Popular wisdom holds that abrasive blasted steel cleaned to a near-white metal finish (NACE 2/SSPC-SP10) or better provides the optimum performance for coatings applied to the substrate. This is certainly true for inorganic silicate coatings such as zinc silicates where the silicic acid from the binder will covalently bond with metallic iron. For organic coatings, however, in particular epoxy-amine coatings, the mechanism of bonding with metallic iron, other than van der Waals forces, is not known, if it exists at all. On the other hand, the mechanism of epoxy-amine coatings binding to iron oxide is well known, and consists of a strong Lewis acid-base interaction which may result in enhanced coating performance properties. This may indicate that applying an epoxy-amine coating over clean (i.e. without soluble salts) rusty steel could provide better corrosion mitigating performance than that achieved by using clean, white metal blasted steel.
This study compares the accelerated corrosion performance of an alkylated phenolic polyamine cured epoxy (epoxy-phenalkamine) coating applied to abrasive blasted steel prepared to a NACE 1/SSPC-SP5 standard cleanliness and to substrates that have been allowed to rust in a variety of conditions, including varying soluble salt compositions and density. Performance properties evaluated included adhesion loss, scribe creep and cathodic delamination away from the scribe on the epoxy-phenalkamine coating. The mechanism of improved anti-corrosion performance of an epoxy-amine coating over rusty steel is examined in detail.
The abrasive blasting of steel prior to applying a coating serves several purposes including, removal of mill scale or previous coatings, removal of contaminants from the surface, and increasing the effective surface area of the substrate for interaction with an applied coating. An abrasive blasted substrate also, albeit to some small degree, may provide for some mechanical anchoring for the coating. It is also assumed that abrasive blasting of a surface will increase the bonding efficiency with an applied coating. In fact, it is conventional wisdom that the cleaner or ”whiter” the steel (as defined by organizations, such as NACE(1), SSPC(2), and ISO(3)), the better the performance the coating will impart to the steel substrate.