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 4 - Cleanliness and Surface Profile
The standard of cleanliness can vary between Sa 2.5 to Sa 3. However, when the initial surface to be blasted has a tenacious scale, or necessary to obtain an Sa 3 white metal finish, the surface of the metal can be overworked and the Rt / Rz um is ultimately reduced. Therefore, over blasting can reduce the optimum strength that can be obtained from the resulting “keyed” surface, to a point that it becomes weaker than intended, potentially allowing adhesion failures to occur.
Continued blasting over time will flatten more existing peaks, than will be created, resulting in lower peak density.
For a given cleanliness and surface profile to be achieved, the characteristics of the start condition and hardness of substrate require to be observed, recorded and qualified. All steels differ according to where they are sourced, manufactured and previously stored. The start condition can vary enormously, varying from a Grade “A” condition with a hard thin blued tenacious scale, to a much thicker black surface scale, through to B, C, and D grades.
Plate steel can be supplied in a wide range of hardness and characteristics depending on the specification of the steel. It can be hardened or softened according to the temperature and composition of the steel. In order to make steel harder, it must be heated to very high temperatures. The final result and exactly how hard the steel will become depends on the amount of carbon in the metal. Only steel that is high in carbon can be hardened and tempered. If a metal does not contain the necessary quantity of carbon, then its structure cannot be broken, and therefore the physical makeup of the steel cannot be altered.
The reason for knowing the hardness of the steel is to formulate the blast process requirement to obtain the optimum adherence characteristics. Often the COSHH sheets are provided by the paint or coatings manufacturer or supplier.
To understand the issues and finding the information it is necessary to explain some of the parameters required.
Plates and sections are produced by rolling steel slabs, blooms or billets (at a high temperature) until the required plate or section size is achieved. This rolling is the mechanical working that refines the grain structure and determines the mechanical properties. The more steel is rolled the stronger it becomes. This effect is all too apparent in material standards, which specifies a reducing level of minimum yield strengths with increasing material thickness. However, although rolling increases the strength it also reduces the ductility of the steel.