Preece Test Comparisons of ZRC and Hot Dip Galvanized Steel
July 18th 1994
Z.R.C. Products Company,
21 Newport Ave.,
Quincy, Massachusetts, 02171.
Attn: Mr. S. Collins
Re: Preece Test Comparisons of Z.R.C. and Hot Dip Galvanized Steel
On July 5th 1994, these laboratories conducted Preece Test evaluations according to the methodology outlined in ASTM A 239 - 73 (78) on two pieces of Hot Dipped Galvanized Steel and two samples of Z.R.C. coated steel for purposes of comparison. The Z.R.C coatings had been allowed to age for 2 months prior to testing, the galvanized steel pieces were of unknown age but several months old as a minimum. Film thickness values of each coupon was determined using a Tooke Paint Film Inspection Gauge and confirmed with a Magnetic Gauge. Samples were selected to represent typical low and high build film applications. The copper sulfate solution was applied to each film by pipette, allowed to remain in contact with the zinc surface for 1 minute, then removed with a cotton swab. The surface of the coupon in the test area was then scrubbed using a stiff bristle brush in distilled water. The procedure was repeated with successive immersions until the zinc coating was removed and a copper deposit of at least 1/8" was observed on the base steel substrate.
The Hot Dip Galvanized Metal pieces were found to bear 0.6 plus or minus 0.1 dry mils of zinc (on the thin coupon) and 4.5 plus or minus 0.25 dry mils of zinc (on the heavy coupon). The low film thickness Z.R.C. coating was 1 .5 plus or minus 0.1 dry mils, the high build film was 3.5 plus or minus 0.25 dry mils in thickness.
Zinc removal from the thin film Hot Dip coupon was very rapid, and the entire area of contact had been stripped of zinc after three immersions. The thicker Hot Dip coupon was more resistant and only showed a 1/8" spot after 20 immersions.
Zinc removal from both of the Z.R.C. films was much slower, and after 50 applications of copper sulfate solution the test was terminated without complete zinc removal in the area of immersion and without the deposition of copper on steel. At this time a cross-section of the contact area was examined microscopically. It was found that very little of the zinc had been removed at all.
The excellent resistance of the Z.R.C. film to the Preece Test compared to Hot Dip Galvanized Steel is most reasonably ascribed to the presence of the monomolecular sheath of polymer (coating binder) which encapsulates the zinc metal spheres in the Z.R.C. film. This acts as an electrical insulator, which controls the magnitude of current flow and preserves the anode in the corrosive environment, thereby prolonging the period of effective service of the Z.R.C. This same insulative layer also serves to inhibit, though not totally prevent the electrolytic reduction of copper sulfate by zinc metal and the consequent conversion of zinc metal to zinc sulfate.
From these results it would seem that the Z.R.C. films would remain resistant to the Preece Test for an extended number of immersion, being orders more resistant to removal than normal Hot Dip Galvanizing films.
Should you require additional interpretation of these data, feel free to contact these laboratories at our convenience.
CLIVE HARE, INC.
Clive H. Hare, President