Removal of surface dust after

blast-cleaning. All dust, residues and debris should be removed from the steel surface before the protective coating is applied. Dust reduces adhesion of paint coatings and encourages attack upon the steel by absorbing moisture.

Unless the dust can be automatically removed with vacuum hand-operated or centrifugal machines, separate vacuum cleaners should be used.

50 © BSI 11-1998

Table 5 — Methods of blast-cleaning

Methods Advantages Disadvantages

Dry methods using compressed air or centrifugal force

Automatic plants based on centrifugal throwing of the abrasive

High production rates, lowest costs, no moisture problems, can be coupled to automatic application of primer, dust problems contained

High capital cost, high maintenance cost, lack of flexibility, i.e. not suitable for recessed areas etc.

Open blasting

based on propelling the abrasive with compressed air

Simple to operate, very flexible and mobile in use in both indoor cabinets or special rooms or on site, low capital and maintenance costs

High cost of compressed air, low efficiency, liable to moisture entrainment from the compressed air, manually operated and a variable profile can result, operator requires protective clothing, serious dust problems

Vacuum-blasting based on propelling the abrasive with compressed air and immediately recycling by suction from the blast-cleaned surface

No dust problems, no special protective clothing for operators, fairly low capital costs

Can be very slow and therefore expensive on awkward profiles and girder sections. Where flat-plate or gun-head automation is possible it may be considered, but liable to moisture entrainment from the compressed air

Wet methods (hydroblasting) Open blasting

based on projecting water at very high pressure

Simple to operate, very flexible and mobile in use, suitable for removing soluble contaminants, at very high pressure can remove mill-scale, no dry dust hazards

Slow if firmly held contaminants are to be removed, dangerous at very high pressure if proper precautions are not taken, limitation of drying surface before painting unless approved water-based or water miscible primers are used, requires availability of water and drainage, operators require protective clothing

Open blasting

based on projecting water at high pressure and entraining abrasive into the water stream

Simple to operate, very flexible and mobile in use, suitable for removing all firmly held contaminants as well as soluble contaminants

Dangerous at very high pressure if proper precautions are not taken, limitation of drying surface before painting unless approved water-based or water-miscible primers are used, particulate dust hazard remains, requires availability of water and drainage, operators require protective clothing

Open blasting

based on injecting low pressure water into a compressed air stream which is carrying an abrasive

As above High cost of compressed air, low efficiency, limitation of drying surface before painting unless approved water-based or

water-miscible primers are used, dust hazard reduced, operators require protective clothing

Open blasting

using steam-cleaning Similar to the above according to whether abrasive is or is not entrained

Table 6 — Classification of abrasives used for cleaning steel

Abrasive Hardness Normal usage Advantages Disadvantages

Chilled iron-grit 60 to 80 RC Captive blasting and open

blasting with recovery systems

Relatively cheap, cleans very quickly, will chip under repeated impact with work surface,

presenting fresh cutting edges

Breaks down fairly quickly. In centrifugal wheel plants, special protection is required to reduce wear on moving parts

Chilled iron-shot 60 to 80 RC Captive blasting

only Relatively cheap, very hard, should break down to grit in use

As chilled iron-grit. Because of ricochet effect is not suitable for open blasting or in open

cabinets High-duty chilled iron-grit or iron-shot 55 to 64 RC Captive blasting and open blasting with recovery

Breaks down less quickly than chilled iron

More expensive than chilled iron, rendered spherical in use, poorer and slower rate of cleaning than chilled iron Heat-treated

chilled iron-grit or iron-shot

30 to 40 RC As high-duty As high-duty As high-duty

Steel grit 60 to 67 RC

47 to 53 RC Captive blasting mainly Does not break down so quickly as chilled iron, causes less wear in centrifugal wheel plant

More expensive than chilled iron, rendered spherical in use and is less efficient, supplied in various hardnesses but at best is not so hard as chilled iron-grit and therefore cleans more slowly

Steel shot 41 to 49 RC Captive blasting

only As for steel grit As for steel grit, tends to hammer-in rather than loosen scale, ricochet effect makes it unsuitable for open blasting Cut steel wire 41 to 52 RC Captive blasting

only As for steel shot and grit, wears down as fairly even sizes

High cost, rendered spherical in use and slower cleaning than chilled iron

Aluminous oxide

(corundum) Not common in the United Kingdom

Extremely hard Expensive, hardness of dust is a danger to machinery unless used in sealed captive blast plant

Copper slag Open blasting

only Cheap, no silicosis hazards Initial particles rather coarse, breaks down to dust very quickly, angular particles tend to embed in workpiece

Iron slag Open blasting

only As for copper slag As for copper slag

Sand Open blasting Cheap In United Kingdom, Factory Inspector’s approval is required, danger of silicosis

52 © BSI 11-1998

14.3.1.5 Standards of blast-cleaning. The four

qualities of blast-cleaning given in BS 7079-A1 are listed as follows:

NOTE 1 The equivalent of the three qualities Sa2, Sa2½ and Sa3 in the Swedish Standard SIS 05 59 00 have similar designations.

BS 7079-A1 should be referred to for the complete requirements for the preparation of steel

substrates.

NOTE 2 Until further Parts of BS 7079 are published the methods of measuring cleanliness given in appendices F and G may be used.

14.3.1.6 Surface profile. The method of assessment

of the abrasively blast-cleaned profile is given in BS 7079-C1 and BS 7079-C2 for qualities Sa2½ and Sa3. Three qualities of profile grades are given: fine, medium and coarse but for most protective coatings it is generally advantageous to have as small an amplitude as can be economically achieved. This helps the avoidance of “rust spotting” which can occur with rough surfaces, where the coating does not completely cover the peaks.

The profile size is largely governed by the type and size of abrasives and by the method of blasting. Fine abrasives clean more quickly and more thoroughly than coarse abrasives, except where it is necessary to crack very heavy mill-scale. In modern automatic plants for plates and sections, the scale can be cracked by steam-cleaning and heating before blast-cleaning.

Profiles needed for sprayed-metal coatings are specified in BS 2569 and tend to be of greater amplitude than those specified for paint coatings. Generally for paint the size of the abrasive particles should not exceed G 17 (see BS 2451).

Automatic plants give the most consistent profiles. For manual operation, angles near the vertical give lower profiles, particularly when large abrasives are used.

Instrumental methods of controlling preparation for painting are described in appendix F.

14.3.1.7 Surface quality of steel. Blast-cleaning is

most effective on steel that has not been allowed to rust.

Where steel has been allowed to rust badly, longer times for blast-cleaning may be required.

Sometimes 1st quality standards cannot be economically achieved. It is therefore advisable to blast-clean steel as soon as is practicable after rolling.

Steelwork should be sound and free from such segregation cracks, laminations or surface flaws as might preclude its satisfactory protection against corrosion, both initially and in service. Surface laminations, shelling, cracks, crevices, inclusions and surface flaws should be removed by chipping and/or grinding before painting or metal-coating. Burrs and sharp edges should be removed before painting. When excessive grinding has been necessary the dressed areas should be re-prepared to the necessary quality, including filling or welding as required.

14.3.2 Acid-pickling. Mill-scale and rust can be

removed by acid-pickling. A particular type of pickling, known as the “Duplex” or “Footner” process has a final treatment in hot 2 % phosphoric acid solution. This leaves a thin phosphate coating on a warm steel surface, to which the paint should be applied immediately. This method is not generally used outside the pipe industry, but large plates for storage tanks have been pickled in this way.

Generally, pickling is done by specialist firms.

14.3.3 Flame-cleaning. In flame-cleaning a high

temperature oxyacetylene flame is passed over the surface to be cleaned. The effect of the heat is to remove scale and rust, partly by differential expansion and partly by evolution of steam from moisture in the rust. After flame-cleaning the surface is wire brushed before painting.

The method may be useful for maintenance work, particularly in damp weather. The first coating should be applied while the surface is still warm and dry. The flame-cleaning of high-strength

friction-grip-bolted joints and the adjoining areas should be totally prohibited.

The method does not remove all rust and scale and is in no way a substitute for blast-cleaning.