As any coatings expert will tell you, good surface preparation is essential for adhesion and performance of protective coatings. Abrasive blasting is commonly used to prepare industrial and marine substrates. This article offers basic information about abrasive blasting standards, various abrasive media, consumption and cleaning rates, and other relevant information that may be of value to contractors and specifiers concerning surface preparation of steel.
Abrasive blasting usually is specified by reference to one of the standards on abrasive blasting. In Europe, this commonly is the Swedish SIS 055900 standard, which is parallel to ISO 8501. These, in turn, are comparable to standards developed by both SSPC and NACE in the US. Table 1 outlines the four degrees of blast cleaning found in each set of these standards and which degrees are comparable to one another.
A blast-cleaning specification will define not only the degree of surface cleanliness to be achieved but also the necessary degree of roughness or surface profile, which provides an anchor pattern for coating adhesion.
|Swedish* SIS 055900/ ISO 8501||SSPC||NACE||Description|
|Sa 3||SP 5, White Metal Blast Cleaning||1||Removal of all visible rust, mill scale, paint, and foreign matter by blast cleaning|
|Sa 2 1/2||SP 10, Near-White Blast Cleaning||2||Blast cleaning until at least 95% of surface is free of all visible rust, mill scale, paint, and foreign matter|
|Sa 2||SP 6, Commercial Blast Cleaning||3||Blast cleaning until at least 65% of surface is free of all visible residue|
|Sa 1||SP 7, Brush-Off Blast Cleaning||4||Blast cleaning of all except tightly adhered residues of mill scale, rust, and coating|
* Also SSPC-Vis 1 Standard
The minimum acceptable profile usually is specified. The profile achieved on a steel surface depends on a number variables, including the condition of steel before blasting (i.e., whether rust, mill scale, or a combination of rust and mill scale are present), the thickness of the steel, the setup of blasting equipment, the method of measuring the profile, and the type of abrasive media used.
Table 2 is a guide to the profiles typically obtained from a variety of common abrasives. A 25-micron profile, for instance, is a measure of the roughness of a blast-cleaned surface based on an average distance of 25 microns between its peaks and valleys. Abrasive suppliers should be consulted for information about the performance of specific types of abrasives.
|Abrasive||25 micron Profile||37.5 micron Profile||50 micron Profile||62.5 micron Profile||75-100 micron Profile|
|Silica Sand||30/60 mesh||16/35 mesh||16/35 mesh||8/35 mesh||8/20 mesh|
|Steel Shot*||S-110||S-170||S-230||S-280||S-330 or 390|
|Garnet||80 mesh||36 mesh||36 mesh||16 mesh||16 mesh|
|Aluminium Oxide||100 grit||50 grit||36 grit||24 grit||-|
|Coal Slag||20/40 mesh (or finer)||12/40 mesh||12/40 mesh||10/40 mesh||10/40 mesh|
* Steel shot alone will not give a good angular anchor pattern and should be used in combination with steel grit for best results.
Once the type of abrasive has been selected for a job, a contractor needs to know how much abrasive material is required and how quickly the specified profile can be achieved. This information is needed to calculate the time and costs involved in completing the project. The following tables illustrate various consumption and cleaning rates.
Table 3 shows how much abrasive (sand) and air are consumed based on different nozzle sizes at various common pressures used for blast cleaning.
|Orifice Size||4.2 bar||4.9 bar||5.6 bar||6.3 bar||7.0 bar||units|
|5 mm||0.84||0.92||1.06||1.15||1.26||Air (m3/m)|
|6.5 mm||1.51||1.71||1.9||2.08||2.27||Air (m3/m)|
|8 mm||2.5||2.83||3.16||3.53||3.84||Air (m3/m)|
|9.5 mm||3.53||4||4.5||4.85||5.5||Air (m3/m)|
|11 mm||4.76||5.44||6.09||6.73||7.11||Air (m3/m)|
|12.5 mm||6.28||7.06||7.85||8.65||9.46||Air (m3/m)|
|16 mm||9.95||11.3||12.6||14.1||15.35||Air (m3/m)|
|19 mm||14.1||16||18||19.4||21.95||Air (m3/m)|
* Electric motor horsepower required to produce indicated cubic metres per hour
Table 4 takes a different approach. It uses one specific nozzle size and pressure rate to compare how various common abrasive materials perform both in terms of their consumption and cleaning rates.
|Abrasive||Abrasive Consumption (kg/m2)||Production Rate (m2/hr)||Comments|
|Silica sand 16/40 mesh||12.7||26||35 micron profile; dusty|
|Crushed flint 12/30 mesh||17.6||15||75 micron profile|
|Staurolite 50/100 mesh||15.1||27||10-15 micron profile; smooth surface|
|Coal slag 16/40 mesh||15.1||21||60-70 micron profile|
|Copper slag 16/40 mesh||15.1||24||50 micron profile|
|Garnet 36 grit*||17.6||20||35 micron profile; very little dust|
|Aluminium oxide 36 grit*||15.1||25||35 micron profile; very little dust|
|G-40 steel grit*||26.8||17||60-70 micron profile; no dust|
*These abrasives can be reused
Finally, Table 5 looks at abrasive consumption and production from still another perspective. It compares how much abrasive is used and how much surface area is cleaned depending on the level of cleanliness specified, given a particular set of parameters.
|Method||Production Rate (m2)||Abrasive Used (kg)|
|Sa 2 1/2||140||5600|
These tables are only guides, of course. Actual consumption and production rates will vary depending on factors such as the source of the abrasive, the type of structure being blasted, the working conditions, and the experience of the blaster, but they provide an indication of what to expect.