Wash primers are used in many areas of industry as metal pretreatments prior to priming and topcoating. They customarily contain hexavalent chromium (Cr[VI]) as well as large amounts of organic solvents (e.g., isopropanol). There are no satisfactory chromate-free wash primers on the market with good adhesion to many primers or topcoat systems of interest to the Department of Defense. Further, the permissible exposure limit (PEL) for Cr(VI) is expected to be revised downward, increasing the need for environmentally safe wash primers.
The objective of this project was to obtain a new wash primer formulation that performs well on primarily cold-rolled steel (CRS) and also on aluminum alloys, that is completely devoid of Cr(VI), HAPs, phosphoric acid and has low volatile organic compound (VOC), without sacrificing corrosion protection performance under typical water-borne and solvent-borne primers and topcoats. Part of the objective was also that the replacement primer can be used without requiring a change to the existing infrastructure, i.e., it should be a drop-in process.
The superprimer (SP) technology, developed in SERDP project WP-1341, was selected as a basis from which a new wash primer could be developed. SPs are mixtures of silanes, water-dispersed resins, anti-corrosion pigments and other ingredients. They can be applied on bare substrates by means of dipping, brushing, spraying or rolling. They do not require a metal pretreatment such as a conversion coating (phosphate or chromate). Their adhesion to metallic substrates is good, due to the presence of the silane in the coating. Their adhesion to a primer or topcoat can also be good. They do not contain Cr(VI) pigments for corrosion protection of the metal substrate. Thus, the project focused on the reduction of the 25μm SP to an 8-10μm wash primer with equal performance.
A number of systems that perform well on the substrates CRS and aluminum alloy AA7075 (AA) have been developed. The preferred system is water-borne and consists of a metal compound, a prehydrolyzed silane and a water-dispersed epoxy resin. It forms a film that is considerably thinner than 8μm, is stable over a long period of time, contains zero VOC, zero HAP and no chromate. It performs well under waterborne and solvent-borne military primers and can rival the performance of the wash primers that meet the DoD-P-15328D specifications on CRS. On AA the performance exceeds the DoD-P-15328D specifications, because the new wash primer etches both CRS and AA, while the DoD-P-15328D wash primers can etch CRS but not AA.
The test protocols used in this project were:
- ASTM B-117 salt spray test; scribed; primed only
- GM9540P cyclic corrosion test (CCT); scribed; primed only
- Electrochemical Impedance Spectroscopy (EIS) of panels exposed in the CCT test; not scribed; primed only
- Exposure at the Battelle subtropical exposure site in Florida; not scribed; topcoated; still in progress
- Copper-Accelerated Acid Salt Spray Test (CASS, ASTM B368); for AA; scribed; primed only; in progress
- Humidity Test (ASTM 2247); not scribed; primed only; in progress
The new wash primer can be applied as a drop-in technology by spraying, dries quickly at room temperature, has very low VOC, provides good adhesion and corrosion resistance and is devoid of chromate. It can be used on several different metals such as CRS and AA7075-T6. It is a one-pack system which is stable for months. It can be applied with conventional painting equipment in conventional paint booths. It works well under several different primers, whether water-borne or solvent-borne. A new water-borne primer with improved properties over water-borne primers that meets the MIL-P-53030C specifications is also available. The new wash primer works effectively with that primer.