"The evaluation of Z.R.C. and hot dipped galvanized zinc films on steel substrates in a cycling environment of salt spray, humidity and freezing conditions, reported in the December 27th 1995 report (#951219) has been continued for an additional 50 cycles. Data after eighty cycles again shows little serious ferric corrosion on any of the eleven panels. Ferric corrosion is generally seen to be localized, although it is more widespread on the hot dipped galvanized panels bearing lower film thicknesses of zinc than on the Z.R.C. panels, or the high weight galvanized panels. Some slight blistering noted on the Z.R.C. panels is not detected on any of the galvanized panels.
Most obvious difference between the two sets of panels is the very heavy copious white zinc corrosion product that characterizes the response of the galvanized panels to the test. This is not seen on the Z.R.C. protected steel, where zinc corrosion remains very light.
This phenomenon is again related to the higher current flow and greater zinc availability in the galvanized films which results in more rapid consumption of the zinc anode. Performance differentials between the two types of coatings over the period of the test are still not yet very marked, and there has been little propagation of the deterioration noted after the original 30 cycles, although the lighter weight galvanizing films are showing more progressive ferric attack. It seems likely that as continued conversion occurs on the galvanized panels, complete anode consumption and loss of galvanic protection will occur rather more quickly on these panels than it will on the Z.R.C. protected panels. Less differential should be apparent in a less demanding (corrosive) environment." Clive H. Hare, Inc. May 14, 1996.
Table I: Evaluation of Z.R.C. and Hot Dipped Galvanized Films on Steel Using Salt Spray/ Humidity/Freezing Cycling Test Protocol Second Series 80 Cycle