The life of paint, powder and other coatings applied to the surface of steel depends largely on how the surface of the steel has been prepared initially.
It essential that all mill scale, rust and previous coatings have been totally removed prior to painting and coating so that the new coatings are able to adhere to the steel. Coating over anything other than a bare white metal surface will lead to premature failure of the coating - rectification will be expensive as the coatings will have to be removed, the steel surface prepared properly and new coatings applied. If the steel has to be dismantled and removed from site then the cost of rectification will be out of all proportion to the initial cost of the project.
Shot blasting will ensure that the steel is cleaned to bare white metal, SA2.5 / SA3, giving the best possible surface essential for the application of paint, powder and other coatings. The standard surface treatment quality is usually SA 2.5.
What is SA 2.5?
Shotblasting , Shot Blasting Standards & Guidelines
Shotblasting is a means of cleaning steel by blowing an abrasive media against the steel using compressed air, or mechanical means to propel the shot.
Shot Types
Historically, the material used for artificial sandblasting was sand that had been sieved to a uniform size, and hence the term 'sandblasting'. The silica dust produced in the sandblasting process caused silicosis after sustained inhalation of dust. Other materials for sandblasting have been developed to replace sand; for example, steel grit, steel shots, copper slag, glass beads (bead blasting), metal pellets, dry ice, garnet, powdered abrasives of various grades, powdered slag, and even ground coconut shells or corncobs, walnut shells, baking soda have been used for specific applications and produce distinct surface finishes.
Cleaning Steel
For coating applications, there are two main surface contaminants that need to be removed. Mill scale is the grey flakey oxide of iron that’s present on hot rolled steel. It only forms at high temperature in the hot rolling process, so is not present in cold rolled steel. For most coating processes this must be removed, otherwise the coating only adheres to the oxide, not to the steel, and adhesion is reliant on this poor bonding of steel and mill scale.
The other oxide is brown, commonly called rust, and must also be removed. The extent of rust depends on time and conditions of storage, and can represent even millimeters thick deposits. It all must come off.
Standards
ISO 8501-1 is the standard that covers blast cleaning, and it covers blast, hand flame and acid cleaning. The best of the methods is shot blasting and has become the industry standard with various grades as represented in the chart below.