The objective of this demonstration was to determine the viability of non-hexavalent chromium sealers for zinc phosphate as a base for organic coatings. By itself, zinc phosphate does not provide adequate corrosion protection. It is a porous coating that requires a sealer to enhance the corrosion protection and provide a good surface for paint adhesion. The current post treatment for zinc phosphate is a chromic acid rinse with hexavalent chromium, a known carcinogen. A successful demonstration of a non-hexavalent chromium sealer will reduce and eventually eliminate the use of hexavalent chromium in phosphating processes resulting in reducing health and environmental risks associated with hexavalent chromium.
The alternative sealers tested and demonstrated, SurTec 580 and Chemseal 100, are considered drop-in replacements for the chromic acid rinse used for sealing zinc phosphate. The product demonstrated at Anniston Army Depot (ANAD) was SurTec 580. It is a trivalent chromium product manufactured by SurTec International (Trivalent Chromium Process [TCP] licensee). When a freshly phosphated surface is immersed in the SurTec 580, there are two reactions that take place. First, a localized chemical reaction at the metal/solution interface which forms a hydroxide. As the pH increases locally, chromium (III) hydroxide (Cr[OH]3) is formed. The actual application of both alternatives is similar to using chromic acid rinse, but with some advantages. In addition to being non-chromate, both processes are carried out at ambient temperature. This reduces the utility burden on the facility. No capital improvements or modifications to the existing phosphate lines are needed to accommodate either of the technologies tested. These products are commercially available.
Two non-chromate sealers for zinc phosphate; SurTec 580 and Chemseal 100, were laboratory evaluated. Based on the laboratory results, SurTec 580 was selected for demonstration on parts processed at ANAD. The M1 Abrams small fuel cap National Stock Number (NSN) 5340-01-460-5277 was the primary candidate part on the phosphate line used in the demonstration.
Some miscellaneous parts and test panels were also included for the purpose of evaluating at the Army Research Laboratory (ARL) laboratory. One chromate sealed fuel cap was tested against one SurTec 580 fuel cap at ARL. The adhesion strength for the SurTec 580 sealed cap was nearly identical to the chromate cap. Both measured within two pounds per square inch (psi) of each other (1672 psi and 1674 psi). Both fuel caps also performed well after completing 60 cycles of cyclic corrosion tests. Although the statistical sample is small, when all data is considered, it is evident that the SurTec 580 compared well to the chromate seal.
The synergy of this project and the revision of Federal Specification TT-C-490 has provided a pathway for the implementation of these and other new pretreatment technologies. All three pretreatments evaluated in WP-200906 met the minimum performance requirements of TT-C-490 Revision F and were assigned a Quality Products Database (QPD) number making them available to any Original Equipment Manufacturer (OEM), or Depot for use on abrasive blasted steel. This is especially useful on contracts issued that must be free of hexavalent chromium.