ACRYLUME® Steel Sheet Performance Research Report

Comparison of ACRYLUME® Steel Sheet with Conventional Chromate-Treated GALVALUME® Steel Sheet

Abstract

Multiple panels of GALVALUME® (zinc-aluminum coated steel sheet) with three different chromated-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 given the tradename ACRYLUME®. These treatments were evaluated in a salt spray, complete water immersion and wet stack test. In all three tests, the acrylic coated ACRYLUME® steel sheet significantly outperformed both conventional chromate passivation treatments.

Evaluation of Post-Treatments

Introduction

The corrosion performance of three types of chromatic-treated GALVALUME® steel sheet was evaluated in a series of accelerated tests. Performance was studied by subjecting treated panels to salt spay exposure, total water immersion and wet stack testing. The treatments evaluated were two conventionally applied chromic acid-based chemical treatments and a chromated acrylic coating that has been given the tradename ACRYLUME®.

Procedure

Each of the materials was prepared similarly and in triplicate for each of the test procedures. Each test required different sized panels and different evaluation times and techniques. Prior to testing, the materials were analyzed to accurately determine the amount of chromium on both surfaces. This was done by using both non-destructive X-ray fluorescence (Porta-Spec) and wet chemical techniques (colorimetry). Table I shows that the ACRYLUME® steel sheet had significantly lower amount of surface chromium than CT2 and a slightly lower amount than CT1.

For the salt spray test, the panels were sheared to 4 inches by 12 inches and the edges were masked. These panels were placed onto a rack which maintained the samples about 20 degrees from vertical. The samples were equally spaced on the rack and then placed into the salt spray chamber. This chamber constantly maintains a 5 percent salt spray mist environment. Visual evaluation of the surface corrosion behavior of these panels occurred each day initially and ultimately three times weekly.

For the water immersion test, the panels were sheared to fit neatly inside Pyrex glass loaf pans. This resulted in panels measuring 4 inches by 7-7/8 inches. The sloped edges of the pans maintained the panel fully submerged at the midpoint of the pan depth therefore allowing 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. At 500 hour intervals the panels were evaluated and the water in the loaf pans was replaced. This test is designed to evaluate the amount of darkening that might occur on a flat roof that experiences water ponding.

The 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. Visual evaluation and rewetting of the panels in each of the stacks occurred every Monday, Wednesday and Friday.

Discussion

Panels were evaluated at various time intervals for surface appearance, edge activity and other cosmetic anomalies. After 30 days in the salt spray chamber, the CT1 and CT2 treated panels showed a great deal of activity and the test was terminated, however, the ACRYLUME® steel sheet panels showed very little corrosion activity.

Panels were exposed in the water immersion test for a total of 2,000 hours (83 days and 8 hours). The CT2 panels were completely covered with corrosion product after only 1,000 hours. At 1,500 hours, the CT1 panels had suffered uniform, light corrosion of the GALVALUME® steel sheet coating with spots of clean, metallic coating still evident. At the test conclusion, the ACRYLUME® steel sheet had exhibited some darkening and evidence of corrosion activity very near the edges (within 2 or 3 centimeters) and regions of whitening. The whitening appears to be due to water uptake by the acrylic coating since the underlying GALVALUME® steel sheet shows little signs of corrosion activity.

The wet stack test was terminated at 750 hours (31 days and 6 hours). Again, the CT1 and CT2 panels had suffered a great deal of attack while the ACRYLUME® steel sheet's acrylic coating protected the GALVALUME® steel sheet's surface from any corrosion activity ("white" or "black" rust). The ACRYLUME® steel sheet panels showed a slight amount of surface activity. However, this light white surface stain appears to be in the acrylic coating only while the GALVALUME® steel sheet's coating is unaffected.

Photographs of representative panels after the termination of each of the corrosion tests are attached.

Conclusions

Under all three corrosion test conditions, the ACRYLUME® steel sheet panels outperformed both the CT1 and CT2 panels. In the salt spray test, the ACRYLUME® steel sheet was superior followed by the CT1 and finally the CT2. However, the corrosion product on the CT1 samples was slightly more voluminous, powdery and white than the dark gray, matte finish corrosion on the CT2 samples. The results of the water immersion test after 200 hours indicated that CT2 is the poorest performer followed by CT1 which is only slightly better than the CT2 but not as good as the ACRYLUME® steel sheet which shows only light activity at the edges and mild whitening of the acrylic. Finally, in the wet stack test, the corrosion product on the CT2 was very dark black while the CT1 showed much lighter corrosion products. In this test, the ACRYLUME® steel sheets performed very well compared with the CT1 and CT2, both of which showed extensive attack of the GALVALUME® steel sheet's coating.

TABLE I
Chromate Film Weights

Chromate Film Weight (mg/ft2)
Material	Side	Porta-Spec	Wet Chemistry
CT2	top bottom	1.32 1.43	1.35 1.44
CT1	top bottom	0.98 1.05	1.05 1.10
ACRYLUME	top bottom	0.87 0.84	0.89*

Salt Spray Tested per ASTM B117 5% Sodium Chloride Solution Mist 30 Day Test Duration

Water Immersion Complete Immersion in Deionized Water 100°F and 100% Relative Humidity 2,000 Hour Test Duration

Wet Stack Test Cyclic Wetting with Deionized Water 100°F and 100% Relative Humidity 750 Hour Test Duration

USS® is a registered trademark of United States Steel
ACRYLUME® is a registered trademark of United States Steel
GALVALUME® is a registered trademark of BIEC International, Inc. and some of its licensed producers

United States Steel

March 4, 1996