Basics of Surface Preparation

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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.

Table 1: How to Specify Blasting
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.

Table 2: Abrasive/Profile Comparative Chart
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 Grit G-80 G-50 G-40 G-40 G-25
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.

 

Table 3: Abrasive Consumption and Air Consumption at specified Pressure Nozzle and Orifice Size
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)
77.5 89 98 108 120 Sand (kg/h)
7 7.5 8.5 9.5 10 HP*
6.5 mm 1.51 1.71 1.9 2.08 2.27 Air (m3/m)
141 160 185 203 224 Sand (kg/h)
12 13.5 15 16.5 18 HP*
8 mm 2.5 2.83 3.16 3.53 3.84 Air (m3/m)
242 274 304 335 368 Sand (kg/h)
20 22.5 25.5 28 30.5 HP*
9.5 mm 3.53 4 4.5 4.85 5.5 Air (m3/m)
346 391 435 477 522 Sand (kg/h)
28 32 36 38.5 44 HP*
11 mm 4.76 5.44 6.09 6.73 7.11 Air (m3/m)
467 533 594 656 718 Sand (kg/h)
38 43.5 48.5 53.5 56.5 HP*
12.5 mm 6.28 7.06 7.85 8.65 9.46 Air (m3/m)
605 685 761 841 917 Sand (kg/h)
50 56 62.5 69 75 HP*
16 mm 9.95 11.3 12.6 14.1 15.35 Air (m3/m)
969 1097 1218 1347 1472 Sand (kg/h)
80 90 100 112 122 HP*
19 mm 14.1 16 18 19.4 21.95 Air (m3/m)
1384 1566 1740 1906 2087 Sand (kg/h)
112 127 143 154 175 HP*

* 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
Table 4: Examples of Abrasive Cleaning Rates
(Newly fabricated steel using a 9.5 mm I.D. orifice nozzle at 7 bar to a Sa 21⁄2 condition)
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)
Table 5: Examples of Cleaning Production Rates
(For a three-person crew day on lightly rusted steel, using 30/40 mesh medium hardness abrasive, 9.5 mm orifice nozzle at 5.6 bar)
Sa 3 90 4500
Sa 2 1/2 140 5600
Sa 2 230 3600
Sa 1 480 3000

 

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.