Before describing the difference between inorganic zinc-rich coatings and organic zinc-rich coatings, an explanation of galvanic protection is necessary. Zinc-rich coatings are used to protect steel substrates from corroding by employing some barrier protection but primarily sacrificial or cathodic protection. A corrosion cell contains four elements (oxygen is assumed to be present): anode, cathode, metallic pathway, and electrolyte. The anode is the part of the metal that corrodes or dissolves. As the anode dissolves, positively charged ions are released through the electrolyte (typically an aqueous solution that may or may not contain ionic contamination such as salts) and absorbed by the more noble region, also known as the cathode. Every metal has a corrosion potential; some corrode more easily than others. When these metals are placed in a list from most active to least active, this is known as a galvanic series. There are other metals that are anodic to steel aside from zinc, however their corrosion product encapsulates the steel, which reduces their sacrificial potential, relative to the steel. Zinc’s corrosion product is often a powder (e.g., zinc oxide) that easily exposes the underlying layer of zinc to continue the galvanic protection. This makes zinc the ideal candidate for creating an optimal protective primer.
SSPC Paint Specification No. 20 describes two types of zinc-rich coatings: Type I – Inorganic and Type II – Organic. There are three versions of Type I, based on the vehicle type, including Type 1A (inorganic post-cured); Type 1B (inorganic self-curing – water reducible) and Type 1C (inorganic self-curing – solvent reducible). It also lists three dust class levels, which is related to the amount of zinc in the dry film. These include Level 1 — equal to or greater than 85%; Level 2 — equal to or greater than 77% and less than 85%; and Level 3 — equal to or greater than 65% and less than 77%.
The type of zinc-rich coating is defined by the binder that is used during the formulation and manufacturing of the coating. Inorganic zinc-rich coatings generally consist of a silicate binder whereas organic zinc-rich coatings can use a wide array of binders including epoxy, polyurethane, and alkyd, etc. Because of the differences in the binders, the performance of inorganic zinc-rich primers versus organic zinc-rich primers varies.
Inorganic zinc-rich primers generally provide better galvanic protection than organic zinc-rich primers, due to the silicate binder reacting with the zinc particles as opposed to encapsulating the zinc particles, as is the case with organic zinc-rich coatings. Because of this reaction, inorganic coatings can be formulated with a higher volume of zinc. A higher concentration of zinc in the dried film will inevitably provide a longer life expectancy of the coating system. This is not to say that an inorganic zinc-rich coating can be applied to any surface and be expected to last. These primers perform best when applied to a Near-white blast cleaned surface (SSPC-SP 10/NACE 2); however, some can be applied to a Commercial blast (SSPC-SP 6/NACE 3) if the service environment is mild. Organic zinc-rich primers are easier to apply and are not as susceptible to dry spray or mudcracking. Inorganic zinc-rich primers are most commonly applied in the shop (they can be applied in the field by skilled applicators) and organic zinc-rich primers are most commonly applied in the field. However, many agencies are switching to all organic zinc-rich systems due to the challenges associated with application of the inorganic formulations.
Topcoating of inorganic zinc-rich primers can be challenging, since they require moisture to cure and are porous. Applying a topcoat to a dry but uncured inorganic zinc primer can result in a cohesive split of the zinc primer, caused by the contractive curing stresses created by the topcoat. Verification of adequate cure by performing a methyl ethyl ketone (MEK) resistance test according to ASTM D4752 can help prevent catastrophic failure. Since inorganic zincs are porous, they outgas when topcoated, so the application of a thin mist coat is often required to seal the surface. Organic zinc primers are substantially less porous, since the binder (epoxy, urethane, etc.) fills the pores created by the zinc loading.
Inorganic zinc-rich primers are typically repaired using organic zinc-rich primers, since the inorganic versions do not have strong adhesive binders and typically do not adhere well to one another once cured.
Kaley Stanczyk is a Project Manager/Chemical Technician in KTA’s Analytical Testing Laboratory. She is a NACE Level 2 Certified Coatings Inspector and a Past Chair of the NACE International Pittsburgh Chapter. She can be reached at firstname.lastname@example.org