6.6.1. Metallic abrasives.

Metallic abrasives are mainly used in facilities where there is an opportunity for recovering and re-using the abrasives (i.e. centrifugal blasting units). The metallic abrasives have three different shapes and are standardised in accordance to ISO 11124 (Specification) and ISO 11125 (Test methods).

Figure 6.4.

Shape of three main type of abrasives

The figure below illustrates the various blast profiles after blast-cleaning.

Figure 6.5

I llustration of blast profiles obtained by different abrasives.

6.6.2. Non-metallic abrasives. (Mineral abrasives, natural and synthetic)

In general, non-metallic abrasives are commonly used for compressed-air abrasive blast-cleaning. Most of them are cheaper than the metallic abrasives. The shape is angular (grit) and they are usually only used once (some can be used twice). In all, nine different abrasives are standardised in accordance with ISO 11126 (Specification) and ISO 11127 (Test methods).

Table 6.6.

Non-metallic abrasives.

Natural abrasives	Synthetic abrasives
Silica sand Olivine sand Stauorite Garnet	Iron furnace slag Copper refinery slag Nickel refinery slag Coal furnace slag Fused aluminium oxide

Due to health hazards, the use of silica containing abrasives is forbidden by law in many countries (especially silica sand may cause silicosis). Slags from metal production of copper, nickel etc. must be free from heavy metals (health hazard) and free from metal particles in the slag which can act as sources for corrosion on blast-cleaned steel.

On zinc, aluminium and stainless steel only non-metallic abrasives are to be used. The reason for this is that blast-cleaned surfaces will be contaminated with the abrasives. Practical experience and research work has shown that approximately 25% (some times even more) of the blast-cleaned surface is covered with grit stuck to the surface. If noble metallic abrasives are used the metals mentioned above may as a consequence suffer from localised corrosion attack.

6.7. Water jetting and hydro blasting

This method employs fresh water under extremely high pressure, from 300 to 3000 bars, and from 20 up to several hundred litres of water per minute. Water jetting is an effective way of removing both rust and old coatings, and will also give a degree of cleanliness similar to what is obtained with blast-cleaning. However, even at the highest pressures, there will be no additional surface roughness. The “anchor pattern” will remain as it was prior to the water jetting process. To be able to increase the roughness addition of abrasives is necessary. Such additions will improve the roughness of steel and old coatings. A high-pressure water blasting and the principle of sand injection are shown in the figures below.

A German standard, STG Richtlinie Nr. 2222, is available for determining the level of cleanliness following high-pressure water blasting. Several other companies have also produced their own guidelines. In near future it is expected that SSPC (Steel Structure Painting Council) will issue a new version of their old standard covering this topic. The new standard will include a comprehensive photo documentation.

Water jetting is a process by which water at a certain high pressure is forced through a nozzle with the purpose of cleaning a surface of old coating and rust. To obtain such cleaning, a nozzle pressure of min. 700 bar (10 000 psi) is required. For pressures lower than 700 bar, the process is termed “Water Cleaning”. The process as such has been known for some 20 years, but it was not until around 1990 that the technology had been developed to offer a reasonably attractive cleaning rate as compared to that of open nozzle blasting.

Definitions regarding pressure have also been proposed by SSPC and NACE (National Association of Corrosion Engineers):

1. Low-pressure water cleaning (LPWC)

Pressures: Less than 34 MPa (340 bar or 5,000 psi).

2. High-pressure water cleaning (HPWC)

Pressures: From 34 to 70 MPa (68 - 680 bar or 5,000 - 10,000 psi)

3. High-pressure water jetting (HPWJ)

Pressures: From 70 - 170 MPa (680 - 1,700 bar or 10,000 - 25,000 psi)

4. Ultra-high-pressure water jetting (UHPWJ)

Pressures: Above 170 MPa (1,700 bar or 25,000 psi)

When conducting a water jetting operation the following guideline is recommended:

• Use machinery and services from suppliers who have experience of similar types of work.

• Before work commences, make sure that each individual with responsibility for the job, or part of the job, has worked out clear procedures for the work. This refers to the equipment to be used.

• Water-jets or water/sand-jets must not have recoil forces higher than 25 kp when hand-operated equipment is used. If excessive forces are unavoidable, the operator’s safety must be ensured.

• The surface to be cleaned must be evaluated on the basis of the available equipment, including machinery, lighting, access, scaffolding, etc.

On the basis of the pressure and amount of water available to the water jetting system, as well as cleaning standard requirements, the surface may be treated using:

• Round-jet nozzle, which has a concentrated, penetrating, and splitting effect. On hard, thick and heavy coatings, this is the best method of breaking up the most tenacious coatings.

• Flat-jet nozzle, which is used for washing down with high rate of removal of material. The area covered by the nozzle is determined by its angle of spread, and this must match the pressure/quantity delivered by the pump. A properly designed nozzle can have a very good chipping effect.

• Rotating nozzle, which is chosen on the basis of obtaining the right rotation speed for the coating that is to be removed. Rotating nozzles have an enormous cleaning capacity when they are properly designed, and are capable of having a milling effect on the surface, even down to bare steel.