6.5.2.5.Wet abrasive blast-cleaning (Compressed-air wet abrasive blast-cleaning).

This method is similar to “Compressed-air abrasive blast-cleaning” (section 6.5.2.3.), but with the addition of liquid (generally clean, fresh water) before or after the nozzle to produce a stream comprising air, water and abrasive. The method is similar to “Moisture-injection abrasive blast-cleaning” (section 6.5.2.4.), but more water is used.

As long as presence of water is permitted this method is suitable for cleaning of all types work pieces (including large structures). It is also applicable to work pieces having different rust grades (see ISO 8501-1) and particularly for pitted and chemically contaminated steel. It may be used in both continuous and intermittent operations, particularly where cleaned surfaces are required to have low levels of residual soluble salts.

For surface preparation during maintenance, it is possible to partially or selectively remove existing coatings by adjusting the pressure and the proportions of air, water and abrasive in the mixture.

The blast-cleaned surface is usually covered with a slurry that impairs visual examination by the operator. This slurry must therefore be removed by dry blasting or water jetting. The surfaces will need to dry before application of paint, and “flash rusting” may occur.

A suitable rust inhibitor may be added to the water. The rust inhibitor shall be compatible with subsequent coatings.

6.5.2.6. Other methods

There are additional methods to those described above available on the market today. They will not be covered in this manual as they usually are used for smaller and more special jobs. This group comprise methods using Sponge, Dry-ice, Sodium bi-carbonate, glass, coconut shell etc.

6.6. Abrasives.

In some paint specifications a required surface profile (anchor pattern profile) will be given. To be able to meet this requirement the size of the abrasive particles must be carefully chosen (See table 6.1.). The surface cleanliness should be checked to see that it conforms to the specification (Table 6.2.). Furthermore, the blasting abrasives should be checked to ensure they are clean and free from dust, salt, oil and other contaminations (Ref. ISO 11125 and ISO 11127). The most common abrasives used in connection with abrasive blast-cleaning can be divided into two groups, e.g. metallic and non-metallic abrasives.

The production rates will be influenced by the type of abrasive selected (See table nos. 6.3. and 6.4.) and consumption of the abrasives and air will vary with the orifice size of the nozzle and pressure at the nozzle (Table 6.5.).

Table 6.1. Abrasive / Profile Comparative Chart

25 Microns Profile 30/60 mesh silica sand G-80 steel grit S-110 steel shot * 80 mesh garnet 100 grit aluminium oxide 20/40 mesh (or finer) coal slag 62,5 Microns Profile 8/35 mesh silica sand G-40 steel grit S-280 steel shot* 16 mesh garnet 24 grit aluminium oxide 10/40 mesh coal slag

37,5 Microns Profile 16/35 mesh silica sand G-50 steel grit S-170 steel shot * 36 mesh garnet 50 grit aluminium oxide 12/40 mesh coal slag 75/100 Microns Profile 8/20 mesh silica sand G-25 steel grit S-330 or 390 steel shot* 16 mesh garnet 10/40 mesh coal slag

50 Microns Profile 16/35 mesh silica sand G-40 steel grit S-230 steel shot* 36 mesh garnet 36 grit aluminium oxide 12/40 mesh coal slag