Acryzinc Research

Comparison of ACRYZINC®
With Conventional Chromate-Treated Galvanize

Abstract
Three sets of galvanized steel with different chromate-based chemical post-treatments were evaluated to determine corrosion performance. The treatments evaluated were two conventionally applied chromic acid-based chemical treatments and a chromated acrylic coating that has been dubbed ACRYZINC®. These treatments were evaluated in salt spray, wet stack and complete water immersion tests. In all three tests, the acrylic coated ACRYZINC® significantly outperformed both conventional chromate passivation treatments.

INTRODUCTION
The corrosion performance of three types of chromate-treated galvanize coated sheet was evaluated in a series of accelerated tests. The testing included subjecting the treated panels to salt spray exposure, wet stack testing and total water immersion. The treatments evaluated were two conventionally applied chromic acid-based chemical treatments and a chromated acrylic coating known as ACRYZINC®.

PROCEDURE
Three chemically post-treated, hot-dip-galvanized materials were evaluated in this study. The samples were dubbed "G1," "G2" and "G3." G1 and G2 are two conventionally passivated commercially available chemical post-treatments used on hot-dip lines. G3 is the code for the ACRYZINC®, a chromated acrylic coating applied to the galvanized substrate at a nominal thickness of 1 micrometer containing 1 milligram per square foot of chromium.

For the salt spray test, the panels were sheared to 4 inches by 12 inches and tested with bare, unmasked edges. The panels were placed onto a plastic rack which held the panels about 20 degrees from vertical. The samples were placed on the rack equally spaced at about a 5-millimeter separation and then placed into the salt spray chamber. This environment created by the chamber envelopes the panels in a 5 percent sodium chloride salt fog. The panels were monitored daily for the five-day test duration.

Panels for the wet stack test were sheared to 4 inches by 6 inches and the edges very lightly deburred. In this test, wet panels are stacked on one another and bolted together in a bundle to simulate a tightly wound finished coil. Each of the panels in the stack were moistened by spraying approximately 3 milliliters of deionized water equally over each surface prior to stacking between the plates. These stacks were then placed in a 100 percent humidity and 100°F environment. The stack was initially wet on the first day and rewet on the third and fifth days of the test. Due to the amount of corrosion activity on the two conventionally treated materials, the panels were removed on the seventh day.

For the water immersion test, the panels were sheared to fit neatly inside Pyrex^TM glass loaf pans. This resulted in panels measuring 4 inches by 7-7/8 inches. The sloped edges of the glass pans kept the panel fully submerged at the midpoint of the pan depth and allowed full water contact on both faces of the panel. The pans were filled with 700 milliliters of deionized water and placed in a constant 100°F environment. The duration of this test, again due to the amount of activity on the conventional panels, lasted for two full weeks.

RESULTS
Panels were evaluated at various time intervals for surface appearance, edge activity and other cosmetic anomalies. The material tested in the salt spray chamber rapidly showed surface corrosion activity. In five days the surface buildup of corrosion products on the conventionally passivated material was so voluminous that the test was terminated. Due to the unmasked edges, the activity on all of the panels is greatest near the exposed upper edge as can be seen in the top photographs in Figures 1 through 3. These figures clearly show the ACRYZINC® outperforming the conventional chemical post-treatments. The shiny spangles can still be seen in most regions of the ACRYZINC®, while the conventionally treated materials were completely covered in white corrosion product. The bulk of the activity on the ACRYZINC® panels is due to the accelerated attack of coated products at uncoated edges.

Photographs of the panels after seven days in water immersion are given in Figures 1 through 3 below the salt spray results. As with the above-mentioned test, the conventionally passivated materials suffered voluminous white rust completely covering the surface while the ACRYZINC® still mainly exhibits shiny, clean spangled regions with small islands of localized attack.

The wet stack test was run for 14 days and again, the G1 and G2 panels suffered a great deal of attack while the ACRYZINC® was well protected. Figures 4 through 6 show both top and bottom surfaces of each material set after the wet stack test. In this test, the ACRYZINC® panels appear virtually unaffected. The G2 treated materials show slightly less attack than the G1 material, but both show much more activity than the ACRYZINC® material.

CONCLUSIONS
Bases on these three comparative corrosion tests, the ACRYZINC® coating is clearly superior in cosmetic corrosion resistance. The light attack of the highly active zinc coating under salt spray and water immersion is to be expected on a galvanized product, especially around the cut edges. The results of the wet stack test show superb resistance to attack compared with the conventional materials. This result alone is extremely encouraging since much of the white rust damage in the field is incurred due to improper storage of the coils or cut to length panels. These corrosion data prove that the acrylic coated galvanized, ACRYZINC®, material exhibits excellent corrosion resistance.

For Use Outside U.S. Steel by Designated Organizations Only
August 11, 1997