Blast cleaning

As well as being the only effective means of removing millscale blast cleaning is the quickest and often the most economical method of surface preparation.

Several levels of surface cleanliness can be produced depending on the rust grade of the steel the abrasive size and shape the velocity of the abrasive and the blasting time.

In addition to cleanliness another important aspect relating to blast cleaning is the surface profile. Factors affecting the profile are: type particle size and distribution velocity and shape of the abrasive. Distance and angle of contact are relevant also.

ISO 8503 Part 1 specifies a Profile Comparator which provides a visual comparison between the blast-cleaned surface. Gradings relate to fine medium and coarse profiles which are repeated for shot-blasted and grit blasted surfaces. In practice the three profiles should be specified as follows:

'Fine' for thin gauge metals or where conventional decorative systems are used.

'Medium' for high-performance coatings such as acrylated rubber paints.

'Coarse' for metal spraying or thick films of specialised coatings.

Blast-cleaning methods

Several methods of blast cleaning are in common use:

Mechanical blasting

Using in-situ plant mechanical blasting involves feeding the steelwork into an enclosed compartment where it is subjected to abrasive (usually steel shot because of the high velocities involved) directed at the surface via revolving 'wheels' (impellers) fitted with guide vanes. By adjusting the 'wheels' various shapes of metal section can be catered for. The abrasive detached millscale and corrosion products etc fall through a mesh floor and the abrasive is separated and recycled. To avoid contamination of the abrasive and the surface abrasive cleaning operations are very important. This method is mainly found where new steel is normally processed - in steelworks shipyards and major steel fabricators. Profiles produced by shot are more rounded and have a lower maximum average height than profiles produced by grit. Control specifications should quote the ISO Shot Profile Comparator.

Pneumatic blasting

With this method an abrasive is contained in a stream of high-pressure air (normally 100 psi). The process can be carried out in an enclosed compartment (known as a booth) or in the open air as the equipment is portable. Normally grit or angular abrasives are used. The ISO Grit Comparator should be specified for profile control.

Where an abrasive-recovery system is used steel grit can be used but this abrasive is too costly for open-air blasting where the abrasive is not recoverable. Cheaper grits are available for open-air blasting generally slags from metal smelting operations but they should be free from silica to meet Health and Safety Regulations. Although sand blasting is sometimes used for maintenance work open-air dry sand blasting is not permitted in the United Kingdom.

Wet blastings

Now widely used especially for maintenance work wet blasting takes various forms - compressed air water and abrasive. It is claimed that wet blasting is superior to dry blasting as far as the removal of soluble salts from contaminated surfaces is concerned. However visual control of the blast-cleaned surface is difficult due to rapid re-rusting. Although rust inhibitors can be used their selection and application must be controlled. One major problem is the removal of wet grit from the surface.