. Therefore, the grit size can be targeted to have a specific impact on the required profile.
A material that is commonly used in surface finishing is steel shot. This is because it is long-lasting. However, it does have some drawbacks. Most importantly, it can produce flash rust, and this is particularly true in moist conditions. Rust can corrode and cause the coating to fail if it is left on the surface when the coating is applied.
Over time, steel shot loses its shape and rounds out as it breaks down over time. This leads to inconsistent profile characteristics on the surface. The coating may suffer from poor adhesion if the profile is not uniform.
Although traditional grit blasting media like steel shot is still prevalent, the industry is increasingly looking to alternate materials, for example, aluminum oxide, for improved coating adhesion and performance. Fused brown aluminum oxide is an abrasive material that is highly resistant to corrosion and chemicals, as well as exhibiting superior abrasion properties (see Table 1). Typically, it is available in macro grit sizes, ranging from 12 to 240 grit. It is also low in iron and therefore does not leave behind impingements that can leave rust on the surface and potentially cause problems in the future (see Table 2).
Table 1. Typical physical properties
||Alpha alumina, in a hexagonal crystal
||Blocky with sharp edges
|Ball mill friability
||50 (14 grit)
||ANSI B74.12-2001, Table 3
|Bulk density (grain)
||ANSI B74.4-1992 (R2007)
Table 2. Typical chemical analysis
|Al2O3 (by difference)
According to a representative from a company that produces thermal spray coatings for the gas and oil industry, other materials cannot match the performance of aluminum oxide. “We’ve tried everything, and aluminum oxide works best,” he says. Aluminum oxide is “very hard and fairly inexpensive. On impact, we need it to create a rough profile – not just to clean like sandblasting, but to actually create little indentations in the surface.”
Quality is imperative, and, typically, a low-cost, low-quality aluminum oxide does not pay off in the long run. “We can buy material made in Brazil or China cheaper, but then we use more so it’s not cost-effective,” he explains. “It produces lots of dust, which is detrimental. It doesn’t have enough size or mass to create a good impact, so it just blows out in the air. Also, the hardness or robustness of the individual particles of [high-quality] aluminum oxide is superior to the others. The others, when they hit the pipe surface, they just tend to shatter, as opposed to embedding and ricocheting to create the best profile.”
Another benefit of aluminum oxide compared to steel shot and other grit blasting materials, is that it can be reclaimed and recycled back to the aluminum oxide manufacturer. Most large-scale blasting operations have an outdoor or indoor system that collects the spent grit. The only manufacturer of aluminum oxide that offers a completely closed-loop, waste-free spent aluminum oxide grit recycling system in North America is Washington Mills. Here, spent grit from the grit blasting operation is picked up and used in the furnaces to produce fused aluminum oxide that is the same as virgin material1. Aluminum oxide is a very attractive blasting material because it can be used in closed-loop recycling and this reduces liabilities and disposal costs.
A New Alternative
To protect the pipes and drilling equipment, the gas and oil industry depends on reliable coatings. If the surface is not prepared effectively, the coatings can suffer from poor adhesion. This leads to poor performance or even failure of the pipe/equipment.
Aluminum oxide is an ideal material for grit blasting materials because it is strong, corrosion-resistant and hardy. It can also be directly incorporated into coatings in order to improve their corrosion resistance and durability.
- McLeod, Don, “Recycling Spent Aluminum Oxide,” Ceramic Industry, October 2008, pp. 29-31.
This information has been sourced, reviewed and adapted from materials provided by Washington Mills.
For more information on this source, please visit Washington Mills.