Blast cleaning

Near White Blast Cleaning, Specification SSPC-SPIO Steel Stmctures Painting Council, Pittsburgh, Pa., 1963. 

Softer metals such as aluminum and its alloys can be blast cleaned using abrasives that are not as hard as those used on steel. Garnet, walnut shells, corncobs, peach pits, glass or plastic beads, and soHd carbon dioxide have been used successfully. 

Anchor Pattern (Surface Profile)—the shape and amplitude of the profile of blast-cleaned or grooved steel, which influences the bond between metallic or paint films and the substrate. 

Application of protective paints consists of surface preparation of steel, priming coat and finishing coats. Wherever possible, steel should be blast-cleaned before painting. Primers thoroughly wet the metal to promote adhesion of finishing paints and carry inhibitive pigments. For example, red lead oxide will minimize the spread of rust on metal surfaces. The total thickness of fmishing coats must be at least 0.125 mm for adequate protection and life. Four coats of paint usually are necessary to achieve this. 

J. L. Goodier, E. Boudreau, G. Coletta, and R. Lueas. Industrial Health and Safety Criteria for Abrasive Blast Cleaning Operations. HEW Publication no. (NIOSH) 75-122. NIOSH (. krthur D. Little), 1974. 

These materials are now widely used for coating both steel and concrete surfaces that are subject to a particularly aggressive environment (e.g. North Sea oil platforms). There is less validity for their use under normal atmospheric conditions since they are relatively expensive and tend to chalk on exposure to sunlight. However, their use as zinc phosphate, pretreatment or blast primers for blast-cleaned steel which is subsequently overcoated by any other paint system is an extremely valuable contribution to the painting of new steel work. 

Shrinkage on curing means that a high standard of visual cleanliness and an exceptionally coarse surface profile is required on blast-cleaned steel. Its main use is for tank linings. 

Dry abrasive blast cleaning should be used on new steelwork where the main contaminant is mill scale. For heavily rusted and pitted steelwork, increased durability can be obtained by the use of wet abrasive blasting where this is practicable. The water will be more effective in removing the potentially destructive and corrosive soluble iron-corrosion products that form at the bottom of corrosion pits. 

This should be carried out using a finer abrasive, lower pressure and a faster movement across the surface with the blast nozzle further from the work than for blast cleaning steel. It provides a roughened, irregular surface with the laitence removed and all holes and voids opened up so they can be more easily sealed. 

Measure, at regular intervals, the air temperature, steel temperature, relative humidity and dewpoint in the area where blast cleaning or painting is to take place. 

Decide whether, in the light of these measurements, the ambient conditions are within the specification and therefore whether blast cleaning or painting can proceed. 

Check that the freshly blast-cleaned surface is of the specific standard (e.g. Sa2, Sa3, etc.) at all points. 

Measure the surface profile of the freshly blast-cleaned surface and ensure that it is within specification. 

Ensure that the newly blast-cleaned surface is primed within the specified overcoating time. 

In some situations, it may be impossible for physical or legal reasons to use blast cleaning or to spray paint when maintenance is required. In such cases, systems that can be easily maintained or very long-life systems should be considered. 

Surface preparation is of prime importance, and optimum performance of modern protection coatings can be achieved only if the surface of the steel has been adequately treated. The method of surface preparation depends on the shape and size of the structure or component. Thus it is preferable to blast-clean an openwork steel structure by manual methods, since with this type of structure automatic blast cleaning would lead to excessive impingement of the abrasive on the machine itself. 

Low-alloy steels can be obtained as structural sections or in sheet form, and must be blast-cleaned to remove the millscale before exposure. Such material has been widely used in North America for highway bridges and for architectural purposes, and also to some extent in the UK and Europe. 

After suitable surface preparation, e.g. blast cleaning, low-alloy steels can be coated by paints, sprayed metal coatings, etc. and there is some evidence that such coatings last longer than on mild steel under similar conditions of exposure. ... 

The anode material must stay firmly attached to the steel insert, which is necessary to conduct the current from the anode to the structure, throughout its design life to remain effective. Consequently surface preparation (by dry blast cleaning ) of the insert prior to casting, to ensure a sound bond with the anode material, is essential. Voids at the insert/anode material interface are undesirable as these will also affect the bond integrity. 

Surface finish is increasingly referred to as surface cleanliness . This can be misleading because the standards refer to the appearance of the blasted steel and do not deal with chemical contamination. Site tests for assessing the level of soluble salts on freshly blast-cleaned surfaces, and which allow the semi-quantitative determination of the chlorides, soluble sulphates and soluble iron salts, are urgently needed. 

Blast-cleaning produces a roughened surface and the profile of that surface is important. The size and nature of the profile is largely determined by the type and size of the abrasive used. To identify and control surface roughness, comparators are available conforming to ISO 8503/1 specifica-... 

Very high standards of surface preparation are necessary, e.g. abrasive blast cleaning to Sa2y, BS7079 Part Al, 1989 for steel components... 

In BS 4232, Surface Finish of Blast Cleaned Steel for Painting, first quality corresponds to SA3,2nd to SA2-5 and 3rd toSA2of the much more extensive Swedish Standards Commission s SIS 055 900-1962, RusI Grades for Steel Surfaces and Preparation Grades Prior to Protective Coaling, Stockholm (1962) see also BS 7079 Part A1 (1989)... 

Note The above systems are for application to steel blast-cleaned to a near-while finish (Second Quality of BS 4232 1967) and immediately shop-primed before fabrication. The shop primer must be thoroughly cleaned and degreased at the lime of painting. 

Weld joints may be coated in the field with FBE powder, utilising a portable blast cleaning/induction heating and powder application system. Alternatively joints may be provided with self-adhesive laminate tapes or heat-shrink crosslinked polyethylene sleeves. 

Blast Cleaning the cleaning and roughening of a surface by the use of natural grit or artificial grit or fine metal shot (usually steel), which is projected on to a surface by compressed air or mechanical means. 

Figure 19.60 illustrates a steel surface which should have been blast cleaned to a high standard before being coated at works with a zinc-rich epoxy primer and on site with two-pack intermediate and finishing coats. After exposure for 18 months in a marine environment, flaking millscale from beneath the paint was observed, and a survey showed that the paint... 

Him thickness varied between 50 and 140/im. Some parts had not been primed. Although the remedial re-blast cleaning of all surfaces had to be carried out in-situ, and a suitable system applied, the resulting protection (Fig. 19.61) after exposure for 4 years was still of the high order expected. The only difference between the two contracts was that full-time specialist inspection was given at every stage of the site remedial work. 

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