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Shotblasting - Surface Profile And Consistency

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Shotblasting of steel plate, profiles, construction, and fabrications is a process to clean, descale, provide a specified surface profile and edge break as a surface preparation.  This process takes place prior to a coating or paint application to maximise the adherence potential and corrosion control.

This is a document made up of five parts  on  “How Do You Make Your Coatings Stick Better?”

Section 2 - Measurement Terms

Section 3 - How to Measure

Section 4 - Cleanliness and Surface Profile

Section 5 - Recognising Steel Processes (for hardness of steel)

 Section 1 - Specified Consistency

A surface with a consistent profile height will provide a key that has a far better coating adherence and performance rather than one that is inconsistent.

By providing and achieving a consistent profile height the shotblasted “keyed” surface is much more uniform.  It potentially enables the distribution of the wet coating more evenly and leads to less hairline cracking and improved adhesion.Steel_Plate

Various stresses in the drying paint film are created as the coating cures and can be created by a prior inconsistent keying of the surface. These paint /coating stresses can cause cracks to develop and allow premature multi-point failures to occur.                                               

Some specifiers require an angular profile, for example, steel grit rather than steel shot must be used for blast cleaning. This is certainly the case when applying a zinc or aluminium thermal spray coating.

Another important point is to achieve a high peak count, which is somewhat harder to attain than uniformity, in many cases.

This item intends to explain how to consider achieving this goal.

Typically proven by academics in the business, the optimum S-Beamconditions for a wide range of standard coatings that will completely wet the surfaces are a 65 ”m profile height and a peak distribution between 50 and 60 peaks per cm. These two independent mechanical properties converge to maximise the surface area, achieve optimum peak count and optimum peak height. The greater the surface area for the coating to bond to, the better!

More peaks provide more surface area. However, if there are too many peaks the valleys become too narrow for complete wetting/saturation.

As peak counts increase more, for standard industrial coatings, the valleys become too narrow.  Eventually the smallest of the pigment particles and limitations of the paint / coating viscosity will not allow the complete wetting / saturation of the surface.  Small air bubbles are likely to be trapped.  The particles get stuck part way down the valley walls and may bridge over causing a bubble and a point of future failure.

Coatings perform better on high peak count surfaces as long as they can wet the surface.