Choosing Abrasives for Surface Preparation of Metals
One of the critical construction and maintenance challenges faced by many industries globally is corrosion. The global cost of corrosion is estimated to be US $2.5 trillion, according to NACE International1 — equivalent to 3.4 percent of the global gross domestic product (GDP) in 2013. These costs do not include work safety and environmental consequences.
In the energy sector, thousands of industrial facilities are constructed primarily of carbon steel. These structures, when unprotected, are vulnerable to corrosion. The application of industrial protective coatings on the surface is generally considered the most economical and effective corrosion protection solution available for carbon steel.
Prior to this procedure, the surface needs to be cleaned and free from contaminants, with the right surface profile (roughness) for effective paint adhesion. This step is the most expensive part of the coating job — crucial to preventing coatings failure and to maximizing the life of the coating system.
Surface preparation is a crucial step to prevent coatings failure and to maximize the life of the coating system.
Although there are multiple surface preparation methods, such as pickling (acid etch), sanding and wire brushing, the most effective method is abrasive blasting.
Abrasive blasting is preferred for several reasons. It effectively:
The next key factor to look at is the blasting medium. A wide variety of options are available in the market, ranging from soft abrasives like baking soda to hard abrasives such as garnet. Selecting the right abrasive for the job should not be based solely on price per ton or habitual practices. In addition to achieving the highest productivity rates, the main criteria should include achieving the correct anchor profile and required surface cleanliness to ensure effective adhesion and long-lasting coating life.
The overall cost per square foot cleaned should also be a prime consideration — not just price per ton of abrasive. Cheap, inferior abrasives generally require more tons per unit area cleaned, more clean-up and disposal handling, and they often yield more dust. Other considerations that are equally important are safety risks of the workers on site and the environment.
In order to compare different blasting abrasives, it is important to understand their physical properties and how they impact blasting performance.
Larger particles will produce a bigger indentation; however, they produce less impact per square foot than the same volume of smaller particles, which work faster. Smaller particles also produce a more uniform surface profile and a cleaner surface. In addition, smaller particles can increase the number of peaks produced per square inch of surface, which can create more surface to adhere to than a deeper surface profile. Therefore, the most efficient approach is to use the smallest particle necessary to achieve the required anchor profile.
Abrasives are classified in three different shape types: rounded, subangular and angular. Rounder particles without cutting edges will pound or ‘peen’ a surface, while sharp particles with points and edges remove surface material on impact. Both angular and subangular abrasives will create angular profiles. As for subangular particles, they present a larger surface area for contact with the surface while still maintaining sufficient angularity for cutting. Moreover, subangular particles are more resilient to breaking down than angular particles and less likely to cause impingement (abrasive splinters stuck in the metal surface).
A harder particle will generally be more aggressive in cleaning the surface and imparting a deeper profile; however, if the particle is friable (the tendency of a solid substance to break into smaller pieces) and shatters on impact, the force on the surface will be reduced. A good example is diamond, which is extremely hard but also brittle.
Abrasives that are both hard and tough (as opposed to friable) provide the best means of transferring energy to the surface during blast cleaning. Tough grains are resistant to breakdown upon surface impact and, hence, there is better conversion of energy from the blast stream into surface cleaning and profile formation. Minimal breakdown also means lower dusting. This improves operator visibility and reduces possible occupational health and environmental impacts.
The higher the density (specific gravity) of an abrasive particle, the more energy it will carry to the surface, compared to a less dense particle of the same size. This is illustrated by the equation e=mv2 — whereby energy equals mass (determined by density) times velocity (provided by air stream) squared — and explains why the finer grades of higher-density abrasives can achieve the same surface profile as coarser grades of lower-density abrasives. The advantage to using finer abrasives is they have exponentially more particle impacts per unit area per second, cleaning faster than coarser abrasives.
In addition, higher-density particles are more likely to fall to the ground after blasting, rather than becoming airborne, so there is less dust and abrasive dispersion, making it is easier to clean up.
Most natural mineral abrasives are inert and pose little if any OHS risk. A notable exception is silica sand (quartz). Free silica dust, including that from shattered silica sand abrasive, is classified as a human carcinogen2 which can cause lung silicosis when inhaled. Heavy metals released from some smelter slag abrasives are also a known environmental hazard, particularly in waterways and marine environments. There are several human health risks associated with over-exposure to heavy metals. Projects using these types of abrasives require more environmental and containment considerations.
Choosing the right abrasive helps maximize productivity and ensures good surface quality to achieve a coating application that lasts. The best results can be obtained by considering the following points:
Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors. Editor’s Note: The equation e=mv2 first appeared as e=mc2 and was revised March 9, 2017.