Coil-coated zinc-alloy steel is a widely used commodity from white-goods manufacture to architectural claddings. Corrosion protection is generally provided to the substrate, and to any exposed cut edges, by a primer coating using an inhibitive pigme that, conventionally, consists of strontium chromate. However, chromate-free formulations containing phosphate-type pigments are now preferred. A common primer formulation contains strontium aluminium polyphosphate (SAPP) and, in previous work, it has been determined that the solubility of such inhibitors has two conflicting effects. On the one hand, free ions released from the inhibitor migrate to the metal substrate and reduce/inhibit corrosion by formation of a thin precipitated salt film containing zinc, phosphorous and oxygen. On the other hand, the dissolution of the inhibitor results in an increased porosity in the coatings, consequently creating more obvious pathways for aggressive species to reach the metal surface and for the transport of corrosion products. Further, insoluble corrosion products, or precipitated inhibitive salt films, in some cases appear to fill the cavities left by the dissolution of the SAPP inhibitor and, consequently, provide potentially improved barrier protection to the metal substrate.
In the present study the leaching behaviour of nano- to micron scale corrosion inhibitor pigment, of strontium aluminium polyphosphate from a polymer coating was followed over 14 days by 4D x-ray nano tomography. It is apparent that SAPP is not a homogeneous material and contains both strontium-rich components, which leach extensively, and aluminium-rich components, which are more resistant to dissolution. Through the investigation of individual pigment particles and their 3D spatial connections in the polymer matrix, the transport routes of the leaching particles in the polymer matrix may be explored.
The figure shows the evolution of an organic coating pigmented with SAPP corrosion inhibitor over the course of immersion in sodium chloride solution: (a) before immersion; (b) after immersion for 1 day; (c) after immersion for 7 days; (d) after immersion for 14 days. The oval regions highlight the dissolution of strontium-rich particles while the square region highlight the aluminium-rich components of the SAP.